Effect of Heat Treatment on Corrosion and Fracture Behavior of SPV50Q High Strength Steel Weldment in H2S-Containing Environment

Abstract

SPV50Q high strength steel is often used to fabricate liquefied petroleum gas (LPG) spherical tanks with larger capacity, and tanks are expected to free post weld heat treatment (PWHT) for avoiding the possible reduction in strength. Sulfide stress corrosion cracking (SSCC), however, has been found in weldment, especially the heat-affected zone (HAZ), in LPG environment contaminated by wet H2S. The failure analysis showed that the existence of welding residual stress in weldment is one of the major factors in the occurrence of cracking. Post welding heat treatment (PWHT) is a feasible method of reducing welding residual stress. Therefore, in order to investigate the effect of heat treatment on mechanical properties, corrosion and fracture behavior of SPV50Q steel weldment, the difference in mechanical properties of the weldment with and without heat treatment at 590°C for 160 min after welding was measured using tensile test and impact test. The corrosion behaviors of base metal (BM), weld metal (WM) and HAZ metal in the weldment were investigated by potentiodynamic polarization in H2S-containing solution. In the same solution, the susceptibility to environmental cracking was evaluated by slow strain rate testing (SSRT). The feature of fracture and the morphologies of cracks were observed by scanning electrode microscope (SEM) and optical microscope (OP). The results indicate that the execution of heat treatment does not greatly change the properties of SPV50Q steel weldment, which can provide technology support for the remanufacturing of the LPG tanks having suffered from SSCC by repair welding following local or integral heat treatment.