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Thermostrengthened alloyed medium-carbon and carbon steels of average, increased and high hardness are widely used in manufacture of welded metal structures for car bodies. HAZ metal should also have corresponding values of yield strength considering the requirements necessary for providing of welded joint full-strength. However, mechanical properties of HAZ metal obtained by means of formation of specific complex of microstructures, generated in process of its cooling (welding modes), in contrast to steel, which acquires necessary complex of mechanical properties as a result of initial heat treatment (quenching and tempering). Preferred formation of martensite structures in HAZ metal of welded joints from given steels and saturation of this area by diffusible hydrogen result in increase of their susceptibility to cold crack formation. In this connection, this work represents the results of investigations aimed at study of effect of welding thermal cycles on nature of structural transformations, hardness, static strength and cold crack resistance of HAZ metal of high-strength alloyed medium-carbon steel of the 30Kh2N2MF type with 0.31 and 0.36 % carbon content. Structure and kinetics of transformation of undercooled austenite was investigated using current methods of physical materials science by means of simulation of phase transformations on Gleeble 3800 machine. It is determined that austenite decomposition in the samples of 30Kh2N2MF steel HAZ, independent on carbon content in it, takes place mainly in area of martensite transformation at cooling rate w6/5 = 2.5—30 °C/s. It is shown that hardness and static strength of HAZ metal in steel with 0.36 % C are provided for the whole studied range of cooling rates. Similar properties of HAZ metal in steel with 0.31 % C can be achieved under condition of cooling at 600—500 °C and rate not less than 10 °C/s. The results of investigations can be used for optimizing welding modes of special equipment and further improvement of modes of steel heat treatment. 15 Ref., 4 Tables, 7 Figures.

Effect of welding thermal cycle on structure-phase transformations and properties of haz metal of alloyed 30Kh2N2MF type medium-carbon steel