Session: MF-03-01 Welding Residual Stress and Distortion Simulation and Measurement-1

Paper Number: 155479

155479 - Characterizing Residual Stress Relaxation of 347ap Weldments Using Neutron Diffraction and Thermomechanical Testing 

Abstract: 

Residual stress (RS) formation is an unavoidable feature of welding processes caused by localized heating, impacting weldment performance. One failure mode associated with welds is stress relief cracking (SRC). In the heat-affected zone (HAZ), elevated temperatures cause precipitate dissolution. During reheating, either from post-weld heat treatment (PWHT) or in-service conditions, relaxation occurs simultaneously with the formation of large carbides at grain boundaries. Reprecipitation creates a thin region along the grain perimeter that is deficient in alloying elements. Stress can be resolved by plastically deforming these weakened regions, though there is a ductility limit. Once reached, cavities form, coalesce, and ultimately lead to intergranular crack initiation. Significant research has focused on understanding precipitate kinetics, the primary cause of failure. Nonetheless, a SRC-susceptible microstructure remains stable in the absence of stress relaxation. Therefore, understanding relaxation is necessary to produce mechanism-based lifetime predictions. Although RS can be modeled, experimental validation is essential for providing confidence in solutions. Neutron diffraction is one technique used to measure RS. 347H is one common creep resistant austenitic stainless-steel used in industry. Driven by an interest in improving performance without significantly driving up costs, previous studies have shown that slight adjustments in base metal compositions or using a dissimilar filler metal can affect cracking susceptibility. In this study, 347AP austenitic stainless-steel pipes were manually gas tungsten arc welded (GTAW) using both 347AP and 16-8-2 filler rods, and RS measurements were taken before and after a PWHT. Additionally, another 347AP/347AP pipe underwent intermittent measurements during thermal exposure. Confidence in neutron diffraction remains a challenge since it does not directly measure RS. Therefore, various methods to analyze stress field results are also explored and compared. Moreover, thermomechanical testing, coupled with digital image correlation (DIC), was used to observe stress relaxation behavior in different weld regions. Obtaining temperature dependent mechanical properties will be crucial in modeling a cross-weld microstructure gradient.

Presenting Author: C Andrew Kocak University of Tennessee

Presenting Author Biography: Andrew Kocak was awarded a bachelor's degree and a master’s degree from the University of Tennessee’s Material Science and Engineering Department and is working towards getting a PhD in the same department. His research is focused on mechanical modeling of welding processes. Currently, he is a board member for the University of Tennessee’s Material Research Society local chapter and recipient of a Graduate Advancement & Training Education (GATE) Fellowship that supports collaborative research between the University of Tennessee and Oak Ridge National Laboratory.

Authors:

C Andrew Kocak University of Tennessee
Yanfei Gao University of Tennessee
Zhili Feng Oak Ridge National Laboratory
Jeffrey R. Bunn Oak Ridge National Laboratory
E Andrew Payzant Oak Ridge National Laboratory
Jorge Penso Shell Projects and Technology