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Thermal-Metallurgical Modeling of Heat-Affected Zone Tempering during Temper Bead Welding
Temper bead welding is a widely used technique in the nuclear industry for field welding repairs where post weld heat treatment (PWHT) is not practical. The heat input from welding is purposefully used to temper the hard microstructures in the heat-affected zone (HAZ) or previously deposited weld metal for improving toughness. The extent of tempering depends on (i) initial microstructure, (ii) thermal cycles exposed during temper bead welding, and (iii) steel chemistry. Developing optimal welding parameters based on experimental tests can be time consuming. The overarching goal of the present research is to establish a thermal-metallurgical model for heat-affected zone tempering that can help aid the procedure development for temper bead welding. ASME Section IX requires the heat input to be used for temper bead welding based on power ratio. For the same power ratio, the linear heat input per unit length can vary significantly. Hence, a specific objective is to study the extent of tempering with varying heat inputs per unit length while keeping the power ratio constant.
For temper bead welding experiment, a three layer bead-on-plate weld was deposited on a SA533 base plate by cold wire gas tungsten arc welding (GTAW) using filler metal 309L. Three sets of welding parameters were evaluated, which yielded low, intermediate and high heat inputs per unit length for the same power ratio. TypeC thermocouples were attached to the base plate through drilled holes to measure the thermal cycles at locations close to the HAZ during welding. Weld samples were prepared metallographically and HAZ micro-hardness distribution was mapped. Gleeble® physical simulation was used to evaluate the extent of tempering by subjecting samples to rapid heating, isothermal holding at different peak temperatures and over different times, and rapid cooling. For the thermal modeling of temper bead welding, a finite element model was built on Abaqus® using the moving heat source approach to calculate the welding thermal cycles. An analytical equation was developed to describe the tempering kinetics from Gleeble testing results. This equation was incorporated into the thermal model to predict the final HAZ hardness.
The maximum hardness of HAZ in SA533 plate was about 500 Vickers and the microstructure was found to be comprised of martensite. After temper bead welding, the HAZ hardness was reduced to below 350 Vickers. The extent of tempering for different heat inputs per unit length (and same power ratio) was analyzed. The predicted hardness using the thermal-metallurgical model was compared to that measured experimentally.
Industry Sponsor: Areva
Faculty: Wei Zhang (OSU)
Graduate Student: Kaiwan Zhang
Industry Contact: Bennett Grimmett, Jeffrey Enneking, Steve Wolbert