scope:

Introduction

Post-weld heat treatment (PWHT) relieves the residual stress at welds. It is beneficial in avoiding brittle fracture (for which the design codes give rules) and environmentally assisted cracking (for which purchasers determine requirements). In fabricating new equipment, PWHT may be planned into the manufacturing process and is generally very cost effective.

However, with existing vessels, carrying out repeat PWHT may present many practical difficulties. There may be restrictions at the site on keeping the components hot over the length of time needed. It may be difficult or impossible to provide sufficient heat. Undertaking PWHT in an oven, which is the standard practice for new fabrication, may not be feasible. The disturbance to adjacent equipment caused by carrying out PWHT may also be unacceptable.

Occasionally the need may arise to weld on a vessel after its original fabrication and heat treatment has been completed. Such a welding operation might be necessary almost immediately, before or during commissioning the vessel, due to a design change; however, the need is more likely to occur in-service. New clips or studs may be needed for insulation supports or new anchors for fire-proofing materials. Components such as doubler plates or nozzles may be needed because of corrosion, or due to a process or mechanical design change. Overlay welding may be necessary to build up the thickness of a corroded part. In the extreme, for a major modification, a full circumferential butt-weld may be needed.

This Chapter indicates the thicknesses of components on which the codes currently permit welding without PWHT. It also discusses the welding techniques by which welds on thicker components can be made without repeat PWHT. It applies predominantly to vessels made in ferritic (i.e. carbon, carbon manganese or low alloy) steels.