Analysis of heat-affected zone microstructures of steel P92 after welding and after post-weld heat treatment

Abstract

Modern thermal power plant operating on fossil fuels has to be highly efficient and should emit low levels of CO2. One of possible ways to fulfil these requirements is to increase thermal power plant peak service temperature. P92 steel enables service temperature of up to 650 °C. It is stronger, more resistant to creep and enables higher service temperature than P91. During welding of steam boiler components made of P92, it is important to consider proper preheating, welding and post-weld heat treatment parameters in order to avoid cracks and diminish sensitivity to creep in service. The research focused on analysis of simulated heat-affected zone (HAZ) microstructures and distribution of carbides. The FG HAZ (fine-grain heat-affected zone) (Tmax is 950 °C) is the most problematic when it comes to occurrence of type IV cracks, as in this zone, the average representation of carbides on PAGB is 72%, while the average representation of carbides on PAGB in the P21 steel is 58%. After the heat treatment, maximum hardness is recorded in the CG HAZ (coarse-grain heat-affected Zone) (Tmax is 1300 °C), where the values reached 248 HV, which was below the maximum allowable value of 250 HV. Other areas of HAZ expressed the values from 218 to 227 HV. Charpy impact test at service temperatures of 650 °C showed that the CG HAZ was the most critical immediately after welding as the energy for crack initiation was 30 J. However, after obligatory post-weld heat treatment, FG HAZ appeared to be more problematic than CG HAZ, so this research showed that the crack initiation energy value increased due to the fine-grain microstructure, yet the crack propagation energy reduced.