Intergranular corrosion stabilized grades

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. 

Reactors are normally made of low-alloy steel (selected per API Publication 941) and clad or weld overlaid with type 347 (UNS S34700) SS. The stabilized grades (i.e., type 947 or 321 [UNS S32100] SS) are normally used for all stainless equipment in these units to avoid intergranular stress corrosion cracking (SCC) during downtime. Downtime corrosion is discussed in the next section of this chapter, Feed-Effluent Exchangers."... 

To prevent intergranular corrosion the most grades have a reduced carbon content and/or are stabilized with titanium or niobium. Stabilized grades can suffer from a spe-... 

This phenomenon is restricted to the stabilized grades of steel, such as 321 and 347. The knife-line attack occurs immediately adjacent to the weld and shows as a thin line of intergranular corrosion. It results from intergranular corrosion like the weld decay. It may be noted that weld decay develops at some distance away fi"om the weld. The following is the mechanism suggested for the knife-line attack of stabilized steels. 

Knife-line attack Intergranular corrosion of an austenitic steel (stabilized grade) extending only a few grains away from the fusion line of a weld during sensitization of steel. 

The susceptibihty to intergranular corrosion of austenitic stainless steels after sufficient exposure to temperatures between 427 °C and 871 °C (800 °F and 1600 °F) unless stabilized or low-carbon grades are used. 

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