Effects of Thermal Aging on Material Properties, Stress Corrosion Cracking, and Fracture Toughness of AISI 316L Weld Metal

Abstract

Thermal aging and consequent embrittlement of materials are ongoing issues in cast stainless steels, as well as duplex, and high-Cr ferritic stainless steels. Spinodal decomposition is largely responsible for the well-known ''748 K (475°C) embrittlement'' that results in drastic reductions in ductility and toughness in these materials. This process is also operative in welds of either cast or wrought stainless steels where d-ferrite is present. While the embrittlement can occur after several hundred hours of aging at 748 K (475°C), the process is also operative at lower temperatures, at the 561 K (288°C) operating temperature of a boiling water reactor (BWR), for example, where ductility reductions have been observed after several tens of thousands of hours of exposure. An experimental program was carried out in order to understand how spinodal decomposition may affect changes in material properties in Type 316L BWR piping weld metals. The study included material characterization, nanoindentation hardness, double-loop electrochemical potentiokinetic reactivation (DL-EPR), Charpy-V, tensile, SCC crack growth, and in situ fracture toughness testing as a function of d-ferrite content, aging time, and temperature. SCC crack growth rates of Type 316L stainless steel weld metal under simulated BWR conditions showed an approximate 2 times increase in crack growth rate over that of the unaged as-welded material. In situ fracture toughness measurements indicate that environmental exposure can result in a reduction of toughness by up to 40 pct over the corresponding at-temperature air-tested values. Material characterization results suggest that spinodal decomposition is responsible for the degradation of material properties measured in air, and that degradation of the in situ properties may be a result of hydrogen absorbed during exposure to the high-temperature water environment.

Evaluation of the thermal aging of δ-ferrite in austenitic stainless steel welds by electrochemical analysis

“…Meanwhile in previous DL-EPR studies of welds 6 , 7 , the electrochemical behavior after thermal aging were estimated based on the austenite peaks, which would be inaccurate to estimate the spinodal decomposition. Also in most cases 7 , no reactivation peaks corresponding to the spinodal decomposition in δ-ferrite appeared, indicating the overall electrochemical response was dominated by the austenite matrix. Therefore, the electrochemical analysis of δ-ferrite phase only was performed as an attempt to directly correlate with the microstructural changes in δ-ferrite.…”

“…Also, DL-EPR analysis of the bulk welds (before selective austenite etching) was conducted for comparison. A concentrated solution of 1 M H 2 SO 4 + 0.1 M KSCN was used for ER347 weld, as previously reported 6 , 7 to intensify the electrochemical response during reactivation scan. However for ER316L weld with better corrosion resistance, a more aggressive solution of 2 M H 2 SO 4 + 1 M KSCN was used to observe the reactivation peaks during reactivation scan.…”

Prognosis methods of stress corrosion cracking under harsh environmental conditions

“…Another harsh environmental condition that can exacerbate SCC is exposure to harsh chemicals. Chemicals such as acids, alkalis, and salts can accelerate the corrosion process and increase the likelihood of SCC [ 9 , 16 ]. This is particularly true in environments where these chemicals are present in high concentrations, such as in the chemical processing industry [ 17 ].…”