Effects of Tempering Temperature and Stress Amplitude on Low-cycle Fatigue Behavior of a Cold-Work Tool Steel

Abstract

Low-cycle fatigue behavior of a newly developed cold-work tool steel containing 0.8C-8Cr-2Mo was investigated. The aim of this study is to examine the fatigue behavior in connection with its microstructure and hardness of the specimens tempered at several temperatures. The specimens were tempered at 473K, 793K and 823K, and then were tested under fully reversed stress amplitudes of 1568MPa and 1176MPa. The reasons of choosing three kinds of tempering temperatures were follows. Firstly, the secondary hardening was remarkable at 793K and the hardness at this was 62HRC while the hardness of specimens tempered at 473K and 823K was almost same of 60HRC. Secondly, amount of retained austenite (γR) were 14vol% and 2vol% for specimens tempered at 473K and at 823K, respectively. At the higher stress amplitude of 1568MPa, the fatigue lives of the specimens tempered at 793K and 823K were almost same, while the life of the specimen tempered at 473K was the shortest. In this case, the fatigue lives were attributed to the time till carbide cracking. Thus, the crack initiation process was influenced by the strength of steels. On the other hand, at the medium stress amplitude of 1176MPa at which the fatigue life was 104-105 cycles corresponding the actual tool lives, fatigue lives of the specimens tempered at 473K were the longest, then with increasing tempering temperature from 793K to 823K, the fatigue lives decreased. Fisheyes were observed on the fracture surface of the specimens tempered at 473K and 793K. The fatigue lives in this fracture morphology were attributed to the crack propagation process until it reached to KfC. Additionally, it was found that γR contributes the increase of the resistance for crack propagation.