ABSTRACT
The heat exchangers (coolers) made of a quenched and tempered (Q&T) High Strength Low Alloy (HSLA) Steel developed multiple cracks emanating from the circumferential weld in the shell after a short time of service. Some of the cracks had grown through the thickness and started to leak. The failure has been diagnosed as Environmentally Assisted Cracking (EAC), which is a general term for brittle mechanical failures that result from a combination between residual tensile stress, susceptible microstructure and corrosive environment. Environmentally assisted cracking includes stress corrosion cracking (SCC) and hydrogen-induced cracking (HIC). Slow Strain Rate Testing (SSRT), which is in connection with 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 SCC susceptibility, the plastic strain-to-failure ratio (EPR) was calculated from the stress-strain curve and the reduction-in-area ratio (RAR) 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&THSLA steel at higher temperatures and in alkaline environments and taking proper preventive measures can help to avoid failures in the future.
