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Weld Decay -
Concept
One of the most widely known effects in heat-affected-zones is that of weld-decay, observed in 18/8 type stainless steels. When these steels are heated within the range of 600-850°C or cooled slowly through/that range, the carbon is precipitated from solid solution mainly at grain boundaries (that is, intergranularly) uniting with chromium to form chromium-rich carbides.
In other words, chromium carbide is formed and precipitated at grain boundaries.
The areas around the weld, thus, impoverished in chromium, have a much lesser resistance to corrosion than the remainder of the metal and, under certain corrosive conditions, these results in a localized intergranular attack, the severity of which depends on
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(i) The time and temperature of exposure, and
(ii) The composition and prior treatment of the stainless steel.
When doing welding of 18/8 stainless steel, a zone of temperature 600+ 850°C exists near the weld running parallel to it and it is in thus zone that the corrosion may occur (and is known weld decay), though no corrosive effect occurs in the weld itself.
Prevention of Weld Decay
A number of methods can be employed to minimize the problem of weld decay:
1. Decrease the carbon content of steel to an amount insufficient (e.g., 0.03%) to form carbides.
2. Add to both parent plate and weld filler rod material, alloying elements such as titanium and niobium (or columbium) to form carbides preferentially at the boundaries and preserve the chromium content.
3. Add molybdenum (usually with Ti and Nb) to reduce precipitation tendency.
4. Solution treats the steel at 10500C after welding to take carbide phase back into solution.
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