Why can high carbon content create brittle zones around a steel weld?

High carbon content significantly impacts the weldability of steel, often necessitating specific precautions when levels exceed 0.3 percent. The formation of brittle zones around a steel weld is primarily a result of the microstructural changes that occur in the heat-affected zone (HAZ) due to the thermal cycles of the welding process.12

When steel is heated during welding, the region adjacent to the fusion zone—the heat-affected zone—undergoes structural changes based on peak temperatures and cooling rates. In steels with higher carbon content, these thermal cycles can promote the formation of harder, more brittle microstructures, such as those containing increased amounts of pearlite or cementite.21

Furthermore, welding induces significant residual stresses due to the contraction of the weld metal and the expansion and subsequent contraction of the heat-affected zone. Because the surrounding unheated metal restricts this movement, these stresses can lead to cracking or brittleness in the joint. To mitigate these issues, fabricators often employ controlled welding techniques or post-weld heat treatment to manage the microstructure and relieve internal stresses.2