Stainless Steel Butt Weld Fittings

Choosing 304 or 316

Both 304 and 316 are austenitic stainless steels with excellent general corrosion resistance, and 316 is the more corrosion resistant of the two because it contains 2 to 3 percent molybdenum. That molybdenum addition markedly improves resistance to chlorides, to pitting and crevice corrosion, and to many organic and inorganic acids. As a rule, 304 is used for general water and mildly corrosive service and for food and pharmaceutical applications, while 316 is specified where chlorides are present, in marine environments, and in most chemical process service. The L grades, 304L and 316L, have a restricted carbon content of 0.03 percent maximum, which prevents the precipitation of chromium carbides at grain boundaries during welding and thereby avoids intergranular corrosion in the heat affected zone. For welded fittings, which by definition have a heat affected zone, the L grades are normally the correct choice, and the dual certified 304/304L and 316/316L materials meet both specifications at once.

Duplex Stainless Steel Fittings

Duplex stainless steels such as S31803 (2205) have a two phase microstructure, roughly half austenite and half ferrite, which gives them roughly twice the yield strength of 316 and much higher resistance to chloride stress corrosion cracking and pitting. This makes them the standard choice for offshore piping, seawater systems, desalination and the oil and gas industry in chloride containing service. The practical cautions are that duplex must be welded with controlled heat input and interpass temperature to preserve the phase balance, and that the ferrite content of the finished fitting should be checked, typically required to be in the range of 35 to 65 percent. Super duplex grades such as S32750 (2507) extend the performance further for the most aggressive seawater and sour service. Where duplex is specified, the fitting and the pipe must both be duplex, and PMI plus ferrite testing should be specified on the order.