The high-strength steels are widely used at manufacturing of welded metal hull structures of the vehicles. The formation of the weld joints of the given steels of predominantly martensitic structures in the HAZ metal and the saturation of this area with diffusion hydrogen leads to the increase of their tendency to cold cracking.
The paper presents the results of studies of the effect of WTC (welding thermal cycles) on the nature of the structural transformations of the HAZ metal high-strength steel with a carbon content of 0.12…0.74 mass%, which affects resistance to cold cracking. With the help of the Gleeble 3800 simulator of the thermal state of welding, the continuous cooling transformation diagrams of the austenite decomposition of the high-strength carbon steels were built; the special features of transformation, the microstructural state and the initial and end phase formation temperatures at different cooling rates were studied and their mechanical properties were determined.
The connection between carbon content, temperature of phase transformations and structure is established: for steels of different purpose, an increase in carbon content in each group (construction: up to 0,12...0,19% C, special: up to 0,26...0,31% C, railways: to 0,58...0,74% C) reduces the temperature of phase transformations by 40...60 °С, which affects the increase of the formation of the critical part of the hardening structures, which leads to deterioration of mechanical properties. In structural steels, phase transformations take place in two stages, initially by a diffusion mechanism with the formation of ferrite-pearlite structures at a cooling rate of 10 °C/sec, and then without diffusion - with the formation of hardening structures at cooling rates above 15 °C/sec. In special steels, a diffusion mechanism with the formation of ferrite-pearlite structures at a cooling rate of 7 °C/sec and a diffusion mechanism for the formation of bainite and martensite structures of different morphologies at a cooling rate above 10 °C/sec. In the railway steels, the transformation takes place by the diffusion and shear mechanism in the ferrite-pearlite zone with the formation of troostite-martensite structures at a cooling rate of 5...30 °С/sec.
The optimal parameters of the thermal cycle of welding at a cooling rate of 5…20 °С/sec have been established, which ensures the formation of ferrite-perlite structures for railway steels and bainite-martensite for construction and armor steels in preventing the formation of cold cracks in the metal of the heat affected zone of high-strength carbon steels with carbon content of 0,12...0,74%.
It has been determined that in order to ensure the strength and ductility of the whole welded joint by forming the optimal structure of high-strength carbon steels, welding at a rate of cooling the metal a zone of thermal impact of 5...10 °C/sec for steel Naxtra 700 is recommended. For all other steels the cooling rate should be 10...20 °С/sec with the use of preheating of the initial metal to temperatures of 200°С.
