Detection of martensite transformation in high temperature compressively deformed austenitic stainless steel by magnetic NDE technique

The present work demonstrates that a magnetic non-destructive evaluation technique can be useful for detecting the presence and extent of ferromagnetic martensitic phase in high temperature deformed 304 austenitic stainless steel.A good correlation between the martensitic transformation and magnetic parameters; saturation magnetization, coercive force and magnetic susceptibility have been obtained. Saturation magnetization was increased depending on the volume percentage of martensite transformation. The volume percentage of martensite was found to be dependent on the temperature and level of plastic strain. At temperatures below 623 K, martensitic transformation was detected after deformation of 10 to 40% plastic strain. A massive increase in martensite phase was observed in the specimen deformed at RT to 40% plastic strain.Compressive deformation at RT formed thermodynamically more stable long and broad shape of martensite. But as the temperature of deformation increases lath shape gradually converted into needle shape.Coercive force was exclusively connected with size, shape and spatial distribution of martensite. For specimens deformed at 523 K coercive force were much higher than those of the specimens deformed at RT. Coercive force decreased remarkably at temperatures above 623 K. Formation of ferromagnetic martensite in a paramagnetic matrix was also accompanied by an increase in magnetic susceptibility. Low magnetic susceptibility at temperatures above 623 K was due to disappearance of martensitic phase.