Phase-hardening and phase-softening in dual-phase steels

The phenomena of phase hardening of a soft phase (ferrite, austenite) and phase-softening of a hard phase (martensite) in dual phase steels were examined on steels with ferrite-martensite and austenite-martensite structures. Phase hardening and phase softening are universal phenomena. The hardening of the soft phase is caused by the increase of dislocation density due to volumetric expansion of surrounding hard martensite phase during transformation. The softening is caused by the less constraining effect of the soft matrix resulting in the decrease of dislocation density in martensite and less density of twinned martensite. The existence of ferrite microareas within martensite islands is also a factor which decreases the microhardness.