Modeling and Optimization of HAZ Characteristics for Submerged Arc Welded High Strength Pipeline Steel

Heat affected zone (HAZ) is the most critical part of welded pipe in oil and gas pipelines as HAZ shows more susceptibility toward embrittlement and weld cracking. It happens because microstructural changes occur due to high heat of welding. Heat input and preheating temperature together control the cooling time of weld which in turn determines the weld microstructure and its mechanical properties. Therefore, in the present study, effect of heat input as well as preheating temperature on the characteristics of the HAZ in submerged arc-welded high-strength low alloy (HSLA) pipeline steel was studied. Hardness and area of HAZ were observed and analyzed as HAZ characteristics. Heat input of the process was varied by changing the voltage, welding speed, wire feed rate and contact tube to work distance. Experiments were designed using central composite rotatable design (CCRD) approach, and using response surface methodology (RSM), process modeling was done. For the purpose of single- as well as multi-characteristic optimization, desirability approach was used. Relationship of heat input and critical cooling rate with HAZ characteristics and its microstructure was also revealed. Inclusion of preheating temperature led to significant improvement in the HAZ characteristics of the weld.