An Investigation on Stress Corrosion Cracking Behavior of API-X70 Pipeline Steel Weldment in a Simulated Soil Solution

In this study stress corrosion cracking behavior of X70 pipeline steel after shielded metal arc welding has been investigated. For this purpose, the alloy was welded by low, medium and high rates of heat input. Microstructures of specimens were studied using light and electron microscopes. Stress corrosion cracking susceptibility was evaluated using slow strain rate tests in C2 simulated soil solution in free corrosion potential. Scanning electron microscopy was used to examine the fracture surfaces. Corrosion of different areas of weld was evaluated by potentiodynamic polarization test method. Results showed that by increasing heat input, tensile strength and hardness of welded metal reduces, larger heat affected zone (HAZ) is produced and the grain size is increased. Slow Strain Rate Test (SSRT) results showed that increasing the welding heat input will reduce the susceptibility to SCC. SEM images obtained from samples confirmed brittle fracture of specimens tested in the corrosive environment. It was also observed that the produced cracks might grow in transgranular mode. Potentiodynamic polarization tests showed the lowest rate of anodic dissolution for welded samples with higher heat inputs. Overall, results showed that by reducing heat input the susceptibility to SCC increases; this is due decrease in areas which are sensitive to crack growth and initiation.