What is the consequence of excessive heat input on weld microstructure?

• A. High heat input increases distortion and coarser microstructure; low heat input risks incomplete fusion.
• B. Low heat input minimizes distortion and improves microstructure.
• C. High heat input reduces distortion.
• D. Heat input only affects color.

Answer: (B)

Excessive heat input changes how the weld cools and how the microstructure forms. When the weld and surrounding metal heat up too much, they cool more slowly. That slower cooling gives grains in the weld metal and in the heat-affected zone time to grow larger, producing a coarser microstructure. A coarser grain structure tends to be less tough and can be more prone to distortion because the heated zone remains softer and shifts more with thermal cycles.

Keeping heat input lower speeds cooling, which tends to refine the microstructure with smaller grains and also reduces distortion. This alignment of finer grains and tighter dimensional changes is why lower heat input is generally favored for better weld properties. Of course, heat input must still be enough to ensure proper fusion, since too little heat can cause incomplete fusion.