Evaluation of Filler Metal 52M (ERNiCrFe-7A) Hot Cracking When Welding on Cast Austenitic Stainless Steel Base Materials

Abstract

Dissimilar metal welds of filler metal 182 (ENiCrFe-3) in the primary loop of pressurized water reactor (PWR) nuclear plants are susceptible to primary water stress corrosion cracking (PWSCC) after decades of service. Repair or mitigation has been routinely accomplished by installing a structural weld overlay (SWOL) on the filler metal 182 weld joint with the more PWSCC resistant filler metal 52M (ERNiCrFe-7A). The typical dissimilar metal joint consists of a low alloy steel vessel nozzle welded to an austenitic stainless steel safe end. The SWOL extends from the low alloy steel nozzle over the safe end and most often onto the adjoining wrought or cast stainless steel pipe. Field experience shows that filler metal 52M is susceptible to hot cracking when welding on certain heats of centrifugally cast stainless steel piping. This report evaluates 52M hot cracking when welding on CASS piping and provides the likely cause and mechanism for the cracking. The synergistic influence of silicon (Si) and sulfur (S) elements on the weld bead shape and dilution that leads to hot cracking is investigated. In addition, studies on the influence and use of the gas tungsten arc welding (GTAW) power ratio parameter for 52M overlays are presented.

The Microstructure of Inconel Alloys Used for Repair Welding

“…Results from EDX microanalysis also support the conclusion that the repair weldments will not have a high susceptibility to DDC. Solidification cracks and DDC occurred in regions enriched with iron and silicon from the melted base material if concentrations of iron and silicon exceeded 32 wt-% and 0.4wt-% respectively [3]. In our experiment, concentrations of iron and silicon in the critical region of the weld metal near the fusion line did not exceed 21 wt-% and 0.3 wt-% respectively.…”

“…Hot cracking is promoted by an excessive amount of low melting impurities such sulfur, phosphorus and silicon. Therefore, hot cracking was observed only in the first and second layers and in the first and second beads if Inconel 52M was welded on the steel substrate [3]. The fracture surface of hot cracks revealed a dendritic egg crate type morphology and an elevated content of Nb and Ti.…”

“…The welding is the source of the heat, and cycles the material through the susceptible temperature range. The DDC can extend from solidification crack tip [3] and can be minimized by controlling the welding process and optimizing the weld metal composition.…”

Evaluation of weldability of nickel-base alloy weld materials with high chromium content

“…On the other hand, pressurized water reactor (PWR) plants use alloy 52 with Cr content of 28.0-31.5%, and research and development on a global scale are continuously being conducted in view of its excellent SCC resistance and weldability. It is generally known that weld materials having high Cr content cause solidification cracking and ductilitydip cracking because the weldability decreases as the Cr content increases (McCracken et al, 2019;Virkkunen et al, 2019;Fink et al, 2017;McCracken and Smith, 2011;Young et al, 2007;Young et al, 2011). Ni-base alloy weld materials, which have both high corrosion resistance (Andresen, 2011;Hayashi et al, 2022) and weldability, are expected to be applied to BWR plants in view of the potential need for repair and replacement of aging operating plants and extended life and improvement in the safety and reliability of new nuclear plants.…”