**Abstract**

During welding, due to the highly localized transient heat input, considerable residual stresses and deformations may occur. Welding residual stresses and welding distortion may greatly impair manufacturing and strength. Residual stresses are internal forces without external forces acting. The total residual stresses superimpose on the stresses from the external load, i.e., the load stresses. Residual stresses are the result of microstructural deformations, e.g., dislocations etc. and can be divided as follows: i) volumetric (or "dilatoric") ii) distortional (or "deviatoric"). Volumetric strain generally is caused by sectioning by thermal expansion, chemical conversion, microstructural transformation or change in state; distortional strain is generally caused by time-independent plastic or time-dependent visco-plastic deformation. There are several methods available to measure the residual stresses or strains nondestructively. One of them is X-ray diffraction. X-rays are diffracted crystal lattices and produce interference phenomena, from which it is possible to draw conclusions relating to the interplanar spacing of the lattice. Other methods are the neutron diffraction method, ultrasonic method and the magnetostriction method. In the magnetostriction or Barkhausen noise method, the stress state is deduced from the value of the local magnetization restraint. The magnetic flux density in a ferromagnetic material subjected to a time-varying magnetic field does not change in a strictly continuous way, but rather by small, abrupt, discontinuous increments called Barkhausen jumps. The jumps are due primarily to discontinuous movements of boundaries between small magnetically saturated regions called magnetic domains in the material. This chapter describes the causes and measurement of residual stress induced during welding.

**Keywords:** arc welding, residual stress, Barkhausen noise, magnetically induced velocity changes, stress dependence
