Microstructures of nickel-base alloy dissimilar metal welds

Abstract

Dissimilar metal welds (DMWs) between low-alloy steels (LAS), stainless steels (SS) and nickel-base alloys are very important in the design of conventional and nuclear power plants (NPPs). They help to reach better performances for high temperature environment but they can promote premature failure of components. Failure is often related to cracking in the heat affected zone of base materials. In this study, a literature review was conducted concerning the behavior of Inconel Ni-base alloys and LAS in DMWs for nuclear applications. It was centered on the metallurgical changes occurring with post-weld heat treatment (PWHT) at the interface of ferritic/austenitic DMWs, on the weldability of Inconel filler metals and on the narrow-gap welding (NGW) technique emerging in the NPP design. The aim was to characterize a NGW present in modern pressurized water reactor (PWR) design, which uses an Inconel filler metal to join the reactor pressure vessel nozzle to its safe-end. In addition, the behavior of Alloy 690 was studied. Eight samples were characterized. A narrow-gap Alloy 52 mock-up manufactured in the SINI project was studied in the as-welded condition and after PWHT. It showed that PWHT resulted in increased carbon depletion in the LAS side and in an extensive chromium carbide precipitation in the weld metal. It was responsible for a sharp hardness peak in the weld metal. Samples from EPRI (Electric Power Research Institute) were characterized for ENVIS project, showing different weld configurations involving Alloy 690 as base metal and Inconel 52, 152 and 52M as filler metals. Differences in the behavior of the filler metals were observed. Higher hardness was found in Inconel 52M, followed by Inconel 152 and 52, respectively. Inconel 152 showed different behavior than Inconel 52 concerning carbon migration. The microstructure of Alloy 690 was characterized and was found to correspond to the literature review.