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Part 1: Reports on Heat Treatment of Steels Used in Boiler and Pressure Vessel Applications - Carbon Migration in Cr-Mo Weldments Effect on Metallurgical Structure and Mechanical Properties

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.

Our results show that the occurrence of the carbon denuded "soft" zone is due to carbon migration, which is driven by elemental differences (especially in chromium) between the weld metal and base metal. The extent of carbon migration depends on the PWHT schedule. OLM characterizations show that in 2 Cr - 1Mo weld metal and 2 Cr - 1Mo base metal weldments the "soft" zone is widest for the A Cr - 1Mo base metal weldment the "soft" zone development begins a short time after PWHT (less than 1 hr at 1350F) in the overlapped region of the heat-affected zone (HAZ), and then spreads along the entire fusion line as time at temperature increase.

Part 2: Reports on Heat Treatment of Steels Used in Boiler and Pressure Vessel Applications - ASME Post-Weld Heat Treating Practices: An Interpretive Report

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 toughness and enhanced weld-ability, 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.

We propose that revisions be made to the Code rules to relate the PWHT requirements to notch toughness and nondestructive examination considerations when a PWHT is not required because of outside considerations (i.e., environmental effects). Such revisions would permit economies in the fabrication of some pressure vessels to take advantage of the effect of high toughness on service performance. Specific research that would be the basis of changes in Code requirements is presented.

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Published: 1995 Number of Pages: 75 File Size: 1 file , 2.6 MB