FEATURES OF MICROSTRUCTURE FORMATION IN THE WELDED PIPE JOINTS DURING HIGH-FREQUENCY WELDING AND SUBSEQUENT LOCAL HEAT TREATMENT

Abstract

The transience of heating and cooling processes during high-frequency welding (HFW) of pipes leads to the formation in the heat-affected zone (HAZ) of a coarse-grained strained bainitic and/or martensitic structure with high hardness. Local or volumetric heat treatment is used to correct the microstructure of the HAZ. The paper presents the results of a study of the structure and properties of welded pipe joints produced at the Vyksa Steel Works. It has been established that the microstructure of the welded joint of pipes made of low-carbon steels (not more than 0.07% C) is mainly bainite of lath morphology, and for steel with a high content of carbon (~0.3%) and manganese ‒ martensite. To estimate the temperature field in terms of the angle and thickness of the pipe wall during local heat treatment a finite element model was created. The paper shows that the use of the model made it possible to optimize the heating parameters during the local heat treatment of pipes of various sizes. The experiments on local heat treatment of pipes with a diameter of 325 mm, made from rolled products 8 mm thick of steel 09Г2С, showed a significant refinement of the initial microstructure of the HAZ, an increase in uniformity and the disappearanc of structural heat-affected zones. In medium carbon steels (~0.3% C) with a high manganese content, the thermal cycle of HFC welding leads to formation of martensite. The local heat treatment in the form of normalization does not allow one to exclude the formation of martensite in the segregation zones due to the high cooling rate. The local heat treatment of welded joints in pipes made of steel 30Г (Russian Standard) in the high tempering mode (650–700 °C), with preliminary cooling of the welded joint to a temperature below the completion of the martensitic transformation (in segregation zones ~50 °C), makes it possible to release the segregation martensite and form the microstructure of sorbitol there.