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Is SS 321 or SS 347 better for high-temperature welding?
Short answer
SS 347 is usually better for welded high-temperature parts. Its niobium carbides are more stable than the titanium carbides in SS 321 and survive the welding arc, giving more reliable resistance to sensitisation and slightly better creep strength. SS 321 is a cost-effective choice for exhausts, expansion joints and general heat service.
SS 321 vs SS 347 at a glance
| Property | SS 321 | SS 347 |
|---|---|---|
| UNS | S32100 | S34700 |
| EN | 1.4541 | 1.4550 |
| Family | Austenitic stainless steel | Austenitic stainless steel |
| C (%) | 0.08 max | 0.08 max |
| Cr (%) | 17.0 – 19.0 | 17.0 – 19.0 |
| Ni (%) | 9.0 – 12.0 | 9.0 – 13.0 |
| N (%) | 0.10 max | — |
| Ti (%) | 5×(C+N) – 0.70 | — |
| Nb (%) | — | 10×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 | 7.93 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 | Exhaust systems and manifolds | Refinery piping and heater tubes |
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.
Why titanium is lost in the arc
Titanium oxidises readily in the welding arc, so SS 321 filler would lose much of its stabilising element. That is why SS 321 base metal is normally welded with SS 347-type filler: the niobium carries across the arc and stabilises the weld metal.
Polythionic acid in refineries
Both grades are used in refinery heaters and hydroprocessing where polythionic acid stress-corrosion cracking can occur during shutdowns. A stabilising heat treatment and shutdown neutralisation procedures are often specified for either grade. See sensitisation and stabilised grades.
Products in both grades
Related guides, comparisons and articles
Frequently asked questions
Are SS 321 and SS 347 interchangeable?
Often, but not automatically. Check the design code, the welding procedure and any project preference for niobium stabilisation before substituting.
Which is easier to polish?
SS 347 generally gives a better polished finish, because titanium nitride inclusions in SS 321 can show as streaks.
What is the main difference between SS 321 and SS 347?
Both are stabilised 18-10 grades. SS 321 uses titanium and SS 347 uses niobium. Niobium carbides are more stable and survive welding, so SS 347 has slightly better weld-zone behaviour and creep strength, and SS 347 filler is used to weld both.
When should I choose SS 321?
Exhausts, expansion joints and general high-temperature service.
When should I choose SS 347?
Critical welded high-temperature parts, aerospace and refinery piping.
Can SS 321 replace SS 347?
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.