Investigation of creep degradation in a Super304H austenitic steel and its nonlinear ultrasonic assessment method

Abstract

ABSTRACT This paper reports the degradation behaviour of Super304H during the creep process. The failure mechanism of this Super304H is the typical intergranular damage, the creep voids and micro-crack are mainly concentrated on the GBs and triple point GBs. Meanwhile, there is a lot of precipitates inside void chain, those particles are the M23C6 phase, which indicate that the coarsening and growth of M23C6 phase are detrimental to the microstructural property of Super304H. Besides, the GND density of Super304H increased remarkably at the creep acceleration period, and the dislocation structure also changed from the pattern of straight-line to the zigzag. In addition, the nonlinear ultrasonic measurement results showed that there is a general upward trend of nonlinear parameter β’ of Super304H during creep. The increase of the nonlinearity effect in Super304H could be due to several factors such as the precipitates precipitation, dislocation density/structure change, and the microdefects formation.