Influence of Microstructure Changes on the Stress Corrosion Properties of U-bend S275 Mild Steel

Abstract Current research investigates the material degradation behavior of thermally processed S275 mild steel under continuous stress in chloride and sulfate media at different pH values (4, 7, and 10). The mild steel samples were quenched at 1000 °C, followed by tempering at 600 °C, followed by U-bending the samples and attaching a bolt and nut to develop continuous stress according to ASTM standards. A U-bend sample without heat treatment was used as a reference. The corrosion behavior of the samples was evaluated using linear polarization resistance and potentiodynamic polarization scans. Optical microscopy, scanning electron microscope, roughness measurements, and Raman spectroscopy have been used to examine the wear behavior of S275 samples. Quenching followed by tempering caused the quasi-polygonal ferrite and granular bainite to convert to martensite. This microstructural change impacted on the material’s mechanical properties and corrosion behavior. Tempering caused a 38% increase in ultimate tensile strength and a 103% increase in yield strength. It also caused a 7–27% increase in corrosion resistance in sulfate media. The pH of the electrolyte solution also influenced corrosion. The corrosion rate of the original and tempered U-bend samples in acidic sulfate media showed approximately a 700% increase in corrosion rate compared to media alkaline and neutral sulfate media samples. The prime reason for the exponential increase in the corrosion rate is the effect of the chemical reaction of highly oxidative species on stress-induced surface defects. Raman analysis confirmed a variety of ferrous oxides and hydroxide formations on the surface of S275 steel.