Toughness Evaluation of Welds in 9Cr-1Mo-V and 9Cr-0.5Mo-V Steels Made Using the Flux-Cored Arc Welding (FCAW) Process

摘要

The toughness of girth welds in 9Cr-lMo-V and 9Cr-0.5Mo-V steel seamless pipe (ASME SA-335 Grades P91 and P92, respectively) made using the flux-cored arc welding (FCAW) process was evaluated. Electrodes from two different suppliers were used for production quality welding of each steel. The welds received post-weld heat-treatment (PWHT) in accordance with the requirements of the ASME Code. The objective of the work was to determine if the fracture toughness of the FCAW welds was acceptable for high-temperature steam piping.Toughness was measured using standard sized Charpy V-notch impact specimens. The specimens were oriented transverse to the weld seam with notch located approximately in the center of the weld metal and parallel to the direction of weld seam. Full-range (lower to upper shelf) Charpy impact energy and shear area curves were developed for each weld joint. These were used to estimate the temperatures corresponding to 30 ft-lb average impact energy. The estimated temperatures were well below the service temperature but were above the typical hydrostatic test temperature.Engineering critical assessments were performed to evaluate the potential of fracture during hydrostatic testing employing the procedures of Standard API 579-1/ASME FFS-1. Minimum fracture toughness values were estimated from the average Charpy impact properties. These were then used to compute minimum critical crack sizes for unstable cracking under static internal pressure loading. It was concluded that crack-like flaws significantly smaller than the minimum predicted critical sizes would be found nondestructive examination techniques (radiography or ultrasonic testing). Thus, the likelihood of subcritical crack growth or fracture during hydrostatic testing is believed to be negligible.