Review of Thermo-Mechanical Testing and Micro-Characterization Studies of Ductility-Dip Cracking in Nickel-Base Alloy 52 Weld Filler Metal

Abstract

Nickel-base Alloy 690 wrought material and Alloy 52 (ERNiCrFe-7) weld filler metal have in recent years become the material of choice in new fabrication and repair of commercial nuclear power plant components. Alloys 690 and 52 are preferred due to improved resistance to primary water stress corrosion cracking (PWSCC) as compared to other nickel-base alloys or filler metals, such as Alloy 600 and Alloy 82 (ERNiCr-3). Nickel-base alloys are commonly used in dissimilar metal joints between quenched and tempered low alloy steel and austenitic stainless steel components in nuclear power primary water systems. Nuclear power industry experience with Alloy 52 filler metal using manual or machine gas tungsten arc welding (GTAW) in multi-pass welds and highly restrained thick section welds has been troublesome. Ultrasonic and radiographic examination of Alloy 52 welds has, in some cases, revealed multiple subsurface micro-cracks in the weld metal heat affected zone (HAZ). Recent laboratory thermo-mechanical testing using modified varestraint test methods and a newly developed Gleeble-based strain-to-fracture test method indicate nickel-base alloys are susceptible to a ductility-dip cracking (DDC) phenomenon during the on cooling cycle of welding. Thermo-mechanical testing also demonstrates that initiation of DDC is dependent on exceeding a specific strain threshold or strain rate in susceptible alloys. Microanalysis and micro-characterization studies indicate that DDC is a solid-state thermo-mechanical phenomenon that occurs most commonly along migrated grain boundaries of single-phase austenitic stainless steel and nickel-base alloys. Though not fully understood, DDC is believed to initiate in the temperature range where material ductility drops concurrent with high shrinkage strains during the on cooling weld cycle. This paper reviews the most current thermo-mechanical laboratory test results and micro-characterization studies of nickel-base alloys for susceptibility to DDC. Alloy 52 weld filler metal is discussed in detail due to its importance to the nuclear power industry. Finally, welding parameters and specific filler metal chemistry to reduce potential for DDC are presented and information for evaluation of specific heats of Alloy 52 for susceptibility to DDC are discussed.