Influence of Weld Factors on Creep-Rupture Cracking at Elevated Temperature

The purpose of this paper is to identify the weld effects which are of primary importance in elevated temperature design. A full-scale Fast Flux Test Facility (FFTF) Intermediate Heat Exchanger (IHX) was tested at Westinghouse to investigate weld effects at elevated temperature. The IHX was subjected to two and a half times the design pressure. In addition, one of the four welded nozzles of the IHX was subjected to 26 severe thermal downshock transients, which were interspersed with 156 hr of creep hold time at 1100°F (593°C). At the end of testing, creep rupture cracks were observed in the weldments at the nozzle to cylinder intersections, whether or not they experienced downshock transients. Detailed three-dimensional inelastic analyses were performed to investigate the effects of welding on the creep-rupture strength of weldments. The analyses suggest that the weldment material property variation contributed to creep-rupture cracking at high primary pressure loading. The weld metal and heat-affected zone had higher yield strength, but lower creep ductility compared to the nozzle base material. The analytical predictions and metallurgical observations suggest that the role of residual stresses on creep-rupture cracking is of secondary importance, and need not be numerically simulated in the elevated temperature design of weldments.