Experimental determination of the annealing effect on critical area of HSLA multi-pass girth weld at natural gas pipelines

Shield metal arc welding on the high strength low alloy steels in pipelines to transport natural gas from Iran is of great importance. In this article the effects of annealing heat treatment on properties of multi pass welding in different situations (6-4:30 , 4:30-3 , 3-1:30 , 1:30-12) with 36 in outside diameter is evaluated by chemical,  metallography, tensile, toughness and hardness. Tensile test results showed the lowest yield strength (Vertical to weld and in position 6-4:30) equal to 348 MPa, and the lowest energy impact (Vertical to weld and in position 1:30-3) equal to 108J. The impact energy alignment to weld had a rate of 12 percent increase before the heat treatment. The amount of hardness variation in different areas and positions is negligible (less than 5 percent). Images of metallographic test made by light and electron microscopes demonstrated that the amount of perlite cap pass weld and heat affected zone near the weld metal were decreased compared to main metal to respectively 29 and 8 percent. The elongation weld of was increased ratio than before the heat treatment in base metal respectively 75 and 23 percent. The increase rate of C, V and Ti in the weld zone according to base metal in situation of 3-4:30 are respectively 0.02, 0.003 and 0.006.