Phase Boundary Migration

When welding, the structural phases in the steel transform – that's normal. But if it happens unevenly, you get a weld with inconsistent properties, which can lead to cracks or distortion. It's important to control the heat input and cooling. If you weld too hot or cool too fast, the structure can become hard and brittle. If it runs too slow, it becomes soft and won't hold. So make sure the weld cools evenly and you don't put in too much heat. Then the structure stays stable and the weld holds.