Martensitic diffusionless transformation
A displacive lattice shear transformation without thermally activated atomic diffusion, occurring upon rapid supercooling of austenite. It produces a tetragonally distorted, extremely hard, and dislocation-rich microstructure.
Martensite forms when you cool austenitic structure too quickly. The lattice then distorts tetragonally, making the material very hard and brittle. In welding, this means: if cooling is too rapid, you get cracks in the weld or in the heat-affected zone, because the structure can no longer relieve stresses. To prevent this, you have to slow down cooling—let the weld cool slowly, for example by preheating or covering with insulating material. This is especially important for high-carbon steels, otherwise you end up with a part that you can't work on without it shattering in your hands.