Effect of degradation on non-linear ultrasonic behaviour of aged centrifugally-cast stainless steel

The effect of degradation on the behaviour of aged centrifugally-cast austenitic stainless steel (CASS) and its non-linear ultrasonic response was studied. The duplex stainless steel CF8A was aged for 2400 h, 6000 h and 8400 h at 450ºC. The CF8A is mainly composed of austenite and δ-ferrite, of which the δ-ferrite will become harder with an increase in ageing time at the test temperature. The experimental results showed that the non-linear ultrasonic parameter increased because of the spinodal decomposition of the δ-ferrite and the formation of precipitates in the aged CASS. In view of the correlation found between the non-linear ultrasonic wave measurement and the mechanical test results, it is suggested that the acoustic non-linearity of duplex stainless steel depends on the microstructure, phase and precipitation.