S. Roychowdhury

Environmental Effects on the Fracture Toughness of a Duplex Stainless Steel (UNS S31803)

Abstract The environmental effect on the mechanical properties of steels, specifically hydrogen embrittlement (HE), has been widely investigated and is one of the major factors responsible for in-service material failure. Testing methods like slow strain rate testing (SSRT) of tensile specimens have been used but the results are qualitative. This study aims at evaluating the effects of hydrogen on fracture toughness of a duplex stainless steel (UNS S31803). The effect of hydrogen on the J-integral and crack tip opening displacement (CTOD) was studied in this investigation. CTOD is a geometric measurement used to calculate the fracture toughness. Cathodically precharged specimens have been tested according to the ASTM E1820. It was seen that this standard fails to identify the initiation J-integral for these tough materials. Stretch zone width (SZW) was estimated from fractographs to estimate the initiation toughness. Results indicate that hydrogen pickup adversely affect the initiation toughness as well a...

IGSCC of Non-Sensitised Austenitic Stainless Steels in BWR Environment – Effect of Deformation Mode on Grain Boundary Susceptibility

Intergranular Stress Corrosion Cracking (IGSCC) of austenitic Stainless Steels (SS) in Boiling Water Reactor (BWR) environment is generic in nature in both the sensitised and the non-sensitized conditions. IGSCC in non-sensitized austenitic SS in the strain hardened condition has been reported without any grain boundary chromium depletion or impurity segregation. The present study ascertains the reason for IGSCC in BWR environment in non-sensitized condition and investigates the effect of nitrogen content in SS on the susceptibility to IGSCC. Two heats of type 304LN stainless steel with 0.08 and 0.16 wt. % nitrogen were used. Strain hardening was done by cross rolling at 200 °C to 20 % thickness reduction (warm rolling) to simulate the weld induced strain in constrained welds. Subsequently, Transmission Electron Microscopic (TEM) examination was carried out on the rolled SS. The deformation mode observed due to warm rolling was predominantly elongated twin and shear band (SB) formation in both the SS, terminating at the grain boundary regions. This resulted in higher stresses and strains making grain boundary susceptible to IGSCC. Presence of more dislocations at grain boundaries is a key feature for such enhancement in the susceptibility of non-sensitized SS to IGSCC. Formation of twins and SB occurred to a greater extent in the SS with higher nitrogen content indicating greater susceptibility to IGSCC in BWR environment. Crack growth studies done in simulated BWR environment at different Dissolved Oxygen (DO) levels showed higher crack growth rates in the SS with higher nitrogen content in the non-sensitised and strain hardened condition, confirming the higher susceptibility of SS with a higher level of nitrogen.

Effect of Nitrogen Addition in 304 L Stainless Steel on the IGSCC Crack Growth Rate in Simulated BWR Environment

Intergranular Stress Corrosion Cracking (IGSCC) in austenitic Stainless Steels (SS) in Boiling Water Reactor (BWR) operating conditions have been reported worldwide. Nitrogen containing Stainless Steel is used in BWRs and it can affect IGSCC behavior. In this investigation type 304L stainless steel with two different levels of nitrogen was evaluated in the sensitized and non-sensitised strain-hardened condition. Experiments were carried out in high temperature water with controlled dissolved oxygen. In the sensitised condition, the Crack Growth Rate (CGR) reduced and in the non-sensitised strain-hardened condition the CGR increased with increase in nitrogen level in SS. Transmission electron microscopic (TEM) investigations of the as-rolled SS and the SS after tensile testing at 288 °C indicated that rolling resulted in higher grain boundary strain which is a possible cause for higher CGR in the SS with higher nitrogen. Nitrogen did not have a noticeable effect on the deformation mechanism, for the SS after tensile testing at 288 °C, and the dislocation structures observed were similar for both the SS.