Martensitic needle microstructure
Martensitic needle structure is an extremely hard and stressed forced-lattice structure formed during rapid cooling. It leads to glass-hard hardness and increases the risk of cracking.
Physicochemical Mechanism of Action
Martensite is a metallurgical phenomenon in which the iron lattice is transformed into a hard structure by shear under fast cooling. This happens without carbon diffusion, leading to a strongly strained forced lattice structure.
Metallurgical Relevance & Welding Risks
Martensite makes the weld extremely hard and prone to cracking. Cooling too fast can turn the structure into martensite, which raises hardness and reduces toughness. This leads to a higher susceptibility to hot cracks and brittle fractures.
Workshop Practice & Instructions
To avoid martensite, it is crucial to ensure controlled cooling of the weld. This can be done using cooling water or special cooling jackets. The critical time t8/5 should be between 5 and 25 seconds to get a balanced structure. With very hard steels, slow cooling is essential.