Isothermal Phase Transformation
Microstructural transformation process that occurs at a constant temperature over a specified duration.
Isothermal phase transformation simply means that the structure changes at a constant temperature. This happens, for example, during quenching and tempering or when holding at an intermediate temperature. In welding, this is relevant because the heat-affected zone gets different structures depending on the cooling behavior – from soft to hard and crack-sensitive. If you hold the part at a specific temperature after welding, you can settle the structure and relieve stresses. With higher-alloy steels, you have to be careful not to drop too quickly through the critical range, otherwise the weld will become hard and brittle. When in doubt, rather slow down the cooling or post-heat, instead of being surprised afterwards.