Study of Ultra-Long Life Fatigue of High Strength Steel with Duplex-Phase of Carbide-Free Bainite and Martensite

Abstract

The ultra-long life fatigue properties of novel high strength steel with duplex-phase of carbide-free bainite and martensite (CFB/M) were studied in this article. The specimens were heat-treated by the following procedure: austenitizing at 900°C for 20 min followed by air cooling and then tempering at 340°C for 2 h. All the fatigue tests were carried out by ultrasonic fatigue testing equipment, at a cycling frequency of 20 kHz. Experimental results show that there is no conventional fatigue limit for the steel studied in this paper. From the S-N curve it could be suggested that it is more reliable to use the fatigue strength at 10 8 cycles than that at 10 7 cycles for the safe-life design. Based on the observations of fracture surface, it is obvious that the fatigue fracture surface can be classified into two types: initiating from surface and interior. A fish-eye mark and an area with a particular morphology surrounding the fracture origin termed ODA (optically dark area) by Japanese scholars are always found in the latter type, but no obvious inclusion is observed inside it. By calculating the values of ΔK at ODA and fish-eye, it is found that the values of Δk fish-eye and ΔK ODA tend to be constant respectively, independent of the stress amplitude, fatigue life and the distance of the initiation site to the edge of specimen, therefore, it is thought that the contribution of hydrogen to the formation of ODA should not be considered.