Martensitic Transformation
Martensitic transformation is a quick phase change from cooling, producing a hard, crack-susceptible structure. It occurs on rapid cooling and raises hardness, but also the risk of cracking.
Physical-Chemical Mechanism of Action
Martensitic transformation is a diffusionless phase transformation process. On rapid cooling (t8/5 < 5 s), the lattice remains in a distorted state, leading to hard and crack-sensitive martensite.
Metallurgical Relevance & Risks in Welding
Martensitic transformation increases the hardness of the weld, but it also makes the weld susceptible to hot cracking. Too rapid cooling can prevent the lattice from fully relaxing, causing cracks.
Workshop Practice & Instructions
To avoid or control martensitic transformation, it is important to regulate the cooling rate. For steels with high carbon content (e.g., HSS), cooling should be slow to obtain a balanced and tough weld. Recommended cooling times vary by steel type, but generally t8/5 should be between 5 s and 25 s.