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.

Stainless fittings: grade, stabilisation and ferrite

Stainless fittings are made to A403 rather than A234, and the grade letter is prefixed with WP in the same way. What matters is the molybdenum in the 316 grades, the stabilisation in 321 and 347, and the ferrite balance in the duplex grades, which is checked on the finished fitting rather than assumed from the plate.

Which fitting is A403, and in which grade

A403 grades carry the WP prefix and the matching pipe grades carry TP: the fitting is WP316 and the pipe it joins is TP316. Both are 316 stainless, and they are ordered together so the chemistry at the joint matches.

A403 covers wrought butt weld fittings. Forged socket weld and threaded fittings to ASME B16.11 are a different material specification: the stainless forged grades follow ASTM A182, and the grade is written F316 rather than WP316. An enquiry for a stainless socket weld elbow is therefore an A182 fitting, not an A403 one.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the stainless butt weld fitting grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A312 TP316 pipe. The L suffix denotes the low carbon version, which can be welded in most service without post-weld solution annealing.

Source: specified minimum yield and tensile strength for each grade, per ASTM A312 / ASME SA312. The standard is the governing document.

• WP304
• WP304L
• WP316
• WP316L
• WP321
• WP347
• WP904L
• S31803 duplex
• S32205 duplex

Stainless Steel Pipe – grades, standards and how the order is fixed

Download Stainless Steel Butt Weld Fittings data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.