Effect Of Heat Treatment On Microstructure And Mechanical Properties Of Mould Steel

Development of China's mold industry is rapid, but compared with the industrialized countries there is still a wide gap, and the die life is generally low. Heat treatment has a significant impact on the performance of the die steel. By heat treatment, die steel can be given the necessary strength and toughness, and the life can be substantially increased. So in order to improve the technological level of China's mold industry and give full play to the potentiality of existing materials, conducting a comprehensive in-depth study is very necessary on the heat treatment process and technology of die steel. Using scanning electron microscopy, optical microscopy, hardness tester and tensile testing machine analyses the influence of quenching and tempering temperature on microstructure and mechanical properties of the P20 and H13 die steel, and microstructure and mechanical properties of P20 steel with ferrite and martensite obtained by intercritical quenching were studied.It is found that for the P20 steel, when the quenching temperature is above 830℃, almost all quenching microstructure is lath martensite, and their microstructures generally coarsen and the hardness increases with the quenching temperature increasing, but the microstructure will not be significantly coarsening until it reaches 890℃.860℃is considered as more appropriate quenching temperature. Precipitating carbides increase and grow up together, and the strength of the materials reduces with the tempering temperature increasing. When 620℃for 30min is selected as tempering temperature, the tempering hardness is from 32.8HRC to 35.8HRC, it can satisfy pre-hardening hardness requirement. In addtion, tempering hardness remains almost constant when the steel is quenched at 830-890℃and tempered at 620℃, which will be benefit for actual production in factory. Based on those, the best pre-hardened process is 860℃×30min quenching+620℃×60min tempering.For H13 steel, the result of orthogonal experiment show that the yield strength is not significantly affected by the first tempering temperature, but other strength indexes of the materials are all significantly affected by quenching and tempering temperature; the plasticity indexes are not significantly affected by quenching and tempering temperature except that reduction of area is significantly affected by second tempering temperature, so quenching temperature is designated as 1040℃by the analysis of microstructure and hardness. Through the analysis of tempering tensile properties, it has been found that when the higher tempering temperature is 600℃in two annealing, better mechanical properties can be got. So the optimum heat treatment process is 1040℃×15min quenching+600℃×2h tempering+580℃×2h tempering.After the intercritical quenching, the stucture and performance of P20 plastic die steel have been researched, and found that when the original organization is different, the ferrite morphology and the number of the undissolved carbide are also different. With the extension of holding time, the ferrite morphology remains unchanged, undissolved carbide reduces, the difference of mechanical properties is decreased. When the steel is quenched at 785～800℃, in general, the mechanical properties of the steel whose original organization is lath martensite is better than the steel whose original organization is pearlite. When the quenching temperature come to 815～830℃, quenching microstructure is mainly martensite, mechanical properties of the steel whose original organization is lath martensite is better.