Microscopic Residual Stresses
Stresses existing between neighboring crystallites or phases possessing distinct elastic or thermal properties.
Residual stresses of the second kind sit in the microstructure, i.e. between the crystallites. They arise from rapid heating and cooling during welding, because the individual grains expand and contract differently. These things are dangerous because they leave internal stresses in the material. Together with shrinkage of the weld, they can lead to cracks, especially if the part is already under load or hard. You can't get rid of them directly, but you can influence them: keep the heat input uniform and don't let the component cool too quickly. If you post-weld treat the weld or stress-relieve the part, that's the surest way to get the stresses out.