International Journal of Pressure Vessels and Piping 188 (2020) 104238 Available online 2 November 2020 0308-0161/© 2020 Elsevier Ltd. All rights reserved. Residual stresses in narrow-groove girth welds and applications for fitness-for-service assessment Shaopin Song a , Pingsha Dong a, * , Myung H. Kim b a Welded Structures Laboratory, Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI, 48109, USA b Naval Architecture and Ocean Engineering, Pusan National University, South Korea A R T I C L E INFO Keywords: RESIDUAL STRESS PROFILE DEFECT ASSESSMENT NARROW GAP GIRTH WELD FINITE ELEMENT MEMBRANE AND BENDING ABSTRACT Narrow gap welds have been increasingly used in pressure vessels and pipe components due to their unique advantages. A recent comprehensive investigation into residual stress distributions in narrow gap is presented in this paper, covering component wall thickness from 1” (25.4 mm) to 10” (254 mm), component radius to wall thickness ratio from 2 to 100, and linear welding heat input from low (300 J/mm) to high (18,000 J/mm). By means of a residual stress decomposition technique, two key parameters that govern through-thickness residual stress distributions in terms of their membrane and bending content have been identified. One is component radius to wall thickness ratio (r/t) and the other is a characteristic heat input density (  Q) having a unit of J/mm 3 . With these two parameters, a unified functional form for representing through-thickness residual stress profile in narrow gap welds is proposed for supporting defect assessment. Its adequacy for estimating residual stress dis- tribution trends is further confirmed by comparing with thermomechanical finite element residual stress analyses for a number of selected narrow gap weld cases. Author statements • All authors have seen and approved the content of the submitted manuscript. • The paper presents original work not previously published in the same form and not currently under consideration by another Journal. • If accepted, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder. 1. Introduction Narrow gap welding has been increasingly used for the construction of heavy wall structures (e.g., thickness above 50 mm [1–10]) in nu- clear, petrochemical and shipbuilding industries. Unlike traditional single-V joint preparation which usually requires 30+ degrees as a bevel angle, the joint preparation for a narrow gap weld has almost vertical sides, similar to an “I” shape. The narrow groove is filled up with either one-pass per layer, two-pass per layer or more, depending on the groove width and wall thickness. As a result, both filler metal and welding time can be significantly reduced. For a wall thickness of 90 mm, reduction of weld volume using narrow gap joint preparation is a factor of 4 compared to a single-V groove, as reported in Ref. [10]. Furthermore, the unique groove shape associated with narrow gap joint preparation also results in a considerable reduction in weld shrinkage and angular distortion due to its reduced plastic zone (shrinkage zone) size and nearly symmetric shape with respect to the mid-wall plane, compared with a conventional V joint preparation. Structural integrity of these heavy structures has always been a major concern as discussed in Ref. [9–13], particularly on recently adopted narrow groove welding for the dissimilar metal in nuclear power plants [9–11]. Discontinuities or defects can be found in the weld area due to stress corrosion cracking, welding defects (e.g., pores, lack of fusion, slags, etc.), dissimilar material induced softener zone, and among others. In performing fracture mechanics based assessment for demon- strating such a structure’s fitness for service (FFS), residual stresses must be considered in fracture driving force calculations in addition to service loads such as operating pressure, as demonstrated by Bryan and Holz [13] on a thick nuclear vessel. Over the past two decades, codified Fitness-for-Service assessment procedures such as BS 7910 [14], API 579 [15], and R6 [16] have incorporated increasingly detailed guidance on welding-induced residual stress profiles for typical welded components. * Corresponding author. E-mail address: dongp@umich.edu (P. Dong). Contents lists available at ScienceDirect International Journal of Pressure Vessels and Piping journal homepage: http://www.elsevier.com/locate/ijpvp https://doi.org/10.1016/j.ijpvp.2020.104238 Received 17 April 2019; Received in revised form 14 October 2020; Accepted 21 October 2020