Effects of heat treatment oil microstructure and properties of 20wt%Cr and 27wt%Cr white cast irons

Abstract

20wt%Cr and 27wt%Cr oast irons were destabilised at 1000°C for 4 hours, followed by air cooling to room temperature. The as-cast microstructure of 20%Cr iron consisted of eutectic M<inf>7</inf>C<inf>3</inf> carbide, primary austenite (γ) dendrites with some pearlite structure and eutectic austenite. The as-cast 27%Cr iron contained eutectic M<inf>7</inf>C<inf>3</inf> carbide, primary austenite dendrites and eutectic austenite which had partially transformed to martensite (α) during cooling in the mold. After destabilisation, the microstructure of both irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide was M<inf>7</inf>C<inf>3</inf> in the 20wt%Cr iron and both M<inf>7</inf>C<inf>3</inf> and M<inf>7</inf>C<inf>3</inf> for the 27wt%Cr iron. Size and volume fraction of secondary carbide in the 20wt%Cr iron were higher than for the 27wt%Cr iron. After destabilisation, the hardness and corrosion resistance were increased for both irons. Anodic polarisation curves used to indicate corrosion resistance showed that the 27wt%Cr iron possessed a slightly wider passive range than the 20wt%Cr iron.