How does heat input influence distortion and residual stress in welded joints?

• A. Higher heat input reduces distortion
• B. Higher heat input increases distortion and residual stress
• C. No effect on distortion or residual stress
• D. Only affects porosity

Answer: (B)

Increasing heat input means more energy goes into the metal per unit length, enlarging the weld pool and heat-affected zone and raising the peak temperature. When metal heats, it expands; as it cools, it contracts. If the heated region is large and the surrounding material constrains this movement, different parts of the joint move by different amounts, causing distortion such as warping or bowing. A bigger heated area and steeper temperature gradients make these differential movements more pronounced, so distortion tends to grow with higher heat input.

Residual stresses are locked-in stresses that remain after the weldment returns to room temperature. They arise because the material yields and plastically deforms during heating and the subsequent contraction during cooling is restrained by surrounding material and fixture constraints. More heat input expands the plastically deformed zone and increases thermal gradients, leading to greater restraint and higher residual stresses in the weld and adjacent metal.

So, higher heat input generally increases both distortion and residual stress. Other options don’t fit because heat input does affect distortion and stresses (not just porosity or nothing at all), and it typically worsens distortion rather than reducing it.