scope:

The occurrence and behavior of a carbon denuded "soft" zone in Cr-Mo weldments was studied to determine its influence on mechanical properties and in-service behavior. Room temperature tensile tests, creep rupture tests and Moire interferometry evaluations were performed in order to characterize the behavior of this unique zone in Cr-Mo weldments. The zone is brought about by chromium level differentials between the weld metal and base metal. Extensive metallography was accomplished using OLM, SEM and STEM techniques. SEM and STEM studies show that in the 2¼Cr-1-Mo weld metal and 2¼Cr-1Mo base metal weldment (A & T condition, austenitized and tempered at 1300°F [704°C] for 2 hr) the "soft" zone is not carbide-free but instead that a significant number of carbides are present. The "soft" zone is believed to be a potential failure location in elevated temperature service.The purpose of this report is to review the present ASME Code requirements, identify inconsistencies, provide recommendations for changes and recommend research that should be the basis of further changes in post weld heat treatment (PWHT) practices. Based on a review of the effects of PWHT practices and the present Code rules, it is concluded that present Code rules are generally applicable to high carbon steels produced without notch toughness requirements. For modern-day steels that exhibit high toughness and enhanced weldability, the present Code rules might impose unnecessary requirements and, for certain steels, might actually produce undesirable effects on the properties of the steel. A review of specific requirements related to time and temperature indicate that some are not consistent with literature data and are in need of revision. Revisions would permit economies in the fabrication of some pressure vessels to take advantage of the effect of high toughness on service performance.Publication of this document - WRC Bulletin No. 407 - Report No. 1 was sponsored by the Welding Research Council