The most technologically important manifestation of SIC is hydrogen-induced cracking (also known as ‘cold cracking’) caused by the numerous sources of hydrogen in both fabrication and service, the relative ease of hydrogen entry into metals, its high diffusiv- ity, and its ability to weaken metallic bonds or form brittle second phases.36-39 Hydrogen cracking in steels and hydride-type cracking of zirconium alloys

30 Cr alloy cb—І a «3.58
O <P (j)G)
Partially coherent grain boundary Cr23C6
Grain boundary
(Cr, Fe)23C6
(a)	0.1um
Tension between carbides
Cracking between carbides
1um
Acc. V Spot Magn Det WD 20.0 kV 3.5 10000x SE 12.7
2 mm
Small cracks coalesce
‘Ductility Dip’ Crack
100 um
(c)
0.1%
-0.1%
О
-0.6%
3.575	3.580
(d)
Figure 10 Illustration of the mechanism of ductility dip cracking in Ni-Cr alloys. (a) Partially coherent (Cr, Fe)23C6 carbides form in reheated weld metal, which have misfit with the grain boundary. (b) Precipitation generates local grain boundary tensile stresses between carbides which (c) results in ductility dip cracking when sufficiently large global stresses are present during welding or are applied during tensile testing. (d) The increased misfit between the carbide and the matrix with increasing chromium concentration helps explain the susceptibility of alloy 690/EN52 and the resistance of A600/EN82 to DDC.

Figure 12 Correlation of the subsolidus cracking susceptibility of selected superalloys with the misfit and kinetics of second-phase precipitation (g or y"). Adapted from Young, G. A., et al. Welding J. Res. Suppl. 2008, 31S-43S; Prager, M.; Shira, C. S. Welding Res. Council Bull. 1968, 128; calculations done with JMatPro, Version 4.1.

have been treated recently in the literature and more extensive reviews are found elsewhere.40’41

However, it should be highlighted that low — strength austenitic alloys are resistant but not immune to hydrogen-induced cracking. Figure 13 shows a hydrogen-induced crack in a Ni-20Cr-3Mn — 2.5Nb-1Fe weld metal (EN82) that was produced by the combination of poor welding practice and the use of hydrogen-bearing shield gas. The 95%Ar-5%H2 shield gas helps minimize surface oxides and interpass grinding but results in ~12wt ppm hydrogen dis­solved in the filler metal. ‘Refuse welding’ or remelt­ing beads in an attempt to improve the tie-in and contour increases the plastic strain in the joint and can trigger cold cracking.42,43