Evaluation of Fatigue Strength in Very-long Life Regime of SNCM439 Steels

Abstract

Nickel chromium molybdenum steels (SNCM439) were fatigued with the rotating bending fatigue testing machine and the ultrasonic fatigue-testing machine (20 kHz) in laboratory air. The duplex S-N curve could not be observed for the ultrasonic fatigue tests. The origins of crack initiation in the long life regime were internal inclusions. The fish-eyes were observed in the specimens broken from internal inclusions. When the number of stress cycles to failure was longer than 8.1×106, the facet region was observed around inclusions. The fracture surface could be divided into three regions. The first region surrounding inclusion was a facet area. The crack propagation in the facet region was Stage A and the crack propagation outside the facet region was Stage B. When the fatigue cracks reached the specimen surface, the fatigue cracks propagated as surface cracks. The crack propagation rate in each region was estimated to be a power function of the stress intensity range. Fatigue life was calculated by integrating with the crack propagation laws determined from the experimental data. The predicted fatigue life agrees with the experimental result.