Abstract Scope |
Introduction: This study investigates the fundamental mechanism of Stress Relief Cracking (SRC) in Grade 11 and Grade 22 steels through in-situ SEM simulation of PWHT. SRC has been extensively studied over decades of research and the general concept for the SRC mechanism is known: precipitate/carbide strengthened grain interiors result in grain boundary strain accumulation during PWHT as the stress relaxation process is occurring. However, finer details on initiation/propagation are disputed. SEM observations will be cross examined with proposed SRC mechanisms from literature to validate or refute past theories and gain a better understanding of the SRC mechanism. Procedure: An environmental SEM, equipped with heating and tensile stages, was used to replicate the thermo-mechanical history of an SRC test, while simultaneously capturing high-resolution images of the crack nucleation and propagation process. Dog bone samples with simulated CGHAZ microstructure were preloaded with tensile stress at room temperature, brought to the PWHT temperature, and held at constant displacement until crack nucleation and failure. Notched samples were used to promote cracking initiation in a controlled location and ensure high magnification imaging would capture the onset of SRC. EDT imaging and EBSD were used for microstructural characterization and quantification of strain accumulation. Results / Discussion: The entire SRC failure process was observed and captured during the in-situ SEM simulations. Cracking initiation was controlled by both intergranular and triple point void formation. Triple point void formation signifies high grain boundary strain and grain boundary sliding. Intergranular voids that formed between triple points signify grain boundary particles acted as localized void nucleation / cracking initiation sites. However, past SRC mechanisms generally only focus on either intergranular particle void nucleation or grain boundary sliding leading to voids at triple points and not both. Current observations suggest cracking initiation is controlled by grain boundary strain accumulation leading to void formation at intergranular particles. The initiation of these voids weakens grain boundaries, accelerating grain boundary deformation, and promoting the initiation of voids at triple points. This results in the observed microstructure with both triple point voids and disconnected intergranular voids. Cracking propagation occurred through the connection of voids. Additionally, propagation also occurred from the expansion of triple point voids into grain boundaries. The main fracture path formed by the connection of propagated crack fragments, with many crack fragments adjacent to the fracture surface. Ongoing testing is being conducted to determine grain interior versus grain boundary strain. EBSD and microscopic DIC will be used to quantify localized strain and relate to the SRC mechanism. Additionally, ongoing testing looks to determine intergranular particle characteristics. Conclusions: In-situ SEM testing allowed to capture the mechanism of SRC crack initiation and propagation. The cracking mechanism for Grade 11 and Grade 22 steel correlates to SRC mechanisms in literature, which rely intergranular void nucleation leading to propagation and failure through the connection of these voids. The impact of the PWHT temperature and applied stress / restraint on the observed SRS mechanism will be explored in future work. |