Which of the following contributes least to the formation of cracks in welds?

The formation of cracks in welds is influenced by several factors, and low carbon content in steel contributes least to crack formation compared to the other options. Carbon is an alloying element that increases hardness, but it does not significantly affect weldability as much as the other factors mentioned.

Rapid cooling can lead to increased thermal stresses and solidification-related cracking, especially in high-carbon steels or in welds with high heat inputs. High heat input can also lead to excessive grain growth in the heat-affected zone (HAZ), increasing the susceptibility to cracking due to changes in material properties. Poor fit-up can create issues such as lack of fusion or incomplete penetration, which can lead to weak points that are prone to cracking under stress.

In contrast, low carbon content generally results in a more ductile and tougher material that is less likely to crack during welding processes. Therefore, considering the specific influences on crack formation in welds, low carbon content is the least contributory factor among the options provided.