INFLUENCE OF HEAT INPUT WELDING RESISTANCE TO CRYSTALLIZATION CRACKS OF STEEL WELDED JOINTS 45L

Welded joints of medium-carbon steels 35L and 45L tend to form hardening structures in the heat affected zone and crystallizational cracks in the weld metal body. The formation of crystallization cracks is caused by contribution of high-carbon base metal in the joint which depends on the amount of welding heat input. The condition for preventing the formation of hardening structures is to ensure the cooling rate of the heat affected zone below the critical value. In our paper we investigate the effect of welding heat input on resistance to formation of crystallizational cracks of the metal in joints affected with automatic two-layer welding in carbon dioxide of 45L steel. The ranges of welding heat input of the first and second layers and the admissible carbon content and cooling rate of the welded joint that exclude the emergence of crystallizational cracks in the metal and in the hardening structures within the heat-affected zone of the welded joint are determined. Admissible carbon compound in the joint metal should not exceed 0,2%; the cooling rate of the welded joint should not be more than 18 ? 190С/s. The first layer is implemented at the welding heat input up to 6000 J/cm and is serves as a low-carbon backup (0,15 ? 0,16% of carbon content) that provides the preliminary heating of the welded joint up to 200 ? 2500C and absence of crystallizational cracks in the joint metal. The second layer is implemented the welding heat input up to 10,000 J/cm; it provides the necessary heat input, thus eliminating the formation of hardening structures in the heataffected zone of steel welded joints of 45L steel. We develop the technology and worked out the welding regimes in carbon dioxide without preheating of 45L steel (8 ? 16 mm thick). The dependences of calculated modes are determined and regimes of two-layer welding without preheating of 45L steel (8 ? 16 mm thick) are carried out. The latter providing the realization of welded joints without crystallizational cracks within the joint metal and with ferrite-pearlite structure in the heataffected zone. The comparative assessment of the strength of welded pipe joints of steel 45L, made basic technology (manual, single-layer welding) and new technology (automatic two-layer welding in carbon dioxide), which showed that the strength of welded joints at two-layer welding in carbon dioxide is 35 ? 40% higher than the those obtained via one-pass welding with SSSI-13/45 electrodes. The new technology is implemented in welding frames of self-propelled chassis Cm-20 with a rear beam and a glass of steel 45L. The durability of the frames has increased 1,5 times, and the endurance limit at 20%.The results of the present studies are recommended for implementation for carbon dioxide welding of gears, sprockets, shafts and other parts made of 35, 40, 45 steel grades.