★ We analyze the nonlinear Lamb wave for ageing behavior study in Fe–Cr–Ni steels. ★ Nonlinear Lamb wave increases with formation of precipitate at early ageing time. ★ It keeps as a plateau with precipitate dynamic balance for a long-term ageing. ★ It reduces at final ageing time for precipitate coarsening and microvoids initiation. ★ Variation of nonlinear Lamb wave follows same trend as that of hardness in samples.
The cumulative second-harmonic analysis of ultrasonic Lamb wave has been performed to study the precipitation kinetics and microvoid initiation of ferritic Cr–Ni alloy steel during the ageing process. Ageing of ferritic Cr–Ni alloy materials have been done at 1223 K and 1173 K for different degradation time intervals and air cooled. The results show that the normalized acoustic nonlinearity of Lamb wave increases with the formation of fine precipitates at the early stage of ageing till about 1000 h and keeps as a plateau with the precipitates dynamic balance for a long-term ageing, and then decreases gradually at the final holding time with the coarsening of precipitates and initiation of microvoids. The results also show that the variation of nonlinear Lamb wave follows the same trend as that of hardness in materials. Therefore, the cumulative second-harmonic of ultrasonic Lamb waves has been found to be strongly sensitive to the precipitates behavior and microstructure evolution during the thermal ageing of ferritic Cr–Ni alloy steel.