Two differently manufactured steel materials are ductile cast iron, Medium Carbon with Mn-Mo alloy steel (P&H-11), and a mild steel combination welded to fabricate a structural component. A typical problem analyzed and discussed related to premature failure under stress and fatigue. It gives guidelines to engineers and fabricators to follow the discipline of pre-heating when they are trying to join two materials with indifferent microstructures especially using hot operations. While the investigation of the heat-affected zone of the plate and casting fillet joint, microhardness was observed at higher sides varying from 400-600 HV, against the recommendation of 350 HV max. The high Hardness at the heat-affected zone has contributed to crack initiation due to high localized stress. Along with microhardness, chemical and microstructural studies were also done to correlate the cause of failure. The effect of higher microhardness has been eliminated through pre-heating during welding. A suitable pre-heating temperature has been established through experimentation for ductile cast iron and mild steel combination of 200 to 250 deg. C was the most optimized and suitable solution.