X-Ray Residual Stress Measurements Correlated with Weld Microstructure Characteristics

Residual stress due to welding can result in brittle fracture, fatigue failure, or stress corrosion cracking in welded structures.  The measurement of residual stresses on the surface is of great importance particularly when crack initiation needs to be evaluated for welded components.  There are several ways to measure non-destructively the residual stresses on the surface, and the reflection method with laboratory X-rays is still useful when soft synchrotron radiation is not available at any time.  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, i.e., there are preferred orientation due to the unidirectional solidification and the grain growth in the heat-affected zone.  Laue spots are not always detected as corresponding to the equator with a zero- or one-dimensional detector in materials such as especially weld metals of austenitic stainless steel and Ni-based alloy.  It is important to establish a measurement method that can evaluate the accurate scattering angle.  To exclude their effect, a method that brooded gauge volume combined with multi-axial rocking has been proposed.  The diffraction peaks were additionally recorded in a two-dimensional detector.  Since the lattice strain measured by X-ray diffraction differs from the mechanical strain, stress analysis is needed to determine the lattice strain's dependence on the diffraction plane.  X-ray elastic constants can be estimated from the elastic stiffness (C_ij for a single crystal) using the Krӧner model.  Weld metallurgical evaluation for measurement accuracy and stress distribution is a key factor.  The relationship between the microstructure evolution and stresses obtained by X-ray diffraction was evaluated as an example of our works.  Materials should be classified according to the accuracy of residual stress measurement by X-ray diffraction.