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Abrasion resistance as a function of substructural changes in steel
Wear 258 (2005) 275–280
Abrasion resistance as a function of substructural changes in steel I.I. Garbar∗ Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel Received 22 December 2003 Available online 30 October 2004
Abstract It is known that there are three different stages of wear rate as a function of the abrasive grit diameter. The first shows only a modest increase in wear rate when the grit diameter increases. Then, in the second stage as the grit diameter further increases, the wear rate rises rapidly. In the third stage, the wear rate becomes independent of the grit diameter or increases only slightly. The main objective of this work is to explain these dramatic changes in wear rate. For this purpose, two-body abrasive wear tests were performed in order to study the structural changes in different steels, depending on the abrasive grit diameter. Special attention was paid to the correlation between the size of the fragments (subgrains) formed in the surface layers and the variations in the wear rate. The former determines work hardening of the metal, and therefore, its wear resistance. X-ray investigation showed that the size of the fragments formed in the surface layers of steels during the wear process is dependent on the abrasive grit and corresponding wear rate. In the first stage, the size of the fragments is relatively small. Then, it becomes larger, and therefore, work hardening of the surface layers decreases by comparison to those under wear with a smaller grit diameter. The dimensions of fragments are maximal and change only slightly in the third stage when we used abrasive paper of a greater grit size. Therefore, in these cases, work hardening of surface layers is minimal and it is expected that wear resistance will also be minimal. © 2004 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Steel; Structure; Work hardening
1. Introduction The influence of abrasive grit size on wear has been investigated in various works [1–4]. Three different stages of wear rate as a function of the grit diameter were observed. The first shows very little wear rate with only a modest increase when the grit diameter increases. In the second stage, as the grit diameter further increases the wear rate rises rapidly. The wear rate then becomes independent of the grit diameter, or increases only slightly in the third stage. The typical curve of this dependence is shown in Fig. 1, where I, II and III correspond to the three stages. The curve in Fig. 1 shows two critical points: A and B, where the wear rate changes dramatically.
∗
Tel.: +972 8 6472813; fax: +972 8 6472813. E-mail address: [email protected].
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.011
According to Suh [3], wear until point A, is no longer caused by an abrasive mechanism but is of a different nature. Point B has been attributed to the factors, such as abrasive deterioration [2], depth of penetration of abrasive particles [3] and others. Along with these reasons, we can also add that the structure formed in the surface layers of metal during the wear process could also be responsible for such dramatic changes in wear rate. Since wear is a special case of material fracture and depends on the material structure, this structure is responsible for work hardening of metal and its wear resistance. It is now well established that the structure of surface layers of metals changes during the friction process and work hardening takes place due to the changes in the dislocation density and distribution. These changes result from a rearrangement of the dislocations in the surface layers in cellular structures, when the interior of the cells become relatively free from dislocations and the walls or boundaries made up
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I.I. Garbar / Wear 258 (2005) 275–280
Fig. 1. Scheme of wear rate dependence on grit size.
of dense dislocation arrays. The dislocations in the walls accommodate the lattice angular misorientation caused by the microscopic strain. As the straining proceeds, the cell structure evolves in a complex manner, usually ending with fragmentation of the metal. The formation of a fragmented structure in the surface layers during friction is characteristic of metals and various studies have shown that the level of fragmentation determines the strengthening of the metal [5–12]. The fragment size indicates the level of work hardening, and can be used instead of the grain size in the classical Hall-Petch equation. The generalized form of the Hall-Petch equation is n b σs = σ0 + KG (1) d where σs is the flow stress, σ0 the friction stress, K the constant, G the shear modulus, b the Burgers vector and d is the size of the microstructural elements. These elements are the cells and the geometrically necessary boundaries (GNB) formed during deformation [13,14]. In this case, according to Hughes [15], Eq. (1) can be written as: 0.5 K1 Gb b σs = σ0 + + K2 G (2) d1 d2 where K1 Gb/d1 and K2 G(b/d2 )0.5 are, respectively, the contributions of the cells and the GNB to the flow stress. Therefore, in all cases if d → min, then σs → max. In other words, the metal flow limit significantly increases and the work hardening of the metal is maximal if the size of the fragments reaches the minimum value inherent to this material, and the mutual disorientation between the neighboring fragments is also maximal. At this point, σs and σUTS will have increased to their maximal values. When this occurs in surface layers of metal during friction, the maximal ability of the metal to undergo wear is reached. Plastic deformation and the corresponding level of work hardening usually increase when the applied stresses increase. Nevertheless, in a recent work [16], it has been shown that fragmentation and therefore the level of work hardening
under friction can vary depending on the friction and wear conditions. Moreover, these properties of the material may not always be proportional to the friction stresses, i.e., under harsher friction conditions, work hardening of surface layers of metal can be less than under moderate ones. Such unexpected results were explained by the difference in the rates of the plastic deformation and the relaxation processes. It is known that material failure takes place when the plastic deformation rate is greater than the rate of the relaxation process [17]. If, under the selected friction conditions, the plastic deformation rate is very high, the structure has no opportunity to evolve even though the stresses are sufficiently high. In this case, work hardening does not occur and wear resistance is minimal. In the case when the rate of plastic deformation is lower than the rate of the relaxation process, the evolution of the fragmented structure in the surface layers occurs until this condition is fulfilled. The minimal size of fragments and correspondingly the maximal level of work hardening can only be reached under these conditions. Therefore, the capacity of the same metal for work hardening and its corresponding wear resistance can be different, depending on the friction conditions. This approach has been used in the present study to explain the regularities of abrasive wear described above. Due to the distinction between the levels of applied stresses during abrasion with different grit size, the rate of plastic deformation should be also different. Therefore, it is thought that the behavior of the curve shown in Fig. 1 can be explained by the varying structural changes in the surface layers.
2. Materials and experimental procedure To study the surface structure, AISI 1020, AISI 1040 and AISI 1080 steel specimens were annealed for 2 h at 950, 910 and 850 ◦ C, respectively, followed by furnace cooling. The hardness of the steel after annealing was HV121, HV154 and HV219, respectively. The specimens were in the form of cylinders of 25 mm diameter and 6 mm height. The base of the cylinder was served as a friction surface of a pin, in pin-disk two-body abrasive tests on a friction machine. Fig. 2 shows the schematic representation of the testing apparatus. Silicon carbide abrasive paper of grit 1200–120 (grit size corresponding to 6.5–116 m) was used for the tests. The tests were conducted under identical conditions, with a 50 N constant load and associated pressure of 0.1 MPa. The sliding distance was 30 m and the velocity was 0.4 m s−1 . To consider the influence of the preceding abrasive machining and to exclude its influence on the structural data and wear results, two different sequences of the tests were conducted. The first sequence of grits was 120 → 180 → 240 → 320 → 400 → 600 → 800 → 1200. The second sequence of grits was reversed 1200 → ··· → 120. The data obtained was then averaged. Before and after both sequences of the tests, the working surface of the specimens
I.I. Garbar / Wear 258 (2005) 275–280
277
Fig. 2. Schematic representation of the testing apparatus: (a) shape and dimension of specimen, (b) pin-disc friction machine. Fig. 3. Wear rate as a function of grit size.
was electropolished to remove the work-hardened surface layers formed accordingly from machining and from abrasion and was studied by X-ray to obtain the initial structural characteristics of the steels. Three series of experiments were carried out for each specimen and for each abrasive grit size, and fresh abrasive paper was used for each test. Wear was determined by weighing the specimens on a chemical balance before the tests and after the particular exposure to friction. Weight loss was determined as an average value of the three series of experiments. Specimens were examined by X-ray diffraction in Cu K␣ radiation after electropolishing and after testing with each grit size, and diffraction lines (1 1 0) and (2 2 0) ␣-Fe were studied. The dimensions of the surface layer fragments were determined on the basis of the extinction effect (for details see [16]). It should be noted that X-ray study represents only the mean value of the data obtained throughout the layer, which contributes to the diffraction pattern. Considering that the dimension of the fragments increases with depth below the friction surface [18], the structure of the surface layer is more fragmented than the data show. Therefore, X-ray results can be used mainly for comparative study of the surface layer structure.
The X-ray study shows that the size of fragments formed in the surface layers of the steels as a result of plastic deformation during the wear process depends on the abrasive grit (Fig. 4). The minimum size of fragments in all specimens is observed after testing with grit 1200 (grit size 6.5 m). Beyond the first “critical” point, the size of the fragments became larger, and, therefore, according to Eqs. (1) and (2), work hardening of the surface layers decreased by comparison to those under wear with the smaller grit size. The substructure corresponding to the second “critical” point shows the maximum size. In steels AISI 1020 and AISI 1040, this size changed only slightly when we used abrasive paper of a greater grit size. In these cases, work hardening of surface layers, according to Eqs. (1) and (2), is minimal and, therefore, wear resistance should also be minimal. The specimen of steel AISI 1080 shows maximal fragment size after undergoing tests with grits 400 and 320 (grit sizes 23 and 36 m correspondingly). Wear under more coarsely grained abrasives leads to some decrease of the microstructural elements in the surface layer. Fig. 5 shows the fragment size dependence on wear rate. One can see that the fragment of minimal size formed in the surface layers of metal under relatively low abrasive wear
3. Results The results of the wear tests are shown in Fig. 3. Three different stages of the wear rate as a function of the grit diameter, similar to those in Fig. 1, can be observed. The first, having only a modest increase in wear rate when the grit diameter increases, can be seen for grits 1200 and 800 (grit size corresponding to 6.5 and 12 m). Then, in the second stage, as the grit diameter increases from 16 to 36 m (grits 600, 400 and 320) the wear rate rises rapidly. Wear rate in the third stage increases slightly (grits 240, 180 and 120) (grit size corresponding to 53, 78 and 116 m, correspondingly). The first “critical” point can be seen under the tests with grits 800 and 600 and the second with grit 240.
Fig. 4. Fragment diameter vs. grit size.
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Fig. 5. Fragment size dependence on wear rate.
conditions (i.e., in tests using a small grit size). This case corresponds to a high level of work hardening. Maximal fragment size and a correspondingly low level of work hardening are observed under a high wear rate. Raj and Pharr [19] proposed the following simple inverse relationship between flow stress and mean fragment size, which can be adequately used for many materials and experimental conditions. d/b = 23
G σs
1 (3)
According to this relation, the data of our study can be represented by the curves in Fig. 6. These graphs show that the maximal flow stresses are observed under abrasive wear with a small grit size.
Fig. 6. Flow stress vs. grit size.
I.I. Garbar / Wear 258 (2005) 275–280
4. Discussion In the previous sections, the level of work hardening was associated with the ability of the metal to undergo wear. Let us consider the connection between the strengthening of materials and their abrasive wear resistance. As stated above, work hardening of surface layers of metals takes place under friction. The structure of the surface layers is changed by the friction process; the surface layers being hardened through these changes. The level of metal strengthening in this process depends on the fragmentation of the surface layers under friction. It is unknown a priori what structure is the critical one under friction, i.e., the type of the structure where the wear process takes place [16]. At the same time, since (according to [4]) abrasive wear resistance is proportional to the hardness of work hardened material of friction surface, it is very important to know the level of plastic deformation, and therefore, the level of work hardening of critical structures of metals under definite friction conditions. The substructural concept of wear resistance suggested in the present research consists in the assumption that wear resistance γ is determined by the initial strength of the metal σ and its strengthening during friction σ, γ = γ(σ + σ)
(4)
This strengthening depends on the ability of the metal to form the fragmented structure under the given friction conditions:
σ = σ(d −n )
(5)
where d is, as in Eqs. (1) and (3), the size of the microstructural elements and 0.5 ≤ n ≤1. According to this concept, the work hardening and the corresponding wear resistance of the same material can vary, depending on the friction and wear conditions. The data obtained in this study show that under two-body abrasive wear increase of the grit size, i.e., increase of the applied stresses during friction, can lead to a less deformed structure of surface layers. In our case, this is the larger dimension of fragments that is formed in the surface layers of steel. These results can explain the existence of the three stages of the wear rate as a function of the grit diameter and the “critical” points A and B (Fig. 1). It would appear that the first stage corresponds to maximal strengthening of metal surface layers are due to formation of a developed fragmented structure. This is the commonly occurring case of plastic deformation due to a relaxation rate that is greater than the rate of plastic deformation. Such a structure corresponds to maximal wear resistance of metal. Stage II in Fig. 1 corresponds to a transition from a relatively low wear rate to a high one. The structure in this stage shows an increase in the size of fragments. Work hardening of this structure, according to Eqs. (1)–(3) and the corresponding wear resistance are less than in stage I. The “critical” point A can be explained by the transition from the conditions when the structure’s strength
279
provides the possibility to withstand friction forces to when the structure’s strength is lower. In the third stage, with course-grained abrasives where the wear rate is high, the structure, in most cases, shows that relatively large fragments are formed in the surface layers. This means that under such wear conditions, the level of plastic deformation and corresponding level of work hardening is lower than those after wear with fine abrasives. It is suggested that in the case of wear with course-grained abrasives the rate of the deformation process is higher than the rate of the relaxation process. This phenomenon leads to the fracture of the material (specifically to wear particle formation under friction) without high plastic deformation. The identical minimal level of work hardening for the metal can explain only the slight dependence of wear rate on the grit diameter noted in the third stage of wear rate. The “critical” point B in Fig. 1 represents the transition to the least wear resistant conditions of the material. It should be noted that such a mode of operation corresponds to the improved machinability of the metal.
References [1] J. Larsen-Basse, Influence of grit diameter and specimen size on wear during sliding abrasion, Wear 12 (1968) 35–53. [2] J. Larsen-Basse, Some effects of specimen size on abrasive wear, Wear 19 (1972) 27–35. [3] N.P. Suh, Tribophysics, Prentice-Hall, Inc., 1986. [4] I.I. Garbar, Correlation between abrasive wear resistance and changes in structure and residual stresses of steels, Tribol. Lett. 5 (1998) 223–229. [5] I.I. Garbar, Yu.V. Skorinin, Investigation of the surface layer structure under friction, Mashinovedenie 5 (1975) 106–109. [6] I.I. Garbar, Yu.V. Skorinin, Metal surface layer structure formation under sliding friction, Wear 51 (1978) 327–336. [7] J.H. Dautzenberg, J.A.B. Van Dijck, The structure of copper rubbed on emery paper at different temperatures, Wear 77 (1982) 217– 227. [8] D. Kuhlmann-Wilsdorf, L.K. Ives, Sub-surface hardening in erosion damaged copper as inferred from the dislocation cell structure, and its dependence on particle velocity and angle of impact, Wear 85 (1983) 359–371. [9] K. Kato, Friction and wear, in: R.W. Cahn, et al. (Eds.), Materials Science and Technology, vol. 6, VCH, 1993, pp. 635–680. [10] D.A. Hughes, D.B. Dawson, J.S. Korellis, L.I. Weingarten, Near surface microstructures developing under large sliding loads, J. Mater. Eng. Perform. 3 (4) (1994) 459–475. [11] C. Perrin, W.M. Reinforth, Work hardening behavior at the worn surface of Al–Cu and Al–Si alloys, Wear 203–204 (1997) 171– 179. [12] I.I. Garbar, The effect of load on the structure and wear of friction pair materials (example of low-carbon steel and copper), Wear 205 (1997) 240–245. [13] D. Kuhlmann-Wilsdorf, N. Hansen, Geometrically necessary, incidental and subgrain boundaries, Scripta Metall. Mater. 25 (1991) 1557–1562. [14] B. Bay, N. Hansen, D.A. Haghes, D. Kuhlmann-Wilsdorf, Evolution of f.c.c. deformation structures in polyslip, Acta Metall. Mater. 40 (2) (1992) 205–219. [15] D.A. Hughes, Microstructure and flow stress of deformed polycrystalline metals, Scripta Metall. Mater. 27 (1992) 969–974.
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[16] I.I. Garbar, Critical structures of metal destruction under the process of wear, Trans. ASME J. Tribol. 122 (2000) 361–366 (also as 99Trib-10). [17] V.V. Rybin, Large Plastic Deformation and Failure of Metals, Metallurgia, Moscow, 1986 (in Russian).
[18] I.I. Garbar, Formation and separation of the fragmented surface structure of low-carbon steel and copper under friction, Wear 198 (1996) 86–92. [19] S.V. Raj, G.M. Pharr, A compilation and analysis of data for stress dependence of the subgrain size, Mater. Sci. Eng. 81 (1986) 217–237.
Abrasion resistance as a function of substructural changes in steel I.I. Garbar∗ Department of Mechanical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva, 84105, Israel Received 22 December 2003 Available online 30 October 2004
Abstract It is known that there are three different stages of wear rate as a function of the abrasive grit diameter. The first shows only a modest increase in wear rate when the grit diameter increases. Then, in the second stage as the grit diameter further increases, the wear rate rises rapidly. In the third stage, the wear rate becomes independent of the grit diameter or increases only slightly. The main objective of this work is to explain these dramatic changes in wear rate. For this purpose, two-body abrasive wear tests were performed in order to study the structural changes in different steels, depending on the abrasive grit diameter. Special attention was paid to the correlation between the size of the fragments (subgrains) formed in the surface layers and the variations in the wear rate. The former determines work hardening of the metal, and therefore, its wear resistance. X-ray investigation showed that the size of the fragments formed in the surface layers of steels during the wear process is dependent on the abrasive grit and corresponding wear rate. In the first stage, the size of the fragments is relatively small. Then, it becomes larger, and therefore, work hardening of the surface layers decreases by comparison to those under wear with a smaller grit diameter. The dimensions of fragments are maximal and change only slightly in the third stage when we used abrasive paper of a greater grit size. Therefore, in these cases, work hardening of surface layers is minimal and it is expected that wear resistance will also be minimal. © 2004 Elsevier B.V. All rights reserved. Keywords: Abrasive wear; Steel; Structure; Work hardening
1. Introduction The influence of abrasive grit size on wear has been investigated in various works [1–4]. Three different stages of wear rate as a function of the grit diameter were observed. The first shows very little wear rate with only a modest increase when the grit diameter increases. In the second stage, as the grit diameter further increases the wear rate rises rapidly. The wear rate then becomes independent of the grit diameter, or increases only slightly in the third stage. The typical curve of this dependence is shown in Fig. 1, where I, II and III correspond to the three stages. The curve in Fig. 1 shows two critical points: A and B, where the wear rate changes dramatically.
∗
Tel.: +972 8 6472813; fax: +972 8 6472813. E-mail address: [email protected].
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.09.011
According to Suh [3], wear until point A, is no longer caused by an abrasive mechanism but is of a different nature. Point B has been attributed to the factors, such as abrasive deterioration [2], depth of penetration of abrasive particles [3] and others. Along with these reasons, we can also add that the structure formed in the surface layers of metal during the wear process could also be responsible for such dramatic changes in wear rate. Since wear is a special case of material fracture and depends on the material structure, this structure is responsible for work hardening of metal and its wear resistance. It is now well established that the structure of surface layers of metals changes during the friction process and work hardening takes place due to the changes in the dislocation density and distribution. These changes result from a rearrangement of the dislocations in the surface layers in cellular structures, when the interior of the cells become relatively free from dislocations and the walls or boundaries made up
276
I.I. Garbar / Wear 258 (2005) 275–280
Fig. 1. Scheme of wear rate dependence on grit size.
of dense dislocation arrays. The dislocations in the walls accommodate the lattice angular misorientation caused by the microscopic strain. As the straining proceeds, the cell structure evolves in a complex manner, usually ending with fragmentation of the metal. The formation of a fragmented structure in the surface layers during friction is characteristic of metals and various studies have shown that the level of fragmentation determines the strengthening of the metal [5–12]. The fragment size indicates the level of work hardening, and can be used instead of the grain size in the classical Hall-Petch equation. The generalized form of the Hall-Petch equation is n b σs = σ0 + KG (1) d where σs is the flow stress, σ0 the friction stress, K the constant, G the shear modulus, b the Burgers vector and d is the size of the microstructural elements. These elements are the cells and the geometrically necessary boundaries (GNB) formed during deformation [13,14]. In this case, according to Hughes [15], Eq. (1) can be written as: 0.5 K1 Gb b σs = σ0 + + K2 G (2) d1 d2 where K1 Gb/d1 and K2 G(b/d2 )0.5 are, respectively, the contributions of the cells and the GNB to the flow stress. Therefore, in all cases if d → min, then σs → max. In other words, the metal flow limit significantly increases and the work hardening of the metal is maximal if the size of the fragments reaches the minimum value inherent to this material, and the mutual disorientation between the neighboring fragments is also maximal. At this point, σs and σUTS will have increased to their maximal values. When this occurs in surface layers of metal during friction, the maximal ability of the metal to undergo wear is reached. Plastic deformation and the corresponding level of work hardening usually increase when the applied stresses increase. Nevertheless, in a recent work [16], it has been shown that fragmentation and therefore the level of work hardening
under friction can vary depending on the friction and wear conditions. Moreover, these properties of the material may not always be proportional to the friction stresses, i.e., under harsher friction conditions, work hardening of surface layers of metal can be less than under moderate ones. Such unexpected results were explained by the difference in the rates of the plastic deformation and the relaxation processes. It is known that material failure takes place when the plastic deformation rate is greater than the rate of the relaxation process [17]. If, under the selected friction conditions, the plastic deformation rate is very high, the structure has no opportunity to evolve even though the stresses are sufficiently high. In this case, work hardening does not occur and wear resistance is minimal. In the case when the rate of plastic deformation is lower than the rate of the relaxation process, the evolution of the fragmented structure in the surface layers occurs until this condition is fulfilled. The minimal size of fragments and correspondingly the maximal level of work hardening can only be reached under these conditions. Therefore, the capacity of the same metal for work hardening and its corresponding wear resistance can be different, depending on the friction conditions. This approach has been used in the present study to explain the regularities of abrasive wear described above. Due to the distinction between the levels of applied stresses during abrasion with different grit size, the rate of plastic deformation should be also different. Therefore, it is thought that the behavior of the curve shown in Fig. 1 can be explained by the varying structural changes in the surface layers.
2. Materials and experimental procedure To study the surface structure, AISI 1020, AISI 1040 and AISI 1080 steel specimens were annealed for 2 h at 950, 910 and 850 ◦ C, respectively, followed by furnace cooling. The hardness of the steel after annealing was HV121, HV154 and HV219, respectively. The specimens were in the form of cylinders of 25 mm diameter and 6 mm height. The base of the cylinder was served as a friction surface of a pin, in pin-disk two-body abrasive tests on a friction machine. Fig. 2 shows the schematic representation of the testing apparatus. Silicon carbide abrasive paper of grit 1200–120 (grit size corresponding to 6.5–116 m) was used for the tests. The tests were conducted under identical conditions, with a 50 N constant load and associated pressure of 0.1 MPa. The sliding distance was 30 m and the velocity was 0.4 m s−1 . To consider the influence of the preceding abrasive machining and to exclude its influence on the structural data and wear results, two different sequences of the tests were conducted. The first sequence of grits was 120 → 180 → 240 → 320 → 400 → 600 → 800 → 1200. The second sequence of grits was reversed 1200 → ··· → 120. The data obtained was then averaged. Before and after both sequences of the tests, the working surface of the specimens
I.I. Garbar / Wear 258 (2005) 275–280
277
Fig. 2. Schematic representation of the testing apparatus: (a) shape and dimension of specimen, (b) pin-disc friction machine. Fig. 3. Wear rate as a function of grit size.
was electropolished to remove the work-hardened surface layers formed accordingly from machining and from abrasion and was studied by X-ray to obtain the initial structural characteristics of the steels. Three series of experiments were carried out for each specimen and for each abrasive grit size, and fresh abrasive paper was used for each test. Wear was determined by weighing the specimens on a chemical balance before the tests and after the particular exposure to friction. Weight loss was determined as an average value of the three series of experiments. Specimens were examined by X-ray diffraction in Cu K␣ radiation after electropolishing and after testing with each grit size, and diffraction lines (1 1 0) and (2 2 0) ␣-Fe were studied. The dimensions of the surface layer fragments were determined on the basis of the extinction effect (for details see [16]). It should be noted that X-ray study represents only the mean value of the data obtained throughout the layer, which contributes to the diffraction pattern. Considering that the dimension of the fragments increases with depth below the friction surface [18], the structure of the surface layer is more fragmented than the data show. Therefore, X-ray results can be used mainly for comparative study of the surface layer structure.
The X-ray study shows that the size of fragments formed in the surface layers of the steels as a result of plastic deformation during the wear process depends on the abrasive grit (Fig. 4). The minimum size of fragments in all specimens is observed after testing with grit 1200 (grit size 6.5 m). Beyond the first “critical” point, the size of the fragments became larger, and, therefore, according to Eqs. (1) and (2), work hardening of the surface layers decreased by comparison to those under wear with the smaller grit size. The substructure corresponding to the second “critical” point shows the maximum size. In steels AISI 1020 and AISI 1040, this size changed only slightly when we used abrasive paper of a greater grit size. In these cases, work hardening of surface layers, according to Eqs. (1) and (2), is minimal and, therefore, wear resistance should also be minimal. The specimen of steel AISI 1080 shows maximal fragment size after undergoing tests with grits 400 and 320 (grit sizes 23 and 36 m correspondingly). Wear under more coarsely grained abrasives leads to some decrease of the microstructural elements in the surface layer. Fig. 5 shows the fragment size dependence on wear rate. One can see that the fragment of minimal size formed in the surface layers of metal under relatively low abrasive wear
3. Results The results of the wear tests are shown in Fig. 3. Three different stages of the wear rate as a function of the grit diameter, similar to those in Fig. 1, can be observed. The first, having only a modest increase in wear rate when the grit diameter increases, can be seen for grits 1200 and 800 (grit size corresponding to 6.5 and 12 m). Then, in the second stage, as the grit diameter increases from 16 to 36 m (grits 600, 400 and 320) the wear rate rises rapidly. Wear rate in the third stage increases slightly (grits 240, 180 and 120) (grit size corresponding to 53, 78 and 116 m, correspondingly). The first “critical” point can be seen under the tests with grits 800 and 600 and the second with grit 240.
Fig. 4. Fragment diameter vs. grit size.
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Fig. 5. Fragment size dependence on wear rate.
conditions (i.e., in tests using a small grit size). This case corresponds to a high level of work hardening. Maximal fragment size and a correspondingly low level of work hardening are observed under a high wear rate. Raj and Pharr [19] proposed the following simple inverse relationship between flow stress and mean fragment size, which can be adequately used for many materials and experimental conditions. d/b = 23
G σs
1 (3)
According to this relation, the data of our study can be represented by the curves in Fig. 6. These graphs show that the maximal flow stresses are observed under abrasive wear with a small grit size.
Fig. 6. Flow stress vs. grit size.
I.I. Garbar / Wear 258 (2005) 275–280
4. Discussion In the previous sections, the level of work hardening was associated with the ability of the metal to undergo wear. Let us consider the connection between the strengthening of materials and their abrasive wear resistance. As stated above, work hardening of surface layers of metals takes place under friction. The structure of the surface layers is changed by the friction process; the surface layers being hardened through these changes. The level of metal strengthening in this process depends on the fragmentation of the surface layers under friction. It is unknown a priori what structure is the critical one under friction, i.e., the type of the structure where the wear process takes place [16]. At the same time, since (according to [4]) abrasive wear resistance is proportional to the hardness of work hardened material of friction surface, it is very important to know the level of plastic deformation, and therefore, the level of work hardening of critical structures of metals under definite friction conditions. The substructural concept of wear resistance suggested in the present research consists in the assumption that wear resistance γ is determined by the initial strength of the metal σ and its strengthening during friction σ, γ = γ(σ + σ)
(4)
This strengthening depends on the ability of the metal to form the fragmented structure under the given friction conditions:
σ = σ(d −n )
(5)
where d is, as in Eqs. (1) and (3), the size of the microstructural elements and 0.5 ≤ n ≤1. According to this concept, the work hardening and the corresponding wear resistance of the same material can vary, depending on the friction and wear conditions. The data obtained in this study show that under two-body abrasive wear increase of the grit size, i.e., increase of the applied stresses during friction, can lead to a less deformed structure of surface layers. In our case, this is the larger dimension of fragments that is formed in the surface layers of steel. These results can explain the existence of the three stages of the wear rate as a function of the grit diameter and the “critical” points A and B (Fig. 1). It would appear that the first stage corresponds to maximal strengthening of metal surface layers are due to formation of a developed fragmented structure. This is the commonly occurring case of plastic deformation due to a relaxation rate that is greater than the rate of plastic deformation. Such a structure corresponds to maximal wear resistance of metal. Stage II in Fig. 1 corresponds to a transition from a relatively low wear rate to a high one. The structure in this stage shows an increase in the size of fragments. Work hardening of this structure, according to Eqs. (1)–(3) and the corresponding wear resistance are less than in stage I. The “critical” point A can be explained by the transition from the conditions when the structure’s strength
279
provides the possibility to withstand friction forces to when the structure’s strength is lower. In the third stage, with course-grained abrasives where the wear rate is high, the structure, in most cases, shows that relatively large fragments are formed in the surface layers. This means that under such wear conditions, the level of plastic deformation and corresponding level of work hardening is lower than those after wear with fine abrasives. It is suggested that in the case of wear with course-grained abrasives the rate of the deformation process is higher than the rate of the relaxation process. This phenomenon leads to the fracture of the material (specifically to wear particle formation under friction) without high plastic deformation. The identical minimal level of work hardening for the metal can explain only the slight dependence of wear rate on the grit diameter noted in the third stage of wear rate. The “critical” point B in Fig. 1 represents the transition to the least wear resistant conditions of the material. It should be noted that such a mode of operation corresponds to the improved machinability of the metal.
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