Characterization of weld zone in ex-service component of power plant: multi-pass weld joint of steam header to drain pocket

Abstract

Structural components used in the steam power plants are often operated under extremely harsh environment, and variable thermal and mechanical load conditions. Steam headers are one such critical component. They are used to carry steam from the boiler to different process sections in the plant. These components are quite prone to creep cavities, hydrogen embrittlement and cracking in the heat affected zone (HAZ) of the weld joints. Therefore, it is mandatory to regularly evaluate their quality condition, including the prediction of their residual life. Herewith, research methods are applied to investigate the imperfections and metallurgical features of a multipass orbital weld joint between the sub-components: steam header and drain pocket; from an ex-service component which has been under operation for more than 290 000 hours, operating under high temperature (approx. 510 °C).

This work is focused on one weld specimen extracted from a strategically relevant position along the orbital weld joint, encompassing the weld zone and base material from both sub-components. The material of both, the steam header and drain pocket sub-components, is a DIN 13CrMo4-5 steel which is a heat resistant low alloy ferritic-pearlitic steel, with Cr and Mo being the strengthening elements. Although the steam header and drain pocket are made from the same material, they are originally obtained via different manufacturing techniques. This is evident from the analysis of the microstructures of the base materials, and its helps to understand the effect of the weld thermal cycles on the base material.

The effect of the thermal cycles from the multipass weld joint, and service loading history, is assessed at the weld zone (fusion zone and HAZ), and base materials of both sub-components, at near- and far-field from the HAZ. The effective chemical composition of the ex-service sub-components was evaluated using Optical Emission Spectroscopy. Vickers hardness measurements are conducted to evaluate the evolution within the cross-section of the weld and for different directions of the sub-components’ base materials. Optical microscopy is implemented to evaluate the base materials of both sub-components. Scanning Electron Microscopy is conducted within the weld zone to characterize the different microstructure features and imperfections. For the inclusions Energy Dispersive Spectroscopy was used to evaluate the contribution from the chemical composition.

Finite element modelling was used to replicate the thermal cycles from the sequence of 21 multiple passes forming the weld joint. The peak temperatures and cooling rates are assessed at zones of interest, namely at the high- and low-hardness zones and zones with high hardness gradients. Weld parameters and joint design are assumed based on the macrograph analysis and literature review. The results on imperfections, metallurgical features and finite element modelling are discussed and analyzed in a systematic approach.