Martensitic shear transformation
Martensitic shear reconstruction is a rapid lattice reconstruction process at low temperatures where atoms suddenly change position. This leads to a heavily distorted crystal lattice and increases the hardness of metals.
Physical-Chemical Mechanism of Action
At low temperatures below the start temperature, a material can suddenly transform from an expanded lattice into a more compact state. This process, called martensitic shear transformation, occurs without diffusion and through collective displacement of atoms. The rapid change in atomic positions leads to a highly distorted crystal lattice structure.
Metallurgical Relevance & Welding Risks
Martensite can occur in weld zones when the material cools too quickly. This leads to significant grain coarsening and increases hardness, which can cause crack-sensitive properties and possible hot cracking. Too-rapid cooling can also make the material glass-hard and no longer formable.
Workshop Practice & Action Guidelines
To avoid martensite, the weld bead should be cooled slowly. This means the cooling process must be controlled and gradual. A typical target is a critical cooling time t8/5 between 5 and 25 seconds. Within this time, the material will have sufficient time to stabilize its lattice structure and avoid martensite.