Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

Abstract

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. Two pipe spools were fabricated using identical pipeline production procedures for manufacturing multi-pass narrow gap welds. Non-destructive neutron diffraction strain scanning was carried out on girth welded pipe spools and stress-free comb samples and also thin slices for the determination of lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses of two pipe spools (in the as-welded condition) were measured through the thickness in the weld material and adjacent parent metal starting from the weld toe. Three line-scans were completed 3mm below outer surface, at mid thickness and 3mm above the inner surface. PWHT was adopted for stress relaxation; one pipe was conventionally heat treated entirely in an enclosed furnace and the other was locally heated by a flexible ceramic heating pad. Residual stresses were measured after PWHT at exactly the same locations as those used for the as-welded condition. Residual stress states of the two pipe spools in as-welded condition and after PWHT were compared and the results were presented in full stress maps. Additionally, through thickness residual stress profiles and the results of one line scan (3mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry’s R6 procedure. The residual stress states of the two pipe spools measured in the as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Challenges of Measuring Residual Stresses in Large Girth Welded Pipe Spools by Neutron Diffraction

“…A six-pass narrow-gap girth weld was located in the middle of the pipe spool. The welding and PWHT procedures were described in a previous publication [14]. At this point, it is worth noting that both furnace and local post weld heat treatment methods were employed in this study but the results concerning local PWHT are discussed in this paper.…”

“…Residual stress values were calculated using hkl specific material properties for the measured planes based on the Kröner model using the software DECcal [14]. The average reported errors range was ± 15 MPa.…”

“…The welding procedure (representative of a procedure used in the industry) was described in the authors' previous work [14]. In this paper, the focus is on the experimental challenges in neutron diffraction procedure used for residual stress measurements.…”