Metallurgical Examination of Premature High-Temperature Fracture in Welded 304H Pipework

Abstract Premature failures of high performance alloys, particularly stainless steels and nickel alloys, at elevated temperatures and after moderate service durations, have imposed a major industrial challenge. This is particularly relevant to the energy sector, in which systems incorporating thick-sectioned, welded solutions are essential. This is a particular concern, because the lack of sufficient understanding of the phenomena creates uncertainties, relating to design and operational parameters and their significance, for obtaining successful service performance and the avoidance of failures. An incident related to fracture in 50mm-think welds, made using 16-8-2 (AWS1 A5.9 ER 16 8 2) filler metal, in 304H (UNS 2 S30409) stainless steel, interconnecting pipework was encountered after approximately two years of refinery service. The piping transferred process gasses, which were mainly a mix of methane, hydrogen and steam. A failure investigation identified the cracking, in shop and site welds, to be consistent with, what has been referred to, commonly, as, ‘stress relaxation cracking’ (SRC). As part of follow-on investigative activities, detailed metallurgical work, i.e. characterization of microstructure and cracking, was carried out in an attempt to better determine the mode and characteristics of the cracking and, particularly, understand the impact of heat-treatments and microstructural stability on the susceptibility to such elevated-temperature cracking. This paper provides a summary of the salient findings of the comparative study carried out on the ex-service, casualty welds, with varied heat-treatments. A discussion on mechanisms contributing to cracking, and potential causes of the phenomena is also presented.