| 其他摘要 | Hot-working tool steels are widely used in steel and metallurgical industries. It is of great significance to produce a nanostructured surface layer on this kind of steels by means of surface mechanical attrition treatment (SMAT). From one side, investigations on microstructure evolutions in such steels containing a large number of second phase particles are helpful to understand the grain refinement mechanisms of dual-phase or multi-phase metals during severe plastic deformation; from another side, it is achievable to significantly lower temperatures and shorten durations of surface thermo-chemical treatments by utilizing the greatly enhanced diffusivity and chemical reactivity in the nanostructured surface layers, so as to optimize traditional surface modification processes and enhance their global performances.
In this work, the most popular hot-working tool steel, H13 steel, was subjected to SMAT. Microstructure evolution and grain nanocrystallization mechanism in the SMAT surface layer, as well as their lower-temperature chromizing behaviors and duplex chromizing behaviors, were systematically investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The microhardness and tribological properties of the surface layer in the duplex chromized SMAT sample were studied by using nanoindentation and tribometry, respectively. The main results are followed:
1. A nanostructured surface layer of about 20 m thick has been fabricated on the H13 steel plate by means of SMAT. The ferrite grain size increases gradually with an increasing depth from the treated surface. The average ferrite grain size of the topmost layer is about 10 nm. The significant grain growth temperature is about 600 ℃.
2. The grain refinement process of ferrite in H13 steel is dominated by dislocation activities. With decreasing depths, i.e. increasing strains and strain rates, the refinement processes involve: the formation of dislocation cells in original ferrite grains and the transformation of these dislocation cells into subgrains at low strains; the formation, thinning and intersecting of lamellar structures with continuously increasing strains; and finally the formation of equiaxed nanocrystallites in the top surface layer due to a large strain with a high strain rate and the multidirectional loading.
3. Evident dislocation activities inside the iron-chromium carbide particles ((Cr,Fe)23C6 and (Cr,Fe)7C3 phases) in H13 steel are observed within the depth of 20 m, where the ferrite grain size was below 100 nm. A large number of carbides promote the grain refinement and nanocrystallization process of the ferrite matrix during SMAT. Compared with the cementite in carbon steels, the harder iron-chromium carbide particles in the H13 steel have less positive effect on the nanocrystallization of the ferrite matrix within the depth of 20 m.
4. Chromizing experiments at lower temperatures (500-700 ℃) show that the diffusion of Cr and formation of Cr-N compounds are greatly promoted in the surface layer of H13 steel by SMAT. A chromized surface layer of about 0.8 m thick is formed on the SMAT sample after the treatment at 550 ℃ for 120 min, while no chromized layer is formed on the coarse-grained (CG) sample even after the treatment at 700 ℃ for 120 min. The thickness of the formed chromized layer in the SMAT sample first increases and then decreases with the increasing chromizing temperature and a maximum value of ~5.2 m in thickness is obtained at 600 ℃.
5. A continuous chromized layer has been produced on the SMAT H13 sample after duplex chromizing treatment at a lower temperature followed by at a higher temperature (950-1050 ℃). The thickness of the chromized surface layer increases markedly relative to that on the CG sample after the same chromizing treatment. The main reason that the duplex chromizing process can greatly increase the thickness of the chromized layer on the SMAT sample is that the thermal stability of SMAT sample is enhanced by numerous dispersive and fine Cr-N compounds formed in the surface layer during the lower-temperature chromizing process, so as to greatly accelerate the diffusion of alloy elements and the formation of the continuous compound layer during the higher-temperature chromizing treatment.
6. After a duplex chromizing treatment at 600 ℃ for 120 min followed by at 1050 ℃ for 240 min, a continuous chromized layer of about 30 m in thickness is formed on the SMAT sample, about 3 times thick of that on the duplex chromized CG sample. In comparison with the duplex chromized CG sample, the surface layer of duplex chromized SMAT sample exhibits smaller grain sizes, higher Cr concentration and hardness, as well as slower gradient variations of microstructure, composition and hardness within a larger thickness. Therefore, the wear resistance is enhanced significantly. |
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