Metallogr. Microstruct. Anal. (2014) 3:477–508 DOI 10.1007/s13632-014-0170-4
FEATURE
Nitriding of Stainless Steels Luiz Carlos Casteletti • Amadeu Lombardi Neto George E Totten
•
ASM International 2014
Because of their corrosion resistance, stainless steels are essential for the modern industrial civilization, especially in the chemical, petrochemical, and food industries. The stainless steel family has the following members: austenitic, ferritic, duplex, martensitic, and precipitationhardening alloys. The last two show higher levels of hardness. The others possess low hardness and consequently low wear resistance, mainly of the tribochemical type—especially the austenitic alloys, which are the most widely used. Among the possible solutions to this problem was the development of appropriate coatings. Thus, a research effort was undertaken with the goal of producing layers of high hardness on these steels to improve their tribological performance without degrading their corrosion resistance. The nitriding and carburizing treatments proved to be the most adequate to attain these properties.
or less than 50% Fe. They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms and heals itself in the presence of oxygen. Other elements added to improve particular characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulfur, and selenium. Carbon is normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic grades. The selection of stainless steels may be based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges, and product cost. However, corrosion resistance and mechanical properties are usually the most important factors in selecting a grade for a given application [1]. Classification of Stainless Steels [1]
Stainless Steels Stainless steels are iron-base alloys containing at least 10.5% Cr. Few stainless steels contain more than 30% Cr
Stainless steels are commonly divided into five groups: martensitic, ferritic, austenitic, duplex (ferritic-austenitic), and precipitation-hardening stainless steels. Martensitic Stainless Steels
This is a preview chapter from the recently published volume Heat Treating of Irons and Steels, Volume 4D, ASM Handbook, Jon Dossett and George Totten, editors. L. C. Casteletti (&) Sa˜o Carlos School of Engineering, University of Sa˜o Paulo, Sa˜o Carlos, SP, Brazil e-mail:
[email protected] A. L. Neto Federal Technological, University of Parana-Londrina, Londrina, PR, Brazil G. ETotten G.E. Totten & Associates, LLC, Seattle, WA, USA
Martensitic stainless steels are essentially alloys of chromium and carbon that possess a distorted body-centered cubic (bcc) crystal structure (martensitic) in the hardened condition. They are ferromagnetic, hardenable by heat treatments, and are generally resistant to corrosion only to relatively mild environments. Chromium content is generally in the range of 10.5–18%, and carbon content is usually in the range of 0.03–0.08 wt% but may exceed 1.2% in certain alloys. The chromium and carbon contents are balanced to ensure a martensitic structure after hardening. Excess carbides may be present to increase wear resistance or to
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maintain cutting edges, as in the case of knife blades. Elements such as niobium, silicon, tungsten, and vanadium may be added to modify the tempering response after hardening. Small amounts of nickel may be added to improve corrosion resistance in some media and to improve toughness. Sulfur or selenium is added to some grades to improve machinability.
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contents. However, duplex stainless steels possess higher tensile and yield strengths and improved resistance to stress-corrosion cracking than their austenitic counterparts. The toughness of duplex stainless steels is between that of austenitic and ferritic stainless steels. Precipitation-Hardening Stainless Steels
Ferritic Stainless Steels Ferritic stainless steels are essentially chromium-containing alloys with bcc crystal structures. Chromium content is usually in the range of 10.5–30%. Some grades may contain molybdenum, silicon, aluminum, titanium, and niobium to confer particular characteristics. Sulfur or selenium may be added, as in the case of the austenitic grades, to improve machinability. The ferritic alloys are ferromagnetic. They can have good ductility and formability, but high-temperature strengths are relatively poor compared to the austenitic grades. Toughness may be somewhat limited at low temperatures and in heavy sections. Austenitic Stainless Steels Austenitic stainless steels have a face-centered cubic (fcc) structure. This structure is attained through the liberal use of austenitizing elements such as nickel, manganese, and nitrogen. These steels are essentially nonmagnetic in the annealed condition and can be hardened only by cold working. They usually possess excellent cryogenic properties and good high-temperature strength. Chromium content generally varies from 16 to 26%; nickel, up to approximately 35%; and manganese, up to 15%. The 2xxseries steels contain nitrogen, 4–15.5% Mn, and up to 7% Ni. The 3xx types contain larger amounts of nickel and up to 2% Mn. Molybdenum, copper, silicon, aluminum, titanium, and niobium may be added to confer certain characteristics, such as halide pitting resistance or oxidation resistance. Sulfur or selenium may be added to certain grades to improve machinability. Duplex Stainless Steels Duplex stainless steels have a mixed structure of bcc ferrite and fcc austenite. The exact amount of each phase is a function of composition and heat treatment. Most alloys are designed to contain approximately equal amounts of each phase in the annealed condition. The principal alloying elements are chromium and nickel, but nitrogen, molybdenum, copper, silicon, and tungsten may be added to control structural balance and to impart certain corrosionresistance characteristics. The corrosion resistance of duplex stainless steels is like that of austenitic stainless steels with similar alloying
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Precipitation-hardening stainless steels are chromium– nickel alloys containing precipitation-hardening elements such as copper, aluminum, or titanium. Precipitationhardening stainless steels may be either austenitic or martensitic in the annealed condition. Those that are austenitic in the annealed condition are frequently transformable to martensite through conditioning heat treatments, sometimes with a subzero treatment. In most cases, these stainless steels attain high strength by precipitation hardening of the martensitic structure. A checklist of characteristics to be considered in selecting the proper type of stainless steel for a specific application includes: • • • • • • • • • • • • • • •
Corrosion resistance Resistance to oxidation and sulfidation Strength and ductility at ambient and service temperatures Suitability for intended fabrication techniques Suitability for intended cleaning procedures Stability of properties in service Toughness Resistance to abrasion and erosion Resistance to galling and seizing Surface finish and/or reflectivity Magnetic properties Thermal conductivity Electrical resistivity Sharpness (retention of cutting edge) Rigidity
General corrosion is often much less serious than localized forms, such as stress-corrosion cracking, crevice corrosion in tight spaces or under deposits, pitting attack, and intergranular attack in sensitized material such as weld heat-affected zones. Such localized corrosion can cause unexpected and sometimes catastrophic failure, while most of the structure remains unaffected; therefore, it must be considered carefully in the design and selection of the proper grade of stainless steel. Corrosive attack can also be increased dramatically by seemingly minor impurities in the medium that may be difficult to anticipate but can have major effects, even when present in only parts-per-million concentrations; by heat transfer through the steel to or from the corrosive medium; by contact with dissimilar metallic materials; by stray electrical currents; and by many other
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subtle factors. At elevated temperatures, attack can be accelerated significantly by seemingly minor changes in atmosphere that affect scaling, sulfidation, or carburization. Despite these complications, suitable steel can be selected for most applications on the basis of experience, perhaps with assistance from the steel producer. Laboratory corrosion data can be misleading in predicting service performance. Even actual service data have limitations, because similar corrosive media may differ substantially because of slight variations in some of the corrosion factors listed previously. For difficult applications, an extensive study of comparative data may be necessary, sometimes followed by pilot plant or in-service testing. Mechanical Properties Mechanical properties at service temperature are obviously important, but satisfactory performance at other temperatures must be considered also. Thus, a product for arctic service must have suitable properties at subzero temperatures even though steady-state operating temperature may be much higher; room-temperature properties after extended service at elevated temperature can be important for applications such as boilers and jet engines, which are intermittently shut down. Fabrication and Cleaning Frequently, a particular stainless steel is chosen for a fabrication characteristic, such as formability or weldability. Even a required or preferred cleaning procedure may dictate the selection of a specific type. For instance, a weldment that is to be cleaned in a medium such as nitric-
hydrofluoric acid, which attacks sensitized stainless steel, should be produced from stabilized or low-carbon stainless steel even though sensitization may not affect performance under service conditions [1]. The compositions of the main families of stainless steels are shown in Table 1.
The Passive Layer in Stainless Steels The presence of chromium causes the occurrence of a chromium oxide layer on the surface of the alloy that is impermeable and adherent to the alloy substrate, protecting it from any further oxidation or corrosion processes. Any scratch on this protective layer exposes the substrate to oxygen, and a new protective oxide layer is produced. One important aspect of this layer is that it must be homogeneous throughout the surface of the alloy. Any differences from place to place could generate a difference of electrochemical potential, which could trigger a corrosion process, meaning that the chromium must be in a homogeneous solid solution in order to perform as expected. This is achieved by a solution heat treatment: heating the alloy to high temperature (above 1,050 C, or 1,920 F) for some time, allowing the diffusion process to homogenize the composition, followed by quenching to prevent the precipitation of equilibrium phases (mostly chromium carbide and nitride). This process is used to dissolve precipitates that are formed during the slow solidification of the alloy in the fabrication process. As a rule, if the alloying elements are not well dispersed throughout the solid phase, properties will degrade.
Table 1 Compositions of some stainless steel alloys Element Family Austenitic Ferritic
AISI
C
Mn
Si
Cr
Ni
Others
304
0.08
2.0
1.0
18.0–20.0
8.0–10.5
–
316
0.08
2.0
1.0
16.0–18.0
10.0–14.0
2.0–3.0 Mo
409
0.08
1.0
1.0
10.5–11.7
0.5
69 %C–0.75 max Ti
430
0.12
1.0
1.0
16.0–18.0
–
–
Martensitic
420
0.15 min
1.0
1.0
12.0–14.0
–
–
440C
0.95–1.2
1.0
1.0
16.0–18.0
–
0.75 Mo
Duplex
UNS S30205
0.03 max
2.0 max
1.0 max
22.0–23.0
4.5–6.5
3.0–3.5 Mo
UNS S32304
0.03 max
2.5 max
1.0 max
21.5–24.5
3.0–3.5
17-4 PH
0.07
1.0
1.0
15.5–17.5
3.0–5.0
0.14–0.2 N 0.05–0.6 Mo 0.05–2.0 N Precipitation hardening
3.0–5.0 Cu 0.15–0.45 Nb
PH 13-8
0.05
0.2
0.1
12.2–13.2
7.5–8.5
2.0–2.5 Mo 0.9–1.3 Al, 0.01 N
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equilibrium phases (called sensitization when chromium carbide precipitates), impoverishing the matrix in chromium and rendering the part corrosion-prone. The homogeneity of chromium in solid solution is thus fundamental to promote the formation of an effective protective passive layer. Difficulties in Stainless Steel Nitriding/Carburizing
Fig. 1 Characterization of the passive layer of an electrochemically polished surface layer. ESCA, electron spectroscopy for chemical analysis. Source: Ref. [2]
Fig. 2 Characterization of the passive layer of a mechanically polished surface layer. ESCA, electron spectroscopy for chemical analysis. Source: Ref. [2]
Under normal conditions, the thickness of the protective passive layer is in the range of 1.5–2.5 nm, composed mostly of chromium oxide/hydroxide and iron/iron oxide. The strong affinity of chromium for oxygen promotes a very stable compound (Cr2O3) that suppresses any other reactions and thus stops corrosion. The chemical stability of the passive layer depends on the type of surface-finishing treatment. Processes such as electrochemical polishing (Fig. 1) increase the surface concentration of chromium, promoting a thicker and more stable passive layer when compared to mechanically processed surfaces (Fig. 2) [2]. More importantly, any increase in temperature above a certain value for some time after the solution heat treatment during the life of the part could cause the precipitation of
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The sensitivity of stainless steels to temperature and the existence of the protective oxide layer are the main problems that set stainless steel nitriding and/or carburizing treatments apart from the surface treatment used in common steels. First, the stainless steel passive oxide layer is so efficient in protecting the substrate against corrosion that it also prevents nitrogen or carbon from diffusing into the metal. Second, the necessity to keep the treatment temperature lower than the precipitation temperature of the equilibrium phases (nitrides and/or carbides) poses some challenges: how to produce atomic nitrogen or carbon, how to remove the oxide layer, and how to maximize the diffusion of nitrogen or carbon into the metal in a low-temperature and thus low-energy reaction. These are the major questions that each nitriding process must answer in order to make the treatment viable. As a consequence, most stainless steel nitriding is done at low temperatures, below 450 C (840 F), and different strategies are used to remove the oxide layer and generate the atomic nitrogen. One exception is the solution nitriding treatment, which is done at very high temperatures (1,050–1,150 C, or 1,920–2,100 F), similar to those used in solution heat treatment. In those temperatures, the oxide layer is permeable, and the N2 in the furnace atmosphere decomposes into atomic nitrogen when in contact with the metal surface. However, at the end of this treatment, quenching is necessary to prevent the precipitation of the equilibrium phases, which is a disadvantage because part distortion is more likely to happen, and the furnace design is complicated [3]. The high concentration of chromium necessary to create the passive chromium oxide layer is also fundamental to stabilize the nitrogen and/or carbon and form the proper protective layer, the expanded austenite (or S-phase), that simultaneously increases the mechanical, tribological, and corrosion properties of the alloy.
The S-Phase The success of plasma and gas nitriding at low temperatures (lower than 450 C, or 840 F) and solution nitriding at high temperatures is associated with the absence of chromium nitride or carbide precipitation and thus with the formation in the surface layer of a particular phase called the S-phase. This nitrogen- and/or carbon-rich phase
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presents higher concentration of the interstitials when treated at lower treatment temperatures and when highchromium alloys are used, resulting in better properties. This happens because of the affinity between chromium and nitrogen (and carbon) and also because at lower homogen temperatures only the interstitial elements are mobile, which allows substitutional alloying elements (such as chromium) to stay in solid solution despite being very reactive with interstitials such as nitrogen and carbon. The solid solution that is formed is said to be in a paraequilibrium state, because only one of the components is allowed to diffuse and thus reach equilibrium. The S-phase was first seen in austenitic stainless steel by Zhang, Bell, and Ichii in the mid-1980s after a low-temperature (400 C, or 750 F) plasma nitriding treatment and close examination by x-ray diffraction (XRD) spectrometry. The detected peaks were not in the ASTM International index, and Ichii gave the newly discovered phase its name: S-phase. Interstitial nitrogen atoms occupy disordered octahedral sites in the fcc crystal lattice of austenite, significantly increasing the lattice parameter as a result of the very high (30–40 at.%) nitrogen content. The expansion caused by the interstitial nitrogen in the austenite layer generates compressive lattice strains, which result in an increase in the stacking fault density. Another consequence of nitrogen supersaturation is related to the crystallography of the S-phase, which displays an anomalous behavior in XRD analysis. The XRD patterns present peaks with anisotropic shift to lower Bragg angles than those expected for austenite, and these promote different expansion among diffracted reflections. Thus, {200} peak shows an expanded austenite lattice larger than that for {111} reflection [4–6]. Presuming local equilibrium between the nitrogen in the gas mixture and the nitrogen in the solid solution (at the surface), the application of an atmosphere with higher nitriding potential corresponds to a higher nitrogen content in the solid solution, a higher lattice parameter increase, and thus increased dislocation of the peaks. Following the new discovery, considerable efforts have been undertaken to better understand the structure, properties, and applications of the phase. The current definition of the S-phase is that of a paraequilibrium phase originating in fcc crystalline structure with strong nitride and/or carbide alloying elements such as chromium and with large amounts of nitrogen and/or carbon in solid solution. The base of the alloy can be iron, cobalt, or nickel. Figure 3 shows the XRD patterns of untreated and plasma-nitrided (PN) AISI 316 stainless steel at 420 C (790 F). It illustrates the displacement of the peaks corresponding to the c-phase to the left, with increasing nitrogen, as well as the broadening of these peaks, compared to the c-phase peaks of the untreated steel [4].
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Fig. 3 X-ray diffraction patterns of untreated and plasma-nitrided (PN) AISI 316 steel showing two broad peaks, S1 and S2, generated from low-temperature nitrided layer. Source: Ref. [4]
Fig. 4 X-ray diffraction patterns of nitrogen and carbon S-phase in comparison with untreated AISI 316 stainless steel. PC plasma carburized, PN plasma nitrided. Source: Ref. [4]
Figure 4 shows the XRD patterns of low-temperature PN, plasma-carburized (PC), and untreated AISI 316 stainless steel samples. It illustrates the displacement of the peaks corresponding to the c-phase to the left, with increasing nitrogen or carbon. The shift is smaller for the PC sample than for the PN sample, indicating less lattice expansion and less distortion in the carbon S-phase. As a consequence, the elastic distortion of austenite lattice combined with massive stacking fault (Fig. 5) result in broader and asymmetric XRD peaks, which increases the difficulty in identifying phases with very small volume fraction, as, for example, chromium or iron nitrides that nucleate in the expanded austenite layer. Another consequence of these crystalline defects is the increase in nitrogen mobility, favoring insertion and diffusion of the interstitial element in the austenite [6]. In the case of PN UNS S31600 austenitic stainless steel samples treated at 450 C (840 F) and analyzed by XRD and transmission electron microscopy (TEM), only TEM was able to verify the presence of fine CrN precipitates, as presented by the bright-field TEM micrograph in Fig. 6, which shows a portion of the nitrided surface with three
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Fig. 5 Thin-foil bright-field transmission electron micrographs showing (a) planar distribution of dislocations and (b) bundle of stacking faults revealed by selection of (111) reflection of UNS S31600 stainless steel. Source: Ref. [6]
Fig. 6 Plan view thin-foil bright-field transmission electron microscopy image showing grains A, B, and C of expanded austenite and their respective selected-area electron diffraction patterns. Some phasedecomposition regions are indicated on the B grain surface (white arrows) and in the grain boundaries (black arrows). Source: Ref. [6]
grains of expanded austenite and their respective selectedarea electron diffraction (SAED) patterns. These particles (the white arrows on grain B) present a lamellar-type microstructure composed by ferrite and cubic chromium nitride, and their formation can be attributed to localized decomposition of expanded austenite during nitriding treatment [6]. This decomposition mechanism is probably controlled by the atomic diffusion of the chromium, which is very sluggish at 450 C (840 F). Thus, only chromiumrich regions would be capable of the transformation, and in these regions the precipitation of CrN would drastically reduce the stability of the austenite promoted by the interstitial. (Nitrogen is considered a very strong austenite former.) Evidence of expanded austenite decomposition was also observed on the grain boundaries, as indicated by the black arrows in Fig. 6 [6].
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The analysis of different regions by SAED (Fig. 6) has systematically shown well-defined spot-type patterns, characteristic of reflections of fcc austenite single crystals with lattice parameters quite close to that determined for ˚ ). the substrate by XRD (3.60 A Electron diffraction analysis shows that each grain maintained the fcc structure with different zone axes. Calculations involving measurements from these patterns allow the expansion of the lattice parameter to be estimated at up to 14.5% [6]. In addition to provoking the formation of diffuse spots in the SAED patterns, the massive introduction of nitrogen caused an extensive twinning in the expanded austenite, due to high elastic strain associated with interstitial supersaturation. These features were usually observed in a set of bundles, as exemplified by the dark-field TEM micrograph in Fig. 7(a), where the low contrast of the stacking faults in the A–A direction is due to very close diffracted beams selected by the objective aperture of the TEM (white arrow). In some regions of the nitrided layer, small, rounded particles (10–15 nm) were found, as shown in Fig. 7(b). The ring-type SAED pattern indicates that there is a larger number of diffracting particles, and there is also a preferential orientation (texture) related to the substrate. Indexing diffraction rings have shown that these particles possess a crystalline structure compatible with cubic chromium nitride (CrN), the volume fraction of which is considerably smaller than the detection limit of the XRD technique, indicating that XRD analysis is not the preferred technique for investigating these phases [6]. S-Phase Layer in Austenitic Stainless Steel The S-phase layer in austenitic stainless steel can be described as a supersaturated metastable phase with massive amounts of interstitial nitrogen in solid solution, up to
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Fig. 8 Influence of nitrogen content on the mechanical properties (hardness, elastic modulus, and toughness) of nitrogen-alloyed stainless steel coatings. fcc face-centered cubic, bcc body-centered cubic. Source: Ref. [8]
Fig. 7 Thin-foil dark-field micrographs showing (a) two sets of stacking faults and (b) 10–15 nm-sized rounded nitride particles found after plasma nitriding of UNS S31600 stainless steel at 450 C (840 F). The selected reflections for imaging are indicated in the respective selected-area electron diffraction patterns. Source: Ref. [6]
65–80 nitrogen atoms per 100 metal atoms (32–40 at.%) (Fig. 8). Carbon S-phase has a lower concentration, approximately 12 at.%. For comparison, the maximum amount of nitrogen in experimental high-nitrogen austenitic stainless steel is below 1.4 wt% [4, 7, 8]. These high concentrations of nitrogen and/or carbon in solid solution are possible because of the affinity of chromium and nitrogen and/or carbon and the low process temperature. The need to decrease the treatment temperature decreases the case depth but assures that there is no precipitation of nitrides and/or carbides (Fig. 9) [9]. Solution nitriding done at 1,100 ± 50 C (2,010 ± 90 F) increases case depth up to 2 mm (0.08 in.) but drastically lowers the nitrogen concentration, resulting in a deep but low-hardness case. Independently of the treatment temperature, chromium increases the solubility of nitrogen and/or carbon in solid solution and thus is fundamental for the formation of the S-phase (Fig. 10, 11) [10, 11]. Other
Fig. 9 Layer thickness versus plasma nitriding temperature for AISI 316, 304, and 321 stainless steels. Source: Ref. [9]
elements that promote nitrogen solubility include titanium, zirconium, vanadium, niobium, manganese, and molybdenum. One side effect of the high chromium content needed for formation of the S-phase is that the diffusion of nitrogen is slower than in pure iron. Also, its diffusion in the austenitic phase is a hundredth of that in the ferritic phase. As a result, the S-phase layer obtained in low-temperature treatment has massive amounts of nitrogen but is shallow. Carbon S-phase is somewhat deeper but still shallow as compared to conventional cementation surface hardening at higher temperatures. The diffusion of nitrogen and, to a lesser extent, carbon performed at low temperatures for the formation of the
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Fig. 10 Effects of chromium and nickel on the nitrogen solubility at 1,100 C (2,010 F) for 20 and 28 wt% Ni with 20Cr austenitic steels, respectively. The nitrogen concentration increases with chromium, while nickel has the opposite effect. Source: Ref. [11]
Fig. 11 Carbon solubility as a function of carbide/nitride formers. Source: Ref. [10]
S-phase does not proceed according to Fick’s laws. The fundamental assumption of those laws, namely that there is no chemical interaction between the solvent (iron–chromium alloy) and the solute (nitrogen and/or carbon), is not true at these low temperatures, at which chromium acts as a ‘‘trap’’ for nitrogen and, to a lesser extent, for carbon. As discussed previously, this interaction of chromium with nitrogen and carbon is fundamental for the formation of the S-phase. High-temperature solution nitriding, on the other hand, is performed at 1,100 ± 50 C (2,010 ± 90 F). Here, the diffusion behaves more in accordance with Fick’s laws, because at these high temperatures only one homogeneous phase is present (as indicated in the phase diagram), and no precipitate is thermodynamically stable; that is, the preference of chromium for nitrogen and carbon does not manifest in these high temperatures. The final result is that the low-temperature treatment (below 450 C,
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Fig. 12 Nitrogen profile of a low-temperature AISI 316L plasmaassisted nitrided sample, with the typical steplike profile. Adapted from Ref. [12]
Fig. 13 Lattice parameter of nitrogen- and carbon-stabilized expanded austenite (S-phase) as a function of the number of interstitial nitrogen or carbon atoms per metal atom (cN or cC). Source: Ref. [13]
or 840 F) does not follow the equilibrium diagram, and the final concentration of nitrogen is up to 800 times the expected equilibrium concentration [10] when chromium is present in high concentrations, as in the case of stainless steel. The resulting profile presents a steplike behavior that can best be described using two complementary error functions (erfc) instead of the one erfc used in the traditional solution of Fick’s second law. Figure 12 shows the typical steplike nitrogen profile of a sample of AISI 316L processed by low-temperature plasma-assisted nitriding. The very high concentration of nitrogen in solid solution causes important changes in the austenitic phase. The resulting phase is still a face-centered structure like austenite but is no longer a cubic lattice because of the distortion caused by the interstitial nitrogen. The resulting structure is classified as a metastable face-centered tetragonal crystal, with an increase in the lattice parameter of up to 10% when compared to the
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Fig. 14 Micrograph of nitrided AISI 316 (673 K for 4 h) showing the S-phase layer above the austenitic matrix. Source: Ref. [14]
Fig. 15 Micrograph of nitrocarburized AISI 316 (673 K for 4 h) showing the S-phase layer above the austenitic matrix. Source: Ref. [14]
substrate, depending on the nitrogen (and carbon) concentration (Fig. 13). Because of the distortion, a very high compressive stress is present in the nitride layer, which is important for the improvement of fatigue strength [13]. The S-phase layer presents much-enhanced mechanical, tribological, and fatigue properties as compared to those of the substrate or of a white layer composed mainly of hard nitrides [6]. Pronounced hardening, up to 1,400 HV for austenitic stainless steels, is typical of low-temperature ion nitriding. The corrosion resistance is as good as or better than that of the austenitic stainless steel substrate. The pitting
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Fig. 16 Micrograph of nitrided AISI 316 (773 K for 22 h) showing the S-phase layer above the austenitic matrix with the nitride layer at the top. Source: Ref. [14]
Fig. 17 Micrograph of nitrocarburized AISI 316 (773 K for 22 h) showing the S-phase layer above the austenitic matrix with the nitrocarbide layer at the top. Source: Ref. [14]
potential is increased in NaCl medium as compared to that of the substrate. This adds up to a very much-enhanced wear resistance in severe corrosive environments in which there is a synergy of wear and corrosion acting simultaneously. Figures 14–17 present the microstructure of PN AISI 316 steel showing the S-phase layer. Figure 18 shows the optical micrographs of nitrided layers produced on AISI 316 steel after 20 h at 400 C (750 F) (Fig. 18a), 500 C (930 F) (Fig. 18b), and 550 C (1,020 F) (Fig. 18c) [15].
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Fig. 18 Optical micrographs showing nitrided layers produced on AISI 316 steel after 20 h at (a) 400 C (750 F), (b) 500 C (930 F), and (c) 550 C (1,020 F). Source: Ref. [15]
Treatment at 400 C produced a case constituted only by the S-phase (Fig. 18a), whereas treatments at 550 C resulted in the formation of a dark sublayer at the top (Fig. 18b) that gradually grows inward with increasing treatment temperature. At 550 C, the S-phase layer was completely suppressed (Fig. 18c). The combination treatment of nitriding ? carburizing, or nitrocarburizing, offers a smoother transition of hardness and residual stress. Due to the greater diffusivity of carbon, such treatment is able to achieve increased load-bearing capacity and to promote greater stability of the outer nitrided layer, which has less tendency to spall and is thus even more beneficial to fatigue strength. Figure 19 shows the chemical composition depth profiling of elements for ASTM F138 austenitic stainless steel
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obtained with the following treatments: PN, PC, and plasma nitrocarburizing (PNC) at 425 C (800 F) [16]. Hardness of the S-Phase The hardness of the S-phase is outstanding, with hardness values for nitrogen and carbon supersaturation in the range of 1,300–1,500 and 700–1,000 HV, respectively, for austenitic stainless steel and up to 2,000 HV for martensitic and precipitation-hardening alloys. Because of the different nature of carbon and nitrogen interactions with chromium, the hardness profile is also different. As expected, hardness increases with nitrogen and carbon in solid solution, but the nitrogen S-phase shows higher hardness with a shallower case and a more abrupt change in
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Fig. 19 Chemical profiles for ASTM F138 steel sample plasma nitrided (PN), plasma carburized (PC), and plasma nitrocarburized (PNC) at 425 C (800 F). Source: Ref. [16] Fig. 21 Lost wear volume. Plasma nitriding (PN) at 400 C (750 F), plasma carburizing (PC) at 450 C (840 F), and plasma nitrocarburizing (PNC) at 400 and 450 C. Source: Ref. [16]
Figure 21 shows the results of wear tests on plasmatreated ASTM F1586 stainless steel samples [16]. Fatigue and Fretting Properties of the S-Phase
hardness as compared to the carbon S-phase. A simple means to minimize this effect is to perform a mixed treatment, such as low-temperature nitrocarburizing. The resulting case will have the best of both worlds—the high hardness of nitriding with the case depth and more gradual hardness profile of the carbon S-phase—better suiting it for load-bearing applications and giving it better wear resistance. Figure 20 shows the hardness depth profiles for ASTM F138 stainless steel obtained with the following treatments: PN (400 C, or 750 F), PC (450 C, or 840 F), and PNC (400 and 450 C) [16].
Fretting fatigue is a problem that takes place when small oscillatory movement between contacting materials causes surface damage, eventually resulting in the development of fatigue cracks in engineering components subjected to a superimposed alternating tensile stress. Because fatigue originates at the surface, the introduction of a hard, wearresistant surface layer containing compressive residual stress is an effective way of preventing fretting fatigue. The production of the S-phase layer with elevated hardness in stainless steels can meet this requirement. A typical treatment for AISI 316 austenitic stainless steel and of 3Cr12 duplex stainless steel consists of plasma nitriding at 400 C (750 F), resulting in increases of 10–20% in the fatigue limit and 15–50% in the fretting fatigue limit, respectively [17]. Fatigue tests carried out in air at room temperature in the low-cycle fatigue range also show a significant improvement of the fatigue life of AISI 316L austenitic stainless steel PN at 400 C (750 F). The fatigue-life increase seems to be directly correlated with the residual compressive stress at approximately 2–3 GPa (0.3–0.4 9 106 psi), as deduced from the experiments. An optimal effect is found at 3 h of nitriding, which also coincides with the highest compressive residual stress [18].
Wear Resistance of the S-Phase
Tribological Properties of the S-Phase
Wear resistance is perhaps one of the properties most improved by the enhanced hardness of the S-phase.
The S-phase layers are very effective in improving the wear resistance of austenitic stainless steel. Figure 22
Fig. 20 Hardness profiles of ASTM F138 steel sample plasma nitrided (PN) at 400 C (750 F), plasma carburized (PC) at 450 C (840 F), and plasma nitrocarburized (PNC) at 400 and 450 C. Source: Ref. [16]
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Fig. 22 Wear volume loss against sliding distance for plasmanitrided and nonnitrided samples of AISI 316 stainless steel. Source: Ref. [19]
shows the results of wear tests on AISI 316 steel plasma nitrided at 420 and 500 C (790 and 930 F) using a pinon-disk tribometer. The sample was rotated against a stationary WC–Co ball of 8 mm (0.3 in.) diameter in the dry sliding condition. The wear volume decreased by more than 2 orders of magnitude [19]. Wear of untreated samples was severe and characterized by strong adhesion, abrasion, and severe plastic deformation, whereas wear of the PN samples was mild and dominated by oxidation wear and microabrasion. Figures 23 and 24 [20] show the results of the pin-ondisk dry wear test for an AISI 316 austenitic stainless steel disk rotating against a stationary steel or alumina ball. The steel samples were plasma carburized at 450 C (840 F) for 5 h, 500 C (930 F) for 5 h, and 500 C (930 F) for 20 h, producing layers free from precipitates, with thicknesses of 15, 25, and 40 lm, respectively. As expected, these treatments have the ability to prevent plastic deformation on the sample surface and the resulting formation of strong adhesions between the surfaces in contact (samples and steel or alumina balls) during dry sliding, with the thicker case performing better than the thinner. As a result, the wear occurred in a gentle way, dominated by the action of microabrasion. The untreated substrate suffered a severe wear process, where surface plastic deformation, adhesion, and abrasion were dominant. The wear rate decreased more than an order of magnitude with the surface treatments. Table 2 shows a comparison of properties between the nitrogen and carbon S-phases. Corrosion Resistance Although austenitic stainless steels are the most commonly used structural alloys for corrosive environments, their
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Fig. 23 Variation of wear volume with applied load for various conditions of treatment. Steel disk sample rotating against a stationary steel ball. Source: Ref. [20]
Fig. 24 Variation of wear volume with applied load for various conditions of treatment. Steel disk sample rotating against a stationary alumina ball. Source: Ref. [20]
resistance to surface degradation during sliding contacts with other materials in such environments is low. The synergistic combination of wear and corrosion, known as corrosion–wear processes, causes severe surface material loss. The presence of the S-phase, with high hardness and good corrosion resistance, can properly solve the problem [21]. The corrosion resistance of the S-phase can easily be seen after exposing the nitrided samples to a strong metallographic etchant (50% HCl ? 25% HNO3 ? 25% H2O) that causes no effect in the S-phase, so that it appears bright under an optical microscope. This is an indication that the nitrided layer is more resistant to the corrosive attack of the acid mixture than is the substrate [22]. The significant increase in corrosion resistance for the nitrogen S-phase in austenitic stainless steel is due to the electron configuration promoted by nitrogen atoms in the metal lattice, which enhances the metallic component of interatomic bonds and provides more homogeneous
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Table 2 Comparison of the properties between the carbon- and the nitrogen-base S-phase [10] Properties
SN
SC
Formation temperature [C (F)]
300–450 C (570–840 F)
400–550 C (750–1,020 F)
Surface interstitial content (at.%)
20–30 (\30)
5–10 (\15)
S-phase layer thickness (mm)
10–20 (\30)
20–40 (\50) 800–1,000
Surface hardness (HV0.05)
1,300–1,500
Hardness depth distribution
Abrupt change
Gradual change
Load-bearing capacity
Low
High
Ductility/toughness
Poor
Good
Residual stresses
High but shallow
Low but deep
Fatigue properties
Low
High
Pitting-corrosion resistance
Very good
Good
Dry sliding wear resistance Corrosion wear in saline solution
Very good Very good
Good Good
Fretting wear in Ringer’s solution
Very good
Good
Erosion-corrosion in silica/saline slurry
Very good
Good
Thermal stability
Low
High
Biocompatibility
Good
Good
distribution of substitutional solutes through short-range ordering of nitrogen atoms [23]. This increases the homogeneity of the distribution of chromium, and this more homogeneous distribution in turn promotes a more stable passive layer because of the decreased probability of local chromium depletion. Also, the strong chemical interaction between nitrogen and alloying elements results in a high thermodynamic stability of nitrogen in austenitic stainless steels. Figure 25 shows the corrosion weight loss of AISI 316 stainless steel samples untreated and plasma nitrided at 420 and 500 C (790 and 930 F) after immersion in 10% HCl water solution for up to 120 h. It is verified that the highertemperature (500 C) PN sample (AS500) had higher corrosion weight loss than both untreated (AS000) and lower-temperature nitrided (AS420) samples. This is due to the presence of chromium nitrides in the layer produced in
Fig. 25 Corrosion weight loss and corrosion rate of various 316 stainless steels in 10% HCl water solution. Source: Ref. [22]
the higher-temperature treatment, which deteriorates the corrosion resistance [22]. Figure 26 presents a comparison between samples characterized by means of polarization curves in 5% NaCl aerated solutions. Nitriding treatments performed at higher temperatures ([723 K) can significantly increase the surface hardness of AISI 316L stainless steel samples, but, as can be seen in the plot, they also decrease the corrosionresistance properties as a consequence of the CrN precipitation. Nevertheless, nitriding treatments performed at lower temperatures (\723 K) avoid the precipitation of CrN particles and allow the formation of layers composed essentially of an S-phase that shows high hardness and very high pitting- and crevice-corrosion resistance [24]. Untreated samples show the typical behavior of a passive material sensitive to pitting corrosion when the polarization potential is higher than a certain threshold. As a result of the high chloride concentration, the pitting branch starts at relatively low anodic potential values (approximately ? 100 mV Ag/AgCl). At the highest polarization potentials, the anodic current density values are quite large and are limited only by concentration polarization phenomena. Morphological analysis on the surface of the corroded samples reveals the presence of many large and deep pits, confirming the plot predictions. As the plot shows, all nitriding treatments, with the exception of nitriding at 773 K, resulted in corrosion performance similar or superior to that of the untreated sample. Crevice corrosion tests carried out on nitrided and untreated samples in 10% NaCl aerated solution at a fixed temperature of 328 K for times up to 60 days are shown in Fig. 27. Examination of the percentages of crevices in
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Fig. 26 Polarization curves in 5% NaCl aerated solution of AISI 316L austenitic stainless steel samples untreated and glow-discharge nitrided at different temperatures. Source: Ref. [24]
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behavior reliability. Samples treated at temperatures ranging from 743 to 773 K are subject to the intergranular corrosion process and show lower localized corrosion resistance in comparison with samples untreated and nitrided at lower temperatures. A glow-discharge nitriding treatment, performed at low temperature, is an effective technique to significantly increase the pitting- and crevicecorrosion resistance and the surface hardness of the AISI 316L austenitic stainless steel [24]. The aforementioned results are in accordance with the pitting-resistance equivalent number (PREN), a parameter commonly used to describe the pitting-corrosion resistance of stainless steels in terms of their alloying elements using formulae, of which the following is, perhaps, the most common: PREN ¼ %Cr þ 3:3 %Mo þ 16 %N The concentration of chromium, molybdenum, and nitrogen in the steel will result in an increased PREN and pitting potentials, but it is expected that nitrogen would have a much greater effect, because it has the largest coefficient [16] in the equation by a wide margin [22]. The Corrosion-Wear Process
Fig. 27 Trend of the percentage of crevices in which corrosion processes are active versus time in the case of AISI 316L austenitic stainless steel samples untreated and nitrided at different temperatures and immersed in 10% NaCl aerated solution at 328 K. Source: Ref. [24]
which corrosion processes were observable versus the immersion time shows that untreated samples appear to be sensitive to crevice corrosion in the tested environment. Nevertheless, samples nitrided at temperatures ranging from 673 to 723 K, even after 60 days of exposure, do not show any significant sign of crevice corrosion attack [24]. As seen in Fig. 27, the corrosion behavior of treated samples in concentrated NaCl aerated solutions depends on the nitriding temperature. Samples treated at 673 K show higher crevice-corrosion resistance than untreated samples, but the S-phase surface layer can be too thin to be consistently effective in increasing the pitting-corrosion resistance. Samples treated at temperatures ranging from 703 to 723 K show a very high pitting- and crevice-corrosion resistance. Moreover, samples nitrided at 703 K appear to provide the best compromise in order to obtain very high corrosion-resistance properties and corrosion-
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When abrasion is acting with corrosion in what is known as the corrosion-wear process, the added effect of high hardness with improved corrosion resistance shows a sharp drop in material loss. This is a complex process because the synergy between abrasion and corrosion is dependent on the abrasion and corrosion properties of each individual material in contact and on the environmental context of the wear process. This is an issue with bearings, valves, and mechanical components working in corrosive aqueous solutions in food processing, with marine components, and with biomedical devices for which lubricants cannot be used or are unstable. There are three main corrosion-wear mechanisms that must be minimized to optimize performance: • •
•
Removal of the coating passive film during sliding contact Galvanic attack of the substrate, which results in blistering and ultimately the removal of the coating during sliding contact Galvanic attack of the counterface material, which leads to abrasion of the coating during subsequent sliding contact
A corrosion-wear process is simulated in the laboratory by using an abrasion test (e.g., pin on disk, reciprocating ball on plate) performed in a corrosive environment. Figure 28 shows the results for a reciprocating ball (Al2O3) on plate (nitrided AISI 316L) in a 3% NaCl solution as a function of the nitrogen in solid solution in the S-phase [21].
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Fig. 28 Corrosion test using a reciprocating ball (Al2O3) on plate (nitrided AISI 316L) under a 3% NaCl solution as a function of the nitrogen (at.%) in solid solution at the S-phase. Source: Ref. [21]
Thermal Stability The S-phase formed by nitriding is the result of massive deposition of nitrogen near the surface where the treatment was performed, thus resulting in a metastable solid solution of nitrogen. As the usage temperature increases, the S-phase formed by nitriding will transform to a compound layer (metal nitrides and a mixture of ferrite and austenite, depending on alloy composition) at temperatures above 450 C (840 F). Figure 29 shows a plot of time against temperature for the transformation of 50% of the S-phase. Figure 30 shows the images obtained by TEM of samples nitrided at 400 C (750 F) and annealed at 400 and 600 C (750 and 1,110 F). The high-temperature annealing at 600 C completely decomposed the S-phase, forming a lamellar structure composed of CrN and a- and cphases. As indicated earlier, the presence of CrN precipitates in the nitrided layer is very detrimental to corrosion resistance. Interestingly, if the CrN is deposited by physical vapor deposition (PVD) rather than precipitated in the case
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during nitriding, the resulting corrosion and abrasion resistances are even better than the S-phase, as shown in Fig. 31, where three samples of AISI 316 in the untreated, nitrided, and CrN PVD-coated condition are compared in a corrosion–wear test [21]. Clearly, CrN possesses very good corrosion and abrasion resistances when not originating from the nitrided case. The main difference between the two layers is that the CrN that precipitates in the nitrided case is a mixture of phases (CrN, ferrite, and austenite) in a lamellar structure (Fig. 30c) [15], whereas the PVDdeposited CrN is a continuous single-phase structure. This difference in morphology and the consequent chemical heterogeneity seems to be responsible for the lower corrosion resistance of the nitrided layer when it is obtained in temperatures above 500 C (930 F).
Thermochemical Nitriding Treatments The thermochemical nitriding treatment, which consists of delivering atomic nitrogen species to the surface of a metallic part for subsequent inward diffusion at appropriate temperatures, has proven to be suitable for the production of the desired layers. It is also possible to use carburizing or nitrocarburizing treatments, in which carbon and/or nitrogen are used to produce a monolayer or dual layers. The conventional gas nitriding process has limited applicability because of two limitations. The first is its minimum processing temperature of 500 C (930 F), which results in the formation of chromium nitride layers, causing a decrease in corrosion resistance to levels inferior to that of the stainless steel base alloy. The second is the fact that additional treatments are needed for the removal of the protective invisible and adherent passive surface film of chromium oxide (Cr2O3), which insulates the stainless
Fig. 29 Plot of time against temperature for the transformation of 50% of the S-phase. Source: Ref. [25]
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Fig. 30 Transmission electron microscopy studies performed on plasma-nitrided specimens at 400 C (750 F) for 20 h and then annealed at 400 C for 20 h (a, b) and 600 C (1,110 F) for 20 h (c, d) showed that low-temperature annealing at 400 C maintained the S-phase layer precipitatefree and preserved the high density of dislocations and microtwins (a). Hightemperature annealing at 600 C, on the other hand, completely decomposed the S-phase, forming a lamellar structure composed of CrN and a- and c-phases (c). Source: Ref. [15]
Fig. 31 Corrosion–wear data after unidirectional testing against WC6%Co balls in 3% NaCl. Source: Ref. [21]
steel from the corrosive environment but also acts as a barrier against the penetration of nitrogen atoms during the nitriding process. Plasma or ion nitriding, on the other hand, can be carried out at lower temperatures, from 350 C (660 F), with the depassivation process performed by sputtering as a first step in the treatment. Additionally, because both the depassivation and the nitriding/carburizing treatment occur in the same chamber, the formation of the undesirable oxide layer is prevented and an additional oxide removal
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treatment is not needed. As a result, this process is becoming the most commonly used process for nitriding and/or carburizing of stainless steels. Another available process is gas nitriding at high temperature (1,100 ± 50 C, or 2,010 ± 90 F), in which the presence of spontaneous dissociated N2 at that temperature provides the atomic nitrogen and the inward diffusion, thus promoting the nitriding treatment. Each of these stainless steel nitriding treatment processes is explained in greater detail in the following. It is important to understand that whatever process is used to promote nitriding, the final result must always be the same: the production of a case composed of the S-phase (or expanded austenite). The quality-control inspection must verify the case depth, the layer hardness, and the absence of precipitates such as chromium nitrides and carbides in the layer. Plasma-Assisted Nitriding Techniques Glow Discharge Plasma nitriding, developed in the mid-1980s, is a surfacehardening process that uses glow-discharge technology to introduce nascent (atomic) nitrogen into the surface of a metal part for subsequent inward diffusion. The process uses a vacuum chamber, with the workpiece as the cathode
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Fig. 32 Schematic of plasma nitriding and nitrocarburizing equipment. 1 and 2, gas introduction; 3, thermocouple; 4, cathode; 5, pressure sensor; 6–8, needle valve; 9, diaphragm valve; 10, solenoid; 11, vacuum pump; 12, sample holder; 13, sample; 14, anode; 15, sealing ring; 16, stainless tube; 17, window; and 18, insulator. Source: Ref. [14]
(negative potential) in a nitrogen/hydrogen atmosphere at pressures between 100 and 1,000 Pa (0.015 and 0.15 psi). A voltage in the range of 500–1,000 V is applied between the metal part and the chamber wall, resulting in the formation of a plasma through which positive nitrogen ions are accelerated against the negative-biased part to be treated. This ionic bombardment heats the piece to the treatment temperature (although chamber heating can also be implemented to heat the part by radiation, a common option for industrial systems), cleans the surface by sputtering, and provides active nitrogen species for the nitriding process [26]. Although the exact mechanism of plasma-assisted nitriding (mostly for direct-current and pulsed treatment) is not modeled to great depth (the Ko¨lbel model does not apply adequately to stainless steel), the large amount of research and development performed by the scientific community has resulted in a robust industrial-strength process that has been adopted throughout the industrial world. Figure 32 shows the schematic of a device used for glow-discharge plasma nitriding. The development of plasma nitriding as an industrial manufacturing process was initially slow because the occurrence of electrical arcing posed a risk of damaging the parts. Advances in the fields of electrical and electronics engineering over the last 30 years, with the introduction of the controlled thyristor circuit breakers to control the suppression of arcs, has contributed to the widespread
industrial acceptance of the plasma nitriding process. In 2003, there were approximately 500 industrial plasma nitriding units in Europe [27]. Figure 33 shows an industrial plasma nitriding installation. The chamber dimensions are 0.9 m (3 ft) in diameter, 1.6 m (5.3 ft) high, with a charge up to 2,000 kg (4,400 lb). Figure 34 shows examples of loaded chambers prior to the direct-current nitriding operation. The coupling of the part being treated in the plasmageneration process has some drawbacks. For instance, the ion bombardment heats differently parts with different area/volume ratio; small parts with larger specific areas will heat more rapidly and at higher temperatures than massive parts, a difference that can affect the outcome of the ion nitriding treatment. The same happens within the part being treated when it possesses regions with large variations in area/volume ratio, but these limitations are overcome by the use of auxiliary resistive heating of the chamber walls. Therefore, batch treatment of parts with sharp changes in size should be avoided. There is also a need to keep a minimum distance between the parts being treated to allow the plasma to pass between them and, in doing so, prevent the hollow-cathode effect (high concentration of secondary electrons), which increases the local temperature. The same effect occurs when a part has a cavity. This requires intensive use of manual labor for the arrangement of parts. Because of this, lots of small parts in
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Fig. 33 Industrial plasma nitriding installation. Chamber dimensions are 0.9 m (3 ft) in diameter, 1.6 m (5.2 ft) high, with a charge up to 2,000 kg (4,400 lb). Courtesy of ISOFLAMA Co
large numbers may not be suitable for this type of treatment. Also, the support region of the workpiece is untreated, because no plasma penetration occurs there; this problem is resolved by using cones to support the parts, so that the support region is reduced to a point (Fig. 34a). Electrical arcing problems, if any, are avoided by the use of pulsed plasma, which prevents the accumulation of electric charges on the tips and edges and thereby keeps parts from being locally damaged by arc discharges. Active Screen More recently, a process called active screen plasma (ASP) has been developed for the purpose of minimizing or avoiding some of the limitations of conventional plasma
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nitriding, such as the edge effect, in which the regions near the edges of the pieces may not be nitrided to the same standard as the rest of the part. In the conventional glowdischarge process, the plasma is produced by the voltage between the wall of the chamber (positive potential) and the workpieces (negative potential). By contrast, in the ASP process, the production of the plasma is produced by the voltage between the chamber (positive) and a screen (negative) placed around the parts being treated. The nitriding species produced on the screen are then transferred and deposited on the part. The part is heated to the treatment temperature by radiation from the active screen, which is under ion bombardment [28, 29]. There are two configurations for this process. In one, the piece is electrically isolated, placed in a floating potential. The other is a mixed system in which the workpiece is also subjected to a negative potential of 100–200 V (much smaller than in conventional plasma nitriding). One of the limiting factors of the floating potential process is the distance between the parts and the screen, which varies depending on the placement and shape of the part. As the nitriding active species in this process are generated on the cathode screen and then transferred to the workpiece surface for the subsequent nitriding reaction, the amount and activity of these nitriding species decreases with increasing distance from the surface of the screen, which may affect the resulting nitrided case. Therefore, the distance between the screen and the parts must be considered if the parts to be treated are placed in a floating potential. Some authors suggest that appropriate positioning of tubes conducting the nitriding gas can minimize the problem. However, there still remains the problem of uneven flow of gas between the different arrangements of parts within the nitriding chamber, which can still result in heterogeneity in the layers produced. Most of the published
Fig. 34 Examples of loaded chambers prior to the nitriding operation. Note the possibility of treating different geometries in the same batch. Courtesy of ISOFLAMA Co.
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process is the high vacuum required [31] and the subsequent cost. Figure 36 shows a schematic of RF plasma equipment. Ion-Beam Implantation
Fig. 35 Schematic of active screen plasma equipment. (a) Floating potential. (b) Anodic potential
works on the subject refer to treatments performed on small parts being treated in lab equipment, with small chambers and a small quantity of parts, where these problems are not usually observed. At the time of this writing (2014), there have been some initial industrial attempts to use the mixed process (in which parts are negatively biased) to minimize these types of problems, but there are very few published papers; moreover, even though large equipment chambers are used, the test specimens are of small dimensions. Another concern is the time to achieve thermal equilibrium between the parts at variable distances from the heating screen and with different relative placements of the parts, which has a significant effect on the radiation heating process. That means increased treatment times and heterogeneity. Therefore, at this time, the process still requires additional studies and development for its full utilization. Figure 35 shows the two possible configurations for the ASP process [30]. Radio-Frequency Plasma Glow-discharge plasma nitriding is carried out in nitrogen– hydrogen atmospheres at pressures in the range of 100–1,000 Pa (0.015 and 0.15 psi). At pressures below 10 Pa (0.0015 psi), the glow-discharge technique is not used, but plasma can still be generated by radio-frequency (RF) excitation, microwave energy, or electrons produced by a heated filament. These low-pressure plasmas diffuse through the treatment chamber with a large number of active species, which increases the nitriding efficiency. The uncoupling of plasma generation from the part being treated allows independent control of the energy and flux of ions against the substrate surface, and the temperature can be kept low [31]. In the RF plasma process, plasma is generated by high frequency, in the range of 13.56 MHz, applied to an inductive antenna inside the vacuum chamber. Heating of the part is accomplished by heating the chamber and the working table with standard electrical resistance. One of the limiting factors for the commercial exploitation of the
Another technique used for nitrogen introduction in stainless steels is bombardment with nitrogen ions with sufficient energy to pass through the surface and penetrate into the material. The main limiting factors for the commercial exploitation of the process are the high vacuum required and the need for moving parts due to the line-of-sight effect. Because the treatment process is ballistic, the kinetic energy of the ion is the most important process parameter, since it is the only cause for the enrichment of the target by nitrogen. Treatment temperature can be very low, because no diffusion is necessary. The resulting case is very shallow and should not be used when load-bearing capacity is needed. Plasma-Immersion Ion Implantation The fundamental difference between plasma nitriding (glow discharge) and plasma-immersion ion implantation (PIII) is that the latter relies on high-energy pulses (up to 45 kV) that promote a low-energy ion implantation in the target but that is still less than the energy used in conventional ion implantation ([100 kV). The PIII process uses a combination of target temperature and pulse voltage to achieve the desired implantation profile [32]. The PIII process can be seen as a modification of the plasma nitriding process that provides control of individual operational parameters and thus improves the plasma treatment. Most systems use an RF plasma at frequencies up to 2.45 GHz to generate a nitrogen microwave plasma, independent of temperature and bias voltage at the target, in the same desired low-pressure atmosphere as the regular RF plasma treatment process. Additionally, the target is connected to a pulsed negative voltage of approximately 45 kV. The RF plasma alone promotes nitriding of the target through the normal surface thermochemical adsorption reactions of nitrogen that occur normally in such a treatment, but during the short time that the negative 45 kV pulse is acting, the positive ions in the plasma are accelerated toward the part at much higher energies typical of the ion implantation process, enhancing the plasma nitriding treatment. The downside is the increased cost of equipment and operation, because the process requires lower pressures from a turbomolecular pump and an RF plasma generator, although a more conventional pulsed direct-current plasma unit can also be used. Figure 37 shows a schematic diagram of such a device.
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Fig. 36 Schematic of radiofrequency plasma equipment. Source: Ref. [31]
This setup allows high concentrations of nitrogen in the near region below the surface that can diffuse farther into the material, controlled by the substrate temperature. A treatment temperature in the range of 200–450 C (390–840 F) can be used, depending on the material being treated. Radiation heating (with heated chamber walls) or conduction heating (with a heated worktable) by standard electrical resistance is used to uncouple the working temperature from the ion bombardment. The resulting layer properties depend on the parameter settings. Initial surface preparation by sputtering is enhanced by high-energy pulses, and less time is needed. Because the process is a mix of plasma nitriding and ion implantation, for treatment temperatures near plasma nitriding (&400 C, or 750 F) and short, low-bias (lowenergy) pulses, the resulting layer is similar to regular plasma nitriding, with cN and case depth in the range of 10–20 lm. High voltage imposes several practical problems and is never desired. High-density plasmas with metal vapor present can lead to arcing. Uniformity of treatment over complex components and in large batches is difficult. The same applies to temperature uniformity. Packing of multiple components for a treatment load and treatment of internal holes is also difficult, as it is with any plasma technique [34]. Currently (2014), PIII is predominantly a research and development tool, able to do niche treatments on specific materials using parameters that are either difficult or impossible for other treatment processes. The big advantage
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of this technique is the large choice of independent tunable process parameters, which allows a level of fine-tuning unrivaled by other nitriding techniques. One last important aspect of PIII is that other ions besides nitrogen can be used and many different characteristics can be achieved in different materials and industry applications. Advantages of plasma-assisted nitriding include: • • • •
• • • • • •
•
It is a clean process with no toxic effluent, because the gases used are nitrogen and hydrogen. It uses a compact and lean system that can be installed near the production line. It causes no significant dirt, noise, or heat pollution. It poses only a small risk of fire, because of the minimum amount of hydrogen used in the chamber under vacuum. Its low process temperatures (from 350 C, or 660 F) cause minimal distortion of the parts. It reduces processing times. The results are absolutely reproducible. The process is computer controllable, which allows a high degree of automation. It involves low operating costs because of its reduced energy costs and low gas consumption [35]. It provides such good control of the structure of the nitrided case that it is possible to produce only the diffusion layer, with no compound layer, for use in duplex treatments (e.g., nitriding ? PVD). Resistance to fatigue is increased by the development of compressive residual stresses in the process.
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Fig. 37 Schematic of equipment for plasmaimmersion ion implantation. RF radio-frequency. Source: Ref. [33]
Table 3 Environment impact comparison between direct-current plasma and low-temperature gaseous nitrocarburizing Type of nitrocarburizing
Plasma
Gaseous
Plasma/gaseous
Amount of gas used (m3/h)
0.6
6.0
10
Total carbon emission via CO/CO2 (mg/m3)
506
137,253
272
Total amount of NOx gas (mg/m3)
1.2
664
553
Output of residual carbon-bearing gas (mg/h)
302
823,518
2,726
Output of residual NOx gas (mg/h)
0.72
3,984
5,533
• •
It is possible to treat only a region of the part by using masks. The installed base of plasma nitriding chambers can readily process stainless steel without any modification.
Non-Plasma-Assisted Nitriding Processes The major advantage of the non-plasma-assisted nitriding process over plasma-assisted processes is that it can be used to treat small parts and large batches, because there is no need to separate them as in plasma-assisted processes. It should also be kept in mind that the support region of bigger workpieces will not be treated (as in the plasma treatment), and when treating batches of small parts, movement should be provided by using rotating retorts. This is necessary to achieve a homogeneous gas distribution. The environmental impact of the gas treatment process is much higher than that of the plasma-assisted process. Table 3 shows a comparison between direct-current plasma and low-temperature gas nitriding, relevant because the gas-nitrided surface layers have essentially the same structure and properties as a direct-current PN surface [27]. Table 4 shows a more general comparison among surface-
treatment processes concerning pollutants and environmental impact [36]. Gas Nitriding at Low Temperatures This is a new treatment process, similar to regular gas nitriding but performed at lower temperatures and thus able to produce the S-phase (without precipitates). A major advantage of gas nitriding is the great ease of batch treatment for small parts, a consequence of the nitriding gas penetration. Although there is no need to arrange the parts as in plasma nitriding, the retorts must be moved during the process to enhance homogeneity. In this surface-hardening process, nitrogen is introduced into the surface of the part by maintaining the metal at the proper temperature and in contact with a nitrogenous gas, usually ammonia, which dissociates into hydrogen and nitrogen in the surface of the material, forming a layer with high nitrogen concentration, the S-phase. Removal of the superficial chromium oxide layer may be accomplished by dry honing, wet blasting, and pickling; chemical reduction in an appropriate atmosphere; submersion in molten salts; sputtering [37]; or any of several proprietary processes to be discussed subsequently. Before
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Table 4 Selected examples of environmental problems within surface engineering sector [36] Technology
Pollutant
Problem area
Gas carburizing
CO2, CO, CH
Greenhouse effect
Cyaniding
NaCN, nitrites
Waste management, water quality
Gaseous ferritic nitrocarburizing
NH3
Air quality
Shot blasting
Noise
Noise
Fluidized bed processing
Mineral dust
Air quality
Degreasing
Trichlorethylene
Stratospheric
Electroplating
Ni, Cu, Cr (e.g. hexavalent Cr salts)
Heavy metals, water management
Plasma nitriding
Nil
–
Physical vapor deposition
Nil
–
Vacuum treatments (coating, spraying, etc.)
Nil
–
Acid pickling Quench hardening
H2SO4, HNO3 Oil
Water quality, acid rain Contaminated land
being nitrided, all stainless parts must be perfectly clean and free of embedded foreign particles [26]. Depassivation in Low-Temperature Gas Nitriding As discussed elsewhere, the two inherent difficulties in nitriding and/or carburizing stainless steel are the passive chromium oxide layer barrier that must be removed and the fact that this must be done at low temperatures: for nitriding and nitrocarburizing, below 450 C (840 F) and for carburizing, below 550 C (1,020 F). The low temperature required to prevent the precipitation of nitrides and/or carbides, together with the high chromium content of stainless steel, results in a case with small depth and high concentration of nitrogen and/or carbon, the S-phase or expanded austenite, which has high wear and corrosion resistance. The recent advent of low-temperature gas nitriding (for the production of the S-phase in stainless steel) as opposed to regular high-temperature (above 550 C) gas nitriding (of regular steel) had to find a way to remove the passive layer in this low-temperature condition. The passive layer forms very quickly over stainless steel, and any method used to remove it must not allow the formation of a new layer, which means that the part being treated must be kept away from an oxidizing atmosphere, preferably in the same chamber where the next step of the treatment will take place. A number of patents have been issued toward this end, and the proposed mechanisms are as follows: •
EP 0588458 (1993) [38]: This patent describes the use of a fluorine-bearing gas as the active agent in a pretreatment in which the passive chromium oxide layer is converted in a fluorinated chromium surface layer that is permeable to carbon and nitrogen atoms, thus allowing the second step of the treatment to take place. This is expensive, corrosive (it deteriorates the surface finish of the part and the furnace), and environmentally aggressive.
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Fig. 38 Gas-phase low-temperature carburization process for 316L austenitic stainless steel. Source: Ref. [40]
•
•
•
U.S. 6165597A (1998) [39]: This patent describes a process in which activation is performed by HCl at 250 C (480 F) according to the chart shown in Fig. 38. After the activation process, the carburizing treatment is performed in a CO ? H2 ? N2 atmosphere. EP 0248431 B1 (1987) [41], EP 1095170 (1999) [42], and WO 2004/007789 A1 (2003) [43]: The process described in these patents uses electroplating to deposit metals that are different from chromium (iron, nickel, ruthenium, cobalt, or palladium) over the stainless steel surface to change the chromium oxide layer into a nonchromium-oxide layer that is permeable to carbon and nitrogen atoms, thus allowing the second step of the treatment to take place. This is expensive and again environmentally aggressive because of the electroplating treatment. EP 1707646 B1 (2005) [44]: The process set forth in this patent uses a gaseous carbon donor compound (such as acetylene, ethylene, propane, butane, and carbon monoxide) with ammonia at temperatures greater than 300 C (570 F) and in a reactor vessel made of metal (such as iron, nickel, cobalt, copper, chromium, molybdenum, niobium, vanadium, titanium,
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Fig. 39 Reactor used to perform gas nitriding. 1, muffle furnace; 2, outer shell; 3, heater; 4, internal container (retort); 5, gas inlet pipe; 6, exhaust pipe; 7, motor; 8, fan; 9, metal-made jig; 10, gas guide cylinder; 11, inverted funnel; 12, vacuum pump; 13, effluent gas combustion facility; 14, carbon donor compound gas cylinder; 15, ammonia gas cylinder; 16, nitrogen gas cylinder; 17, hydrogen gas cylinder; 18, flowrate meter; 19, gas control valve. Source: Ref. [44]
and zirconium), the catalytic action of which will generate HCN. The HCN will depassivate the stainless steel surface through the following reactions:
•
2NH3 þ C2 H2 ! 2HCN þ 3H2 Cr2 O3 þ 6HCN ! 2CrðCNÞ3 þ 3H2 O The carbon and nitrogen in 2Cr(CN)3 diffuse inward and form the nitrocarburized case, leaving a clean surface. The dewpoint must be kept below 5 C (40 F) to keep the metal surface being treated from repassivating, which means that the flow of gas must be continuous to eject excessive water formed during the depassivation and nitrocarburizing reaction. After the depassivation step, ordinary gas nitriding with NH3 ? H2 ? N2 atmosphere can be carried out as desired. The exhaust gas is burned and decomposed at the exit as nitrogen and CO2. This patent also discusses the action of acetone, which is commercially used to dissolve and stabilize acetylene and is present to some extent every time acetylene is used: 2ðCH3 ÞCO ! 2CH3 þ CO 5Cr2 O3 þ 6CH3 ! 10Cr þ 6CO þ 9H2 O CO þ NH3 ! HCN þ H2 O As can be seen, the acetone can also be used to reduce the passive oxide layer and will also produce HCN. Again, water is a by-product that must be discarded to keep the dewpoint below 5 C (40 F) inside the reactor. Figure 39 shows a schematic of a possible reactor to perform such a treatment.
•
EP 1712658 A1 (2005) [45]: This patent describes a process that uses an amino resin (such as melamine, urea, aniline, and formalin resin) that, when heated, decomposes into carbon, nitrogen, hydrogen, HCN, and NO and reacts with the passive layer to eliminate it. This happens during the warmup to the nitriding/ carburizing temperature, without the need to hold the part at any specific temperature for any length of time. Once the nitriding and/or carburizing temperature is reached, the surface of the material will be depassivated, and the treatment can be performed as desired. The resin can be applied to the surface of the material (as a varnish) that is then placed in the furnace for treatment, or, alternatively, the material and the resin can both be placed in the furnace for treatment, without the need to paint the part with the resin. WO 2006/136166 A1 (2006) [46]: The process described in this patent uses an unsaturated hydrocarbon gas (alkenes, CnH2n; or alkynes, CnH2n-2) such as acetylene. The decomposition of the unsaturated hydrocarbon is used to depassivate the surface and perform the carburizing treatment. The treatment temperature must be kept low to prevent the formation of soot on the surface of the part being treated, which effectively slows down the carburizing process and prevents control of the carbon content in the steel. To suppress the tendency for sooting, the temperature must be lowered, which results in even longer treatment times (14 to 72 h) (WO 2011/009463 A1). After this first depassivation process, nitriding using NH3 ? H2 atmosphere (or mixed with the hydrocarbon for nitrocarburizing) can be performed
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500
•
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as desired. Reactions should follow as seen in some of the previous processes. WO 2011/009463 A1 (2011) [47]: The process set forth in this patent uses a compound with nitrogen and carbon (nitrogen/carbon compound) that, when heated, should depassivate the surface of the part being treated and preferably also perform nitriding and/or carburizing. It is stated that HCN is not used but may evolve (by the decomposition of the nitrogen/carbon compound) during the course of the treatment. Treatment temperature should preferably be below 500 C (930 F) to prevent the precipitation of nitrides and/or carbides. The alloy being treated can be iron (stainless steel), nickel, cobalt, or titanium base. Interestingly, the article to be nitrided should be heated to the highest nitriding and/or carburizing temperature possible—below 450 C (840 F) for nitriding and nitrocarburizing and below 500 C (930 F) for carburizing—but when the nitrogen/carbon compound is urea, the nitrogen/carbon heating temperature should be lower than 250 C (480 F) and thus lower than the nitriding/carburizing temperature. Such a procedure is said to improve the surface activation, greatly improving the kinematics of the reaction, and thus decreasing the treatment time needed to obtain a certain case depth.
As can be seen, the low-temperature gas nitriding/ carburizing process, like most industrial processes, has come a long way. The U.S. 6165597A (1998) process that uses HCl has seen success in low-temperature carburizing but at the added cost of an aggressive depassivation process. The EP 1707646 B1 (2005) process uses the HCN generated during the treatment to perform the depassivation and part of the treatment at the same time, somewhat similar to the EP 1712658 A1 (2005) process, which activates the surface during the warmup by using an amino resin such as urea. Both processes generate some HCN that is discarded by combustion without any additional environmental impact. The other depassivation processes are adaptations of these, but with more knowledge about the S-phase (or expanded austenite), the resulting process should be more reliable. The curious exceptions are the processes that use electroplating to change the passive chromium oxide layer into some other type that is not impermeable to nitrogen and carbon: EP 0248431 B1 (1987), which uses iron; EP 1095170 (1999), which uses iron; and WO 2004/007789 A1 (2003), which uses nickel, ruthenium, cobalt, or palladium). These processes are expensive, and electroplating presents environmental problems, but it is nonetheless another option for solving the problem. Each process should eventually find some niche application.
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Any of the aforementioned methods for removing the passive layer will allow the posterior nitriding and/or carburizing treatment to be carried out at the desired temperature and with the benefits of a batch process, without the need to keep the parts from touching each other. In most cases, the environmental impact is small when compared with cyanide salt baths but much higher when compared to plasma-assisted processes. The gas and energy consumption are much higher. Another point of concern is the safety of the process, which needs careful consideration in order to avoid explosion hazards. Low-temperature gas nitriding with acetylene surface activation This process [46] uses a mixture of C2H2, H2, NH3, and N2 to promote nitriding, carburizing, and nitrocarburizing in temperatures ranging from 370 to 430 C (700 to 805 F) for up to 20 h. The resulting carbon- and/or nitrogen-expanded austenite has similar properties to the PN layers. An important benefit of this treatment is that no additional surface treatment is needed to remove the passive layer, because the highly reactive C2H2 is said to be able to reduce the oxide layer during the nitrocarburizing, carburizing, and nitriding treatments. As explained, this is possible because the triple bonds between the carbon atoms are more unstable than single bonds decomposing at lower temperatures: or carburizing, below 550 C (1,020 F), and for nitriding and nitrocarburizing, below 450 C (840 F). The reaction is as follows [48]: C2 H2 þ H2 ½C þ 1=2 H2
with
KC ¼ ðpC2 H2 =pH2 Þ;
where [C] is the amount of carbon dissolved into the metal and in equilibrium with the atmosphere, resulting in a high carburizing potential. The carburizing potential (KC) controls the amount of carbon that is possible to incorporate into the steel. Here, the atomic carbon has a double duty: first, to depassivate the surface, and second, to serve as a source of carbon for carburizing. At this time, the exact mechanism of surface depassivation is still unknown, but the presence of H2 is important for the reaction to take place. This first depassivation step is necessary for nitriding and/or carburizing. Careful temperature control (decrease in temperature) is necessary to prevent soot deposition at the metal surface, which would be detrimental to the treatment. An important advantage of the C2H2 ? H2 mixture over the CO–CO2–H2–H2O mixture is the absence of oxidizing species that could repassivate the surface during the treatment [46]. In a nitriding treatment in a NH3–H2 atmosphere, the following reactions are expected [49]: 1=2
N2 ½N
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Fig. 40 Atomic concentration of nitrogen as a function of nitriding potential (Kn) and depth. Source: Ref. [50]
NH3 1=2 N þ 3=2 H2 resulting in: NH3 ½N þ 3=2 H2 ; where [N] is the nascent nitrogen. The resulting nitriding potential is defined as follows: Kn ¼ pNH3 =pH2 3=2 The nitriding potential controls the amount of nitrogen in the metal, as shown in Fig. 40. Figure 41 shows nitrided layers on AISI 316 under different nitriding potentials for a 22 h treatment. Figure 41(a) at Kn = 0.293 bar-1/2 and Fig. 41(b) at Kn = 2.49 bar-1/2 show the resulting difference in layer depth. Figure 42(a) shows a carburized layer on AISI 316 heat treated at 773 K for 4 h using Kc = ? and subsequently nitrided at 713 K for 18.5 h with Kn = ?; Fig. 42(b) shows a nitrocarburized sample treated at 693 K for 19 h with 10% Ar ? 54% NH3 ? 22% H2 ? 14% C2H2, with the resulting double layer. Some examples of nitrocarburizing and carburizing treatment processes are presented in Ref. [46]. What little information is available, mostly from patents, shows that low-temperature stainless steel gas nitriding is in the initial stage of industrial implementation. More scientific publications would help to better understand the process applicability and to attest to its reproducibility in different situations. Gas Nitriding at High Temperatures—Solution Nitriding High-temperature nitriding or solution nitriding (1,100 ± 50 C, or 2,010 ± 90 F) is a new treatment available to produce cases with nitrogen concentrations in the range of 0.4 to 0.9 wt%, which is low compared to plasma nitriding treatments at low temperature (up to
16.9 wt% N), but the resulting case is up to 100 times deeper (2 mm, or 0.08 in.). The lower nitrogen concentration implies a lower surface hardness (360 HV for solution nitriding of austenitic steels). Martensitic stainless steels present a maximum surface hardness of 700 HV after solution nitriding [52] versus 900–2,000 HV for plasma nitriding [5, 53]. It is also necessary to quench at the end of the treatment to prevent the obtained metastable layer from dissociating in the more stable CrN, to avoid the sensitization reaction (the precipitation of chromium carbide), and to prevent the formation of other composition-dependent phases (austenite and/or ferrite) that can also precipitate, to the detriment of corrosion resistance in austenitic stainless steels or of the martensitic structure in martensitic stainless steels. The higher temperature is needed to enhance nitrogen diffusion and decrease the time necessary for case formation, but it has consequences: •
•
•
Grain growth is a problem, although it is reported that a careful selection of the material to be treated can lessen the problem. Distortion is caused by quenching, which limits its application in the case of parts with complex geometries. The reactor design is difficult, given the high operational temperatures (*1,100 ± 50 C, or 2,010 ± 90 F) with pressures up to 2 bars (200 kPa) of pure N2 (needed to improve treatment efficiency and nitrogen concentration in the case). The equilibrium concentration of atomic nitrogen at the alloy surface is determined by the following equation [3, 5]: 1=2
ln½N ¼ ln pN2 ln fNX DH0 =RT þ DS0 =R; where fXN is the activity coefficient of the alloy (constant), and H0 and S0 are the standard enthalpy and
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Fig. 41 Micrographs of nitrided AISI 316 (nitrided at 718 K for 22 h). (a) Kn = 0.293 bar-1/2. (b) Kn = 2.49 bar-1/2. Source: Ref. [51]
Fig. 42 Micrographs of (a) carburized and subsequently nitrided AISI 316 and (b) nitrocarburized AISI 304. Treatment specifications: (a) carburizing: 773 K for 4 h at Kc = ?, subsequent nitriding: 713 K for 18.5 h at Kn = ?; (b) 693 K for 19 h with 10% Ar ? 54% NH3 ? 22% H2 ? 14% C2H2. Source: Ref. [51]
•
entropy for the chromium–nitrogen system, from the DG0 (Gibbs standard free energy). As seen in the equation, nitrogen concentration increases with pN2 and with a negative DH0 (in this case, the heat of solution of chromium–nitrogen), but the term is divided by T, meaning that an increase in temperature decreases the nitrogen concentration in equilibrium. The increase in temperature is needed because it improves diffusion, decreases treatment times, and prevents the precipitation of nitrides and/or carbides. However, to keep the solubility of nitrogen stable at the desired level, an increase in nitrogen pressure is needed when the temperature is increased [5]. This pressure increase is an additional problem in furnace design, worse when operating at 1,100 ± 50 C (2,010 ± 90 F). The need for quenching at the end of the treatment further complicates the reactor design, increasing equipment price, operation, and maintenance. That is probably the reason why the available reactors are small compared to plasma nitriding chambers. Figure 43 shows a time–temperature transformation (TTT) curve for the precipitation of Cr2N, from which it can be estimated that, to avoid CrN precipitation, the temperature must drop from 1,100 to below 900 C (2,010 to below 1,650 F) in approximately 20 s. That is feasible only in relatively thin parts (large area/ volume ratio) that cool sufficiently fast to cope with the
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Fig. 43 Time–temperature transformation curve for precipitation of CrN in stainless steel
TTT curve time limitations to avoid precipitates, and only for a small number of parts at a time. Berns [3, 52] presents a list of successful applications that includes flanges, ball bearings, molds, and impellers, all in martensitic steels. There are some examples with austenitic steels, such as impellers and disks among other cavitation-resistant parts. The available literature does not adequately explain the consequence of the resulting grain coarsening and how to effectively lessen the problem. Additional toughness testing is needed to verify the effectiveness of the treatment.
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Table 5 Industrial processes for S-phase production Trade name
Process
Owned by
Kolsterising
Carburizing
Bodycote
Nivox
Plasma carburizing
Nitruvid, France
Expanite
Gas nitriding/carburizing
Expanite A/S, Denmark
NV-Pionite
Gas carburizing
Air Water Ltd., Japan
Swagelok
Gas carburizing
Swagelok, United States
Palsonite
Salt bath nitriding
Nibin Parkerising, Japan
Solnit
High temperature gas nitriding
Ipsen, Germany
Industrial Processes for Low-Temperature Nitriding/ Carburizing and New Developments for the Treatment of Stainless Steel As discussed in previous sections, the S-phase may be produced by many laboratory processes, including diffusion-based processes using liquid and gas phases, plasma treatment, and ion implantation. However, with the exception of the pulsed plasma nitriding treatment, which is already firmly established, only a few industrial processes have been developed over the last 17 years, due to technical, economic, and environmental constraints. Table 5 presents some of these processes [10]. Nitriding in Fluidized-Bed Reactor at Low Temperatures [54] Fluidized-bed reactors are well known for their high values of heat- and mass-transfer coefficient, which allow them to provide high-temperature and composition uniformity. These qualities are desirable in a great number of industrial processes and also in nitriding media used to increase the surface hardness of steel by the diffusion of nitrogen in its crystal lattice while keeping distortion to a minimum. A fluidized-bed reactor is a complex dispersed system usually composed of a granular solid phase (sand or alumina, depending on the operating temperature) and a gaseous fluid that provides the fluidization of the solid phase. The solid granular material is thus suspended in a gas cushion, and the system behaves as a fluid with a pseudodensity, occupying the whole volume of the chamber, propagating waves, and flowing continuously, much like boiling water in a pan. Heating can be accomplished in a number of ways: by electric heaters or burners heating the gas phase before entering the fluidized bed or by combustion in the fluidized bed. Temperature can be automatically controlled within ±3 C (±5.5 F). The resulting system shows the outstanding property of homogeneity in temperature and composition, much like a molten salt bath but without the environmental and safety problems associated with high-temperature molten salts or metals.
Fluidized-bed reactors can be made as large as needed without performance degradation, so part size is not a problem. Energy efficiency is good because the system is well insulated and the mass- and heat-transfer coefficients are high; thus, less treatment time is needed. In nitriding treatment furnaces, the heating is done electrically, and the fluidizing gas composition is chosen to provide the optimum chemistry for the desired surface treatment (usually a mixture of NH3 ? N2, with CH4 added when nitrocarburizing is desired). The granular solid is considered inert and does not react with the part being treated but is fundamental in establishing the fluidized bed and its outstanding properties. So far, a number of difficult steps have been addressed toward a highly efficient nitriding treatment: temperature and atmosphere composition control with an homogeneity equal to or better than most competing processes, low environmental impact, no size restrictions, and good energy efficiency. The main difficulty for this nitriding method is that the material to be treated (stainless steel) must be depassivated before the nitriding treatment in order to remove the Cr2O3 layer that protects against corrosion but also against nitriding. This can be achieved by acid pickling and/or abrasive blasting. However, neither of these is as efficient as sputtering prior to plasma nitriding to remove the oxide layer within the same chamber that will be used for the posterior nitriding treatment, which completely isolates the part from atmospheric O2 and promotes the best possible surface properties for nitrogen diffusion into the metal substrate. This fact alone offsets the initial advantage of low environmental impact and increases operational costs over those of plasma nitriding. Also, the resulting layers tend to be shallower than those produced by plasma nitriding in the same treatment time. Some of these effects are also due to another peculiarity of stainless steel nitriding: the need to keep treatment temperature below 450 C (840 F) to prevent the precipitation of chromium nitride (and chromium carbide, which starts to precipitate at approximately 500 C or 930 F). The lower treatment temperature increases the efficiency of
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Fig. 45 Case thickness for fluidized-bed nitrided 316L samples after 8 h of treatment. Source: Ref. [55]
Fig. 44 Percent NH3 dissociation versus temperature in a fluidizedbed furnace. Source: Ref. [54]
the Cr2O3 barrier against nitriding; even after the acid treatment, there is still some oxide layer left at the steel surface. The result is lower diffusion rates and lower nitriding potential (less dissociation of NH3, which is the mean supplier of atomic nitrogen needed for the nitriding treatment), as shown in Fig. 44. The NH3 dissociation effect is controlled by changing the composition of the atmosphere, but the oxide layer would dissociate or become permeable only at much higher temperatures, which would cause the precipitation of nitrides and carbides in the case. Control of Nitriding Potential at Low Temperatures Because the treatment temperature must be kept, ideally, below 430 C (805 F), the dissociation of NH3 and thus the nitriding potential is controlled by the addition of H2, N2, and CH4 (which dissociates into carbon and hydrogen and also promotes nitrocarburizing) in the treatment atmosphere. The addition of H2 and CH4 decreases the NH3 dissociation, and the addition of N2 increases the NH3 dissociation according to the following relationship: %N ¼ k ½pðNH3 Þ=pðH2 Þ3=2 ; where k is the equilibrium constant, and p is the partial pressure of NH3 and H2. The final result is that, after 8 h of nitriding treatment, the fluidized bed is able to produce a case approximately 8 lm at its thickest, as shown in Fig. 45. In contrast,
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plasma nitriding of 316L samples for 8 h would result in a layer thickness on the order of 18 to 20 lm. If the treatment temperature could be increased, as in regular nonstainless steel nitriding, the results would be much more favorable, but at this point there are more disadvantages than advantages in using this process for the nitriding of stainless steels. Salt Bath Nitriding Molten salt baths are regularly used for carbonitriding treatments of carbon and low-alloy steels, normally using temperatures above 550 C (1,020 F). Recent developments on low-temperature (below 450 C, or 840 F) salt baths have shown potential uses in stainless steel nitriding. As has been seen, low temperature is important in stainless steel thermochemical treatments to prevent the precipitation of chromium nitrides and carbides that could render the stainless steel susceptible to corrosion. Molten salt baths offer many features that are very desirable in metal heat and thermochemical treating: heatand mass-transfer coefficients high enough to ensure homogeneity and efficiency, protection from atmospheric oxidation, simple parts arrangement, and the availability of different salt compositions for specific treatments and temperatures. The major disadvantage of this technique is the use of cyanide salts, which are toxic and promote nitrocarburizing instead of nitriding. Low-temperature nitrocarburizing produces properties very similar to nitriding but somewhat better load-bearing capacity, so this is a disadvantage only if nitriding alone is a must. Environmental impact is improved because cyanates are much more environmentally friendly and have substituted for cyanides in nitriding and nitrocarburizing treatments. However, they are somewhat more complicated to control
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and require regeneration from time to time to keep process properties stable. The present state of research and development on lowtemperature salt bath nitriding and nitrocarburizing is as follows: •
•
Treatment in temperatures above 500 C (930 F) shows very good kinetics but causes the precipitation of chromium nitrides and carbides. This is discussed later in this article. Reference [56] reports salt bath nitriding of AISI 304 stainless steel in temperatures as low as 430 C (805 F), producing a 6 lm case composed of the S-phase with some nitrides after a 4 h treatment. Longer treatment times result in increasing quantities of nitride precipitation and thus should be avoided. This treatment uses a complex salt bath composed of M2CO3 (M = potassium, sodium, or lithium) ? CO(NH2)2 ? trace quantities of unspecified components. The concentration of CNO- in the resulting bath was above 40%; it is reported that it was able to remove (reduce) the Cr2O3 surface layer present in the AISI 304 samples and thus promote the nitriding treatment. The atomic nitrogen used in the nitriding reaction originates from the following CNO- dissociation reaction: 4CNO ! CO2 3 þ 2CN þ CO þ ½N
Carburizing is also promoted by the liberation of atomic carbon species according to the following CO dissociation reaction: 2CO ! CO2 þ ½C
•
In contrast, a 4 h treatment at 430 C (805 F) in plasma nitriding/nitrocarburizing will produce a case with 10–20 lm thickness of high-hardness S-phase with no carbide or nitride precipitation in AISI 304 samples. Eventually, if desired, longer treatment times can be used to increase case depth without any precipitation reactions. Reference [57] describes low-temperature (450 C, or 840 F) nitriding treatments being developed using molten KNO3 ? ultrapure N2 atmosphere as the nitriding medium. It is stated: ‘‘The thermal decomposition of KNO3 on heating may liberate nascent (atomic) nitrogen before the formation of molecular nitrogen (N2). The nascent nitrogen can diffuse into the steel coupon.’’ This goes somewhat against the literature about safe use of molten salt baths, which states: ‘‘It is important that the temperature of molten nitrate salts should not be permitted to exceed 550 C (1,022 F) since, at temperatures not far above this point, nitrates
•
will decompose with liberation of oxygen, and this reaction may occur with extreme explosive violence’’ [58]. How the decomposition of the nitrates would occur at lower temperatures and in a controlled way is not explained. According to Ref. [57], the best results were obtained at 450 C (840 F) with prior sample polishing for a 3 h treatment time. The case depth obtained was in the range of 200–400 nm, and the corrosion resistance was improved. The polishing operation was crucial in decreasing (although not eliminating) the passive layer barrier against nitrogen diffusion into the metal lattice. It is suggested that the nitrogen concentration at the layer should be approximately 0.07 wt% when the treatment is performed at 600 C (1,110 F), a treatment temperature known to be very detrimental to corrosion resistance, probably because of the precipitation of chromium carbides and/or nitrides. One can estimate an even lower nitrogen concentration for treatments performed at 450 C (840 F). In contrast, low-temperature PN AISI 304 can have up to approximately 16 wt% N on the formed S-phase layer. Unpolished samples showed minor improvement in corrosion resistance, suggesting that the as-received material passive layer posed a stronger barrier against the nitriding reaction. At any rate, a case depth of 200–400 nm is useless for the improvement of wear resistance. The commercial processes Tenifer and Tufftride are based on salt bath nitriding, originally using cyanides but now using cyanates to avoid the environmental problems. The chemical activities of the cyanate bath are more difficult to control, but eventually these initial problems were overcome, and nitriding systems for tool steels (extrusion dies) have been operated at temperatures as low as 430 C (805 F). As the chromium content increases, the Cr2O3 passive layer becomes an increasing problem, and some type of pretreatment for its removal is recommended [59]. A variation of these processes [60] uses two additional steps of oxidation after the nitriding treatment, with a polishing operation between them. This is said to improve corrosion resistance even when nitriding in temperatures above 450 C (840 F), which precipitates chromium nitrides and carbides.
At the time of this writing (2014), low-temperature salt bath nitriding of stainless steel is, at best, a work in development. The commercial treatment processes do generally work for nonstainless steels, but process mechanisms and case properties for stainless steels are very scarce in the literature. Additional research and development is needed to understand and control the reactions and develop them into robust industrial processes.
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Table 6 Comparative table of advantages and limitations of plasma and gaseous processes Process
Advantages
Limitations
Plasma
Reduced processing times; reduced consumption of treatment gas and energy; low environment impact
Use of skilled manpower for the arrangement of loads and determining the most appropriate parameters operation to eliminate or minimize problems such as opening arcs, hollow cathode effect or edge effect (the pulsed treatment or active screen process can be used to overcome these problems if they are present)
Gaseous
High temperatures: thicker layers
High temperatures: high operating costs (gas and energy); layers with low hardness; application in specific situations (cavitations, ball-bearings, knives); necessary quench limits treatment to parts with favorable area/ volume ratio; available furnace relatively small for industrial applications (40 9 40 9 60 cm, or 16 9 16 9 24 in.)
Low temperatures: batch process of small parts (with movable retort)
Low temperatures: few data available, mostly in patents only. (The few complete scientific publications are not enough to attest reproducibility and technical viability.) Careful process operation to avoid explosion hazard; high gas consumption
Final Remarks Deposition of S-phase layers over stainless steels (and also over nickel–chromium and cobalt-chromium alloys) can be performed by surface-engineering technologies based on ion beam, gaseous, plasma, and PVD processes. This allows the design of multifunctional surfaces to meet the ever-increasing demand for higher performance and longer component life. The resulting increased surface wear and corrosion resistance, together with excellent tribological, tribochemical, and fatigue properties, are unprecedented and much needed for demanding applications in health, nuclear, chemical, food processing, and general engineering industries. However, the full potential of S-phase surface engineering will not be realized until all the scientific challenges and technological barriers are addressed in the future. These challenges include determining the atomic structure of the S-phase (i.e., the configuration and chemical bonding between interstitial and substitutional solute atoms); the crystal structure requirement for the formation of the S-phase (whether, for example, the fcc structure is essential for the formation of the S-phase and, if so, why it cannot be formed in non-fcc materials); learning whether S-phase can be formed only in alloys based on such 3d transitional metals as iron, cobalt, and nickel; and why the maximum S-phase layer thickness decreases in the order of iron, cobalt, and nickel, which have the outer electron structures 3d64s2, 3d74s2, and 3d84s2, respectively [10]. Because the S-phase is a metastable phase, its safe use requires TTT diagrams. However, the isothermal transformation (TTT) diagram of S-phases depends on the type and amount of both the interstitial (carbon and nitrogen) and the substitutional (chromium, nickel, molybdenum, etc.) alloying elements. There is also little information on the mechanical stability of the S-phase, although it is known
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that some thermodynamically metastable phases can decompose into stable phases under mechanical stress [29]. Although the S-phase shows a desirable combination of properties, such as high hardness and corrosion resistance, the layer thickness is much thinner than those of other nitrided/carburized steels, which translates into a lower load-bearing capacity. The only exception is the solution nitriding treatment done at high temperatures followed by a low-temperature treatment. Such a duplex treatment is complicated by the fact that quenching is needed at the end of the first treatment to prevent the precipitation of nitrides and/or carbides, meaning that distortion and microstructure transformation will occur [29]. At this time, low-temperature plasma-assisted nitriding/ carburizing is an established industrial process with a very well-known application envelope, provided by the great number of scientific publications and from various authors. The initial limitations concerning the process have been overcome by pulsed plasma and new developments such as PIII and ASP. At this moment, low-temperature gas nitriding promises unparalleled performance for batch processing of small parts, but more research will be needed to increase its applicability. The other processes discussed in this article are still in the initial development phase and will need more work to achieve industrial applicability (Tables 6, 7).
Future Directions A better understanding of the nature of the S-phase is necessary in areas such as the atomic and electron structure of the S-phase, the bonding between interstitial and substitutional elements, the effect of crystal structure of the substrate materials on the formation of S-phase, and the effect of substitutional alloying elements on the
100–1,000
Low
Consumables cost
PCN = 76%H2–20%N2– 4%CH4
PN = 75–80%H220–25%N2
PC = 5–40
Medium
100–1,000
PN = 75–80%H220–25%N2
2–20
Low
0.1–0.5
PN = 75–80%H2-20–25%N2
3–6
Medium
1 atm
NH3 ? N2 ? H2 ? C2H2
PCN = 10 (9 h)
PC = 15 (22 h)
PN = 17 (3–5 h)
High
2 atm
N2
Up to 2,000
Medium– high
1 atm
–
6
PCN = 1,350
PNC = 1,500
PN = 2–20
Austenitic = 360
PC = 800
PC = 700–1,000
Pressure (Pa)
Gas
Layer thickness (lm)
– 1,200
4
430
Salt bath
–
4–24
1,050–1,150
High temperature
Martensitic = 700
–
4–22
430–450
Low temperature
Gaseous
PN = 1,200
45 kV pulses
PN = 1,300–1,500
Hardness (HV)
1,300–1,600
400–1,000
Potential (V)
5–20
400–1,000
5–20
5–20
250–400
Plasma immersion Ion Implantation (PIII)
1,100–1,300
350–450
350–450
Temperature [C (F)] Treatment time (h)
Active screen
DC or Pulsed
Characteristics
Plasma nitriding (PN) Plasmacarburizing (PC) Plasma nitrocarburizing (PNC)
Process
Table 7 Process parameters of stainless steels nitriding and/or carburizing treatments
–
Medium
1 atm
NH3 ? N2
8
1,500
8
450
Fluidized bed
Metallogr. Microstruct. Anal. (2014) 3:477–508 507
123
508
supersaturation and metastability of the S-phase. With such understanding, new and improved treatments are expected to solve some of the problems reported and increase the application of the S-phase surface treatment [10].
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