Structure and mechanical properties of welded joints of high-strength low-alloy steel for arctic purposes

Abstract

The work is devoted to the research of cracks causes in welded joints of high­strength steel for arctic purposes based on the study of the structure and mechanical properties of the weld metal and the zone of thermal influence. Consumers of machine­building products make increasingly high demands on welded joints of metal structures. This necessitates the use of rolled steels for their production, which have increased mechanical and special properties. When welding MAGSTRONG W700 type steels, cracks are observed in local sections of welded joints. It was established that the structure of the weld metal of welded joints of MAGSTRONG W700 steel is characterized by the presence of columnar crystals with a hardness of 312 – 323 HV. The metal structure in the overheating area of thermal influence zone is characterized by the presence of enlarged primary grain, as well as batch formations of bainite and bainite-martensite with hardness of 338 – 352 HV. The level of temporary resistance to rupture of the metal in thermal influence zone is 618 – 627 MPa. Depending on the test temperature, values of the impact strength of the metal in thermal influence zone vary from 62 to 86 J/cm2. MAGSTRONG W700 steel has good resistance to the formation of hot cracks during welding (UCS = 20,3), however, it has an increased tendency to form cold cracks (CE = 0,48). Analysis of the data obtained showed that destruction of welded joints of the studied steel occurs due to its unsatisfactory weldability. Such weldability is due to a complex chemical composition, as well as a whole set of factors (such as the formation of unfavorable structures in the metal of welded joints under the influence of thermal welding cycles, a complex picture of welding stresses, the level of which exceeds the temporary resistance to metal rupture). Also, the structure of the weld metal has a large­crystalline structure, which significantly weakens the connection.