Change of magnetic property during austenitizing process of Fe-Cr-Ni-Al-C alloy

Abstract

Effects of austenitization temperatures and holding time on magnetization, J (T), of Fe-17.5 mass%Cr-2.0 mass%Ni-0.8 mass%Al-0.5 mass%C based alloy were investigated in order to clarify the heat treatment condition for austenitizing and quantify the relation between magnetic properties and microstructures. The alloy exhibited high J value of 1.46 T after annealing at 1053 K. On the other hand, the J values of the alloy after austenitizing at temperatures between 1273 and 1473 K lowered with increasing austenitization temperature, describing s-shape curves. The J value of the alloy after austenitizing at temperatures above 1423 K for 240 s was nearly equal to zero magnetization. In addition, they also lowered with the increase of the holding time between 1 s and 240 s. The decrease in J values by austenitizing was closely related to the dissolution of the M₂₃C₆ particles into austenite (γ) matrix phase, which proceeds at higher temperature for longer time. The increase in chromium and carbon concentration in γ matrix phase due to the dissolution of M₂₃C₆ particles resulted in the decrease in martensite start (M_s) and finish (M_f) temperatures, and the increase in the paramagnetic retained γ fraction (X_γR). The following equation was introduced on the relation between magnetization and retained γ fraction.

$$ {{J_{\rm \gamma}}/{J_{\rm \alpha }=0.49{\rm X}_{{\rm \gamma R}}^{\rm 2}-1.50{\rm X}_{{\rm \gamma R}}+0.99}} $$

J_γ means the J values of the alloy after austenitizing at temperatures between 1273 and 1473 K, while J_α means that of the alloy after annealing at 1053 K (=1.46T).