What is brittle fracture, and which welding metallurgical conditions increase its risk?

• A. A ductile fracture with large plastic deformation; risk increased by high carbon steels.
• B. A fracture with little plastic deformation; risk increased by high carbon hardenable steel, poor HAZ microstructure, insufficient preheat, or rapid cooling.
• C. A fracture caused by excessive heat input alone.
• D. A defect due to gas porosity.

Answer: (B)

Brittle fracture is a type of failure that occurs with little to no plastic deformation—the material breaks suddenly and can crack and propagate quickly without noticeable prior stretching. In welding, this risk rises when the steel is prone to becoming very hard and stiff or when the weld heat-affected zone (HAZ) has a brittle microstructure. High carbon hardenable steels are more likely to form hard, brittle phases, especially if they are not tempered properly, which reduces toughness. A poorly refined or coarse HAZ microstructure, including untempered martensite, further lowers toughness and makes cracks propagate under stress. If preheat is insufficient, or cooling after welding happens rapidly, the weld and surrounding metal cool too quickly, encouraging brittle transformations and locking in high residual stresses that drive sudden fracture. Rapid cooling also promotes brittle microstructures like martensite, increasing susceptibility. In contrast, a long-lasting ductile fracture involves substantial plastic deformation before failure, which is not what’s described here; porosity is a welding defect related to gas entrapment, not the fracture mode itself, and heat input alone does not determine brittle fracture without the accompanying microstructural changes.