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What is the difference between SS 347 and SS 347H?
Short answer
SS 347H controls carbon to 0.04–0.10% and requires a coarse grain size, which raises creep-rupture strength for superheater, reheater and other pressure parts above about 500 °C. Standard SS 347 is used for welded equipment that operates hot but below the creep range, where stabilisation against sensitisation is the main need.
SS 347 vs SS 347H at a glance
| Property | SS 347 | SS 347H |
|---|---|---|
| UNS | S34700 | S34709 |
| EN | 1.4550 | 1.4961 |
| Family | Austenitic stainless steel | Austenitic stainless steel |
| C (%) | 0.08 max | 0.04 – 0.10 |
| Cr (%) | 17.0 – 19.0 | 17.0 – 19.0 |
| Ni (%) | 9.0 – 13.0 | 9.0 – 13.0 |
| Nb (%) | 10×C – 1.00 | 8×C – 1.00 |
| Tensile strength | 515 MPa (75 ksi) | 515 MPa (75 ksi) |
| Yield strength | 205 MPa (30 ksi) | 205 MPa (30 ksi) |
| PREN | — | — |
| Density | 7.96 g/cm³ | 7.96 g/cm³ |
| Magnetic | Essentially non-magnetic when annealed; cold work can make it slightly magnetic. | Essentially non-magnetic when annealed; cold work can make it slightly magnetic. |
| Typical use | Refinery piping and heater tubes | Superheater and reheater tubes (A213 TP347H / TP347HFG) |
Which should you choose?
There's no universal "winner": the right choice depends on service conditions, codes and cost. Share your conditions with us for a recommendation.
Fine-grain 347HFG
Modern superheaters often specify TP347HFG, a fine-grained version with better resistance to steam-side oxidation. It is a deliberate exception to the coarse-grain rule for H grades and must be ordered by that name.
Niobium and carbon balance
SS 347H needs enough niobium relative to carbon (at least eight times the carbon) to stay stabilised despite its higher carbon. Check both on the certificate.
Products in both grades
Related guides, comparisons and articles
Frequently asked questions
Is SS 347H used in refineries?
Yes, for heater tubes and hot piping in the creep range, alongside SS 321H.
Can SS 347 tube replace SS 347H in a boiler?
No. The boiler design relies on SS 347H creep strength.
What is the main difference between SS 347 and SS 347H?
SS 347H controls carbon to 0.04–0.10% with a coarse grain for higher creep-rupture strength. It's the standard for superheater and reheater tubes, while SS 347 is used for general stabilised service.
When should I choose SS 347?
Welded equipment at moderate high temperature.
When should I choose SS 347H?
Boiler tubes and creep-range piping.
Can SS 347 replace SS 347H?
Only when the design, code and specification allow it. Strength, corrosion resistance and temperature limits differ, so substitution must be approved by the design authority.