The Short Answer
ASTM A234 is not a single material; it is a specification that covers wrought carbon steel and alloy steel pipe fittings for moderate and high temperature service. Grade WPB, which accounts for the great majority of fittings ordered worldwide, is a carbon steel. Grades WP5, WP9, WP11, WP12, WP22 and WP91 in the same specification are alloy steels. So "A234 carbon steel" is accurate shorthand for WPB and WPC, but it is incomplete as a description of the specification.
The specification covers fittings made from pipe, plate, bar or forgings, including elbows, returns, tees, crosses, reducers, caps, stub ends and other shapes produced by forging, forming or machining.
Carbon Steel Grades and Chemical Requirements
| Element | WPB (max unless noted) | WPC (max unless noted) |
|---|---|---|
| Carbon | 0.30 | 0.35 |
| Manganese | 0.29 to 1.06 | 0.29 to 1.06 |
| Phosphorus | 0.050 | 0.050 |
| Sulfur | 0.058 | 0.058 |
| Silicon | 0.10 minimum | 0.10 minimum |
| Chromium | 0.40 | 0.40 |
| Molybdenum | 0.15 | 0.15 |
| Nickel | 0.40 | 0.40 |
WPB fittings can be produced from pipe that already meets the specification, such as A106 Grade C, or from plate, because the composition windows are wide enough to accommodate both routes. For the alloy grades the chromium and molybdenum windows are much tighter, which is what gives them their elevated temperature strength.
Tensile Requirements
| Grade | Tensile strength, ksi (MPa) | Yield strength, min, ksi (MPa) |
|---|---|---|
| WPB | 60 to 85 (415 to 585) | 35 (240) |
| WPC | 70 to 95 (485 to 655) | 40 (275) |
Because the yield strength of WPB is the same as that of A106 Grade B pipe, a WPB fitting welded into an A106 Grade B line is strength matched to the pipe at ambient temperature. Where a higher strength line pipe such as API 5L X52 or X60 is used, a matching high yield fitting from the high yield wrought fitting specification, with grades from WPHY 42 through WPHY 70, is usually selected so that the fitting does not become the weak point in the run.
Alloy Grades and Where They Are Used
The alloy grades in A234 extend the same product forms to elevated temperature service. WP11 and WP22 fittings are used in steam and process piping where plain carbon steel would suffer from excessive oxidation or creep, and WP91 is used in high energy steam lines. These grades come in more than one class of heat treatment in some cases, and the class is quoted along with the grade on the purchase order because it changes the tensile range that applies.
Related fitting specifications cover the other service conditions: low temperature carbon and alloy steel fittings for low temperature service use grades such as WPL6; stainless steel fittings are covered by the austenitic specification with grades such as WP304, WP304L, WP316, WP316L, WP321 and WP347; and duplex and super duplex fittings are covered by the separate duplex specification with UNS designations such as S31803 and S32750.
Dimensions, Heat Treatment and Marking
Dimensions of butt welding fittings follow ASME B16.9 for the standard long radius, three diameter and other listed shapes, while short radius elbows are dimensioned to ASME B16.28. Wall thickness is stated as a nominal wall or schedule matched to the connecting pipe. Heat treatment requirements vary by grade and class and are stated in the specification; carbon steel grades are in many cases supplied without a separate heat treatment, while several alloy grades require a specific normalising, annealing or tempering treatment.
Every fitting is required to carry marking that identifies the manufacturer, the specification and grade, the size and wall thickness and the heat number, so that the fitting can be traced back to the material test certificate. For pressure service, that traceability is the item most often checked during a delivery inspection, along with hardness checks on alloy and low temperature grades and visual inspection of formed surfaces for laps or cracks.
Frequently Asked Questions
Q: Is ASTM A234 WPB a carbon steel?
A: Yes. WPB is the carbon steel grade within ASTM A234, with a maximum carbon content of 0.30 percent and a minimum yield strength of 35 ksi (240 MPa).
Q: Does ASTM A234 cover stainless steel?
A: No. Austenitic stainless steel fittings are covered by a separate specification with WP304, WP316 and similar grades. A234 covers carbon steel and alloy steel grades only.
Q: Can A234 WPB be used at low temperature?
A: Not for low temperature service. Low temperature carbon and alloy steel fittings use a dedicated specification with grades such as WPL6, which are impact tested for that duty.
Q: What is the difference between WPB and WPC?
A: WPC permits slightly higher carbon and has a higher tensile range, 70 to 95 ksi (485 to 655 MPa) against 60 to 85 ksi (415 to 585 MPa) for WPB, with a correspondingly higher minimum yield strength of 40 ksi (275 MPa).
Q: How should a fitting be ordered?
A: Quote the specification, grade, class where applicable, product form, nominal size, wall thickness or schedule, end type and quantity - for example A234 WPB, 90 degree long radius elbow, NPS 6, Schedule 40, butt welding ends. Include the inspection certificate requirement as well.