Thermal cycles and peculiarities of decomposition of austenite in laser-hybrid welding of steels of strength class K52 and K60

Laser-hybrid welding (LHW) is an advanced high-performance technology for obtaining all-in-one connections. Phase transformations, thermal welding cycle and microstructures of a welding seam and of a heat-affect zone (HAZ) influence on the quality of welding joint of LHW. This article defines experimentally the thermal cycles and shows the results of kinetics of austenite decomposition at the usage of technology of LHW combined with the multi-arc automatic welding under flux. There were defined cooling rates influencing the change of properties of heat-affected zone of welded joints of steel in tube sorts with strength class K52 and K60. First part of the paper presents objects of investigations, chemical composition and technologies under which the welded joints were obtained. The second part of the article shows a method of investigation which reflects samples and operations with instruments, equipment with the help of which parameters of thermal cycles and cooling rates were fixed. The third part shows the peculiarities of formation of structure-phase composition of welded joints with the help of tables, diagrams and microstructures of heat-affected zones at various cooling rates. We found that in the result of LHW in HAZ the decomposition of austenite in the studied steels is 350-360 HV, which increases the probability of formation of hardening structures in welded joints and may lead to crack formation. It is revealed the normative value of hardness can be provided if the metal cooling rate at LHW do not exceed 20 °C/c.