Finite element analysis of the localized post-weld heat treatment of a Pelton runner

Abstract

Pelton and Francis runners are critical components of the turbines of hydroelectric power generation plants. When damaged in service, they must be welded to restore their shape and performance. The most common material for these components is DIN G-5CrNi13–4 or ASTM A743 grade CA6NM stainless steel, which can experience issues during welding such as high hardness in the heat-affected zone (HAZ) due to the formation of fresh martensite, hydrogen-induced cracking and high residual stresses at and around welds. Post-weld heat treatments (PWHT) are used to temper the martensite in the weld metal and the HAZ and reduce the level of residual stresses. When the regions to be treated represent a small fraction of the total volume of the runner, it is not economical to heat treat the entire part in a large furnace. Therefore, localized heat treatments constitute a cost-effective solution. In this work, a numerical model has been developed to predict the performance of an in situ PWHT device under different operating conditions. The model takes into account the calculation of the power supply requirements for the heat treatment of a Pelton runner, the effect of changing the dimensions of the heating elements, and the relevance of the environmental conditions of the repair site. Taken together, these results are important in anticipating possible adjustments required to follow the expected heat treatment curve. The potential of this model is also demonstrated by using the estimated transient power supply to determine the performance of the control system of the PWHT device and reduce the number of runs required for its tuning. This methodology can be extended to the PWHT of other geometries and materials.