Effect of Tempering Temperature on Very High Cycle Fatigue Behavior in Low-Alloy Steel, SNCM439

Abstract

In order to investigate the influence of the tempering temperature on a S-N characteristic in very high cycle fatigue regime, cantilever-type rotary bending fatigue tests up to 109 cycles were performed in air at room temperature using hour-glass shaped specimens heat treated at four kinds of tempering temperature in low-alloy steel, JIS SNCM439. Specimens tempered at low temperature of 433K and 573K showed a step-wise S-N curve, and were ruptured by internal fracture mode at high cycle fatigue regime above 106cycles. On the other hand, surface fracture mode only appeared on the specimens tempered at high temperature of 773K and 893K, and the fatigue limit existed clearly. Granular bright facet (GBF) area was observed in the vicinity of a non-metallic inclusion which was origin of an internal fracture. Properties of the GBF area were verified from the experimental facts that the roughness of the GBF area was larger than that outside the GBF in a fish-eye, the size and the distribution of convex particles on the GBF area were same as those of spherical carbide particles in the matrix, and a high density of carbon was detected on the GBF area by EPMA. It was suggested that a decohesion of spherical carbide from the matrix for the formation of the GBF area was affected by the size of the carbide which became large with decreasing the tempering temperature.