Non-destructive evaluation of plastic strain in DP590 steel using magnetic incremental permeability method under high temperature

In precision mechanical processing, workpiece surfaces often experience combined high temperature and mechanical loads, leading to microstructural changes and degradation of mechanical properties. This necessitates accurate characterisation of material state evolution under such thermo-mechanical conditions for process optimisation and quality improvement. This study investigates the magnetic incremental permeability (MIP) method for non-destructive evaluation of plastic strain in DP590 steel under high temperature. An experimentally constructed high-temperature MIP platform was employed to measure specimens with plastic strain up to 12.22% across temperatures from room temperature to 500 °C. The results reveal a distinct power-law relationship between key MIP features and plastic strain at room temperature, with the mean MIP value μmean achieving an excellent fit (R² > 0.96) and demonstrating high sensitivity, particularly for strains below 2%. A critical transition temperature of approximately 300 °C was identified: below this threshold, MIP effectively characterises plastic strain, while above it, the signal becomes predominantly temperature-driven due to thermally activated microstructural changes. In-situ HT-CLSM revealed accelerated carbide evolution in pre-strained specimens during thermal holding, providing a microstructural basis for the magnetic response. This work validates the MIP method under coupled high-temperature and plastic strain conditions, supporting in-process non-destructive monitoring in high-temperature machining environments.