Fusion welding leads to residual stresses and strains (distortion) via thermally induced stresses, solidifica­tion shrinkage, and phase transformations. Thermal stresses arise from the large temperature gradients inherent to fusion welding and from differences in the coefficients of thermal expansion (CTE) between materials that make up the weldment (Table 1). Thermal contraction generates stress and distortion during on-cooling, with the maximum residual stress often being the flow stress at which the lowest tem­perature distortion occurs.5,45

Dissimilar metal welds are regions of special con­cern for nuclear power systems as the residual stres­ses are often higher than for similar metal welds.18 For example, in ‘safe end’-type welds, the CTE dif­ference between a pressure vessel low-alloy steel and a corrosion-resistant austenitic alloy leads to higher stresses in these welds and in fact, these locations are known to be at increased risk of stress corrosion cracking for this reason.46—48 Solidification shrinkage is a lesser effect, as the liquid cannot support appre­ciable stress but can affect the local bead contour, the deformation at the weld face, and lead to stress raisers (e. g., concave beads or cracks).5

Phase transformations can also have appreciable effects on the sign, magnitude, and distribution of residual stresses in welds. Becker et al. have shown that for accurate prediction of the residual stresses in pressure vessel-type steels, it is critical to account for the on-cooling phase transformations that occur from welding.49 Furthermore, phase transformations dur­ing postweld heat treatments and from service expo­sure must also be considered. For example, nickel alloys can be susceptible to the development of

=1500 mm; BC +	5tep’= 5Wi;Grid8’00 x 100
=1500 um; E123; Step = 5 um; Grid800 x 100
= 1500 mm. BC + GB; Step = 5 mm. Grid800 — 100
=1500 um; E123; Step = 5 um; Grid800 x 100
GB; Step = 5 um; Grid800 x 100
0.5 cm
Figure 13 Example of a hydrogen crack produced in EN82H from the use of 95%Ar-5%H2 shielding gas and abusive welding practice (refuse welding). The top figures show the crack in cross-section and the corresponding electron backscatter diffraction strain maps. The bottom fractographs show the intergranular/interdendritic nature of the hydrogen crack.

Figure 14 Illustration of some welding defects in commercially available Ni-30 wt% Cr alloys. (a) Lack of fusion defects in an SMAW (left) and a GTAW (right), (b) unmelted NbFe2 alloying addition in an SMAW (left) and a slag inclusion from an SMAW (right) and (c) surface (left) and internal (right) oxides from a laser weld.

short — and long-range order. The ordered structure typically has a smaller lattice parameter than the bulk alloy and can lead to increased residual stresses with service exposure.10’50-54 Another point to note is that the welds typically have considerable texture that can be a significant factor in both the macroscopic and microscopic (intergrain) stresses and strains in welds.