Grade 347H Stainless Steel Pipe Fittings: The Niobium-Stabilised Grade for High-Temperature Creep Service
Both 347H (Nb-stabilised) and 321H (Ti-stabilised) resist sensitisation and provide high-temperature creep strength. The reason 347H is often preferred over 321H for welded piping: titanium burns off in the welding arc (due to its high affinity for oxygen and nitrogen at arc temperatures), so 321H welded joints cannot use matching ER321 filler — ER347 filler is specified instead, defeating part of the purpose of choosing 321H. With 347H, ER347 or ER347H filler matches the base metal perfectly — the niobium is retained through the welding arc.
347 vs 347H: The Carbon Difference
| Property | Grade 347 (S34700) | Grade 347H (S34709) |
|---|---|---|
| Carbon | ≤0.080% | 0.040–0.100% |
| Niobium | 8×C min, ≤1.0% | 8×C min, ≤1.0% |
| Grain size | Not specified | ASTM No. 7 or coarser |
| Creep strength at 650°C | Lower | Higher — controlled C and grain size |
| Sensitisation resistance | Nb carbides prevent Cr depletion | Same — Nb stabilisation applies to both |
| Max continuous service | ~700°C | ~900°C (with creep design) |
Why Niobium (347H) vs Titanium (321H)
Both 347H (Nb-stabilised) and 321H (Ti-stabilised) resist sensitisation and provide high-temperature creep strength. The reason 347H is often preferred over 321H for welded piping: titanium burns off in the welding arc (due to its high affinity for oxygen and nitrogen at arc temperatures), so 321H welded joints cannot use matching ER321 filler — ER347 filler is specified instead, defeating part of the purpose of choosing 321H. With 347H, ER347 or ER347H filler matches the base metal perfectly — the niobium is retained through the welding arc.
Applications for 347H Pipe Fittings
- Ethylene cracker transfer line piping (600–750°C, cyclic thermal duty)
- Steam reformer outlet headers at the cooler end (below the range for 800H)
- Hydrocracker and hydrotreater high-temperature effluent piping (425–600°C with H₂ and sulphur)
- Petroleum refinery furnace piping in the range 500–700°C where 316H is insufficient
- FCC regenerator piping in the mid-temperature range
When to Step Up to Incoloy 800H
Above approximately 750°C or in highly oxidising environments where the Cr₂O₃ film of austenitic stainless begins to thin rapidly, step up to Incoloy 800H (N08810) — which has better oxidation resistance above 750°C. At continuous temperatures above 900°C or in carburising environments, Incoloy 800HT or Inconel 601 become the appropriate grades. 347H is the correct material for the 425–750°C range where sensitisation resistance AND creep strength are both required.