ASTM A234 WPB and ASTM A403 WP304 / WP316 are the most commonly specified material grades for wrought butt weld fittings (elbows, tees, reducers and caps made to ASME B16.9). WPB is carbon steel; WP304 and WP316 are austenitic stainless steels. The choice is driven mainly by the corrosiveness of the fluid, the service temperature and the pipe material the fittings are welded to.
Side-by-side comparison
| Property | A234 WPB | A403 WP304 | A403 WP316 |
|---|---|---|---|
| Specification | ASTM A234 | ASTM A403 | ASTM A403 |
| Material | Carbon steel | Austenitic stainless (18Cr-8Ni) | Austenitic stainless (Cr-Ni-Mo) |
| Chromium (Cr), % | — | 18.0–20.0 | 16.0–18.0 |
| Nickel (Ni), % | — | 8.0–11.0 | 10.0–14.0 |
| Molybdenum (Mo), % | — | — | 2.00–3.00 |
| Carbon (C), max % | 0.30 | 0.08 | 0.08 |
| Manganese (Mn), % | 0.29–1.06 | 2.00 max | 2.00 max |
| Tensile strength, min | 415 MPa (60 ksi) | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield strength, min | 240 MPa (35 ksi) | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| Typical matching pipe | ASTM A106 Gr B, A53 Gr B | ASTM A312 TP304 | ASTM A312 TP316 |
ASTM A234 WPB: carbon steel fittings
ASTM A234 covers wrought carbon steel and alloy steel piping fittings of seamless and welded construction for moderate and high temperature service. WPB is the carbon steel grade and is matched to ASTM A106 Grade B and A53 Grade B pipe, which share its 415 MPa minimum tensile and 240 MPa minimum yield strength. It is the usual choice for oil and gas, steam, water and general process lines where corrosion is controlled by the fluid, a corrosion allowance or coatings. The same specification also covers alloy grades such as WP11 and WP22 for higher temperatures, and WPC for higher strength.
ASTM A403 WP304 and WP316: stainless steel fittings
ASTM A403 covers wrought austenitic stainless steel fittings. WP304 is the general-purpose 18-8 grade. WP316 adds 2–3% molybdenum, which improves resistance to pitting and crevice corrosion in chloride environments such as sea water, brines and many chemical processes. Low-carbon versions (WP304L, WP316L) are used where welds must resist intergranular corrosion; they have lower minimum strength than the standard grades. Fittings are matched to ASTM A312 pipe of the same grade.
A403 fittings are also designated by class: WP-S(seamless), and WP-W, WP-WX and WP-WU (welded, with WX meaning all welds radiographed and WU meaning all welds ultrasonically examined).
How to choose
- Match the fitting grade to the pipe: WPB with A106 / A53 Grade B carbon steel pipe, WP304 / WP316 with A312 stainless pipe of the same grade.
- Choose stainless where the fluid would corrode carbon steel, where product purity matters, or for cryogenic service; choose WP316 over WP304 where chlorides are present.
- Choose WPB for non-corrosive service where its higher yield strength and lower cost are an advantage.
- Wall thickness and dimensions are set separately: by the pipe schedule (see the pipe schedule chart) and ASME B16.9.
This guide is a general reference summarised from the named standards. Always design, specify and inspect to the current edition of the applicable standard and your project specification. Published 2026-10-19.