Page 113 - Quick Guide to Welding and Weld Inspection by S.E. Hughes, Clifford Matthews

Fracture Modes and Welding Defects

Weld decay
Weld decay is a form of intergranular corrosion that occurs
in the HAZ of unstabilised stainless steels. Within the
temperature range of around 600—850 8C chromium comes
out of solution (Fig. 7.6) to join with free carbon and form
chromium carbides. The chromium was in the grain to help
prevent corrosion so corrosion can now occur where it has
been depleted. Once this chromium depletion occurs the
depleted area is said to be sensitised (meaning it is susceptible
to corrosion) and will corrode in the presence of an
electrolyte. The critical region is usually in the HAZ parallel
to the weld toes (Fig. 7.7) and once the area is sensitised,
corrosion can lead to rapid failure. Weld decay can be
avoided by:

. Using low carbon grade stainless steels. These steels have
less carbon available to form chromium carbides.
Reducing the carbon content also reduces the tensile
strength, however, so 304L is used instead of 304 and 316L
instead of 316. The L indicates low carbon grade stainless
steel which may contain about 0.3% carbon rather than
0.8% carbon.
. Using stabilised stainless steels instead of unstabilised
grades. To prevent the loss of strength associated with
using low carbon grades of stainless steels, stabilised

Figure 7.7 Weld decay

97

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