**2.1. Basic mechanism of welding residual stresses**

Complex heating and cooling cycles encountered in weldments lead to transient thermal stresses and incompatible strains produced in region near the weld. After heat cycles of welding diminished, the incompatible strains remain and provoking locked stresses or frequently termed as welding residual stresses. In general, term of residual stress deal with those remaining stress in a structure even though no external load applied (Masubuchi, 1980). Several terms having similar meaning with residual stress were found in some literatures, namely: internal stress, initial stress, inherent stress, reaction stress, lock-in stress, etc. In term of welding process, residual stress are the remaining internal stresses after welding and cooling down to room temperature.

There are two basic mechanisms to explain how residual stress produced by welding process, namely: the structural mismatch and the uneven distribution of non-elastic strain composed by plastic and thermal strains.
