Quantitative evaluation of case depth in induction-hardened steels via principal component analysis of first-order reversal curve diagrams

The case depth of induction-hardened SCM435 and S45C steel rods was evaluated through analysis of first-order reversal curve (FORC) diagrams. Two distinct FORC peaks at low and high coercivities were observed, corresponding to the magnetic responses of the core and hardened layer, respectively. Principal component analysis (PCA) of the FORC diagrams revealed that the data variability was primarily captured by the first and second principal components, with the first component exhibiting a strong linear correlation with case depth. These results indicate that PCA of FORC diagrams provides a robust, quantitative, and nondestructive approach for evaluating case depth in surface-hardened steels.