Grade selection is the most critical step in buttweld fitting procurement. Wrong grade = failures, leaks, and safety incidents. This guide walks through the selection process used by experienced piping engineers.
Temperature is the first filter. Standard carbon steel WPB is rated to 427°C. Below -29°C you need LTCS. Above 427°C you need alloy steel. Above 650°C you need nickel alloys. Map your minimum and maximum design temperatures before anything else.
What fluid does the fitting carry? Seawater → needs duplex or CuNi. Chloride-containing process fluid above 60°C → duplex minimum. Sulfuric or hydrochloric acid → Hastelloy C276. Caustic soda → Nickel 200. Dry gas → carbon steel or alloy. General water/steam → SS 304 or 316.
Your piping code (ASME B31.3, B31.8, NORSOK, etc.) may mandate specific grades for your service. Power plant boiler piping under ASME Section I has strict material requirements. Offshore NORSOK specifies duplex as minimum for process piping. Always check the applicable code first.
The fitting must match the pipe material. If your pipe is ASTM A312 TP316L (SS 316L), your fittings must be ASTM A403 WP316L. If your pipe is A335 P91, your fittings must be A234 WP91. Mismatched grades create galvanic cells, different thermal expansion, and code non-compliance.
Once you have technically acceptable grades, optimize for cost. For non-critical general service, WPB is always the most economical choice. For corrosive service, upgrading from 304 to 316L adds 20–30% cost but may eliminate costly maintenance. Duplex costs more upfront but eliminates the risk of SCC failures that cost far more to repair.
| Service Condition | Recommended Grade | Standard |
|---|---|---|
| General service, -29°C to 427°C | WPB Carbon Steel | ASTM A234 WPB |
| Low temperature, -46°C | LTCS WPL6 | ASTM A420 WPL6 |
| Low temperature, -73°C (LNG) | LTCS WPL3 | ASTM A420 WPL3 |
| High temp 427–550°C | WP11 Alloy Steel | ASTM A234 WP11 |
| High temp 550–600°C | WP22 Alloy Steel | ASTM A234 WP22 |
| Ultra-high temp 600–649°C | WP91 Alloy Steel | ASTM A234 WP91 |
| General corrosive, non-chloride | SS 304 / 304L | ASTM A403 WP304 |
| Marine, pharmaceutical, chloride | SS 316 / 316L | ASTM A403 WP316 |
| High-temp stainless (425–900°C) | SS 321 or SS 347 | ASTM A403 WP321/347 |
| Offshore, sour gas, seawater | Duplex S31803 | ASTM A815 |
| Deepwater offshore, NORSOK | Super Duplex S32750 | ASTM A815 |
| Seawater cooling, marine | CuNi 90/10 (C70600) | ASTM B466 |
| Aggressive chemical (HCl, Cl₂) | Hastelloy C276 | ASTM B366 |
| Extreme high-temp, versatile | Inconel 625 | ASTM B366 |
Tell us your service conditions — fluid, temperature, pressure, environment. We'll recommend the correct grade and provide a quote.