ISSN 10678212, Russian Journal of NonFerrous Metals, 2011, Vol. 52, No. 1, pp. 75–81. © Allerton Press, Inc., 2011. Original Russian Text © V.I. Kostikov, Zh.V. Eremeeva, V.Yu. Dorofeev, N.N. Zherditskaya, 2010, published in Izvestiya VUZ. Poroshkovaya Metallurgiya i Funktsional’nye Pokrytiya, 2010, No. 1, pp. 3–9.

THEORY AND PROCESSES OF FORMING AND SINTERING OF POWDER MATERIALS

Formation of Structure and Properties during the Thermal Treatment of Powder Steels with Different CarbonContaining Components V. I. Kostikova, Zh. V. Eremeevaa, *, V. Yu. Dorofeevb, **, and N. N. Zherditskayaa a

Moscow Institute of Steel and Alloys, National Research Technology University, Leninskii pr. 4, Moscow, 119049 Russia *email: eremeeva[email protected] b South Russian State Technical University (Novocherkassk Polytechical Institute), ul. Prosveshcheniya 132, Novocherkassk, 346430 Russia **email: [email protected] Abstract—The effect that thermal treatment has on the formation of the structure of hotstamped powder carbon steels with different carboncontaining components (CCCs) is determined. Processes of thermal treatment such as quenching, tempering, and annealing are investigated. The inheritance of the initial struc ture was observed at all steps of thermal treatment. Keywords: powder carbon steels, carboncontaining components, quenching, tempering, annealing. DOI: 10.3103/S1067821211010111

Improving the physical–mechanical properties of powder steels can be achieved not only by changing their composition and production practice, but also by directional thermal treatment (TT). The results of TT are affected by factors both inherent to cast steels and specific ones caused by the characteristic properties of the initial materials used and technologies for their processing [1, 2]. The possibility of applying different types of TT is based on the fact that each powder par ticle is compact metal in which the same transforma tions take place as in large volumes of metal during heating and cooling. However, the features of the ther modynamic state of powder metal do not allow us completely evaluate its structural changes using the regularities observed for compact cast steels. The specifics of the formation of the structure and properties of hotdeformed powder steels obtained from a charge with different carboncontaining com ponents (CCCs) is related to the features of phase transformations and to the character of structures aris ing under their effect. These phenomena can proceed both directly in the course of cooling the samples after hot stamping (HS) (then they are similar to hightem perature thermomechanical treatment (HTMT)) and during the TT of the materials already cooled after HS [3, 4]. In this work, the effect of a cooling rate after hot stamping on the structure formation of powder steels was determined. Water, oil, or air was used as a cooling medium. In the first case, the effect of the HTMT is achieved with a simultaneous increase in the strength

and plasticity of samples containing all types of CCCs and any amount of carbon (Tables 1, 2). As should be expected, the highest properties are characteristic of the steels of eutectoid composition, which were obtained with the application of artificial special low ash carbon (ASLC) and hightemperature pitch (HP). Steel samples after HS were heated at a rate of 5– 6 K/min to a temperature of 850–875°C; held for 15– 20 min; and cooled Vcool at a rate of 450–500 (in water), 100–150 (in oil), and 30–45 (in air) K/s. The mechanical properties of steels that contain pen cil lead (PL) and ASLC within the charge are improved as Vcool rises (like in cast steels), remaining much higher in the second case. An increase in the cooling rate of the samples doped with pyrocarbon (PC) from 150 to 450 K/s has almost no effect on their properties, which allows us to use oil as a cooling liq uid. Cooling in air did not allow us to obtain a struc ture characteristic of hardened steel in any of the cases (Table 2). The structure of the samples obtained from the charge containing PL after hot stamping consists of largelamellar pearlite; upon heating, it transforms into largegrain austenite. The tendency of PC to seg regate causes the formation of austenite with a differ ent carbon saturation of grains, which results in an intensified growth of martensite crystals during quenching. The coherence of austenite and martensite in ASLCcontaining and HPcontaining steels is vio lated because of the appearance of dislocations at the interface of these phases during martensite transfor 75

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Table 1. Properties of powder steels cooled after HS in various media CCC type in the charge

Carbon content in steel, %

Cooling medium Air

Water*

σB, MPa

δ, %

HRC

σB, MPa

δ, %

HRC

PL

0.5 0.8 1.2

515 755 1320

19 16.5 5

20 18 33

750/1.46 900/1.20 1125/0.85

29/1.53 19/1.15 16/3.2

25/1.25 34/1.89 34/1.03

ASLC

0.5 0.8 1.2

775 975 1510

22 10 8

20 22 38

925/1.19 1275/1.31 1800/1.19

37/1.68 26/2.6 18/2.3

(185)/1.13 28/1.27 38/1

HP

0.5 0.8 1.2

750 1125 2004

22 17 13

22 25 44

1525/2.03 1850/1.64 2220/1.11

31/1.41 22/1.29 15/1.15

31/1.41 43/1.72 55/1.25

* The denominator represents the ratio of the values of the parameter during cooling in water and in air.

Table 2. Effect that the cooling rate during TT has on the structural formation of different regions and properties of hot stamping samples of steel 80p for different CCCs in the charge CCCs type in charge

Cooling medium

νcool, K/s

Structure

HRC (HRB)

Surface

Core

Surface

Core

σB, MPa

PL

Water Oil Air

450–500 100–150 30–45

M+A+B M+A+B FP + P + C

M + A + FP M+A+S FP + P + F

35 27 (89)

29 20 (86)

1575 1125 800

ASLC

Water Oil Air

450–500 100–150 30–45

M+A M + A + FP P+F+A

M + A + FP M+A+S P+F

42 35 (92)

33 33 (88)

1800 1500 1075

HP

Water Oil Air

450–500 100–150 30–45

M+A M+A B+P+C

M+A M+A P+F

50 35 (95)

38 33 (90)

2075 1750 1325

Note: M is martensite, A is austenite, P is perlite, B is bainite, FP is fine pearlite, S is sorbite, C is cementite, and F is ferrite.

mation, and the rapid growth of martensite grains is stopped. The martensite structure with inclusions of residual austenite and bainite is observed on the surface of sam ples of the eutectoid composition, which were obtained from the PLcontaining charge, after annealing in water. Most martensite needles have a rough surface, which is caused by the precipitation of fine carbide particles of the Fe3C type. In its core, bainite is absent, but an increased amount of fine pearlite appears. The fracture has a brittle transcrystal line river character. Residual graphite inclusions are the centers of cracking. Quenching microcracks were observed in largeacicular martensite; they intersect martensite plates or are located in the places of joint with cementite inclusions (Fig. 1). The occurrence of cracks indicates significant quenching stresses.

The surface and core of the HPcontaining sam ples after quenching in water have a martensite struc ture with austenite inclusions; no microcracks were found. Fine martensite needles of different etching abilities and single carbide particles are mainly found, destruction proceeds along the martensite plate, and the fracture is brittle and stony (Fig. 2). It is character ized by pits that are divided into honeycombs; conse quently, the pit surface is formed during destruction along the finedispersed subinterfaces. In the case of the ASLCcontaining samples quenched in water, we observed spearshaped large martensite needles which were formed in the initial period of transformation and fine needles formed dur ing the following cooling. A significant amount of resid ual austenite was also found, and it increased as the

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(а)

(b)

(c)

(d)

Fig. 1. Microstructure of PLcontaining powder steel after quenching in water. (a) Martensite with carbide precipitations (×1000), (b) fractography of the fracture (×3000), (c) coal replica with a fineperlite structure (×5800), and (d) fine structure and carbide precipitates (×105).

(а)

(b)

(c)

(d)

Fig. 2. Microstructure of PLcontaining powder steel after quenching in water. (a) Fine foil with twins of martensite needles (×19000), (b) coal replica with globules of residual austenite (×7200), (c) fractography of the fracture (×1500), and (d) coal replica (×5200). RUSSIAN JOURNAL OF NONFERROUS METALS

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(а)

(b)

(c)

(d)

Fig. 3. Microstructure of ASLCcontaining powder steel after quenching in water. (a) Martensite (×15000), (b) fine foil with dis locations in austenite (×105), (c) coal replica with martensite and residual austenite (×5800), and (d) fractography of the fracture (×3000).

(а)

(b)

(c)

(d)

Fig. 4. Microstructure of PCcontaining powder steel after quenching in water. (a) Twin martensite needles (×1000), (b) globules of residual austenite (×14000), (c) fractography of the fracture (×2000), (d) coal replica (×4000).

quenching temperature increases. The destruction is interparticle and brittle; the fracture is stony (Fig. 3). During the investigation of fine foils obtained from the samples after HS and TT (followed by cooling in water and containing PC in the charge), large packets of martensite crystals were found. Their apparent sec

tion in the foil plane has a uniquiaxial shape. In the case of the present treatment, they completely inherit the substructure of initial austenite. We can see on a replica that the sizes of twin martensite needles are sig nificant. These samples are characterized by brittle destruction (Fig. 4).

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After cooling in air, none of the samples of steel 80p (which were obtained from charges containing various CCCs) had the martensite–austenite structure. The quenching in oil of PLcontaining materials resulted in the formation of a martensite–bainite structure with a large fraction of residual austenite, the coarse grains of which are divided into subgrains of various shapes; they have a developed dislocation structure. The sorbite sites with the precipitation of platelike cementite appear possibly as a result of self tempering. The fracture is viscobrittle. The ASLCcontaining samples cooled in oil con tained martensite needles with fine spheroidized cementite inclusions and sorbite. The fracture has a mixed transcrystalline and intercrystalline character. In this case, for the PCcontaining steel, the trans formation of austenite into martensite is realized most completely. Microfractogramms reflect the mixed character of the fracture; it is brittle, intergranular, and with viscous tornoff regions. The features of the structure of thermally treated hotdeformed powder steels obtained from charges containing different CCCs affected their mechanical properties. As the carbon concentration increases, the hardness and strength of powder steels rise similarly to cast steels; ASLCcontaining and HPcontaining samples are characterized by higher values of these parameters. The reduced properties of PCcontaining steel can be explained by decarbonization in the course of heating for quenching. The lower character istics of PLcontaining steel are likely to be caused by the appearance of “soft” spots of residual austenite and by a nonuniform carbon distribution in the sample bulk (see Table 2). Tempering at t = 150–300°C was performed for 1.0–1.5 h. As the temperature increases, the strength and hardness of steels containing any types of CCCs decrease; this decrease is less substantial if ASLC and HP are applied in the charge (Fig. 5). This is related to the fact that small martensite needles are less prone to softening. The PCcontaining samples have somewhat lower values of these parameters at all tempering tem peratures, which is a result of selective structural inho mogeneity. The latter is most characteristic of PLcontaining steels, for which σb and HRC were minimal. The structure of the PLcontaining samples that were formed during lowtemperature tempering con tains the cementite and ferrite agglomerations, and the structure of the ASLCcontaining samples contains fine carbide precipitates in the range of the initial mar tensite needles. Large and fine martensite needles are found in PCcontaining steels after tempering at 150°C. An increase in temperature results in the isola tion of cementite inclusions in these needles. Tempering at 300–700°C was performed with holding for 30–45 min. We found the decomposition of martensite, the occurrence of return processes, RUSSIAN JOURNAL OF NONFERROUS METALS

σb, MPa 1600

79

(a)

1400 1200 HP ASLC

1000 After 800 quenching

PC PL

600 400

100 200 300 400 500 600 t0, °C

HRC 120 100

(b) HP ASLC PC PL

80 60 40 20 0

After quenching 100

δ, % 16 14 12 10 After 8 quenching 6 4 2 0 100

200

300

400

500

(c)

600 t0, °C HP ASLC PC PL

200

300

400

500

600 t0, °C

Fig. 5. Effect that the tempering temperature has on the properties of steel 80p obtained from the charge containing different CCCs. (a) Ultimate tensile strength, (b) hard ness, and (c) specific elongation.

recrystallization, and carbide transformation for all steels, which affects the characteristics of the material. The most intense decrease in strength and increase in relative elongation and impact strength take place at t = 500°C. In regards to the extent of the effect on mechanical properties, the CCCs can be ranged as HP–ASLC–PC–PL. This is determined by the sizes, morphology, and subgrain structure of initial austen ite. Cementite inclusions within ASLCcontaining and HPcontaining materials become spherical; finer particles are dissolved, and coarse ones transform into globules. In the PLcontaining samples, we observe an intense growth in coarse carbide particles. Results of investigations carried out using trans mission electron microscopy for fine foils show that

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1 2 3 4 5 6

0

30

60

90

120 τ, min

Fig. 6. Effect that the annealing time (tann = 1100°C) has on the ultimate tensile strength of steel 80p containing dif ferent CCCs. (1, 5) HP, (2) ASLC, (3) PC, and (4, 6) PL. (1–4) Heating before HS for 30 min; (5, 6) heating for 10 min.

the initial state of austenite substantially affects the character and rate of its decomposition during tem pering. In the HPcontaining and ASLCcontaining sam ples, the formation of carbides, their coagulation, and spheroidization occur at t0 = 400–500°C; in PCcon taining materials they occur at 450–550°C; and in the PLcontaining sample, they happen only at 600– 700°C and spheroidization is incomplete. This is related to the slower dissolution of the initial PC and PL particles, which become the reason for the appear ance of coarsegrain austenite and martensite that appear during quenching, as well as numerous defects. Tempering of the HPcontaining and ASLCcon taining steel at 600–700°C results in the uniform structure of fine sorbite with fine spherical grains of cementite. In PCcontaining materials, the sphe roidization of cementite occurs only partially. The microstructure of the PLcontaining samples is nee dleshaped; while on the fracture surfaces, sites with the intergranular destruction and intragranular cleav age, which alternate with small regions of viscous pre cipitation, prevail. The ASLCcontaining samples are characterized by transcrystalline destruction, which alternates with the sites of the punctulated profile. To decrease the chemical and structural inhomoge neity, spheroidize the residual pores, and stabilize the structure, the diffusion annealing of the prismatic samples obtained according to the optimal operating conditions was carried out. Heating was performed in dissociated ammonia to a temperature of 1100°C with holding for τ = 30, 60, 90, and 120 min. A relatively high temperature of annealing exerted no noticeable negative effect on the properties of the samples due to the hereditary fineness of steels. Annealing substantially affects the strength and plasticity of steels. The stabilization of these charac

teristics comes at different durations depending on the CCC content in the charge (Fig. 6), which is appar ently caused by the unequal degrees of the “nonequi librium” of the material. This is, in turn, related only to the CCC nature, because all residual technological parameters were identical. An improvement in the properties of HPcontaining and ASLCcontaining samples is observed at τ < 60 min and further they remain almost invariable. Such annealing mode retains the structure of finegrained perlite and, there fore, can be recommended for practical application. For PCcontaining samples, an annealing time of 60 min is necessary; at the given holding, the sphe roidization of perlite also takes place. The PLcontain ing samples require longer diffusion annealing, because partially spheroidized perlite is formed and the mechanical properties improve only at τ = 90–120 min. To obtain the uniform structure of granular perlite, the value of τ should be large. The shape of cementite grains changes during annealing in all cases, which is related to the repacking of iron atoms during the phase transformation. Because cementite has a larger specific volume than ferrite, the generation of vacancies, the rate of which increases under the influence of deformation, as well as the presence of carbon is necessary for repacking. The fact of a simultaneous increase in strength and plasticity during annealing of the materials seems interesting. This can be explained by the healing of defects arising in the course of the hot recompacting of moldings, the improvement of the quality of fusing on the thus formed contact surfaces, and the decrease in the softening action of pores due to their healing and spheroidization. It is noteworthy that, as the annealing duration increases, the properties of steels with all CCCs improve and the ranking of the carboncontain ing components, which is discovered at other steps of obtaining and treating hotdeformed steels, remains (namely, HP–ASLC–PC–PL). This indicates that the positive qualities of unconventional CCCs are also inherited at this technological step. CONCLUSIONS The HTMT effect is achieved during the accelerated cooling of hotdeformed powder steels obtained from charges containing different CCCs. This is related to the austenite structure formed upon heating and char acterized by fineness, which is inherited by highquality fineneedle martensite. In the course of the thermal treatment of powder steel, the martensite structure is formed only during cooling in water. With the use of oil for these goals, such a structure is revealed only for PC containing and HPcontaining steels. No quenching structure arose after cooling in air. The characteristics of CCCs affect the formation of the structure and properties of steels during temper ing, which manifests itself in the inheritance of their

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features laid at all previous technological steps. There fore, the properties of ASLCcontaining and HPcon taining steels tempered at all temperatures improved with respect to the PLcontaining materials. Diffusion annealing of the ASLCcontaining and HPcontaining samples, which are characterized by the homogeneity and fineness of the structure, leads only to the additional refinement of perlite. Therefore, we can either not perform this operation at all, or we can restrict ourselves to holding for 30 min at 1100°C. Only prolonged annealing for 2 h at t0 = 1100°C was favorable for the formation of a structure with separate regions of granular perlite for PLcontaining samples. A homogeneous structure requires an even longer time for annealing.

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REFERENCES 1. Ermakov, S.S. and Vyaznikov, N.F., Poroshkovye stali i izdeliya (Powder Steels and Products), Leningrad: Mashinostroenie, 1990. 2. Metallovedenie i termicheskaya obrabotka stali: Spravo chnik (Physical Metallurgy and Thermal Treatment of Steel: Handbook), Bernshtein, M.L. and Rakhshtadt, A.G, Eds., Moscow: Metallurgiya, 1991, vol.1. 3. Gurevich, Yu.G. and Rakhmanov, V.I., Termicheskaya obrabotka poroshkovykh stalei (Thermal Treatment of Powder Steels), Moscow: Metallurgiya, 1985. 4. Dorofeev, Yu.G., Marinenko, L.G., and Ustimenko, V.I., Konstruktsionnye poroshkovye materialy i izdeliya (Con struction Powder Materials and Products), Moscow: Metallurgiya, 1986.

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