Body-centered cubic matrix

Ferrite has a body-centered cubic lattice and can only absorb a little carbon. When welding, it transforms into austenite at high temperature, which in turn becomes martensite on rapid cooling – that makes the weld hard and prone to cracking. If you want to keep the structure under control, you have to watch the cooling rate and the carbon content. For critical steels, it helps to keep heat input low and preheat if necessary, so that no hard, brittle structures form.