Crystallite Growth

If you put too much heat into the weld or cool the part too slowly, the grains in the structure grow a lot. The grain becomes coarse and the steel loses toughness and strength. Especially with high-strength steels, you have to be careful to keep the heat input within limits and not stay at temperature too long. Once the structure is coarsened, you can't get it out by normal welding – the part is usually scrap. So: work cleanly, don't over-preheat, and don't keep the weld unnecessarily hot.