Effects of heat treatment on microstructure and corrosion resistance of 18wt%Cr-3wt%C-5wt%W-0.9wt%Mo iron cast by a semi-solid process

Abstract

Microstructurc and corrosion rcsistancc of a 18wt%Cr-3wt%C-5wt%W-0.9wt%Mo iron cast by a semi-solid proccss using a sloped cooling plate method were studied. Effects of heat treatments including tempering, destabilisation, and dcstabilisation plus tempering, were investigated. Microstructural characterisation was performed by scanning and transmission electron microscopy. The results revealed that primary austenite had a spheroidal form instead of a dendritic shape. Radiating clusters of eutcctic structure were obtained. M<inf>23</inf>C<inf>6</inf> secondary carbides precipitated during destabilisation heat treatment within primary austenite, which was later transformed to martensite during air cooling. In as-cast condition, W and Mo were mainly associated with the eutectic carbide. Destabilisation led to a dissolution or transformation of these phases. The best corrosion resistance, as tested by a potcntiodynamic technique in 0.5 M sulfuric acid solution, was obtained from the iron in the as-cast iron. Details of chemical attack in particular conditions were discusscd.