X-ray diffraction measurement of residual stresses on the surface is of great importance particularly when crack initiation needs to be evaluated for welded components. The measurement with laboratory X-ray is however known to produce markedly different stress results than mechanical methods because the microstructures in the welds change during the welding thermal cycle. Weld metallurgical evaluation for measurement accuracy and stress distribution is a key factor, and the relationship between the microstructure and stresses obtained by X-ray diffraction is evaluated. Materials should be classified according to the accuracy of residual stress measurement by X-ray diffraction. Copyright © 2013 ASM International® All rights reserved.