On the Hardness Change of 18-8 Austenitic Steel during Thermal Fatigue under Conditions of Transient Temperature Gradient

Abstract

This study was performed by measuring micro Vickers hardness, to make certain of the material changes which had occurred in 18-8 austenitic steel in the course of thermal fatigue under conditions of transient temperature gradient. The thermal fatigue and cyclic tests were carried out by means of the previously mentioned thermal fatigue testing machine of high-frequency induction-current heating and water-cooling type (average heating rate being at 200°C/sec, and the holding time at maximum temperature being 5, 10 and 20sec). After testing to a certain number of thermal cycles, the hardness changes across the cross-section of wedge-shaped specimen were measured, and the effects of cyclic thermal strain and cyclic heating on the changes of its hardness during thermal fatigue, were studied in comparison with the results of prementioned Coffin's type thermal fatigue test. The conclusion is drawn as follows.In the case of thermal fatigue originating from the transient temperature gradient (temperature variations being at 800 and 600°C, and the holding time at the maximum temperature being 5sec), the hardness change by thermal cycling or precipitation hardening showed the values of 3 to 18per cent of total hardness change during thermal fatigue, and the hardness change by thermal strain cycling or the resultant of strain hardening and its softening indicated the values of 67 to 90per cent. These hardness ratios varied as the holding time at the maximum temperature was prolonged. It was found, however, that the hardness change during thermal fatigue was, after all, as it was the case with the results of Coffin's type thermal fatigue, the consummation of these hardness changes.