Owing to its fine structure and high strength to weight ratio, high strength low alloy (HSLA) steel, API X80, is most preferred grade for application in oil and gas pipeline fabrication. Coarse grain heat affected zone (CGHAZ) formed during fabrication of pipelines using high heat input welding operation like submerged arc welding (SAW) is the crucial area for failure during the application of welded structure. Large detrimental changes occur in mechanical behavior and microstructural characteristics of HAZ due to thermal cycles of the welding operation. In this study, CGHAZ built during first-pass of SAW and when it gets reheated during second-pass as well as third-pass is investigated at each stage for mechanical properties (impact toughness and hardness) as well as microstructural changes. Peak temperature of CGHAZ is determined using upper critical temperature limit of steel. For identifying the lower and upper critical temperature for this steel, dilatometry tests were performed and values were found in well agreement with theoretically determined ones. Comparative analysis of mechanical properties as well as microstructure in simulated HAZ with parent material is also discussed. It is found that subsequent reheating of CGHAZ (formed during first-pass of welding) because of second and third-pass of welding has beneficial effect. Work of this study will guide the engineers to put the limit on heat input in terms of number of passes during welding of such high strength pipeline steels.