Dimensional wear, also known as phase change wear, mainly describes the microstructure evolution and work hardening in the normal direction of the wear surface.
After the impact abrasive wear test, the microhardness changes of the two steel grades from the wear surface to the interior of the matrix are shown in Figure 1. In the microhardness test, the microhardness shall be measured from the place closest to the wear surface (30pm), perpendicular to the wear surface, once every 100pm along the straight line until the microhardness value falls back to the value of the matrix, repeated three times, and the average value of the points at the same position on the three straight lines shall be taken as the final measurement result. It can be seen from the figure that the microhardness of the experimental steel of the two steel types is the highest at the wear surface and gradually decreases to the interior of the matrix. Fig. 1A shows that the microhardness of bainitic Martian multiphase wear-resistant cast steel falls back to the matrix hardness at 1030pm. For mnl3cr2 steel, it needs a distance of 2000-2500 PM to fall back to the hardness of the matrix.
Under the conditions of 1J, 2J and 4J impact energy, the microhardness of the wear surface of bainitic Martian multiphase wear-resistant cast steel reached 747hv, 795hv and 829hv respectively, which increased by 197hv, 245hv and 279hv respectively compared with the average hardness of 550hv. For mnl3cr2 steel, the microhardness of wear surface reaches 652hv, 671hv and 729hv respectively, which is increased by 310hv, 329hv and 387hv respectively compared with 342hv of matrix. In contrast, the work hardening effect reduces the microhardness difference between mnl3cr2 steel and experimental steel from 208hv before impact abrasive wear test to no more than 124hv after impact abrasive wear test. Combined with the wear weight loss results, it can be considered that mn13cr2 steel has stronger work hardening ability than the experimental steel, but the wear resistance is worse.
Fig. 2 is the transmission microstructure of bainitic Martian multiphase wear-resistant cast steel under different strengthening mechanisms after three body impact abrasive wear test. Fig. 2A shows the bright field and dark field transmission electron microscope of twin martensite, which proves the generation of twin martensite; As shown in Fig. 2B, high density dislocations appear in the sample after the wear test, and dislocations propagate, slip, annihilate, reorganize and form dislocation walls; Figure 2C shows the typical lamellar retained austenite (determined by diffraction spots) between parallel martensitic laths. The strength of retained austenite structure is lower than that of martensite structure. Therefore, when the experimental steel is impacted in the three body impact abrasive wear test, the lamellar retained austenite perpendicular to the impact direction is most likely to deform and then transform into martensite.
The XRD results shown in Fig. 3 also show this transformation process. The content of retained austenite is calculated by the formula:
Where V is the volume fraction of retained austenite, i α And I γ Cumulative intensity of diffraction peaks of martensite or bainite and austenite, K α And K γ Are the correlation coefficients of martensite or bainite and austenite respectively, taking K γ／ Ｋ α Is 2.55.
The volume fraction of retained austenite in non worn experimental steel and worn steel under 1J, 2J and 4J impact load is 10.1%, 8.0%, 6.2% and 4.8% respectively. When the impact load increases to 4J, the volume fraction of retained austenite decreases monotonously, which indicates that in the wear test, Higher impact load provides higher deformation and energy, so it is easier to induce the transformation of retained austenite.
The TEM photo in Fig. 4 shows the work hardening mechanism of mnl3cr2. The main structural characteristics of mnl3cr2 are dislocation entanglement and stacking fault, and no obvious deformation twins are found. The diffraction spot in the upper right corner of Fig. 4 also shows that there is no twinning relationship.
Figure 5 shows the XRD results of mnl3cr2 before and after the wear test, which is shown in figure (111) γ、 （２００） γ、 （２２０） γ、 （３１１） γ、 （２２２） γ From the five austenite diffraction peaks, it can be seen that the content of austenite has almost no change, and no martensite peak is found on the samples before impact abrasive wear test and after 1, 2 and 4J impact energy impact abrasive wear test, which also shows that martensitic transformation is not the main structural feature and hardening mechanism of mnl3cr2 experimental steel, It should be entangled dislocation and stacking fault. Therefore, it is considered that although mnl3cr2 has obvious work hardening, its work hardening ability has not been fully brought into play under the impact conditions set in this study, which is also confirmed that there is no obvious change in the wear resistance of mnl3cr2 under different impact energies of 1-4j.