Techno-economical comparative evaluation of diverse local heat treatment strategies for thick-walled pipe welds

Abstract

The localised heat addition and rapid temperature fluctuations during welding induces significant levels of residual stresses in thick-walled pipe welds. Residual stress so induced by welding, need to be stress relieved; in these thick walled pipes that are extensively used in power plant applications. While thermal stress relieving (SR) serves as a pivotal intervention, field-welded components and sizable weldments, exceeding furnace capacities, necessitate resistance-based, local post weld heat treatment (PWHT). Nevertheless, the local heating of pipes always results in a finite Through-Thickness Temperature Gradient (TTG). Though, heat treatment parameters are governed by standards, to limit TTG for effective SR; observations indicate limitations of these parameters in achieving desired TTG for specific pipe dimensions. Prior studies specify two alternatives viz. widening the heated band and reducing the heating rate to achieve the desired TTG. However, the preferred approach of widening the heat band for larger pipes; necessitate exorbitantly large capacity power sources and expose a wider zone of weldment to critical transformation temperatures which pose practical limitations and attract industrial apprehensions. Thus, to take informed decisions, the relative efficacies of alternatives over the standard governed approach in terms of relief and redistribution of residual stress, capacity of power source required and energy consumed are evaluated through Finite Element (FE) simulation. Valuable insights from FE results, considering the merits, demerits, logical advantages and financial implications facilitating selection of appropriate SR strategy in industrial applications for successfully relieving stress in difficult to SR pipes (thick-walled, SA335P91) are determined and discussed in detail.