Welds are often located at mechanical-stress concentrations, and welds can be regarded as a form of metallurgical notch due to degraded local material properties, defects, and residual stresses. The combination of high stresses, reduced material properties, and probable defects often leads to failure by fracture, fatigue, stress corrosion cracking, or even creep cavitation.

Although developing materials for demanding applications is essential, joining these materials will be a major difficulty. Welds and other methods of joining are inevitably the weak point due to metallurgical inhomogeneity, defects, residual stresses, and dissimilar mechanical properties. Well-characterized residual stresses are essential for assessing the structural integrity of welds. In a similar manner to the stress redistribution that may occur with high temperature, irradiation can change the residual stress distribution in the weld (Fig. 4.1).

Residual stresses may occur between layers in composites such as the com­monly-used austenitic (stainless steel) cladding on ferritic pressure vessels (Fig. 4.2). These layered composites have two forms of stress, any residual stress due to the bonding process, and a thermal mismatch which is a function of the different coefficients of thermal expansion, and the difference between the bonding temperature and the current (measurement or operational) temperature. In some cases the cladding is too thin for residual stress measurements and only the residual stresses in the base material can be measured [5].

The welds and heat-affected zones (HAZs) are areas of concern for SCC because of the presence of as-fabricated flaws, high residual stresses, elevated plastic strains, chemical heterogeneity, and microstructural differences relative to base metals. Dissimilar metal welds are critical areas in nuclear-power systems due to higher residual stresses than for similar-metal welds, and additional thermal stresses during operation due the different coefficients of thermal expansion.

Fig. 4.2 Residual stresses in base material only in Stellite-clad steel specimens. Reprinted with permission from (H. Kohler, K. Partes, J. R. Kornmeier, F. Vollertsen, Phys. Procedia 39, 354 (2012)) [5]. Copyright (2012) Elsevier

Many of the materials of interest for nuclear-power systems are difficult for neutron diffraction due to large grain size in the weld (stainless steels, U, Zr), low scattering (Zr, Ti), or strong attenuation (W). Hexagonal and orthorhombic crystal structures (Ur, Zr) can complicate residual stress measurement due to type II (inter-granular) stresses from elastic, thermal, and plastic anisotropy which are superimposed on the type I macroscopic stresses.

Dissimilar metal welds (austenitic to ferritic) or bonding system (e. g. copper — tungsten composites for the plasma facing component in fusion systems) requires measurement of different reflections necessitating a different instrument configuration for each material.

Inevitably components will need to be modified or repaired and weld repairs complicate an already complex residual stress field. Repair welds are of special interest as they are often made without post-weld heat treatment, producing welds with higher levels of residual stress (and sometimes hydrogen) than conventional welds. Nearly

half of repair welds made on high-energy components in the power-generation industry subsequently fail. Repair welds are more complex than normal fabrication welds as the repair may have significant stop/start thermal fields, may possess further transformation stresses, and overlay an existing residual stress field. Edwards et al. [6] and Bouchard et al. [7] made neutron-diffraction measurements of residual stresses in typical repair welds for the nuclear industry. Figure 4.3 shows a good comparison between residual stresses measured by deep hole drilling and by neutron diffraction for a short weld-repair in a 20 mm thick 316 stainless vessel.