Stress Relaxation Cracking (SRC) Susceptibility Analysis in UNS S34751 Stainless Steel Weld Microstructures for Petrochemical Piping Applications

Abstract UNS S34751 and UNS S34709 austenitic stainless-steel alloys encompass thermomechanical and creep properties needed for use in chemical processing pipe applications with 900-1200°F (482-665°C) operating temperatures. UNS S34751 alloy has been demonstrated in literature to improve sensitization resistance compared to UNS S34709, a precursor for polythionic acid stress corrosion cracking (PA-SCC), due to lower carbon content (0.01 wt.%) and higher niobium-to-carbon (Nb/C) ratio with lower overall niobium content. The addition of nitrogen in UNS S34751 alloy provides comparable thermomechanical properties to UNS 34709. The heat affected zone (HAZ) and weld metal (WM) of UNS 34709 welds have been documented in the open literature to exhibit SRC or reheat cracking, complicating designed component service life in literature and industry. The goal of this work is to compare the SRC susceptibility of S34751 welds to S34709 welds, including matching and alternative weld fillers. 2024’s paper 1 included a testing matrix of stress relaxation tests (SRT), initial SRT results of S34751 single pass simulated HAZ and WM of S34751-W34751 gas tungsten arc (GTA) pipe welds, and analysis of potential detrimental phases using thermodynamic and kinetic calculations. Initial results showed better SRC resistance in S34751 HAZ and WM than in S34709 HAZ and W34710 WM at 700-800°C with initial stresses of 400-650 MPa. In this follow-up paper, a comparative analysis between S34709 and S34751 HAZ and FZ at 600-800°C with longer hold times (up to 348 hours) and similar stresses is provided. In addition, SRT analysis of S34751 welds with alternative filler S16880 and extracted transverse weld samples centered on the true HAZ is included. A deeper investigation into microstructural development of intergranular and intragranular secondary phases in both matching and alternative weld HAZs and FZs is provided to supplement the SRT results.