3 mm Fig 26 More extensive cracking observed in tougher but softer medium-carbon steel 60 um Fig. 27 Micrograph showing cracking along white-etching localized shear zone Example 5: Identification of Abrasive Wear Modes in Martensitic Steels. Wear modes experienced by a given component can be quite diverse, depending on wear environment and component function. The abrasive wear nodes experienced by steels in typical applications may be broadly divided into three categories: low-stress wear modes are shown in Fig. 28. Wear rates of a given material depend strongly on wear mode and althougle scratching, high-stress gouging, and impact or indentation. Scanning electron micrographs of these differe material may perform well in one mode of wear, it may not perform well in another
Fig. 26 More extensive cracking observed in tougher but softer medium-carbon steel Fig. 27 Micrograph showing cracking along white-etching localized shear zone Example 5: Identification of Abrasive Wear Modes in Martensitic Steels. Wear modes experienced by a given component can be quite diverse, depending on wear environment and component function. The abrasive wear modes experienced by steels in typical applications may be broadly divided into three categories: low-stress scratching, high-stress gouging, and impact or indentation. Scanning electron micrographs of these different wear modes are shown in Fig. 28. Wear rates of a given material depend strongly on wear mode and although a material may perform well in one mode of wear, it may not perform well in another
25 um 25 Fig. 28 Scanning electron micrographs showing the three modes of abrasive wear typically found in steels: (a) low-stress scratching, (b) higher-stress gouging, and(c)impact or indentation For martensitic steels, wear rate is most strongly dependent on material hardness. Figure 29 shows approximate lines of normalized wear rate versus hardness for the different wear modes. Data is plotted against Vickers hardness rather than Rockwell C, because Vickers hardness directly relates to material yield strength, while the ockwell C scale has no such direct correlation. Figure 29 shows that low-stress scratching has the strongest correlation with hardness. As the severity of wear increases, the dependence on hardness decreases. For gouging and impact wear, other material attributes, such as toughness and work-hardening ability, begin to play a larger roll in dictating wear rate. This difference in dependencies leads to materials that perform well in one wear mode but not in others. Figure 30 shows some materials that change the ranking when wear mode is changed. While the 50 HRC ground-engaging steel performs as expected, given its hardness in low-stress Thefileisdownloadedfromwww.bzfxw.com
Fig. 28 Scanning electron micrographs showing the three modes of abrasive wear typically found in steels: (a) low-stress scratching, (b) higher-stress gouging, and (c) impact or indentation For martensitic steels, wear rate is most strongly dependent on material hardness. Figure 29 shows approximate lines of normalized wear rate versus hardness for the different wear modes. Data is plotted against Vickers hardness rather than Rockwell C, because Vickers hardness directly relates to material yield strength, while the Rockwell C scale has no such direct correlation. Figure 29 shows that low-stress scratching has the strongest correlation with hardness. As the severity of wear increases, the dependence on hardness decreases. For gouging and impact wear, other material attributes, such as toughness and work-hardening ability, begin to play a larger roll in dictating wear rate. This difference in dependencies leads to materials that perform well in one wear mode but not in others. Figure 30 shows some materials that change the ranking when wear mode is changed. While the 50 HRC ground-engaging steel performs as expected, given its hardness in low-stress The file is downloaded from www.bzfxw.com
scratching, as the severity of wear environment increases, it becomes the favored material. In contrast to the 50 HRC steel, the steel composite material performs very well in low-stress scratching(due to the presence of hard phases)but more poorly as the wear severity increased, due to its low material toughness 14 12 Impact 30040050060 700 800900 Hardness, HV Fig. 29 Bands of normalized wear rate versus hardness for low-stress scratching, high-stress gouging and impact wear Low-stress scratching shows the strongest dependence on hardness, while impact abrasion shows the least. The scatter in the impact abrasion data suggests a growing contribution of material attributes other than hardness sCratching 1.goUging iMpact 0.8 0.6 0.4 SteelE Steel B Steel/45%TiC (50 HRC) 50 HRC) Fig. 30 Correlation of hardness with wear rate for three materials. The two 50 hrc materials both exhibit the same low-stress scratching wear resistance. However, as the wear severity increases, the steel designed for ground-engaging tools (steel A) exhibits moderate improvements in gouging wear and significant improvements in impact abrasion resistance. In contrast, due to the hard phases, the composite material performs better than would be expected, given its bulk hardness; however, due to its low fracture toughness, it performs significantly worse in more severe wear modes. The dependency of the wear performance on a given material or wear mode emphasizes the need to know the wear mode being experienced by a component. Unfortunately, components returned from field service rarely reveal information as clearly as seen in Fig. 28. Such surface features are frequently damaged by corrosion Therefore, another means of identifying wear modes is desired. One such method used for martensitic steels is to examine the polished and etched cross section of the wear surfaces. Figure 31 shows cross sections taken through the surface of components experiencing the three wear modes discussed. In low-stress scratching, there is essentially no subsurface microstructural modification of the steel. Even the few microns just below the surface show little or no evidence of plastic deformation(Fig. 31a). As the stress and severity of the abrasive
scratching, as the severity of wear environment increases, it becomes the favored material. In contrast to the 50 HRC steel, the steel composite material performs very well in low-stress scratching (due to the presence of hard phases) but more poorly as the wear severity increased, due to its low material toughness. Fig. 29 Bands of normalized wear rate versus hardness for low-stress scratching, high-stress gouging, and impact wear. Low-stress scratching shows the strongest dependence on hardness, while impact abrasion shows the least. The scatter in the impact abrasion data suggests a growing contribution of material attributes other than hardness. Fig. 30 Correlation of hardness with wear rate for three materials. The two 50 HRC materials both exhibit the same low-stress scratching wear resistance. However, as the wear severity increases, the steel designed for ground-engaging tools (steel A) exhibits moderate improvements in gouging wear and significant improvements in impact abrasion resistance. In contrast, due to the hard phases, the composite material performs better than would be expected, given its bulk hardness; however, due to its low fracture toughness, it performs significantly worse in more severe wear modes. The dependency of the wear performance on a given material or wear mode emphasizes the need to know the wear mode being experienced by a component. Unfortunately, components returned from field service rarely reveal information as clearly as seen in Fig. 28. Such surface features are frequently damaged by corrosion. Therefore, another means of identifying wear modes is desired. One such method used for martensitic steels is to examine the polished and etched cross section of the wear surfaces. Figure 31 shows cross sections taken through the surface of components experiencing the three wear modes discussed. In low-stress scratching, there is essentially no subsurface microstructural modification of the steel. Even the few microns just below the surface show little or no evidence of plastic deformation (Fig. 31a). As the stress and severity of the abrasive
event increase, microstructural modification becomes more evident. This may be manifested as a deep, plastically deformed layer(evident in structural deformation) or in white-etching layers(Fig. 31b). The white etching layers are believed to be extremely fine-grained ferrite, with all alloying elements in solution(Ref 33 34, 35, 36, 37). In most cases, the formation of white-etching layers indicates a higher-stress abrasive event However, it is also reflective of the material condition. Figure 32 shows two cross sections that were exposed to the same high-stress abrasive conditions. The material exhibiting white-etching layer formation is approximately 20 HRC softer than that with only slight surface plasticity. Impact abrasion is manifested in the appearance of severe plastic deformation at the surface and the presence of white-etching localized shear bands below the surface(Fig. 31c). The shear bands are created in a single impact event and are again the result of very high strains leading to what is believed to be the same structure as observed on the surface of components experiencing gouging wear(Ref 33, 34, 37). Identification of these wear modes may be used to help guide materials selection and processing to improve the wear resistance of component 115m (c) 25 um Thefileisdownloadedfromwww.bzfxw.com
event increase, microstructural modification becomes more evident. This may be manifested as a deep, plastically deformed layer (evident in structural deformation) or in white-etching layers (Fig. 31b). The whiteetching layers are believed to be extremely fine-grained ferrite, with all alloying elements in solution (Ref 33, 34, 35, 36, 37). In most cases, the formation of white-etching layers indicates a higher-stress abrasive event. However, it is also reflective of the material condition. Figure 32 shows two cross sections that were exposed to the same high-stress abrasive conditions. The material exhibiting white-etching layer formation is approximately 20 HRC softer than that with only slight surface plasticity. Impact abrasion is manifested in the appearance of severe plastic deformation at the surface and the presence of white-etching localized shear bands below the surface (Fig. 31c). The shear bands are created in a single impact event and are again the result of very high strains leading to what is believed to be the same structure as observed on the surface of components experiencing gouging wear (Ref 33, 34, 37). Identification of these wear modes may be used to help guide materials selection and processing to improve the wear resistance of components. The file is downloaded from www.bzfxw.com
Fig. 31 Metallographic cross sections of the worn surface regions showing(a)no microstructural modification in low-stress scratching, (b) white-etching layers in high-stress gouging, and(c) subsurface white-etching shear bands resulting from impact abrasion 你,情 10 um (b) 55 um 么zx23 A Fig. 32 Metallographic cross sections of wear surfaces of two materials that experienced the same high stress abrasion.(a) At 60 HRC, the material exhibited no white-etching layers, while(b)at 20 HRC points, softer, significant gouging and white-etching layer formation is observed. Footnote **The authors would like to recognize Donald Sherman of Caterpillar Inc. for his original work in this Investigation References cited in this section 30. F. Borik and D L. Sponseller, Gouging Abrasion Test for Materials Used in Ore and Rock Crushing Part 1: Description of the Test, J. Mater., Vol 6, 1971, p 576-589 31.S. Kalpakjian and S.R. Schmid, Manufacturing Engineering and Technology, 4th ed, Prentice-Hall Inc.,2001,p627-635
Fig. 31 Metallographic cross sections of the worn surface regions showing (a) no microstructural modification in low-stress scratching, (b) white-etching layers in high-stress gouging, and (c) subsurface white-etching shear bands resulting from impact abrasion Fig. 32 Metallographic cross sections of wear surfaces of two materials that experienced the same highstress abrasion. (a) At 60 HRC, the material exhibited no white-etching layers, while (b) at 20 HRC points, softer, significant gouging and white-etching layer formation is observed. Footnote * *The authors would like to recognize Donald Sherman of Caterpillar Inc. for his original work in this investigation. References cited in this section 30. F. Borik and D.L. Sponseller, Gouging Abrasion Test for Materials Used in Ore and Rock Crushing, Part 1: Description of the Test, J. Mater., Vol 6, 1971, p 576–589 31. S. Kalpakjian and S.R. Schmid, Manufacturing Engineering and Technology, 4th ed., Prentice-Hall, Inc., 2001, p 627–635