Diffusionless phase transformation
A structural microstructural change through abrupt lattice distortion without thermally activated atomic position exchange, as occurs during martensite formation. It leads to extremely hard, dislocation-rich crystal structures.
The thing is: with rapid cooling, the structure can flip without the atoms being able to diffuse properly. That's called diffusionless transformation, and the result is a very hard but also brittle and crack-prone state – especially in steels with more carbon. When welding such steel, you have to control the cooling. Don't let the part cool too fast, otherwise you'll get cracks or distortion. The trick is to manage the heat so that the structure returns to a tough state instead of tipping into that hard, critical range.