Effect of microstructure on mechanical properties of C-Mn weld metal

Carbon steel conforming to ASTM A516 Grade 70 was welded using manual electrodes of type E7018, with different heat inputs (by using different electrode diameters) and interpass temperatures. Mechanical properties of the weld metals, such as tensile strength and charpy impact strength, were evaluated in the as-welded and postweld heat-treated states. Microstructural studies were made after 2% Nital etch. The results showed that with an increase in heat input and interpass temperature, the acicular ferrite content of the weld metal decreases, while grain-boundary ferrite, ferrite side plate and polygonal ferrite contents increase. The underlying passes which were refined showed equiaxed ferrite grains and were relatively coarser with increase in heat input and interpass temperature. Postwelding heat treatment did not lead to any significant changes in microstructures. The structures containing more acicular ferrite were found to be relatively stronger and tougher than those containing more grain-boundary ferrite and ferrite side plates.