| Thermal fatigue (TF) is a leading failure of hot working die steels. It has beenreported that, among the all failures of hot working die steels, 60~70% failureshave been caused by TF. In our country, 40~50 thousand tons of hot work die steels,worthy of thousands of million RMB, have been used every year. Therefore, it is ofgreat interest for the development of the economy to improve the TF property ofhot working die steels. The TF research of hot work die steel is a representativeorientation in the field of the TF research. In the past years, much of the researchon the TF of hot work steels has been focused on forging die steels, while less workhas been carried out on the TF behavior of cast die steel. Especially in the recentyears, the studies on cast die materials have been widely and rapidly developed. Itis interesting to note that more and morecast die steels replace forging die steels asdie materials because of their shorter produce cycles and lower expense. Therefore,the study on the TF behavior of cast hot work die steel not only has a theoretic butalso a practical value. The new type cast high Cr hot work die steel, which is developed by JilinUniversity, is used as the raw material. Optical microscope (OM), scanningelectron microscopy (SEM), chemical composition analysis, impact toughness tests,hardness tests and TF tests have been carried out to study effects of themicroelements on the microstructure, mechanical behavior, TF of new type casthigh Cr hot work die steel and the mechanisms on TF resistance. By analyzing the experimental data, some conclusions have been drawn asfollows:(1) When the contents of Y,Ce and N are 0.01wt. %,0.03wt. % and 0.06wt. %, Respectively, the properties of the new type cast high Cr hot work die steel reach to the optimization. Compared to the steels without the microelements, the structure is finer; the distribution and morphology of the inclusions are greatly improved, and their size is also reduced 2 times; the roughness and hardness are increased 150% and 14%, respectively; under the thermal cycling temperatures of 650℃ ~ 20℃, the TF life is as one time long as H13, and as 861.5 times as the steels without the microelements.(2) It has been found that when only Y is added to the steel, compared to the unmodified steels, the propagating rate of TF crack is decreased one time and furthermore the decrease of the hardness during the TF cycles is reduced 43%, and as a result, the TF property is improved one time; when Y content gets to 0.01%wt.%, the properties reach to the optimization where the roughness and hardness are increased 98% and 14%, respectively.(3) It has been found that when only Ce is added to the steel, compared to the unmodified steels, the propagating rate of TF crack is decreased one time and furthermore the decrease of the hardness during the TF cycles is reduced 39%, and as a result, the TF property is improved one time; when Ce content gets to 0.03%wt.%, the properties reach to the optimization where the roughness and hardness are increased 104% and 12%, respectively.(4) It has been found that when only N is added to the steel, compared to the steels without N, the propagating rate of TF crack is decreased one time and furthermore the decrease of the hardness during the TF cycles is reduced 43%, and as a result, the TF property is improved one time; when N content gets to 0.06%wt.%, the properties reach to the optimization where the roughness and hardness are increased 100% and 13%, respectively.(5) The ability to oxidation of new type cast high Cr hot work die steel are better than those of the unmodified and 8407 steels. With the tempering for 20 hours at 630℃, after modified, compared with the unmodified and 8407 steels, the ability to oxidation of new type cast high Cr hot work die steel is increased 47.86% and 53.07%,...
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