Phase Diagram

A phase diagram shows you which microstructural phases a material has at certain temperatures and compositions. When welding, you need it to understand what happens to the material in the heat-affected zone – for example, whether it becomes martensitic and hard or brittle. If you don't control the cooling, you can mess up the microstructure and get cracks or hardness problems. So check beforehand where your weld lands in the diagram and choose preheating or heat treatment accordingly.