Welded stainless steels, intergranular

A well-known problem is intergranular corrosion of welded stainless steel. The reason for this problem is that welding takes the temperature up to the range where carbide precipitation occurs. The area that is most liable to intergranular corrosion is usually located some millimetres from the fusion zone of the weld, where the temperature is kept within the critical range for the longest time intervals. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

A somewhat similar phenomenon is knife-line attack which may be observed after welding titanium or niobium stabilised austenitic stainless steels. In this case there is a very narrow band of severe intergranular attack along the interface between the parent metal and the fusion zone. During welding, the parent metal immediately adjacent to the fusion zone is heated to just below the melting point and both chromium carbides and niobium or titanium carbides dissolve completely. On cooling rapidly, the conditions are such that when relatively thin sections are welded, neither chromium carbide nor niobium or titanium carbide have time to precipitate. If the weld is now... 

Intergranular corrosion (weld decay) and stress corrosion cracking are problems associated with the use of stainless steels, and must be considered when selecting types suitable for use in a particular environment. Stress corrosion cracking in stainless steels can be caused by a few ppm of chloride ions (see Section 7.4.5). 

If austenitic stainless steel parts exposed to conditions that promote intergranular corrosion are to be fabricated, hard faced, overlaid, or repaired by welding, these parts shall be made of low-carbon or stabilized grades. 

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

Susceptibility of Stainless Steels to Intergranular Corrosion due to Welding... 

When welding conditions and the corrosive environment lead to intergranular corrosion in an austenitic stainless steel such as type 304, the following alternatives may be considered as possible solutions to the problem ... 

Substitute a low-carbon stainless steel such as type 304L that can usually be welded without developing intergranular precipitation to the extent that it becomes susceptible to corrosion. The low-carbon stainless steels are somewhat more expensive, and care must be exercised that there is no carbon pickup during welding. 

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