The influence of welded joint microstructure on the corrosive properties of X70 steel

Hydrogen sulfide dissolved in aqueous electrolyte solutions causes susceptibility to stress corrosion cracking in most structural steels. This is one of the reasons for failure of the oil and gas field equipment and occurrence of the emergency situations. When transporting media containing hydrogen sulfide, most of the damage occurs in the welded joint area. In this study, the authors have established the influence of the microstructure of individual sections of the weld metal, its heat-affected zone, and the base metal on the corrosion resistance of a welded joint of a low-alloy pipe steel (grade X70) in the environment that mimics the operating conditions of the main oil and gas pipelines in the High North regions. The temperature fields and thermal cycles were calculated for submerged multi-arc welding in order to simulate the microstructure of the most problematic areas with the reduced level of mechanical properties and corrosion resistance. The method of controlled resistive heating and subsequent cooling was used to simulate the welding process. The simulation of the welding thermal cycle was performed using the Gleeble 3800 complex for physical simulation of thermomechanical processes. The results of sulfide stress cracking tests are presented. A metallographic analysis and microhardness measurements of the welded joint areas and base metal were carried out. It was shown that the microstructure of the overheated areas in the heat-affected zone is the part of the welded joint that is most susceptible to sulfide stress cracking. It has been established that studying just the welded joint and the base metal for susceptibility to corrosion cracking is insufficient in terms of assessing the reliability of a welded joint, which indicates the need to revise the testing procedure. In this study, samples of welded joints of thick-walled large-diameter pipes, including the base metal, heat-affected zone, and fusion zone, were studied. The areas that are most susceptible to the formation of corrosion cracks were determined and simulated.