Characteristics of an autogenous single pass electron beam weld in thick gage CA6NM steel

Abstract

CA6NM is a low carbon martensitic stainless steel that is widely applied in hydroelectric turbine manufacturing. Using conventional fusion welding techniques, the fabrication of a thick section in CA6NM requires a V-groove joint design and multiple passes to achieve the required penetration. However, exposure to a substantial heat input through this process renders large fusion and heat-affected zones, microstructural variations, as well as distortion of the assembly, which pose difficulties for the manufacture and performance of the component. The application of a high energy density technique, namely electron beam (EB) welding, was used in the present work to penetrate a 60-mm-thick section in CA6NM with a single pass without filler metal. To prevent cracking in the weldment, the CA6NM was heated to 100–170 °C before EB welding using a zonal preheat treatment, which was applied in situ using a defocused beam. The as-welded CA6NM exhibited a narrow fusion zone (FZ) and a series of distinct yet very small heat-affected zones (HAZ) with different microstructural characteristics. As compared to the base metal (BM) hardness of 289 ± 4 HV, the EB weldment exhibited a hardness maximum of 425 HV in the HAZs and an average hardness of 395 ± 6 HV in the FZ. Post-weld heat treatment (PWHT) was effective in lowering the hardness in the FZ of the EB weldment to a mean value of 346 ± 13 HV.