Development of a field metallographic replica procedure for CA6NM stainless steel hydraulic runners

Abstract

Pelton and Francis hydraulic runners, alongside other components of hydroelectric turbine generators, are typically made in soft martensitic stainless steels, especially grade CA6NM from ASTM A743 standard, also known as 13-4.  These hydromechanical components may experience deterioration during service due to the occurrence such as cracks or fractures, as well as wear associated with cavitation or erosion.  Commonly, defects are repaired using conventional

welding processes employing electrodes with chemical composition, microstructure, and mechanical properties like those of CA6NM.  However, these weld repairs not only lead to residual stresses but also to weld deposits and heat-affected zones (HAZ) with martensitic microstructures that make the components susceptible to hydrogen-induced cracking (HIC) or brittle fracture in service.  Therefore, it is necessary to carry out post-weld heat treatments (PWHT) to reduce the level of residual stresses and transform the fresh martensite in the HAZ into softer and tougher tempered martensite.  PWHT of Pelton or Francis hydraulic runners is costly due to the large dimensions and weights of these components, in addition to the logistical challenges associated with disassembly, transportation, and the consequence loss of revenue.  When weld repairs are minor, the costs of PWHT become economically unfeasible.  As an alternative, localized PWHTs (not global) can be performed on-site with appropriate equipment.  However, doubts persist regarding how to verify the effectiveness of such localized PWHT.  To ensure that these localized PWHTs achieve, at least, the improvement of the microstructure of the weld metal and the HAZ, hardness measurements can be carried out on-site and metallography can be performed in the field using the non-destructive technique of metallographic replicas.  This work describes the methodology used to validate a procedure for acquiring and evaluating metallographic replicas on CA6NM steels and surfacing welds applied with EC410NiMo electrode under the following conditions: as-welded, with quenching, and quenching and tempering. The microstructures obtained with conventional laboratory metallography methods for AISI 4140 steel and EC410NiMo welds were compared with the replicas to establish the effectiveness of the technique for applications in hydroelectric turbine generator components. The procedure proposed by the study allows obtaining replica micrographs on CA6NM steel or similar welds with EC410NiMo of sufficient quality to identify the main constituents.