AbstractThis paper describes an ECCC Working Group 3C project on the creep performance of IN617 parent material and fabrication and repair welds in as-manufactured, aged, and service exposed conditions. The aims were to provide data for design of high temperature plant, to investigate the effects of service ageing, and to clarify the potential risk of relaxation cracking should an as-welded weldment enter service. A multi-member Italian consortium undertook creep testing of a new IN617 parent and weld, while Laborelec carried out parallel testing on an ex-service IN617 weld from a GT combustion system component provided by Powergen. The project also included comparison tests on the new weld after thermal pre-ageing to simulate service conditions at both moderate and higher temperatures.The results show that IN617 welds are weaker in creep than the parent material. Pre-ageing at high temperature causes a slight reduction in creep strength, but pre-ageing at a more moderate temperature actually causes a sli...