FURTHER DEVELOPMENT OF HEAT-AFFECTED ZONE HARDNESS LIMITS FOR IN-SERVICE WELDING

摘要

Welding onto an in-service pipeline is frequently required to facilitate a repair (Figure 1) or to install a branch connection using the "hot tapping" technique (Figure 2). Welds made in-service cool at an accelerated rate as the result of the ability of the flowing contents to remove heat from the pipe wall. These welds, therefore, are likely to have hard heat-affected zones (HAZs) and a subsequently increased susceptibility to hydrogen-assisted cracking. During the evaluation of procedure qualification welds, HAZ hardness is often used as an indicator of the susceptibility to cracking. A widely used value below which it is generally agreed that hydrogen cracking is not expected to occur is 350 HV. Both the US standard API 1104 Appendix B and the Canadian standard CSA Z662 indicate that procedures for in-service welding that produce HAZ hardness greater than 350 HV should be evaluated with regard to the risk of hydrogen cracking. A previous paper on this topic is referenced in CSA Z662 as guidance for performing this evaluation. The present paper describes the further development of HAZ hardness acceptance criteria that can be used to evaluate welds during the qualification of procedures for welding onto in-service pipelines. This further development involved taking the effect of material thickness (restraint level) into account and further validation.