| H13 steel,which was medium carbon alloy hot work die steel of series about 5% Cr,has been widely used at home and abroad.It showed a lack of hot strength at temperature 600 ℃ and above.Comparison of research findings at home and abroad,it was found that increasing the content of Mo and reducing the content of Cr were important alloying ideas for developing new type of high strength and hot working die steel.On the basis of H13 steel,two new types of hot work die steel C1 and C3 steel were developed by decreasing Cr content to 3%,increasing Mo content,decreasing V content and adding Ni elements.Compared with H13 steel,different heat treatment system was worked out to determine the best heat treatment process of new steel.At the same time,the effect of alloying elements on the microstructure and properties of steel was systematically studied,1.By thermodynamic calculations,it was found that the critical point temperature of C1 and C3 steels reduced by changing the alloying element content.The temperature range for transformation from ferrite to austenite was extended,and the solution temperature of alloy carbides reduced.2.The peak value temperature of quenching hardness of C1 steel and C3 steel were 1050℃ and 1030℃,respectively.The peak values of quenching hardness were 61.4HRC and 61.2HRC,which was higher than that of H13 steel about 58.5HRC.According to the quenching grain size and the peak value of hardness,the best quenching temperature of C1 steel was1030 ℃ and that of C3 steel was 1000℃.3.When the test steel were quenched at the optimum quenching temperature and the tempering at different temperatures,there was secondary hardening phenomena,and the peak temperature of the secondary hardening was 510 ℃.Because a large amount of M2 C carbides containing Mo were dispersed due to higher Mo content,the secondary hardening effect of C1 was enhanced,resulting the higher hardness peaks.Tempering at 600℃,the tempering hardness of C1 steel reached 50.6HRC,which were 5.5HRC and 6.1HRC higher respectively than that of H13 steel and C3 steel.At the same time,because of the excessive content of Mo,the hardness of C1 steel was 42 HRC after 24 h heat preservation at 600℃,higher than that of H13 steel about 37 HRC.The hardness of C1 steel could be up to 38 HRC after holding 8h at 650℃.C1 steel has higher temper resistance.4.As the tempering temperature raised,the impact energy of the three test steels begun to rise above 510 ℃.The impact energy of H13 steel,C1 steel and C3 steel after tempering at 600 ℃ were 66 J,46J,47 J,respectively.Due to the addition of Ni elements,the toughness of C1 steel and C3 steel grew faster at high temperature.The impact toughness of C1 steel and C3 steel was basically the same as that of H13 steel at 700 ℃.The tensile properties of three kinds of test steels were compared.The increasing of Mo element,the strength of C1 steel improved,because a large amount of M2 C precipitated and hindered the recovery of martensite lath dislocations.The yield strength of C1 steel after tempering at 600 ℃ was 1579 Mpa,258 Mpa higher than that of H13 steel,and that of C3 steel was 1278 Mpa.At this temperature,the tensile strength of C1 steel was 1807 Mpa,262Mpa higher than that of H13 steel,and that of C3 steel was 1493 Mpa.C1 steel has higher strength. |
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