Intergranular cementite
Grain boundary cementite is a brittle precipitation of iron carbide that forms along austenite grain boundaries during slow cooling and severely reduces toughness.
Physical-Chemical Mechanism of Action
Grain boundary cementite forms during slow cooling of high-carbon steels. In this process, iron carbide condenses at the austenite grain boundaries and forms a network-like structure that is brittle.
Metallurgical Relevance & Welding Risks
The development of grain boundary cementite can significantly reduce the toughness of steel. It leads to brittle fracture paths and increases susceptibility to cracking, especially under mechanical stress. This can cause premature failure of the weld.
Workshop Practice & Action Instructions
To avoid grain boundary cementite, the cooling of the steel after welding should be accelerated. This can be achieved by using cooling water or special cooling gases. The optimal cooling time is typically less than 30 seconds. For heavy loads or high-carbon steels, cooling should be even faster to prevent the formation of grain boundary cementite.