This paper deals with the effects of high—temperature ausform hardening and soft—nitriding on the struture,substructure,and fatigue strength of 40Cr steel.The test results showed that in the austenite a kind of polygonal-- substructure willl be formed in the proceolure forging soaking--quenching,and subsequently quenched martenside instead.After soft—nitriding at 570℃,a stable polygonal structure with carbides can be observed.This stable structure provides diffusion channels for C and N atoms. The depth of ...