Cryogenic Treatment And The Properties Of High-chromium Cast Iron

In this paper, the effects of cryogenic treatment on the properties of High-Chromium Cast Iron (short for HCCI) were studied. By the means of the Rockwell hardness test, repeated impact-fatigue test, surface fatigue test and scratching abrasion test, the influence of the various technique processes including cryo-treating sequence, elevated temperature quenching, time-lapse cryo-treating and tempering on the performance of HCCI have been investigated. And microscopic structure and mechanism have been discussed.The results showed that the proper cryo-treatment can improve the hardness and repeated impact-fatigue resistance of HCCI remarkably. The process of quenching-cryotreating-tempering had better general mechanical performance than that of cryotreating-quenching-tempering. Holding in the quenching temperature, the hardness and the times of the repeated impact-fatigue had a certain boost, but the change was smaller after 1.5 hours. Scratching abrasion resistance had the reverse trend and surface fatigue resistance reached a peak value when holding 1.5 hours. The method of slow cooling after the quenching temperature resulted in worse mechanical performance and wear resistance than cooling in the air directly. There were evident affects of time-lapse cryo-treat after quenching on the properties of HCCI. Hardness of the alloy deceased rapidly with the interval extending, but the trend was minishing after 2 days. The repeated impact-fatigue times dropped in a slower rate. Surface fatigue resistance was the other way round the rule of the hardness, and the weightlessness was largest in method of direct cryo-treat following quenching. After slow-cooling from the quenching temperature and cryo-treating, the hardness of HCCI appeared up and down in the conditions of different tempering temperature, the highest value existed in 450℃. Repeated impact-fatigue resistance worked on the opposite operation. 350℃ and 250℃ are the worst and best temperature for the scratching abrasion resistance respectively. The weightlessness of surface fatigue arrived to most on the medium tempering temperature of 350℃ and 450℃, the optimal wear resistance came forth when tempered on 500℃.The main function mechanism of cryo-treatment acting on the HCCI were retained austenite decomposition, ultra-fine disperse carbides precipitation and martensite micro-decomposition in the matrix. Therein, both retained austenite decomposition and carbides precipitation were the key factors in the cryo-treating process. The extent of carbides precipitated, the amount and stability of retained austenite before cryo-treating would affect the properties of the cryo-treating HCCI directly.