Solidification Structures and Microsegragation of Fe-Ni-Cr Alloys

Abstract

The solidification phenomena in 10kg ingots of Fe-Ni-Cr alloys were investigated by observation of macroand micro-structures and determination of micro-segregation.
1) The phase changes of Fe-Cr-Ni alloys are classified into the following four kinds.
A) Liquid→Liquid+δ→δ (α) -α+γ (solid transformation)
B) Liquid→Liquid+δ→Liquid+δ+γ→α+γ
C) Liquid→Liquid+γ→Liquid+γ+δ→γ+α
D) Liquid→Liquid+γ→γ (no solid transformation)
Both types B and C have pseudo-columnar zone, which is composed of equiaxed structure microscopically but is macroscopically observed as columnar structure. This zone is formed with equiaxed structure, which is detached from the columnar crystals by microscopic thermal stagnation on the thernary peritecticeutectics, and is followed by γ formation which has previously begun during solidification.
2) Micro-segregation of Fe, Cr, and Ni may be estimated from the isothermal curves in the phase diagram by JENKINS et al.
Segregation index and effective distribution coefficient of Cr and Ni in this work agreed with those by others. Si and Mn segregate more extensively with increasing Ni content. In case of γ-primary (types C and D), Cr, Si, and Mn concentrate more extensively in the interdendritic region than in the center of dendrite, but Fe and Ni over 50% are higher at the center.