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Three-body abrasive wear of 0.98% carbon steel
802
Wear, 162-164 (19Y3) 802-810
Three-body
abrasive wear of 0.98% carbon steel
S. Das, B. K. Prasad, A. K. Jha, 0. P. Modi and A. H. Yegneswaran Regional Research Laboratory, Council of Scientific and Industrial Research, Hoshangabad Road, near Habibganj Naku, Bhopal, 462026 (MP) (India)
Abstract Low stress abrasion studies of a hypereutectoid steel have been carried out using a rubber wheel abrasion test apparatus. Hardness of the steel was changed by subjecting the specimens to different heat treatment cycles. Abrasion tests were conducted at various loads and wheel speeds using crushed silica sand as the abrasive medium. Wear rates of the steel in all test conditions decreased significantly during the running-in period prior to attaining steady state values, which was considered to be due to the abrasion-induced work hardening of the regions close to the abraded surface. Results showed that the increase in bulk hardness of the steel specimen caused a linear increase in wear resistance. Furthermore, up to a bulk hardness of about 400 HV, the rate of increase was higher than that above 400 HV. Increase in the applied load caused lower wear resistance, while speed did not show any definite trend. One of the material removal mechanisms, in particular, was found to be microcutting as indicated by continuous grooves on the wear surface and generation of machining chips in the debris. Micropitting was found to be another wear mechanism as evidenced by the formation of craters on the wear surface and flake-shaped particles in the wear debris.
1. Introduction Many studies of the two-body abrasive wear behaviour of carbon steels have been reported [l-12]. Three-body abrasion, however, has received less attention despite its commercial and industrial significance [13, 141. The influence of bulk hardness on the abrasion resistance of steels has been examined in earlier studies [2-6, 10, 13-161. A linear relationship between the bulk hardness and the abrasion resistance of steel has been observed in most of the investigations [2-6, 10, 13, 141 and a nonlinear relationship was suggested in another study [15]. In addition to this, it has also been suggested that the microstructure and fracture properties alter the wear behaviour significantly [3, 6-8, 14, 17, 181. It has also been reported that steels of various chemical compositions, heat treated to the same hardness, do not possess the same abrasion resistance [8-111. This indicates that the bulk microstructure plays an important role in controlling the abrasive wear behaviour of steels. In addition to the bulk microstructure, the study of abrasion-induced subsurface changes also throws light on the operating wear mechanisms [12, 141. The influence of the applied load and the nature and the type of the abrasive on the abrasion resistance of steels has also been studied [13, 19, 20).
0043-1648/93/$6.00
An examination of the preceding indicates that very few of the above-mentioned three-body abrasive wear studies [20] have been conducted on the standard rubber wheel abrasion test apparatus. In view of the above observations, the present study has been undertaken to examine the abrasion behaviour of a hypereutectoid steel in different heat treated conditions using an ASTM standard rubber wheel abrasion test (RWAT) apparatus. The influence of applied load and sliding speed has also been studied.
2. Experimental details
2.1. Material A hypereutectoid steel (Fe-O.98 C-O.38 Cr-0.8 Mn-0.08 S-O.06 P) was chosen as the test material for studying the abrasion behaviour.
2.2. Heat treatment cycles Specimens were subjected to various heat treatment cycles (HTC) in order to produce a range of bulk hardness. Table 1 presents the heat treatment cycles along with the resulting bulk hardness values.
0 1993 - Elsevier
Sequoia.
All rights reserved
S. Das et al. / Three-body abrasive wear of 0.98% carbon steel TABLE 1. Bulk hardness of the steel after various heat treatments Heat treatment
Bulk hardness (W
cycles
(1) Annealing: austenitisation at 860 “C for 1 h followed by furnace cooling
187
(2) Normalising: austenitisation by air cooling
as in (1) followed
240
(3) Hardening: austenitisation by ice-water quenching
as in (1) followed
912
(4) Hardening plus tempering: hardening as in (3) followed by tempering at 300 “C for 2 h
590
(5) Hardening plus tempering: hardening as in (3) followed by tempering at 500 “C for 2 h
404
2.3. Abrasion test
Abrasion tests were carried out on metallographically polished surfaces using a Falex RWAT apparatus as per ASTM G6.5-81 specifications [21]. Crushed silica sand particles (size: 212-300 pm) were used as the abrasive medium. The test procedure has been discussed in detail elsewhere [14]. Applied loads in a specific case (annealed steel) were 49,71 and 91 N, while wheel rpms adopted were 150 r.p.m. (1.79 m SK’), 273 r.p.m. (3.26 m s-l) and 400 r.p.m. (4.45 m s-l). The influence of the bulk hardness on abrasion resistance was studied at an applied load of 49 N and 273 r.p.m. speed. Weight loss measurements were made at regular test intervals (each test interval corresponding to a sliding distance of 400 m) until a steady state wear loss was obtained. Tests from the second interval onwards were conducted on the same wear track, i.e. on preworn surfaces. Wear rates were computed from weight loss measurements. 2.4. Microscopy Specimens for microstructural examinations were polished by standard metallographic procedures, etched with 0.5% nital reagent and observed in both optical and scanning electron microscopes. Abraded surfaces, subsurfaces and debris were examined in the scanning electron microscope. 2.5. Hardness Bulk hardness measurements were made using a Vickers hardness tester. Six observations (on average) were reported in each case.
3. Results 3.1. Microstmcture
Figure l(a) shows a typical microstructure of the annealed steel. It clearly reveals the lamellar pearlitic
803
structure along with the gram boundary network of primary cementite. The normalised steel showed identical structure with fine pearlite lamellae, Fig. l(b). The microstructure of the hardened steel consisted of the normal martensite needles, Fig. l(c). Low temperature (300 “C) tempering of the hardened steel was not observed to bring about any significant morphological changes in martensite and the structure remained needle-shaped, Fig. l(d). On the other hand, high temperature (500 “C) tempering was found to form tempered martensite, Fig. l(e). 3.2. Wear behaviour Figure 2 shows the wear rate of steel in various heat treated conditions as a function of the number of test intervals at an applied load of 49 N and 273 r.p.m. speed. It may be noted that the wear rate decreased with the increase in the number of test intervals until the steady state wear rate was attained. Figure 2 also shows that the wear rates of hardened and hardened plus tempered steels were significantly lower than those of the annealed and normalised ones. The steady state wear resistance (inverse of steady state wear rate) has been computed from Fig. 2 and plotted in Fig. 3 as a function of bulk hardness. Figure 3 shows that the wear resistance increased linearly with bulk hardness. Further, up to a bulk hardness of about 400 HV, the rate of increase of abrasion resistance was higher than that above the mentioned hardness. The influence of sliding speed and applied load on the abrasion resistance of the annealed steel may be seen in Fig. 4. The figure indicates that the wear resistance decreased with load. The wheel revolutions per minute value was not found to affect the wear resistance of the steel significantly at 71 and 93 N. However, at an applied load of 49 N and lower wheel speed (150 r.p.m.) the wear resistance decreased, while the remaining two speeds (273 and 400 r.p.m.) offered comparable wear resistance. 3.3. Wear swjace The wear surface of the annealed steel tested at 49 N and 273 r.p.m. reveals continuous grooves and a considerable extent of micropitting on the wear surface, Fig. 5(a). A higher magnification micrograph clearly shows evidence typical of the change in the direction of the wear grooves, Fig. 5(b). The normalised steel tested under identical conditions showed wear surface similar to that of the annealed one. Figure 5(c) demonstrates a typical wear surface of the normalised steel showing a severely scratched region (arrow marked). The wear surface of the hardened steel depicts continuous grooves with practically no pitting, Fig. 6(a). Micropitting to some extent along with grooves were observed in the case of the hardened plus tempered
804
S. Das et al. I Three-body abrasive
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Fig. 1. Microstructure of the steel in various heat treated conditions: (a) annealed, (b) normalised, (c) hardened, (d) hardened plus tempered (300 “C) and (e) hardened plus tempered (500 “C).
(HTC No& Table 1) steel, Fig. 6(b). Occasionally, severely damaged regions were also observed, Fig. 6(c). Figure 7(a) shows the abraded surface of the annealed steel at 49 N load and 150 r.p.m. speed. It may be seen that the grooves are not well-defined and considerable micropitting of the surface has been caused by the abrasive particles. A magnified view clearly reveals the micropitting and few grooves in this case, Fig. 7(b). The wear surface of the steel tested at 49 N load and 400 r.p.m. speed showed relatively less pitting and wefl-
defined grooves, Fig. 7(c). Higher load (91 N) and speed (400 r.p.m.) caused the formation of deeper grooves on the abraded surface with very little pitting, Fig. 7(d). 3.4. Subsurface studies Figures 8(a)-8(d) show the transverse section of the abraded surface of annealed steel at 49 N load and 273 r.p.m. speed. The propagation of microcracks parallel and perpendicular to the sliding direction can be
S. Das et al. / Three-body abrasive wear of 0.98% carbon steel
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Fig. 2. Wear rate vs. number of test intervals: 0, annealed; A, normalised; 0, hardened; 0, hardened plus tempered (300 “C); A, hardened plus tempered (500 “C).
51 0
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Fig. 3. Wear resistance
SW
600
700
800
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900
(b)
( HV I
as a function
(4
of bulk hardness.
(c) Fig. 5. Wear surface: (a) annealed steel showing considerable amount of pitting; (b) annealed steel indicating a change in the direction of grooves; (c) normalised steel indicating a severely scratched region (arrow marked). 1.0 t
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Fig. 4. Wear resistance at various applied loads and speeds: 0, 150 r.p.m.; A, 273 r.p.m.; qi, 400 r.p.m.
seen in Fig. 8(a). Fragmentation of the pearlite lamellae in the regions close to the abraded surface may also be seen in Fig. 8(a) (top portion). Figure 8(b) clearly reveals the microcracks (arrow marked) and the broken pearlitic structure in the subsurface regions and the bulk microstructure consisting of normal lamellar pear-
806
S. Das et al. I Three-body abrasive wear of 0.98% carbon steel
found to be attached to the bulk, Fig. 8(e), while the hardened plus tempered (500 “C) steel revealed occasional attachment of the transfer layer to the bulk, Fig. 8(f). The heavily deformed layer in the subsurface region may also be seen in Fig. S(f) (top portion). 3.5. Debris analysis A range of shapes and sizes of wear debris was found in the case of annealed steel. These were micromachining chips, large flakes and fine particles (regions marked A, B and C respectively in Fig. 9(a)). In the case of normalised steel, the debris was found to be similar in nature to that of the annealed one. The debris produced in the case of hardened steel consisted mainly of machining chips (arrow marked) with a few flaky particles, Fig. 9(b). However, hardened and tempered steel generated equiaxed debris particles with a few machining chips, Fig. 9(c).
4. Discussion
Fig. 6. Wear surface: (a) hardened steel showing mainly continuous grooves; (b) hardened and tempered (HTC No.5, Table 1) steel depicting micropitting to some extent together with grooves, and (c) hardened plus tempered specimen showing severely damaged regions.
lite. Flow lines indicative of plastic deformation of the subsurface layer may be noted in Fig. S(c) (top portion), while a severely deformed layer attached to the bulk is shown in Fig. 8(d) (arrow marked). In the case of the hardened steel, practically no transfer layer was
It is well established that the hardness of a (metallic) material plays an important role in controlling its abrasion resistance [2-6, 10, 13-161. Usually the softer material wears faster and then, due to plastic deformation, the surface work hardens. This subsequently results in improvement in the wear resistance [14]. Similar behaviour of steel during abrasive wear has been observed in the present investigation, Fig. 2. The above tendency was clearly noticed in the case of the annealed and normalised specimens, while the same behaviour was not as prominant in the hardened and hardened plus tempered samples, Fig. 2. The results are also consistent with the subsurface analysis, Fig. 8. In this instance, severely deformed regions (indicating their work hardening) attached to the bulk have been observed for annealed (Figs. 8(a)-8(d)) and hardened plus tempered steels (Fig. 8(f)). The presence of flow lines (Fig. 8(c)) and broken microstructural constituents (Figs. 8(a), 8(b) and 8(f)) along with the structureless region (Fig. 8(d)) just below the wear surface indicates considerable deformation of the regions. An earlier investigation has suggested that severely deformed subsurface layers indicate considerable microstructural changes due to abrasion and also possess significantly higher hardness [14]. The increased rate of wear resistance up to 400 HV followed by a reduction in the rate of the increase (Fig. 3) is in agreement with the nature of the wear surface and debris particles generated. For example, the wear surface of the annealed steel (bulk hardness 187 HV) showed considerable amount of micropitting and few grooves (Figs. 5(a) and 5(b)). It has been suggested that micropitting is caused by the rolling
S. Das et al. / Three-body abrasive wear of 0.98% carbon steel
807
(d) Fig. 7. Abraded surface of annealed steel showing: (a) and (b) a lot of micropitting with few grooves at low load (49 N) and low speed (150 r.p.m.); (c) less pitting and well-defined grooves at 49 N and 400 r.p.m. speed, and (d) deeper grooves at 91 N load and 406 r.p.m.
tendency of the abrasive particles, while their sliding action produces continuous grooves [13, 15, 221. Types of debris particles generated due to rolling are flakes, while sliding action produces machining chips. The debris particles of the annealed steel consisted mainly of large flakes with a few machining chips (Fig. 9(a)) indicating that, in this case, rolling of the abrasive particles was more predominant than their sliding. The subsurface of the annealed steel revealing a large transfer layer attached to the bulk and plenty of microcracks (Figs. 8(a)-8(d)) also suggests the formation of large flaky debris particles (Fig. 9(a)) leading to poor wear resistance (Fig. 3). On the contrary, samples having bulk hardness more than 400 HV showed better wear resistance (Fig. 3). The wear surface of the hardened steel showed mainly grooves with practically no micropitting (Fig. 6(a)). The transverse section revealed no attachment of transfer layer to the bulk in this case (Fig. 8(e)). Above observations suggest the formation of more machining chips in the debris and fewer flakes (Fig. 9(b)) causing improved wear resistance
of the hardened steel (Fig. 3). In the case of the hardened plus tempered steel, the wear surface showed pitting to some extent in addition to the grooves (Figs. 6(b) and 6(c)). The subsurface analysis also showed occasional attachment of the transfer layer to the bulk (Fig. 8(f)) and the debris consisted of equiaxed particles together with machining chips (Fig. 9(c)). This agrees with the relatively poorer wear resistance of hardened plus tempered steel than that of the hardened one (Fig. 3). Thus, the above observations suggest that a mixed mode of wear mechanisms, such as microcutting due to sliding and plastic deformation caused by the rolling action of the abrasive particles, operates during the abrasive wear of the steel. At low speed i.e. at 150 r.p.m., there is every possibility that the abrasive particles will get enough time to interact with the specimen surface and roll over it. Such a situation would cause crater formation on the wear surface (Figs. 7(a) and 7(b)) offering poor wear resistance (Fig. 4). On the other hand, higher speeds would give rise to the predominantly sliding action of
808
S. Das et al. I Three-body abrasive wear of 098%
carbon steel
(d)
Fig. 8. The transverse section of the abraded surface of annealed steel showing: (a) subsurface cracks propagating parallel and normal to the sliding directions and broken pearhtic structure in the transfer layer, (b) a magnified view clearly revealing the cracks (arrow marked) and changed microstructure in the transfer Iayer (top} and the lamellar pearlitic structure in the bulk; (c) the presence of flow lines in the transfer layer (top portion); (d) a severely deformed region attached to the bulk (arrow marked); (e) practically no attachment of transfer layer to the bulk of hardened steel occurs; and (f) occasional attachment of the transfer layer ;o the bulk in the case of hardened pius tempered (500 “C) steel.
the abrasive and lead to the formation of continuous grooves on the wear surface (Fig. 7(c)). Such a situation would give rise to improved wear resistance (Fig. 4).
Higher load would further decrease the probabili~ of the rolling action of the abrasive and lead to the formation of deeper grooves on the wear surface by
S. Das et al. f Three-body abrasive wear
of 0.98% carbon steel
809
steel samples in this study consisted of micropitting caused by crater formation due to the rolling of abrasive particles and microcutting due to the sliding action of the abrasive. However, the predominance of any of the mechanisms would depend on the bulk hardness, which is indirectly controlled by the microstructure of the steel. At lower hardness (less than approximately 400 HV), micropitting will be the dominant material removal mechanism, whereas at higher hardness values microcutting would be the dominant wear mechanism. As regards bulk hardness, a hardness level of about 400 HV may offer the optimum wear resistance for the steel, since this would also give better toughness and hence improved impact resistance.
5. Conclusions 1. The wear rate of the specimens decreased with the number of test intervals until a steady state value was attained. This was probably due to the subsurface work hardening. 2. The wear resistance increased linearly with the increase in bulk hardness at a specific applied load and speed. The slope of the wear resistance versus bulk hardness curve changed at approximateiy 400 HV. 3. The wear resistance of the steel decreased with an increase in the applied load. The wheel speed did not show any appreciable influence on the wear resistance. However, at lower speed and low load the wear resistance of the sample was found to be less than that at higher speed. 4. At any bulk hardness, the material removal was caused by both microcutting and micropitt~g mechanisms. However, at lower bulk hardness (less than about 400 HV) wear was mainly caused by micropitting, while higher hardness caused the microcutting mechanism to mainly control the wear process. Further, at a lower speed, material removal was caused mainly by micropitting, while higher speeds favoured microcutting as the main operating wear mechanism.
Acknowledgment Fig. 9. Wear debris: (a) annealed steel showing machining chips, flakes and fine particles (regions marked A, B and C respectively); (b) hardened steel consisting mainly of machining chips (arrow marked); and (c) hardened plus tempered specimen revealing equiaxed debris particles with a few machining chips.
Authors are grateful to Professor T. C. Rao, Director, Regional Research Laboratory (CSIR), Bhopal, India, for his encouragement and granting permission to publish this paper.
the sliding action causing relatively poor wear resistance (Fig. 4). Above observations inferred that the micromechanisms of material removal during the abrasive wear of
References 1 L Xu and N. F. Kennon, Wear, 148 (1991) 101-112. 2 K. Hokkirigawa and K. Kato, Wear, 123 (1988) 241-251.
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S. Das et al. I Three-body abrasive wear of 0.98% carbon steel
3 J. Larsen-Badse and K. G. Mathew, Wear, 14 (1969) 199-205. 4 M. M. Khrushchev, Wear, 28 (1974) 69-88. 5 M. J. Murray, P. J. Mutton and J. D. Watson,J. Lubr. Techno/., Trans. ASME Trib., 104 (1982) 9-15. 6 M. A. Moore, Wear, 28 (1974) 59-68. 7 K. H. Z. Gahr, Relation between abrasive wear rate and the microstructure of metals, Proc. Znt. Conf: Wear Mater., American Society of Mechanical Engineers, New York, 1979, pp. 266-274. 8 P. J. Mutton and J. D. Watson, Wear, 48 (1978) 385-398. 9 M. M. Krushchov and M. A. Babichev, Friction and Wear in Machinery, 12 (1958) 5. 10 J. Larsen-Badse, Trans. Metall. Sot. AZME, 236 (1966) 1461. 11 M. M. Krushchov and M. A. Babichev, Friction and Wear in Machinery, 19 (1965) 1. 12 0. Vingsbo and S. Hogtnark, Wear of steels, in D. A. Rigney (ed.), Fundamentals of Friction and Wear of Materials, ASM, 1981, 373.
13 14 15 16
17 18 19 20 21 22
M. Misra and I. Finnie, Wear, 85 (1983) 57-68. B. K. Prasad and S. V. Prasad, Wear, 151 (1991) 1-12. L. Fang and Q. D. Zhou, Wear, 151 (1991) 313-321. J. Larsen-Badse and B. Premaratne, Effect of relative hardness on transition in wear mechanisms, Proc. Znt. Conf: Wear Mater., American Society of Mechanical Engineers, New York, 1983, pp. 161-166. K. H. Z. Gahr and D. V. Daone, Met. Trans. A, II (1980) 613-620. N. Prasad and S. D. Kulkami, Wear, 63 (1980) 329-338. E. Rabinowicz, L. A. Dunn and P. G. Russel, Wear, 4 (1961) 345-355. T. H. Kosel and N. F. Fiore, J. Mater. Energy Systems, 3 (1981) 7-27. Standard practice for conducting dry sand rubber wheel abrasion tests, ASTM Standard G65-81, 1981. Y. L. Wang and Z. S. Wang, Wear, 122 (1988) 123-134.
Wear, 162-164 (19Y3) 802-810
Three-body
abrasive wear of 0.98% carbon steel
S. Das, B. K. Prasad, A. K. Jha, 0. P. Modi and A. H. Yegneswaran Regional Research Laboratory, Council of Scientific and Industrial Research, Hoshangabad Road, near Habibganj Naku, Bhopal, 462026 (MP) (India)
Abstract Low stress abrasion studies of a hypereutectoid steel have been carried out using a rubber wheel abrasion test apparatus. Hardness of the steel was changed by subjecting the specimens to different heat treatment cycles. Abrasion tests were conducted at various loads and wheel speeds using crushed silica sand as the abrasive medium. Wear rates of the steel in all test conditions decreased significantly during the running-in period prior to attaining steady state values, which was considered to be due to the abrasion-induced work hardening of the regions close to the abraded surface. Results showed that the increase in bulk hardness of the steel specimen caused a linear increase in wear resistance. Furthermore, up to a bulk hardness of about 400 HV, the rate of increase was higher than that above 400 HV. Increase in the applied load caused lower wear resistance, while speed did not show any definite trend. One of the material removal mechanisms, in particular, was found to be microcutting as indicated by continuous grooves on the wear surface and generation of machining chips in the debris. Micropitting was found to be another wear mechanism as evidenced by the formation of craters on the wear surface and flake-shaped particles in the wear debris.
1. Introduction Many studies of the two-body abrasive wear behaviour of carbon steels have been reported [l-12]. Three-body abrasion, however, has received less attention despite its commercial and industrial significance [13, 141. The influence of bulk hardness on the abrasion resistance of steels has been examined in earlier studies [2-6, 10, 13-161. A linear relationship between the bulk hardness and the abrasion resistance of steel has been observed in most of the investigations [2-6, 10, 13, 141 and a nonlinear relationship was suggested in another study [15]. In addition to this, it has also been suggested that the microstructure and fracture properties alter the wear behaviour significantly [3, 6-8, 14, 17, 181. It has also been reported that steels of various chemical compositions, heat treated to the same hardness, do not possess the same abrasion resistance [8-111. This indicates that the bulk microstructure plays an important role in controlling the abrasive wear behaviour of steels. In addition to the bulk microstructure, the study of abrasion-induced subsurface changes also throws light on the operating wear mechanisms [12, 141. The influence of the applied load and the nature and the type of the abrasive on the abrasion resistance of steels has also been studied [13, 19, 20).
0043-1648/93/$6.00
An examination of the preceding indicates that very few of the above-mentioned three-body abrasive wear studies [20] have been conducted on the standard rubber wheel abrasion test apparatus. In view of the above observations, the present study has been undertaken to examine the abrasion behaviour of a hypereutectoid steel in different heat treated conditions using an ASTM standard rubber wheel abrasion test (RWAT) apparatus. The influence of applied load and sliding speed has also been studied.
2. Experimental details
2.1. Material A hypereutectoid steel (Fe-O.98 C-O.38 Cr-0.8 Mn-0.08 S-O.06 P) was chosen as the test material for studying the abrasion behaviour.
2.2. Heat treatment cycles Specimens were subjected to various heat treatment cycles (HTC) in order to produce a range of bulk hardness. Table 1 presents the heat treatment cycles along with the resulting bulk hardness values.
0 1993 - Elsevier
Sequoia.
All rights reserved
S. Das et al. / Three-body abrasive wear of 0.98% carbon steel TABLE 1. Bulk hardness of the steel after various heat treatments Heat treatment
Bulk hardness (W
cycles
(1) Annealing: austenitisation at 860 “C for 1 h followed by furnace cooling
187
(2) Normalising: austenitisation by air cooling
as in (1) followed
240
(3) Hardening: austenitisation by ice-water quenching
as in (1) followed
912
(4) Hardening plus tempering: hardening as in (3) followed by tempering at 300 “C for 2 h
590
(5) Hardening plus tempering: hardening as in (3) followed by tempering at 500 “C for 2 h
404
2.3. Abrasion test
Abrasion tests were carried out on metallographically polished surfaces using a Falex RWAT apparatus as per ASTM G6.5-81 specifications [21]. Crushed silica sand particles (size: 212-300 pm) were used as the abrasive medium. The test procedure has been discussed in detail elsewhere [14]. Applied loads in a specific case (annealed steel) were 49,71 and 91 N, while wheel rpms adopted were 150 r.p.m. (1.79 m SK’), 273 r.p.m. (3.26 m s-l) and 400 r.p.m. (4.45 m s-l). The influence of the bulk hardness on abrasion resistance was studied at an applied load of 49 N and 273 r.p.m. speed. Weight loss measurements were made at regular test intervals (each test interval corresponding to a sliding distance of 400 m) until a steady state wear loss was obtained. Tests from the second interval onwards were conducted on the same wear track, i.e. on preworn surfaces. Wear rates were computed from weight loss measurements. 2.4. Microscopy Specimens for microstructural examinations were polished by standard metallographic procedures, etched with 0.5% nital reagent and observed in both optical and scanning electron microscopes. Abraded surfaces, subsurfaces and debris were examined in the scanning electron microscope. 2.5. Hardness Bulk hardness measurements were made using a Vickers hardness tester. Six observations (on average) were reported in each case.
3. Results 3.1. Microstmcture
Figure l(a) shows a typical microstructure of the annealed steel. It clearly reveals the lamellar pearlitic
803
structure along with the gram boundary network of primary cementite. The normalised steel showed identical structure with fine pearlite lamellae, Fig. l(b). The microstructure of the hardened steel consisted of the normal martensite needles, Fig. l(c). Low temperature (300 “C) tempering of the hardened steel was not observed to bring about any significant morphological changes in martensite and the structure remained needle-shaped, Fig. l(d). On the other hand, high temperature (500 “C) tempering was found to form tempered martensite, Fig. l(e). 3.2. Wear behaviour Figure 2 shows the wear rate of steel in various heat treated conditions as a function of the number of test intervals at an applied load of 49 N and 273 r.p.m. speed. It may be noted that the wear rate decreased with the increase in the number of test intervals until the steady state wear rate was attained. Figure 2 also shows that the wear rates of hardened and hardened plus tempered steels were significantly lower than those of the annealed and normalised ones. The steady state wear resistance (inverse of steady state wear rate) has been computed from Fig. 2 and plotted in Fig. 3 as a function of bulk hardness. Figure 3 shows that the wear resistance increased linearly with bulk hardness. Further, up to a bulk hardness of about 400 HV, the rate of increase of abrasion resistance was higher than that above the mentioned hardness. The influence of sliding speed and applied load on the abrasion resistance of the annealed steel may be seen in Fig. 4. The figure indicates that the wear resistance decreased with load. The wheel revolutions per minute value was not found to affect the wear resistance of the steel significantly at 71 and 93 N. However, at an applied load of 49 N and lower wheel speed (150 r.p.m.) the wear resistance decreased, while the remaining two speeds (273 and 400 r.p.m.) offered comparable wear resistance. 3.3. Wear swjace The wear surface of the annealed steel tested at 49 N and 273 r.p.m. reveals continuous grooves and a considerable extent of micropitting on the wear surface, Fig. 5(a). A higher magnification micrograph clearly shows evidence typical of the change in the direction of the wear grooves, Fig. 5(b). The normalised steel tested under identical conditions showed wear surface similar to that of the annealed one. Figure 5(c) demonstrates a typical wear surface of the normalised steel showing a severely scratched region (arrow marked). The wear surface of the hardened steel depicts continuous grooves with practically no pitting, Fig. 6(a). Micropitting to some extent along with grooves were observed in the case of the hardened plus tempered
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S. Das et al. I Three-body abrasive
wear
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carbon steel
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Fig. 1. Microstructure of the steel in various heat treated conditions: (a) annealed, (b) normalised, (c) hardened, (d) hardened plus tempered (300 “C) and (e) hardened plus tempered (500 “C).
(HTC No& Table 1) steel, Fig. 6(b). Occasionally, severely damaged regions were also observed, Fig. 6(c). Figure 7(a) shows the abraded surface of the annealed steel at 49 N load and 150 r.p.m. speed. It may be seen that the grooves are not well-defined and considerable micropitting of the surface has been caused by the abrasive particles. A magnified view clearly reveals the micropitting and few grooves in this case, Fig. 7(b). The wear surface of the steel tested at 49 N load and 400 r.p.m. speed showed relatively less pitting and wefl-
defined grooves, Fig. 7(c). Higher load (91 N) and speed (400 r.p.m.) caused the formation of deeper grooves on the abraded surface with very little pitting, Fig. 7(d). 3.4. Subsurface studies Figures 8(a)-8(d) show the transverse section of the abraded surface of annealed steel at 49 N load and 273 r.p.m. speed. The propagation of microcracks parallel and perpendicular to the sliding direction can be
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Fig. 2. Wear rate vs. number of test intervals: 0, annealed; A, normalised; 0, hardened; 0, hardened plus tempered (300 “C); A, hardened plus tempered (500 “C).
51 0
I
loo
100
3W
COO tURDNE55
Fig. 3. Wear resistance
SW
600
700
800
I
900
(b)
( HV I
as a function
(4
of bulk hardness.
(c) Fig. 5. Wear surface: (a) annealed steel showing considerable amount of pitting; (b) annealed steel indicating a change in the direction of grooves; (c) normalised steel indicating a severely scratched region (arrow marked). 1.0 t
I
0
25
I
1
1
50
75
loo
LOADeN
Fig. 4. Wear resistance at various applied loads and speeds: 0, 150 r.p.m.; A, 273 r.p.m.; qi, 400 r.p.m.
seen in Fig. 8(a). Fragmentation of the pearlite lamellae in the regions close to the abraded surface may also be seen in Fig. 8(a) (top portion). Figure 8(b) clearly reveals the microcracks (arrow marked) and the broken pearlitic structure in the subsurface regions and the bulk microstructure consisting of normal lamellar pear-
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S. Das et al. I Three-body abrasive wear of 0.98% carbon steel
found to be attached to the bulk, Fig. 8(e), while the hardened plus tempered (500 “C) steel revealed occasional attachment of the transfer layer to the bulk, Fig. 8(f). The heavily deformed layer in the subsurface region may also be seen in Fig. S(f) (top portion). 3.5. Debris analysis A range of shapes and sizes of wear debris was found in the case of annealed steel. These were micromachining chips, large flakes and fine particles (regions marked A, B and C respectively in Fig. 9(a)). In the case of normalised steel, the debris was found to be similar in nature to that of the annealed one. The debris produced in the case of hardened steel consisted mainly of machining chips (arrow marked) with a few flaky particles, Fig. 9(b). However, hardened and tempered steel generated equiaxed debris particles with a few machining chips, Fig. 9(c).
4. Discussion
Fig. 6. Wear surface: (a) hardened steel showing mainly continuous grooves; (b) hardened and tempered (HTC No.5, Table 1) steel depicting micropitting to some extent together with grooves, and (c) hardened plus tempered specimen showing severely damaged regions.
lite. Flow lines indicative of plastic deformation of the subsurface layer may be noted in Fig. S(c) (top portion), while a severely deformed layer attached to the bulk is shown in Fig. 8(d) (arrow marked). In the case of the hardened steel, practically no transfer layer was
It is well established that the hardness of a (metallic) material plays an important role in controlling its abrasion resistance [2-6, 10, 13-161. Usually the softer material wears faster and then, due to plastic deformation, the surface work hardens. This subsequently results in improvement in the wear resistance [14]. Similar behaviour of steel during abrasive wear has been observed in the present investigation, Fig. 2. The above tendency was clearly noticed in the case of the annealed and normalised specimens, while the same behaviour was not as prominant in the hardened and hardened plus tempered samples, Fig. 2. The results are also consistent with the subsurface analysis, Fig. 8. In this instance, severely deformed regions (indicating their work hardening) attached to the bulk have been observed for annealed (Figs. 8(a)-8(d)) and hardened plus tempered steels (Fig. 8(f)). The presence of flow lines (Fig. 8(c)) and broken microstructural constituents (Figs. 8(a), 8(b) and 8(f)) along with the structureless region (Fig. 8(d)) just below the wear surface indicates considerable deformation of the regions. An earlier investigation has suggested that severely deformed subsurface layers indicate considerable microstructural changes due to abrasion and also possess significantly higher hardness [14]. The increased rate of wear resistance up to 400 HV followed by a reduction in the rate of the increase (Fig. 3) is in agreement with the nature of the wear surface and debris particles generated. For example, the wear surface of the annealed steel (bulk hardness 187 HV) showed considerable amount of micropitting and few grooves (Figs. 5(a) and 5(b)). It has been suggested that micropitting is caused by the rolling
S. Das et al. / Three-body abrasive wear of 0.98% carbon steel
807
(d) Fig. 7. Abraded surface of annealed steel showing: (a) and (b) a lot of micropitting with few grooves at low load (49 N) and low speed (150 r.p.m.); (c) less pitting and well-defined grooves at 49 N and 400 r.p.m. speed, and (d) deeper grooves at 91 N load and 406 r.p.m.
tendency of the abrasive particles, while their sliding action produces continuous grooves [13, 15, 221. Types of debris particles generated due to rolling are flakes, while sliding action produces machining chips. The debris particles of the annealed steel consisted mainly of large flakes with a few machining chips (Fig. 9(a)) indicating that, in this case, rolling of the abrasive particles was more predominant than their sliding. The subsurface of the annealed steel revealing a large transfer layer attached to the bulk and plenty of microcracks (Figs. 8(a)-8(d)) also suggests the formation of large flaky debris particles (Fig. 9(a)) leading to poor wear resistance (Fig. 3). On the contrary, samples having bulk hardness more than 400 HV showed better wear resistance (Fig. 3). The wear surface of the hardened steel showed mainly grooves with practically no micropitting (Fig. 6(a)). The transverse section revealed no attachment of transfer layer to the bulk in this case (Fig. 8(e)). Above observations suggest the formation of more machining chips in the debris and fewer flakes (Fig. 9(b)) causing improved wear resistance
of the hardened steel (Fig. 3). In the case of the hardened plus tempered steel, the wear surface showed pitting to some extent in addition to the grooves (Figs. 6(b) and 6(c)). The subsurface analysis also showed occasional attachment of the transfer layer to the bulk (Fig. 8(f)) and the debris consisted of equiaxed particles together with machining chips (Fig. 9(c)). This agrees with the relatively poorer wear resistance of hardened plus tempered steel than that of the hardened one (Fig. 3). Thus, the above observations suggest that a mixed mode of wear mechanisms, such as microcutting due to sliding and plastic deformation caused by the rolling action of the abrasive particles, operates during the abrasive wear of the steel. At low speed i.e. at 150 r.p.m., there is every possibility that the abrasive particles will get enough time to interact with the specimen surface and roll over it. Such a situation would cause crater formation on the wear surface (Figs. 7(a) and 7(b)) offering poor wear resistance (Fig. 4). On the other hand, higher speeds would give rise to the predominantly sliding action of
808
S. Das et al. I Three-body abrasive wear of 098%
carbon steel
(d)
Fig. 8. The transverse section of the abraded surface of annealed steel showing: (a) subsurface cracks propagating parallel and normal to the sliding directions and broken pearhtic structure in the transfer layer, (b) a magnified view clearly revealing the cracks (arrow marked) and changed microstructure in the transfer Iayer (top} and the lamellar pearlitic structure in the bulk; (c) the presence of flow lines in the transfer layer (top portion); (d) a severely deformed region attached to the bulk (arrow marked); (e) practically no attachment of transfer layer to the bulk of hardened steel occurs; and (f) occasional attachment of the transfer layer ;o the bulk in the case of hardened pius tempered (500 “C) steel.
the abrasive and lead to the formation of continuous grooves on the wear surface (Fig. 7(c)). Such a situation would give rise to improved wear resistance (Fig. 4).
Higher load would further decrease the probabili~ of the rolling action of the abrasive and lead to the formation of deeper grooves on the wear surface by
S. Das et al. f Three-body abrasive wear
of 0.98% carbon steel
809
steel samples in this study consisted of micropitting caused by crater formation due to the rolling of abrasive particles and microcutting due to the sliding action of the abrasive. However, the predominance of any of the mechanisms would depend on the bulk hardness, which is indirectly controlled by the microstructure of the steel. At lower hardness (less than approximately 400 HV), micropitting will be the dominant material removal mechanism, whereas at higher hardness values microcutting would be the dominant wear mechanism. As regards bulk hardness, a hardness level of about 400 HV may offer the optimum wear resistance for the steel, since this would also give better toughness and hence improved impact resistance.
5. Conclusions 1. The wear rate of the specimens decreased with the number of test intervals until a steady state value was attained. This was probably due to the subsurface work hardening. 2. The wear resistance increased linearly with the increase in bulk hardness at a specific applied load and speed. The slope of the wear resistance versus bulk hardness curve changed at approximateiy 400 HV. 3. The wear resistance of the steel decreased with an increase in the applied load. The wheel speed did not show any appreciable influence on the wear resistance. However, at lower speed and low load the wear resistance of the sample was found to be less than that at higher speed. 4. At any bulk hardness, the material removal was caused by both microcutting and micropitt~g mechanisms. However, at lower bulk hardness (less than about 400 HV) wear was mainly caused by micropitting, while higher hardness caused the microcutting mechanism to mainly control the wear process. Further, at a lower speed, material removal was caused mainly by micropitting, while higher speeds favoured microcutting as the main operating wear mechanism.
Acknowledgment Fig. 9. Wear debris: (a) annealed steel showing machining chips, flakes and fine particles (regions marked A, B and C respectively); (b) hardened steel consisting mainly of machining chips (arrow marked); and (c) hardened plus tempered specimen revealing equiaxed debris particles with a few machining chips.
Authors are grateful to Professor T. C. Rao, Director, Regional Research Laboratory (CSIR), Bhopal, India, for his encouragement and granting permission to publish this paper.
the sliding action causing relatively poor wear resistance (Fig. 4). Above observations inferred that the micromechanisms of material removal during the abrasive wear of
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S. Das et al. I Three-body abrasive wear of 0.98% carbon steel
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M. Misra and I. Finnie, Wear, 85 (1983) 57-68. B. K. Prasad and S. V. Prasad, Wear, 151 (1991) 1-12. L. Fang and Q. D. Zhou, Wear, 151 (1991) 313-321. J. Larsen-Badse and B. Premaratne, Effect of relative hardness on transition in wear mechanisms, Proc. Znt. Conf: Wear Mater., American Society of Mechanical Engineers, New York, 1983, pp. 161-166. K. H. Z. Gahr and D. V. Daone, Met. Trans. A, II (1980) 613-620. N. Prasad and S. D. Kulkami, Wear, 63 (1980) 329-338. E. Rabinowicz, L. A. Dunn and P. G. Russel, Wear, 4 (1961) 345-355. T. H. Kosel and N. F. Fiore, J. Mater. Energy Systems, 3 (1981) 7-27. Standard practice for conducting dry sand rubber wheel abrasion tests, ASTM Standard G65-81, 1981. Y. L. Wang and Z. S. Wang, Wear, 122 (1988) 123-134.