PECULIARITIES OF MICROSTRUCTURE AND IMPACT TOUGHNESS OF METAL OF WELDED JOINTS OF PIPES OF HIGH-STRENGTH STEEL WITH NIOBIUM AND MOLYBDENUM

摘要

The work describes the results of investigation of microstructure and impact toughness of metal of welded joints of gas-and-oil pipeline pipes of steel category X65-X80 with different content of niobium and molybdenum. Investigated were welded joints of longitudinal pipes of 820-1420 mm diameter with 17.5- 36.0 mm wall thickness, manufactured by using the traditional technology with applying the double-sided submerged multiarc welding. Optical and electron scanning metallography, as well as standard impact bend tests were used. The negative effect of increased content of niobium on structural characteristics of metal of HAZ and weld was found, which was intensified in the presence of molybdenum. Taking into account the niobium susceptibility to segregation at the grain boundaries, the additional alloying with molybdenum, decreasing the temperature of transformation, leads to the formation of unfavorable structural constituents and phases in the metal of welded joints, and also their clustering along the grain boundaries, in particular in reheating regions. To provide high impact toughness and crack resistance of welded joints of pipes of high-strength microalloyed steel, it is necessary to limit in it, in addition to carbon, the content of niobium (±0.05%) and molybdenum (±0.20%), as well as to use the welding consumables, providing the mass share of molybdenum in weld of not more than 0.30%. Results of the work were applied in industrial production of pipes of category X65-X80 at pipe welding enterprises of Ukraine and Russia.