Observations of Structural Change During Austenitizing

Abstract

Structural changes during austenitizing of a Ni-Cr-Mo-V steel, which has martensitic structure initially, were observed.
Austenite nucleated at A_c1 temperature grows with orientation, therefore, shape of residual ferrite becomes acicular. The final structure includes acicular pattern derived from acicular shaped residual ferrite in austenitic matrix. This is the structure that have previously been named “DEC”. Variations of heat-treating condition induce the changes of distribution of acicular pattern.
Equiaxed grains formed in DEC, which have been named “LEC”, increase with increasing austenitizing temperature, and the increasing rate is smaller in the early stage of its formation and larger in the later stage. Variations of heat-treating condition induce the changes of increasing rate of LEC in the later stage of its formation.
It was shown that, in the case when coarser austenitic grain is formed, relatively rougher distribution of acicular pattern in DEC, larger increasing rate of LEC in later stage of its formation, and larger LEC size are obtained.
It was suggested that examinations on fine austenitic grains which constitute the DEC is important for the explanation of these phenomena.