| Nowadays, mould technology has been a more important symbol that canweigh one country's production ability. Recently, the total amount of mould steelvalues has been 600 ~ 650 hundred million dollars all over the world, and theexpenditure on the mould steel of our nation was about 360 hundred million RMBin 2002. In 2003 the expenditure increased by 25% which was up to 450 hundredmillion RMB. It has been estimated by experts that the increasing extent can be30% in the future, and the automobile and motorcycle will be the most importantparts that may possess about 60% in all expenditure among all of the mould steelmarkets. Thermal fatigue (TF) is a leading failure of hot working die steels. It hasbeen reported that 60~70% failures have been caused by TF among all the failuresof hot working die steels. Therefore, it is of great importance for the developmentof the economy to improve the TF property of hot working die steels. The TFresearch of hot work die steels is a representative orientation in the field of the TFresearch. During the past years, much of the research on the TF of hot work diesteels has been focused on forging die steels, while less work has been carried outon the TF behavior of cast die steels. Especially in the recent years, the studies oncast die materials have been developed widely and rapidly. It is interesting to notethat more and more cast die steels replace forging die steels as die materialsbecause of their shorter produce cycles and lower expense. Therefore, the study onthe TF behavior of cast hot work die steels has not only a theoretic but also apractical value.Under the condition of the chemical composition is unchangeable, heattreatment is the best way for change the microstructure of steel. It can improve theperformance of steel with different heating process and cooling manner. This is abasic proof for heat treating process. in a word, the heat treatment is a keytechnology for improving the life of hot work die steel. The new type of high Cr hot work die steels (HHD steel), which is developedby Jilin University, is used as the raw material. Optical microscope (OM), scanningelectron microscopy (SEM), Transmission Electron Microscope(TEM), chemicalcomposition analysis, impact toughness tests, hardness tests and TF tests have beencarried out to study the effects of the strengthening and toughening heat treatmenton the microstructure, mechanical behavior, TF of new type high Cr hot work diesteel and the mechanisms on TF resistance. At the same time, the betterstrengthening and toughening heat treatment technology of HHD steel has beenoptimized, which is used to guide the practical production. The research was emphasized on the effect of optimizing heat treatment on theproperty of HHD steel. By analyzing the experimental data, some conclusions havebeen drawn as follows:(1) The continuous cooling transformation curve (the CCT curve) of HHD steel has been determined for the first time, which provides the important basis for attaining the perfect microstructure in the theoretic studies and practical application. The result showed that experimental steel had good harden ability; the transformation temperature of the various phases was: Ac1-820℃, Ac3-880℃, Ms-310℃and Mf-190℃; the time of the carbide deposition was later, and it deposited until the cooling rate reached to 0.53℃/s (the cooling time was 30 minutes), and the least critical cooling rate was 0.53℃/s in which the steel can obtain all martensite structure.(2) The pretreatment may eliminate effectively the composition segregation phenomenon and reduce the network carbide in the boundary of the grain. At the same time, it may get the finer grains and obtain the uniform high temperature tempering martensite microstructure. Furthermore, the pretreatment can improve the obdurability and the resistance of thermal fatigue (TF) of the HHD steel. After the pretreatment process, the mechanical properties of the HHD steel were improved significantly: compared the samples pretreated with those pretreated by the conventional anneal and applied without pretreatment, the impact toughness of the steel was increased by 19% and 55%, respectively; the hardness of the HHD steel after the pretreatment was 26~28HRC, which ensures the processability; the hardness of the samples tempered was less difference compared with those without pretreatment; however, the optimizing pretreatment TF life was as 1.5 times as the steel without pretreatment. By pretreated with optimizing pretreatment technique, the HHD steel had an optimal TF property and was as 1.4~1.6 time as the forging 8407 steel.Analyzing the effect of pretreatment on hardness, impact toughness and TF property, the optimal pretreatment process have been optimized as 1080℃ +880℃.(3) The microstructure of the HHD steel treated by the pretreatment process was lathing martensite after the quenching process. The proper increasing of the quenching temperature was favorable to attain the lathing martensite structure and the sub-structure of the dislocation martensite, which can attain the higher strength and toughness after the temper and improve the resistance of the TF. In the quenching temperature range from 980℃to 1160℃, the quenching hardness increased with the increasing of the quenching temperature. However, the peak hardness value was not found in the chosen quenching temperature range. As the quenching temperature increased, the impact toughness increased firstly and reduced subsequently, and its peak value was obtained near 1080 ℃. Compared with the samples quenched at 1000 ℃and 1160℃, the impact toughness of the sample quenched at 1080 ℃was increased by 39% and 63%, respectively. The TF resistance of the sample quenched at 1080℃was better than the other quenched samples. Compared with the 8407 steel, the length of the main crack of the 8407 steel was as1.25 times as the sample quenched at 1080℃, and the attenuation rate of the hardness was as 3 times as the sample quenched at 1080℃. Based on the research of the effect of the quenching temperature on the hardness, impact toughness and thermal fatigue property of the HHD steel, it can be concluded that its appropriate quenching temperature was oil quench at 1080℃.(4) After the optimizing pretreatment and quenching, the microstructures of the HHD steel were tempering martensite, a small quantity of aretained austenite and quantities of dispersion and nanometer carbide, which makes the steel to obtain not only the higher hardness but also better toughness and TF property. In the temper temperature range from 520℃to 660℃, the temper hardness of the HHD steel decreased with the increasing temper temperature, and "the secondary hard"was found near 570℃; the impact toughness increased firstly and reduced subsequently, and its peak value was obtained near 580 ℃. The TF resistance of the samples tempered at 580℃+560℃was the best of all eight temper samples. Considering the effect of temper temperature on the hardness, impact toughness and TF resistance, it can be concluded that its appropriate temper temperature was 580℃+560℃.(5) It is found that the thermal fatigue crack initiation was mainly influenced by... |
PDF Full Text Request