Carburization and tensile behavior of Alloy 617 in impure helium containing a part-per-million level of CH4 at 950 °C.

The carburization and tensile behavior of Alloy 617 corroded at 950 °C for 100 h in the impure helium containing methane were investigated. In helium containing 20 ppm CH 4 , Alloy 617 exhibited slight carburization and held its plasticity. However, in helium containing 200 ppm CH 4 , Alloy 617 was carburized to a depth of over 20 % and lost plasticity. In addition, the stress-strain curves in the plastic phase at room temperature exhibited a serrated flow phenomenon in both two environments, primarily caused by dynamic interactions between carbide-induced crack pinning and stress-concentration-induced crack unpinning. [Display omitted] • Accurate control and real-time detection of gas levels in the ppm range are achieved in this study. • The decomposition of methane at 950 °C is only related to itself and is not affected by the alloy. Carburization occurs only when free carbon from methane decomposition is deposited on the surface of the alloy. • The microstructure of Alloy 617 exhibited dramatic differences in the environments containing different levels of CH 4 , with a relatively smooth surface when the CH 4 content was 20 ppm, but a rough surface covered by a large amount of carbon and carbide when the content increased to 200 ppm. • The ductility of Alloy 617 is highly sensitive to the CH 4 content. The fracture mode of the alloy shifted from ductile fracture to brittle cleavage fracture when the content of methane increased from 20 ppm to 200 ppm. • Alloy 617 showed stress serrations in the plastic deformation stage at room temperature. The main reason for this phenomenon may be the dynamic interaction between carbide-induced crack pinning and stress concentration-induced crack debonding. [ABSTRACT FROM AUTHOR]