WP-W, WP-S and WP-WX markings describe how a wrought stainless steel pressure fitting was manufactured and examined under ASTM A403. They do not identify the alloy grade or wall thickness by themselves. A complete fitting specification must combine the class marking with the grade, size, schedule, dimensional standard and certification requirements.
Key takeaways
- WP-S identifies seamless construction.
- WP-W and WP-WX identify welded construction with different radiography requirements.
- The marking should be checked against the material certificate and purchase order.
What ASTM A403 controls
ASTM A403 covers wrought austenitic stainless steel fittings for pressure piping applications. It includes material and manufacturing requirements for elbows, tees, reducers, caps and other fittings produced from forgings, bars, plate or tubular products.
The grade appears separately from the construction class. A marking such as WP316L-S, for example, communicates both the alloy grade and seamless construction. The schedule or wall thickness and dimensional standard must still be confirmed.
Steelmor stocks stainless steel buttweld fittings in common grades and schedules, supplied with EN 10204 3.1 material certification.
What WP S means
WP-S fittings are manufactured by a seamless method from seamless starting material. They contain no welded seams in the finished fitting. Seamless construction is often specified where the project standard or service conditions call for it, but the marking does not automatically make the fitting suitable for every pressure or temperature.
The designer must still confirm the material grade, size, wall thickness, forming process, heat treatment and applicable piping code. A seamless fitting can be unsuitable if another part of the specification is wrong.
What WP W means
WP-W identifies a welded fitting manufactured under the welding and examination requirements of ASTM A403. Radiography applies to the construction welds required by the standard, but buyers should not read WP-W as a simple statement that every weld in the starting material and finished fitting received full radiography.
This distinction becomes important when a project specification demands a defined examination level. The purchase order should state any supplementary non-destructive testing rather than rely on an informal interpretation of the marking.
What WP WX means
WP-WX identifies welded construction with all welds radiographically examined as required by the standard. It provides a clearer basis for projects that require full radiographic examination of welded seams.
Full radiography does not replace the rest of quality control. Material identity, heat treatment, dimensions, surface condition and mechanical properties remain part of acceptance. The examination report should also remain traceable to the supplied fitting.
How ASME B16 9 fits into the specification
ASME B16.9 covers dimensions, tolerances, ratings, testing and markings for factory-made wrought buttwelding fittings. ASTM A403 controls the stainless steel material and construction requirements, while B16.9 supplies the dimensional framework commonly used to connect the fitting into the piping system.
The two standards answer different questions. Writing A403 without the dimensional standard can leave geometry unclear. Writing B16.9 without the material specification does not define the stainless steel grade or construction class.
What buyers should verify before acceptance
Check the fitting marking against the purchase order and material certificate. Confirm manufacturer, standard, grade, class, size, schedule, heat number and required examination. The heat number should link the physical fitting to the certificate.
Treat the class marking as one part of the specification
WP-S, WP-W and WP-WX help buyers understand construction and examination, but none is a complete purchasing description. Combining the correct class with grade, schedule, dimensions and traceability gives the fabricator a fitting that can be installed with confidence.
For help matching buttweld fittings to your piping specification, contact Steelmor.