BCC lattice

The body-centered cubic lattice (bcc) has one atom in the center and one at each corner. That makes the material tough, but not as ductile as other structures. When welding, it's important to control the cooling. If the part cools too fast, the structure can become hard and brittle, forming martensite-like structure that causes cracks. So you have to choose the heat input so that the part cools slowly enough, preferably with preheating or post-heating, then the structure stays stable and the weld holds.