347/347H Stainless Steel
Type 347 is a non-heat treatable, austenitic, electric furnace processed grade of stainless steel very similar in composition to Type 321. The major difference between Type 321 and Type 347 is that rather than having an addition of titanium, Type 347 has columbium added to it. Tantalum occurs in nature in conjunction with columbium and therefore it may be said that both are additives to this alloy. Type 347 can withstand more severely elevated temperatures than Type 321, as the resulting columbium carbide is more stable and insoluble than the titanium carbide. The only drawback in the use of Type 347 as opposed to Type 321 is that it is not recommended for use in radioactive services as radioactive tantalum has a much longer half-life than columbium. It is non-magnetic in the annealed state.
Type 347 is mainly used where corrosion resistance and sustained operations at temperatures between 800° F. and 1600° F. is of the utmost importance. It is also a superior grade for use on heavy welded objects, which cannot be re-annealed.
Resistance to intergranular corrosion and carbide precipitation is excellent with this grade. Its general atmospheric corrosion resistance is good, being similar to that of Type 302.