Structure and properties of welded joints of reinforcing bars of A500C strength class

The article presents the reasons of strength reduction of welded joints of reinforcing bars of A500C strength class. The joints were made of reinforcing steel with a diameter of 12 mm, with one of the joints made of thermomechanical hardened steel (sample 1), and the other – of hot-rolled steel without further processing (sample 2). It was established that the structure of welded joint 1 is characterized by the presence of products of tempering of martensite – martensite-bainite structure with hardness of 327 – 339 HV. Characteristic needle and packet formations are observed. The weld metal (core) has a structure identical to the structure of heat-affected zone on the overheating area. The structure of welded joint 2 is represented by more expressed zoning. The boundary is traced between the weld metal (core) and the heat-affected zone. In plane of the section, the cast core is observed as a thin light layer of 30 – 40 microns thickness and with hardness of 180 – 190 HV; it consists of ferrite, not fully subjected to post-welding heat treatment. Also slag inclusions present in all volume of the welded joint metal. In the heat affected zone, in the area of overheating the widmanstatten bainitic structure is also observed. Metal hardness of the heat-affected zone is at the level of 250 – 265 HV. The most likely reasons for the reduced strength of welded joints are increased fragility of the weld metal and the zone of thermal influence, due to the high hardness, more than 300 HV, as well as the presence of slag inclusions in the weld metal of the joint (core), which act as stress concentrators and under external loads are a source of destruction.

reinforcing bars, spot welding, cast core, heat-affected zone, metal structure, welding defects, slag inclusions, hardness

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