Enhancement of corrosion resistance and mechanical properties of API 5L X60 steel by heat treatments in different environments

Steel pipelines are commonly used in the petroleum industry, where high strength and durability values are required. In oil installations, the problems of corrosion, especially with crude oil, appear clearly and dramatically in dealing with API 5L series. Improving the corrosion protection and mechanical properties is a genius goal for steel manufacturers, and the heat treatment processes are familiar in this field. In this study, experimental investigations were executed to find the heat treatment influence on the corrosion rate, microstructure, and mechanical properties behavior of API 5L X60 pipeline steel in the presence of three different environments: seawater, freshwater, and crude oil. Two austenitizing temperatures, 900 degrees C and 800 degrees C, were prepared, followed by quenching at 600 degrees C, 450 degrees C, and 300 degrees C, respectively, where the corrosion rate was estimated by the mean weight loss method. In this study, more than 60% of the corrosion rate in both seawater and freshwater was improved at 900 degrees C, austenitizing tempered at 300 degrees C, for the selected pipeline specimens, while only 22% reduction was achieved in crude oil. Different microstructure phases appeared in the heat-treated specimens such as tempered martensitic at 300 degrees C and fine-grained ferrite with polygonal ferrite at 600 degrees C. The microhardness values decreased with increasing the tempering temperatures, while impact toughness values increased with tempering temperature increasing from 300 degrees C until it reached 450 degrees C and then returned to decrease again in 600 degrees C.