Effect of base metal composition on toughness in a heat-affected zone of large diameter welded linepipes

The problem of toughness in a heat affected zone of welded joints of submerged arc welded pipes with high wall thickness is discussed in this article. The greatest toughness failure was observed in coarse grains of the heat affected zone (CGHAZ) adjacent to the fusion line. This is due to significant structural changes in the welded joint area.It is shown that an increase of the welded pipes wall thickness inevitably leads to the increase of the weldingheat input and the decrease of the post welding cooling rate. Thus, during welding of pipes with wall thickness more than 23 mm the cooling rate decreases to 10 °C/s leading to severe CGHAZ embrittlement. Thus, to ensure the operational reliability of thick-walled large diameter pipes it is necessary to make fundamental changes in the welding technology which help significantly reduce the heat input or develop the chemical composition of the base metal which is less sensitive to the welding thermal cycle.The article describes the effect of separate chemical elements on toughness in the heat affected zone of SAW-welded large-diameter linepipes. The negative effect of silicon and molybdenum on HAZ ductile properties was shown. Additional steel alloying by the use of vanadium also leads to significant embrittlement in HAZ coarse grains. To achieve high toughness of welded joints in large diameter pipes it is necessary to eliminate vanadium from the chemical composition of steel and to reduce the silicon content to about 0.1 %. Molybdenum alloying should be performed at a minimum level to achieve desired strength properties.