On the Change of Hardness in 18-8 Stainless Steel During Thermal Fatigue

Abstract

Thermal fatigue and thermal cycling tests of 18-8 stainless steel were performed to study the effects of cyclic thermal strain and cyclic heating on the change of its hardness during the thermal fatigue. In this study, the thermal fatigue and thermal cycling tests were carried out by means of Coffin's type thermal fatigue testing machine, under conditions of high-frequency induction-current heating, (the heating rate being at 67°C/sec and holding time at maximum temperature 5sec), and water cooling (the minimum temperature being at 20°C). After testing to a certain number of thermal cycles, the hardness of the specimens was measured by means of micro Vickers hardness tester, and the relations between these hardness changes and grain size number were investigated. The conclusion can be drawn as follows.In the range of temperature variations from 600 to 800 deg (C), the hardness change by thermal cycling or precipitation-hardening showed the values of 5 to 40 per cent of total hardness change of thermal fatigue, and the hardness change owing to thermal strain cycling or the resultant of strain hardening and its softening indicated the values of 50 to 90 per cent. Consequently, in the final stage of thermal fatigue, the hardness change was considered as summing of these hardness changes, but in the initial stage, the effect of softening of the material by recrystalization and grain growth was obvious.