Tribol Lett (2009) 34:167–175 DOI 10.1007/s11249-009-9425-7
ORIGINAL PAPER
Wear Mechanisms at High Temperatures: Part 2: Temperature Effect on Wear Mechanisms in the Erosion Test H. Winkelmann Æ M. Varga Æ E. Badisch Æ H. Danninger
Received: 15 January 2009 / Accepted: 19 February 2009 / Published online: 5 March 2009 Springer Science+Business Media, LLC 2009
Abstract In previous investigations on wear mechanisms at high temperatures made in High Temperature-Single Impact Test (HT-SIT) and High Temperature-Continuous Impact Abrasion Test (HT-CIAT), predominant wear mechanisms were identified and correlations to different material parameters could be presumed. In order to confirm these correlations, four different alloys which are promising to be used in high temperature applications like a sinter grate have been studied in the High Temperature-Erosion Test (HT-ET) by the use of different impact angles and different impact energies. Especially the change of wear mechanism caused by increasing testing temperature was analysed in detail. Characterisation of microstructure and wear behaviour has been done by optical microscopy (OM) and scanning electron microscopy (SEM). Results obtained by the use of the different measurement techniques were linked and set into comparison to calculate the volumetric wear of the specimen. Predominant wear mechanisms were determined using OM in the mode of cross-section images and SEM. The results indicate that material parameters such as hardness and hard phase content can be correlated to the erosion wear rates at different impact angles. The test results indicate that at higher temperatures, the material fatigue becomes a major wear-determining factor. The test results also confirmed that there is a critical impact energy for each material above where the wear rate increases significantly. Test results with thermally aged materials H. Winkelmann (&) M. Varga E. Badisch AC2T Research GmbH, Viktor Kaplan-Straße 2, 2700 Wiener Neustadt, Austria e-mail:
[email protected] H. Danninger TU Wien, Institute of Chemical Technologies and Analytics, Getreidemarkt 9, 1060, Vienna, Austria
also show that a better heat-resistant matrix reduces the material fatigue thus resulting in lower wear rates. Keywords High temperature Wear Erosion Material fatigue Critical impact energy Thermal ageing
1 Introduction Various materials have been studied in order to achieve increase in the lifetime of a sinter gate. In general, different iron-based alloys are used for this component [1]. The demands on materials which are used as a sinter grate are temperature and oxidation resistance, impact and abrasion resistance. These material demands and the operating cycle of the sinter grate are explained in detail in the introduction of ‘‘Wear mechanisms at high temperatures—Part 1: Wear mechanisms of different Fe-based alloys at elevated temperatures’’ [1]. Mechanical and abrasive wear properties of the steel and other complex alloys depend upon their microstructure and chemical composition [2–13]. A high ductility and good interfacial carbide–matrix bonding is necessary to achieve a high impact resistance [1]. In the case of high impact application, therefore martensitic materials behave best [14]. For abrasion resistance, hard phases and high hardness are the important parameters, especially important being that the hardnesses of the hard phases and/or the matrix is higher than the hardness of the abrasive [15–18]. For temperature resistance and oxidation resistance, a highly alloyed matrix especially such as austenite behaves well [19, 20]. To meet the loading conditions of the sinter grate and to find correlations between high temperature wear and hard phase content, different kinds of matrices and coarseness of the microstructure have been investigated in the previous
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mechanisms at higher temperatures, these materials were also tested in annealed condition. During this ageing process, the samples were kept at 750 C for 24 h at ambient atmosphere. Microstructural characterisation has been done through optical microscopy (OM) and scanning electron microscopy (SEM ? EDS). The matrix microstructure was revealed by etching procedure. Quantitative analysis of the microstructure was carried out by the use of Imtronic Image C Software. Hardness measurements were performed through standard Vickers hardness technique HV5 for macroscopic hardness. HV0.1 was used to determine the hardness of each phase in the microstructure, e.g. hard particles and metallic matrix. Quantitative wear characterisation has been done by gravimetric mass loss of the testing specimen during wear testing. Qualitative characterisation of worn surfaces and worn edges has been carried out by evaluating of macroscopic and cross-sectional images and by SEM investigations. Alloy A (Fig. 1a) is a heat-resistant austenitic steel with 0.08% C, *25% Cr and *20% Ni. The hardness of this alloy was determined as 175 HV5. Austenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength, when compared to typical carbon steel. Austenitic steels have fcc lattice structure which provides more gliding planes for the flow of dislocations, which, combined with the low level of interstitial elements, gives this material its good ductility but significant work hardening. Alloy B is a standard wrought M2 high-speed steel (HSS). Microstructure in Fig. 1b exhibits a martensitic microstructure with fine primary hard phases. Total content of carbides is approximately 15–20%. The hardness of this alloy is measured as 880 HV5. Alloy C shows a dense and uniform distribution of very hard
article (Wear mechanisms at high temperatures—Part 1 [1]). It could be shown by the use of HT-SIT (High Temperature-Single Impact Test) and HT-CIAT (High Temperature-Continuous Impact Abrasion Test) that there exists a material-dependent critical impact energy for Fe-based materials; however, the quantitative verification was not possible for ductile materials. In addition, it was observed in [1] that hard phases are important for abrasion resistance; however. at high temperatures, the wear rates can increase significantly when the matrix cannot backup the hard phases any more. Due to high application temperatures, material ageing itself is an important aspect since the material is often used in thermally aged condition in application. The aim of this study was to explain the high-temperature behaviour of different Fe-based alloys in the High Temperature-Erosion Test (HT-ET) for quantitative determination of material dependent critical impact energies. Detailed explanation of changes in wear mechanisms caused by thermal-ageing effects was also the focus of these investigations.
2 Experimental 2.1 Materials and Characterisation In this study, an austenitic stainless steel, a standard M2 tool steel (HSS), a complex Fe–Cr–C–Nb–Mo–W–B alloy with fine microstructure and a hypereutectic Fe–Cr–C– Mo–Nb alloy with coarse microstructure were investigated. All the materials and their chemical composition are given in Table 1. Typical microstructures of the alloys are shown in Fig. 1. For a better understanding of the different wear
Table 1 Chemical composition and hardness of the Fe-based alloys investigated Low hard phase content
High hard phase content
A Austenite 1.4841
B Tool Steel 1.3343
C Complex Fe–Cr–C–Nb–Mo–W–B alloy; fine microstructure
D Hypereutectic Fe–Cr–C–Mo–Nb alloy; coarse microstructure
Chemical composition [wt%] Fe Base
Base
Base
Base
C
0.08
0.9
1.3
5.5
Cr
24.8
4.1
15.4
21.0
Ni
19.8
–
–
–
Si
1.7
0.25
0.5
0.8
Mn
1.2
0.3
0.2
0.2
Nb
–
–
4.2
7.0
B
–
–
4.2
–
Others (Mo, V, W)
–
13.2
11.5
10.0
175
880
1020
880
Hardness [HV5]
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Fig. 1 Microstructure of asreceived alloys versus thermally aged at 750 C for 24 h: a A: austenite, (a1) A*: aged A, b B: M2 tool steel, (b1) B*: aged B, c C: complex Fe–Cr–C–Nb–Mo– W–B, (c1) C*: aged C, d D: hypereutectic Fe–Cr–C–Mo–Nb and (d1) D*: aged D
complex carbides and carbo-borides (Fig. 1c) with hardness values between 1200 and 1900 HV0.1. The hard phases were identified as Fe/Cr carbo-borides at a volume content of 52% at a size of 10–100 lm and Nb carbides and Mo/W carbo-borides at a volume content of approximately 5% in blocky shape [14]. The hardness of the alloy is very high which is reflected by a macro hardness of 1020 HV5. Alloy D consists of primary Fe/Cr carbides with a micro hardness of roughly 1600 HV0.1 in a ledeburitic matrix (Fig. 1d). The content of Fe/Cr carbides is 57.1% with size of 30–200 lm. The chemistry of the Fe/Cr carbides for hypereutectic Fe–Cr–C alloys in literature is referred to as M7C3 structure [21–24]. The hardness values of the ledeburitic matrix which are determined to be about 800 HV0.1 are close to results shown by Fischer and Buytoz [25] and [26]. In addition, small and evenly distributed primary Nb carbides at a volume content of approximately 5% can be detected. These are supposed to be of major important features for increasing the resistance against erosion and abrasion due to their high hardness. The hardness of the material D is 880 HV5.
2.2 High Temperature-Erosion Test (HT-ET) The wear behaviour of the materials investigated under 2-body erosion at high temperature has been tested in a conventional centrifugal four-channel accelerator where up to 20 specimens can be tested simultaneously under identical conditions as shown in Fig. 2. Five samplesof each material were tested, and each material was tested in two independent test runs. The remaining specimen slots were filled with dummies. The surfaces of the specimen before the test were cleaned to remove dust using ethanol and compressed air. After cleaning and drying, the specimen weight was determined. Before the erosion test, the furnace with installed specimens was heated to the required test temperature. After erosion test, the furnace was cooled before the samples were removed. The duration of heat-up cycle is about 1 h and cool-down sequence about 30 min depending on the temperature required. The test itself lasts about 1 h. In the erosion tests, 6 kg of abrasive was used. After the test, the specimens were cleaned to remove abrasive
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3 Results and Discussion 3.1 Microstructure and Hardness Changes Due to Annealing at 750 C for 24 h
Fig. 2 Schematic representation of the erosion rig
Table 2 Erosion test conditions Parameter
Value
Impact velocity
80 m/s
Impact angles
30, 90
Erodents
Silica sand, 0.1–0.3 mm, angular Silica sand, 0.7–1.0 mm, angular
(Ekin = 0.24 mJ)
(Ekin = 18.11 mJ)
Tungsten carbide, (Ekin = 0.16 mJ) \0.2 mm, spherical Test temperatures 25, 300, 500, 650 C Total weight of erodent 6 kg
dust and the weight loss due to the erosion was determined. For all the experiments, the particle velocity was set constant at 80 m/s whereas impact angle was kept variable between 30 and 90. Three different abrasives were used. The erosion test conditions are summarised in Table 2. The erosion rate was determined as a volume loss of the specimen per mass of abrasive particles (mm3/kg). Volumetric erosion is defined as the volume loss of the sample while 1 kg of abrasive hits the sample. An accuracy of 0.1 mg was obtained for the targeted weight loss measurements. Morphologies of the eroded samples are examined by SEM to understand the material removal mechanism. The eroded samples are then sectioned parallel to the particle flow along a plane perpendicular to the eroded surface. The sectioned surfaces were then polished and studied in the SEM in order to assess the nature of subsurface deformation and formation of surface layers.
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The materials also have been investigated in thermally aged condition. Therein they have been annealed at 750 C for 24 h. Microstructures of the as-received and annealed materials investigated are compared in Fig. 1 from which differences in carbide distribution and carbide size can be identified. The hardness of the materials are given in Table 3, where the calculated hardness loss is also listed. From Fig. 1, it can be seen that for alloy A* no significant change in microstructure can be detected which is in good agreement with hardness results which are the same as with alloy A (as-delivered). In alloy B, the M2 steel, through the same thermal treatment, the hardness drops around 67% from 880 HV5 (*65 HRC) to 290 HV5. The reason for this softening effect can be found by significant microstructure changes caused by spheriodised annealing of the martensitic matrix. Alloy C is the hardest among the alloys investigated; because of, on the one hand, the high content of borides, boro-carbides and carbides, and on the other hand, the relatively hard fine-crystalline matrix. After annealing, the hardness is reduced to 570 HV5 which indicates a loss in hardness of *44%. This softening effect can be explained by coarsening of grains in the ferrite matrix at these temperatures. For Alloy D, the hardness is reduced from 880 HV5 to 770 HV5 because of annealing which means a total reduction of 12%. For the loss of hardness, mainly the transformation of the martensitic– austenitic matrix into ferrite structure is responsible. However, the high content of hard phases which are still present is responsible for high hardness even after annealing. 3.2 HT-Erosion Test Results and Discussion The materials investigated show different wear behaviour in the HT-ET. Especially this test is useful to build up fundamental understanding on the different wear mechanisms at high temperatures, since by varying the impact energy and angle, different loading conditions can be adjusted. HT-SIT and HT-CIAT results of previous Table 3 Hardness values of the materials investigated as-received compared to 24 h annealed at 750 C Material
A
B
C
D
Hardness [HV5]
175
880
1020
880
Material (annealed)
A*
B*
C*
D*
Hardness [HV5] (annealed condition)
175
290
570
770
Loss of Hardness [%]
0
67.0
44.0
12.5
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Fig. 3 Volumetric erosion of material B depending on impact energy in HT-ET with impact angles of 30 and 90
investigations [1] indicated that for each material a defined critical impact energy exists beyond which the wear rate significantly increases. Using different abrasives with different particle mass during the HT-ET, attempts were made to corroborate this theory. For material B, this effect can be shown clearly in Fig. 3. Volumetric erosion in multiphase materials is generally higher at 90 impact compared with oblique impact for all the impact energies since generally at 90 impact, a higher percentage of the particle energy is transmitted into the material [27]. The critical impact energy is detected to be approximately 0.2 mJ where a significant increase in volumetric erosion can be observed. The HT-CIAT results indicated in [1] that at high temperatures, material fatigue becomes an important parameter. This effect could be confirmed through the HTErosion tests (Fig. 4) for materials with high hard phase content. It can be observed that at room temperature, volumetric erosion increases when the impact energy of one single particle increases (Table 4). But at higher temperature (650 C), volumetric erosion decreases with increase of the impact energy of single particles (Table 5). The
Table 4 Volumetric erosion [mm3/kg] at 20 C for different particle impact energies Alloys
Material C Material D
Impact energy of abrasive 18.11 mJ
0.24 mJ
Oblique impact 30
31.06
23.92
Normal impact 90
47.52
31.46
Oblique impact 30 Normal impact 90
36.00 82.20
23.16 51.24
Table 5 Volumetric erosion [mm3/kg] at 650 C for different abrasive impact energies Alloys
Material C Material D
Impact energy of abrasive 18.11 mJ
0.24 mJ 64.67
Oblique impact 30
36.55
Normal impact 90
34.14
67.15
Oblique impact 30
72.01
79.8
Normal impact 90
100.73
157.91
Fig. 4 a Room temperature erosion versus b erosion at 650 C at different particle impact energies for material C and D
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feeding of abrasive particles through the nozzle results in lower frequency of impacts of coarse abrasive particles compared to that of the feeding of the fine ones. This holds both for oblique and for 90 impact. The fact that many slight impacts with the same total energy deposited on the sample cause more wear than less heavy impacts indicates, that at higher temperatures, material fatigue is the weardominating parameter. The tests in the HT-CIAT indicated in [1] show that materials with low hard phase content tend to form more developed composite layers during high temperature loading, than those with high hard phase content. In Figs. 5, 6 and 7, SEM micrographs of worn samples after HT-erosion tests can be seen. It was observed that for material B tested at room temperature mainly particle sticking occurs (Fig. 5), whereas in Fig. 6 it can be seen,
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Fig. 7 SEM micrograph of worn surface of alloy C as-received, and tested at 650 C and oblique impact
Fig. 8 Erosion rate of material A in as-received and in annealed condition at 90 impact angle depending on testing temperature Fig. 5 SEM micrograph of worn surface of material B as-received, and tested at 20 C and 90 impact
Fig. 6 SEM micrograph of worn surface of material B* in annealed condition tested at 650 C and 90 impact
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that at 650 C material B* forms a composite layer, while at material C (Fig. 7) also at 650 C that just particle sticking occurs. It was observed that the tendency to form composite layer during erosion increases for materials with lower hard phase content, at high temperature and under impact angle close to normal. Since material properties can change when the material is exposed to high temperatures for a long time, the materials have also been tested in annealed condition (Fig. 1, Table 3). When comparing the HT-Erosion wear rates of the different materials in annealed condition to the same material as-delivered, some, at first surprisingly, results become quite obviously retrospective that at high temperatures, material fatigue becomes the wear-dominating parameter (Figs. 8, 9, 10 and 11). In material A, there are no significant micro-structural changes during annealing, and thus the unchanged result is trivial to explain (Fig. 8). Material B is spheriodise annealed through annealing (Fig. 1b1); lower hardness basically leads to
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Fig. 9 Erosion rate of material B in as-received and in annealed condition at 30 impact angle depending on testing temperature
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Fig. 11 Erosion rate of material D in as-received and in annealed condition at 30 impact angle depending on testing temperature
Fig. 12 Correlation coefficient: hardness–volumetric erosion rate Fig. 10 Erosion rate of material C in as-received and in annealed condition at 90 impact angle depending on testing temperature
more wear as long as hard phase cracking or decohesion of hard phases does not become the wear-dominant effect. Thus, the total wear is increased in annealed condition (Fig. 9). The wear rate is increased with temperature at both annealing conditions for material C. But since the matrix changes through thermal ageing did not weaken the matrix-carbide bonding, the hard phases still give good wear resistance, so the total wear remains approximately the same in material C* annealed condition (Fig. 10). The wear rate of annealed D* is increasing significantly with increasing temperature if compared to material D in as-received condition. Due to multiple impacts, carbides break and become unclenched, and so fall out of the matrix. This wear mechanism is escalated in material D* annealed condition (Fig. 11). Since the hardness is an important material parameter in the field of tribology, the correlation coefficient rxy (Eq. 1) of hardness to volumetric erosion rate at different temperatures was calculated for the two different impact angles (Fig. 12). The coefficient ranges from -1 to 1. A value of 1 shows that a linear equation describes the relationship perfectly and positively, with all the data points lying on
the same line and with Y increasing with X. A score of -1 shows that all data points lie on a single line but that Y increases as X decreases [28]. P ðxi xÞðyi yÞ rxy ¼ ð1Þ ðn 1Þsx sy rxy correlation coefficient: hardness (xi)–volumetric erosion rate (yi) sx, sy are the standard deviations of measurement data x; y are the mean values of measurement data It can be seen in Fig. 12 that at oblique impact (30) all the correlation coefficient values are negative, which means that at higher material hardness volumetric erosion is reduced. Also at oblique impact at room temperature, a better correlation coefficient can be detected, since elevated-temperature hardness usually is different from roomtemperature hardness. However, at normal impact (90), positive correlation coefficient values can be observed at all conditions, which means that volumetric erosion rate increases with hardness. That mathematically means that some other factors (e.g. ductility) have positive influence on the volumetric erosion. That means that at 90 erosion, the wear rate cannot necessarily be correlated to the hardness of the material and that the wear rate is increased with hardness.
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4 Conclusions The aim of this study was to investigate the temperature and thermal ageing effect on the wear mechanisms in the High Temperature-Erosion Test (HT-ET). The investigations have been done at two different impact angles and also by using different impact energies of abrasives. Following conclusions can be drawn: •
•
•
•
•
For ductile materials, there is a critical impact energy beyond which the wear increases significantly. This critical impact energy exists at both impact angles of 90 and 30 oblique impact. At high temperatures, for materials with high hard phase content, material fatigue is the wear-dominating process, which leads to the conclusion, that good matrix behaviour at high temperatures is very important for low wear rates. Especially the hard phase bonding is important, due to paradontose effect (due to multiple erosion impacts, carbides can fall out of the matrix during erosion if the bonding between carbide and matrix is very weak). The wear resistance of materials with a high-alloyed matrix (A, C) remains at the same level when tested in as-received or annealed conditions. Materials with a less-alloyed martensitic matrix (B, D) have a higher wear rate in annealed condition (B*, D*), because during annealing the martensitic matrix is spheriodise annealed and behaves worse under these test conditions. Hardness is an important factor to decrease wear at oblique erosion, but at 90 erosion, a high hardness is not necessarily beneficial for wear resistance. Erosion wear rate actually increases with hardness under these testing conditions (80 m/s, which is above critical impact energy). Ductile materials such as tool steel and austenite can form a composite layer during erosion testing. This effect is more pronounced at higher temperature and at less hard phase content.
Acknowledgements This study was funded by the ‘‘Austrian KplusProgram’’ and has been carried out within the ‘‘Austrian Center of Competence for Tribology’’. The authors are also grateful to Bo¨hlerEdelstahl for providing steels and performing heat-treatment procedures for different steel types, and Castolin Eutectic for help in the manufacturing of the welding samples. Katsich Christian is acknowledged for help in performing tests and wear quantification.
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