Stress Corrosion Cracking Assessment of a Quenched and Tempered HSLA Steel in Two Alkali Environments- Welded Un-welded Condition

摘要

This paper is a continuation of work that shows results to help better understand the behavior of a quenched and tempered (Q & T) high strength low alloy (HSLA) steel. The first set of results was presented in NACE-2014. Experiments showed multiple through-thickness-cracks emanating from the circumferential weld on the shell-side after a short time of service in the heat exchangers.. The failure has been preliminary diagnosed as Environmentally Assisted Cracking (EAC), which is general term for brittle mechanical failures that result from a combination between residual tensile stress, susceptible microstructure and corrosive environment. Slow Strain Rate Testing (SSRT), which is conducted in a high temperature-high pressure autoclave, has been carried out using a Constant Extension Rate Test (CERT) machine. Stress corrosion testing of this Q&T HSLA steel has been carried out from 25 to 175 °C at a constant strain value (10~(-4) s~(-1)) in two alkaline environments. After the fracture of the specimen, Scanning Electron Microscope (SEM) was used to study the crack morphology as a function of the applied strain. To calculate the stress corrosion cracking (SCC) susceptibility, the plastic strain-to-failure ratio (EpR) was calculated from the stress-strain curve and the reduction-in-area ratio (RAR) calculated from the fracture surface of the broken specimens. Results indicate that SCC susceptibility increased with increasing temperature and the percentage of salt in the solution. Understanding the SCC behavior of Q&T HSLA steel at higher temperatures and in alkaline environments and taking proper preventive measures can help to avoid failures in the future.