Research On Modification And Heat Treatment Of Hot Working Die Steel

As a traditional hot working die steel in China,3Cr2W8V steel only contains carbon from0.3%to0.4%, but it contains many carbide-forming elements such as tungsten, chromium, vanadium, as that it is eutectoid steel. Large number of coarse net-like eutectic carbides often form in the last solidified part attributed to the the segregation of alloy elements during the solification, which lead to the stress concentration and early fracture. In addition,3Cr2W8V steel works in bad condition. Severe stress exists in mould cavity surface under high temperature and pressure metal liquid. So crackle and thermal fatigue form, and mould becomes invalid. It is an urgent problem to increase the life of mold by improving the toughness and thermal fatigue resistance. In this article, we studied the effects of modification treatment and heat treatment on microstructure and properties for the problem on3Cr2W8V steel. The main contents are summarized as follows:The effects of RE modifier, K/Na modifier and RE-K/Na-Ti modifier to3Cr2W8V steel was analyzed by methods of metallurgical microscope, FSEM and XRD etc. The results show that:Both of rare earth and K/Na modifier can be effective in grain refinement and improvement of the carbide morphology and distribution. The grain size of3Cr2W8V steel becomes particularly small after RE-K/Na-Ti compound modification, and the carbides are transferred from net-like to flakes and block-like distributing in the matrix dispersed. The effect of modification is best when the ratio of modifier is0.3%RE and0.6%K/Na-Ti. When the3Cr2W8V steel after multi-modification via heat treatment, the secondary carbides precipitated from the matrix are more fine dispersed, that the toughness and thermal fatigue resistance are significantly improved, and the life of mold greatly improved.The subject investigates the effect of different methods of heat treatment, quenching temperature and temper temperature on the structure and property of3Cr2W8V steel. By research and analysis, the conclusion is that after normalization, spheroidizing,1100℃quenching and640℃+620℃twice temper,3Cr2W8V steel which is ameliorative via RE-K/Na-Ti compound modification will get the temper troosite and grainy carbonide microstructure and the decomposed carbonide is distributed in substrate in dispersion.Because rare-earth element and K/Na have strong deoxidization and desulfuration ability, after0.3%RE and0.6%K/Na-Ti modification, the amount of oxygen and sulfur in3Cr2W8V hot mould steel decreases. The rare-earth element in compound modification forms spherical RE-O-Al-S occluded foreign-substance with the oxygen and sulfur in mould steel. Spherical occluded foreign-substance does not dissevers substrate severely, so the crackles have little tendency to produce and spread.The results of mechanical properties tests show that, the hardness of3Cr2W8V hot work die steel can reach to HRC47.6, and the impact ductility can reach to34.5J/cm2, when it has been multi-modification and heat treatment.