| 4Cr5MoSiV1, namely H13steel by AISI standard in the United States, is a kind of air cooling hardening hot work die steel. Compared with4Cr5MoSiV steel, H13has higher thermal strength and hardness, and has good toughness, thermo fatigue and wear resistance properties with normal temperature. When air-quenched at a lower austenitizing temperature, H13has small deformations and produces less scale on surface, resisting the molten aluminium erosion. This steel is widely applied in manufacture of hot extrusion dies, mandrel, harmmar dies of die forging harmmar, dies of forging press, dies for precision forging machine and die-casting dies of aluminium, copper and their alloys.According to National Natural Science Foundation of China (No.51274031) titled "Investigation on Nanoscale Precipitates in Hot Work Die Steel and Comprehensive Strengthening Mechanism of Steel". The research work in this thesis is mainly concerned about the composition, fraction and particle size distribution of precipitates in H13during different heat treatment and their effect on mechanical properties. The main conlusions are shown as follows,1) V8C7and M23C6mainly precipitate from H13during ESR process, the shapes of which are square-like and sphere-like and their sizes are around200nm. It is found through calculation that the types and relative amount of H13precipitates during solidification of ESR is V8C7:VC:V2C:Cr7C3:Cr23C6:MoC: Mo2C=1.19:1.22:0.28:4.33:5.36:1.65:1.56.2) Deploying thermodynamic calculation, electrolysis+chemical phase analysis and carbon replica+TEM methods, the research on composition and particle size distribution of H13ingot after forging annealing and their effect on the yield strength are carried out. It is found that the particle size of precipitates decreases with increase of volume fraction both from center to surface and from upside to bottom. The possible reason is the cooling speed is faster on surface and bottom during ESR. The types of precipitates are mainly VC, M6C and M7C3, the average size of which are243.5nm. The precipitation amount is1.44μ m-2. The contribution of precipitates to the yield strength is311.32MPa.3) Deploying the same methods, study precipitates of H13after quenching and tempering. The precipitation types after quenching are mainly VC, M6C and a little M23C6, the total average size of which is150.5nm. The precipitation amount is0.67μ m-2. The contribution of precipitates to yield strength is around19%. The precipitation types are VC, M6C, M3C and M23C6after tempering, the average of which are82.6nm. The precipitation amount is5.37μm-2. The contribution to the yield strength is28%. The total average size is88.3nm after20hr tempering, the precipitation amount is6.5μm2. The contribution to the yield strength is39%.4) Research on precipitates of different quenching temperature, tempering temperature and holding time. The effect of precipitates on mechanical properties of H13is also studied. It is shown that the suitable process route of heat-treatment is quenching at1373K for30min, then cooling to room temperature in the air; tempering at863K for4hr, air cooling. From this treatment, precipitates is finely distributed and has suitable amount. H13has the optimal comprehensive mechanical property.5) The research on comprehensive strengthening mechanism during different tempering process is carried out. The result shows that the consequence of contribution is refinement strengthening, precipitation strengthening, solid solution strengthening and dislocation strengthening. |
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