Characterization of aging behaviour in M250 grade maraging steel using eddy current non-destructive methodology

The aim of the study is to investigate the effect of aging of solution annealed M250 grade maraging steel on the microstructure, room temperature hardness and eddy current (EC) parameters. Specimens of M250 maraging steel were subjected to solution annealing at 1093 K for 1 h followed by aging at 755 K for various durations in the range of 0.25-100 h. The hardness was found to increase continuously with aging due to precipitation of intermetallics reaching a maximum and decreased there after due to reversion of martensite to austenite. The aging treatment led to three major microstructural changes, i.e. decrease in dislocation density and quenched-in point defects during initial aging (up to 0.25 h), increase in volume fraction of intermetallic precipitates at intermediate duration and a systematic increase in reverted austenite at longer duration (beyond 40 h). These microstructural changes alter the electrical resistivity/magnetic permeability which, in turn, influence the induced voltage in eddy current coil. Two EC parameters, viz. magnitude and phase angle of the induced voltage were studied for heat treated specimens. The study revealed that a good correlation exists between EC parameters and microstructural changes in M250 maraging steel specimens