Microstructural characterization of the heat-affected zones in grade 92 steel welds: Double-pass and multipass welds

The microstructure in the heat affected zone (HAZ) of multipass welds typical of those used in power plant made from 9 wt.% chromium martensitic Grade 92 steel is complex. There is therefore a need for systematic microstructural investigations to define the different regions of the microstructure across the HAZ of Grade 92 steel welds manufactured using traditional arc welding processes in order to understand possible failure mechanisms after long term service. In this study, the microstructure in the HAZ of an as-fabricated two-pass bead-on-plate weld on a parent metal of Grade 92 steel has been systematically investigated and compared to a complex, multi-pass thick section weldment using an extensive range of electron and ion-microscopy based techniques. A dilatometer has been used to apply controlled thermal cycles to simulate the microstructures in distinctly different regions in a multi-pass HAZ using sequential thermal cycles. A wide range of microstructural properties in the simulated materials were characterised and compared with the experimental observations from the weld HAZ. It has been found that the microstructure in the HAZ can be categorized by a combination of sequential thermal cycles experienced by the different zones within the complex weld metal, using the terminology developed for these regions based on a simpler, single pass bead on plate weld, which have been systematically categorised as Complete Transformation (CT), Partial Transformation (PT) and Over Tempered (OT).