Retained Austenite in Spray Formed High Chromium White Cast Iron

Abstract

High-chromiun white cast irons are widely used for applications which require high wear and corrosion resistance. In this work it was investigated the microstructure of spray formed high chromium white cast iron alloys with the following compositions: 2.4%C-15.3%Cr; 3.6%C-14.5%Cr; 2.9%C-18.8%Cr. A gas-metal mass flow ratio of 0.23 and nitrogen were used for spray processing. The spray formed deposits, the overspray powders (particles that bounce off the deposit and do not participate in its consolidation) and conventionally cast samples were characterized by optical microscopy, scanning electron microscopy and x-ray diffraction. The deposits showed a microstructure formed by fine M 7 C 3 carbides (<50um in length) for the three compositions. The matrix microstructure changed with composition; the 2.4%C-15.3%Cr alloy showed mainly an austenitic matrix whereas the 3.6%C-14.5%Cr and 2.9%C-18.8%Cr alloys showed a matrix with larger amounts of martensite. These microstructures are quite different from the eutetic M 7 C 3 carbides (>300um in length) and a matrix of austenitic dendrites and some martensite, which are observed in conventionally cast alloys. Overspray powders of all compositions studied showed a microstructure composed mainly of austenite. The formation of carbides was suppressed. It was observed that the overspray powders presented a higher amount of retained austenite than the spray formed deposits and the conventionally cast material; the rapid solidification of the droplets enhanced the solid solution of the elements carbon and chromium in austenite, leading to a depression of the Ms temperature of the alloys to below room temperature, thus avoiding the austenite to martensite transformation.