Microstructure of weld heat-affected zone of 12Cr–Mo steel

Abstract

Detailed studies have been made of the transformations and microstructure of the weld heat-affected zone (HAZ) of a class 212Cr–Mo steel, particular attention being given to δ-ferrite. The investigation covered a wide range of welding heat inputs and involved real and simulated HAZ material. To obtain the phasetransformation data extensive use was made of a weld simulator and dilatometer. The weld H AZ consisted of three regions: secondary-hardened martensite, homogeneous martensite, and duplex δ-ferrite and martensite adjacent to the fusion line. The amount of δ-ferrite increased with increasing weld heat input. It was shown that δ-ferrite forms above 1320°–1360°C, depending on the heat rate and remains untransformed on cooling to room temperature for the normal range of weld cooling rates. Detailed metallography showed that δ-ferrite forms at austenite grain boundaries, involves redistribution of allo ying elements and is associated with interphase precipitation of carbides. However, in amounts up to 16·6% it does not affect the subsequent decomposition of austenite. From these findings a representative C–C–T diagram is produced to describe the transformation occurring in the HAZ of this 12Cr-Mo steel.