Effects of Si on microstructure and phase transformation at elevated temperatures in ferritic white cast irons

Abstract

The effects of Si on microstructure and phase transformation at elevated temperature of ferritic 31wt.%Cr-1.1wt.%C white cast irons with up to 3wt.%Si have been studied. Applications of these irons include parts requiring heat resistance at elevated temperature. The irons were produced by sand casting. The microstructure in as-cast condition and after being subjected to high temperature (700 to 1000°C) was investigated by light microscopy, X-ray diffraction, and electron microscopy. The results revealed that the as-cast microstructure consisted mainly of primary ferrite dendrites and eutectic (ferrite+M7C3). Si promotes M7C3-to-M23C6 transformation in the irons subjected to transformation at elevated temperature, but no sigma phase was found. The extent of M7C3-to-M23C6 transformation increases proportional to the increasing transformation temperature, holding time and Si content in the irons. For the iron with 1.0wt.%Si content after holding at elevated temperatures, martensite was also found, which could be attributed to carbon accretion effects in eutectic ferrite. Si was incorporated in M23C6 such that M23C6 containing Si can show darker contrast under SEM-BEI as compared to M7C3; this is the opposite to what has been observed for the cases of typical M23C6 and M23C6 containing Mo or W. The results obtained are important to understand the change in properties of ferritic, high chromium irons containing Si subjected to elevated temperature.