Residual ferrite

Residual ferrite is austenite that did not transform during cooling and remains in the weld. This happens when the heat input is insufficient or the material cools too quickly. Residual ferrite makes the weld harder and more brittle, increasing the risk of cracking. So you have to ensure that the weld stays at temperature long enough and doesn't cool too fast, then the austenite transforms cleanly. If you choose the parameters correctly and control the cooling, you'll get the microstructure under control.