Study on Weldability of EN 10025-6 S550 QT Steel

Cracks developed in the 90 mm thick EN 10025-6 S550 QT steel plates assembled to form a box unit. After assembly the surface cracks generated longitudinally along the elongated grains while preheating or performing root run. Root cause analysis was conducted to investigate the reason for the failure and also how to specify the “conditions of materials on delivery” during procurement is generated. Various tests such as visual test, chemical composition, inclusion rating, microstructure, dye penetrant test and hardness tests were conducted. The investigation revealed that the failure cracks could be due to temper embrittlement due to the segregations of impurities such as tin, arsenic, phosphorus and antimony, etc. in the grain boundaries and resulted in ductile to brittle transformation when exposed in the temperature. It is observed that, depends on the concentration of Ti and N in steel, coarse and cuboidal TiN particles of several micrometers in size act as potential sites for cleavage crack initiation. Furthermore, during the steel making process if sulfur is not properly controlled, then large MnS inclusions can also form during solidification. Soft MnS inclusions elongate during the subsequent hot rolling process, which deteriorate ductility and impact toughness. Hence, it is essential to specify the J factor value and inclusion rate during the procurement of steel.