Article
Two elbows of the same size and grade can carry very different prices. Often the reason is the method of manufacture: one is seamless, the other welded. Both are legitimate products under the same ASTM standard, but they are not interchangeable in every service.
How seamless fittings are made
Seamless fittings start as seamless pipe. Elbows are hot formed over a mandrel, tees are bulged or extruded, and reducers and caps are pressed. There is no longitudinal weld anywhere in the fitting. Seamless fittings are usually available up to about 24" and are the first choice for high pressure, high temperature and critical service.
How welded fittings are made
Welded fittings are formed from welded pipe or from plate that is pressed into shape and welded. This route is economical for larger sizes and thinner walls, and it is the only practical route for very large diameters.
ASTM A403 classes
| Class | Meaning | Weld examination |
|---|---|---|
| WP-S | Seamless construction | Not applicable |
| WP-W | Welded construction | Radiography of welds made by the fitting maker; pipe welds made with filler metal radiographed |
| WP-WX | Welded construction | 100% radiography of all welds |
| WP-WU | Welded construction | 100% ultrasonic examination of all welds |
| CR | Made to MSS SP-43 (light wall) | Lower test requirements; for low-pressure service |
How to choose
- Critical, high pressure, cyclic or toxic service – seamless (WP-S), or WP-WX / WP-WU if the size makes seamless impractical.
- General process and utility service – WP-W is widely accepted.
- Large diameters and thin walls – welded is often the only option.
- Low-pressure light-wall stainless systems – CR fittings to MSS SP-43 may be acceptable.
What the marking should show
A stainless fitting should be marked with the manufacturer's identification, grade and class (for example WP316L-S), size, schedule and heat number. If the order needs WX but the fitting is marked W, it does not meet the order, however good it looks.
Frequently asked questions
Is a seamless fitting always better than a welded one?
For critical service seamless is preferred, but a correctly made and radiographed welded fitting (WP-WX) is fully acceptable in most process applications.
What does WP-WX mean?
It is an ASTM A403 welded fitting with 100% radiography of all welds.
Why are welded fittings cheaper?
They can be made from welded pipe or plate, which costs less than seamless pipe, especially in large sizes.
Key takeaways
- S = seamless, W = welded with RT, WX = welded with 100% RT, WU = 100% UT.
- Match the class to the service, not just the price.
- Check the class marking on the fitting matches the order.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.