The as-quenched microstructures of twelve commercial s teels were observed using optical microscope and scanning electron
microsco pe.It is found that the
packet martensite formed in medium and high carbon stee ls quenched from high temperature is not lath martensite, but rather fine
plate martensite. The formation mechanism of the packet martensite was explored in detail from mar tensite crystallography. Remaining the twi n relationship between the adjacent martensitic plates may significantly lower t he free energy barrier of martensite nucleation and growth, which may be realize d through changing the direction of homogeneous shear of austensite and the aust enitic direction,which is parallel with martensite platelet(i.e. double alterati on). The packet mechanism of twin boundary proposed is a common f ashion of transformation of fine plate martensite, that is to say, a new nucleus with twin relationship will form at two sides of the old nucleus along the same habit plane for diminishing remarkably the boundary energy and the volume strai n energy, then forming packet plate martensite.