Microstructural and crystallographical study of carbides in 30wt.%Cr cast irons

The microstructure and crystallography of carbides in 30wt.%Cr–2.3wt.%C cast irons with up to 3.7wt.%V additions have been studied by scanning electron microscopy and transmission electron microscopy. In a 30%Cr–2.3%C iron with no vanadium addition it was found that after destabilisation, M23C6 secondary carbides were precipitated within the prior austenite dendritic matrix and that duplex core–shell structures were formed in the eutectic carbides. The M23C6 secondary carbide possessed the Kurdjumov–Sachs orientation relationship with the martensite matrix suggesting a possible cube–cube orientation relationship between the M23C6 carbide and the austenite matrix with 〈1 1 0〉M23C6//〈1 1 0〉γ growth direction. The thickening of secondary carbide rods appeared to occur by the translation of steps on {1 1 1} planes in 〈1 1 2〉 directions. Vanadium additions to the 30%Cr–2.3%C cast irons produce ferritic matrix structures. Increasing the V content gave finer eutectic carbide structures, and also increased the volume fraction of eutectic carbides, which was in the range of 20–26%. The eutectic carbides in vanadium containing alloys had the stoichiometry Fe(1.6–1.8)Cr(4.9–5.7)V(0.7–1.3)C3. These eutectic carbides possessed an orientation relationship with the ferrite matrix close to {0 2 2 1}M7C3//{3 1 0}ferrite, {1 1 2 0}M7C3//{1 1 2}ferrite, 〈1 1 0 2〉M7C3//〈1 3 1〉ferrite.