The Behavior of Thermal Fatigue Crack Initiation in Smooth Bar of Low-Alloy Steels

Abstract

In order to investigate the behavior of thermal low-cycle fatigue crack initiation in a smooth bar of low-alloy steels, out-of-phase and in-phase thermal fatigue tests were carried out at a temperature range from 573K up to 823K in an air environment, and fatigue cracks generating in the interrupted and failed specimens were observed by means of optical and scanning electron microscopes. Also, by using a Cr-Mo-V forged steel, load-controlled low-cycle fatigue tests at 823K were performed inside a scanning electron microscope (SEM) with a vacuum environment of about 7mPa for the purpose of in situ observation of the surface cracking behavior.
The results obtained were summarized as follows:
(1) In general, the low-alloy steels showed such a character that failure life (N_f) in out-of-phase thermal fatigue was shorter than that in in-phase at low strain ranges. This was found from the results of the interrupted tests to be due to earlier crack initiation in out-of-phase thermal fatigue.
(2) The fracture mode in both out-of-phase and in-phase thermal fatigue was of transgranular type, but the characteristic of crack initiation was different. Surface cracks generated much more easily in out-of-phase, while cracking in in-phase was apt to occur from internal inclusions, such as MnS.
(3) The fatigue test results inside the SEM indicated that surface oxide scale cracked so easily (below 0.01N_f) under cyclic loading. Furthermore, many surface oxide scale cracks were observed in the out-of-phase thermal fatigue failed specimens, and the fatigue cracks in the underlying metal were also found to correspond to the oxide cracks. On the other hand, such oxide crack was hardly observed in the in-phase thermal fatigue failed specimens. These findings led to the environmental effect that the surface oxide cracks accelerated the early nucleation of fatigue cracks.