Microscopic Residual Stresses
Inhomogeneous stresses between adjacent crystallites or phases within a microstructural matrix due to differing expansion coefficients.
Micro-residual stresses arise at the level of crystal grains when adjacent areas contract differently during cooling. This happens, for example, when different microstructural constituents or phases in the steel behave differently. These stresses remain in the workpiece after welding and can impair toughness.
If you cool the weld too quickly or the heat input is uneven, you're asking for trouble. In the worst case, it can lead to cracks that often only show up later. So make sure the cooling isn't too rapid and the heat can distribute well. For critical materials, it helps to keep an eye on the interpass temperature and not overheat the weld unnecessarily.