Search for: All records

Abstract UNS S34751 and UNS S34709 austenitic stainless-steel alloys contain thermomechanical properties required for use in chemical processing pipe applications with 900–1200°F (482–665°C) operating temperatures. UNS S34751 alloy has demonstrated improved sensitization resistance compared to UNS S34709, a precursor for polythionic acid stress corrosion cracking (PA-SCC), due to lower carbon (C) content (0.01 wt.%) and higher niobium-to-carbon (Nb/C) ratio with lower overall niobium content. The addition of nitrogen (N) in UNS S34751 alloy provides similar thermomechanical properties compared to UNS 34709. Additionally, stress relaxation cracking (SRC) susceptibility in UNS 34709 welds has been documented thoroughly in literature and industry, which poses a problem for long term service life, while UNS S34751 welds have potential for improved SRC resistance without the need for post weld heat treatment (PWHT). In this paper, a literature review of S34751 is explored, and testing matrix of experimental SRC tests using a Gleeble 3500® thermomechanical simulator is developed for S34751 gas tungsten arc welded (GTAW) pipe samples in comparison to S34709 welds. Additionally, initial thermodynamic and kinetic CALPHAD calculations have been completed to analyze potential detrimental phases in S34751 in comparison to S34709, e.g., z-phase. SRC testing has been mostly completed in S34709 welds made with W34710 (E347-16) and S16880 (E16.8.2-15) weld filler, respectively, and SRC comparisons to S34751 are in progress. Current results show higher resistance to SRC in S34751 HAZ and FZ than S34709 FZ and W34710 FZ at 800°C. In the following year, a full comparative analysis between S34709 and S34751 HAZ and FZ, in addition to welds with alternative filler S16880, is planned, including SRC testing at 600–750°C temperatures, metallurgical characterization of intergranular and intragranular precipitates, and additional thermodynamic analyses to complement microstructural observations. Final conclusions on SRC susceptibility comparisons between S34751 and S34709 welds, including alternative fillers, will be made. 

Abstract UNS N06693 is a Ni-base alloy that provides metal dusting corrosion resistance in steam generator pipes with operating temperatures above 500°C. A crack failure occurred in a 6.5mm thick similar weld pipe joint, located at both fusion zone and heat affected zone, after about 10 years in service and 2 months after weld repair in adjacent weld, which warranted an investigation into possible root causes of failure. This study investigates the potential failure mechanisms that may arise during service (such as stress relaxation cracking, stress corrosion cracking, ductility dip cracking, and creep failure) for UNS N06693 in order to understand the observed cracking behavior. In this year, preliminary fractography, metallurgical characterization, thermodynamic and kinetic CALHAD simulations, and investigation into potential contributing factors (e.g., weld procedure specifications (WPS) and post weld heat treatment (PWHT)) to failure have been completed. The fracture surfaces indicate brittle, intergranular failure, such that no shear lips were observed, and radial lines (crack propagation) were primarily observed in weld fusion zone. Metallurgical characterization near the fracture surface is conducted to reveal the contributing factors to failure, such as intermetallic phases (e.g., Cr-rich α-phase) and distribution of carbide particles (e.g., intergranular chromium carbides), that may contribute to reduced cracking and sensitization resistance. Blocky, intergranular Cr-rich precipitates, either Cr-rich α-phase or Cr-rich M23C6., are observed behind secondary cracks. Based on the initial findings, contributing factors for failure considered are increase in tensile residual stresses due to nearby repair field weld and grain boundary embrittlement due to coarse, blocky Cr-rich phase that likely developed during initial PWHT and within the 10-year service window. In the following year, a more in-depth metallurgical characterization, discussion on contributing causes and possible mitigation strategies for improving microstructural stability and performance-based weldability (e.g., weld procedure and PWHT design), and conclusions with root cause analysis will be provided.