347 Stainless Steel (UNS S34700)
18-8 stabilized with niobium for welded and high-temperature service — the refinery and power-plant stainless for hydrogen, hydrocarbon and steam hardware.
What 347 Stainless is.
Type 347 is 304 stabilized with niobium (columbium) at ten times the carbon content. Niobium carbides form in preference to chromium carbides, so the alloy resists sensitization during welding and in service between 800 and 1500 °F, and it holds its creep strength a little better than 321 at the top of that range. It is the stabilized grade most often found in refinery, petrochemical and power piping specifications.
We machine 347 fasteners from certified A276 bar to ASTM F593 Alloy Group 3 conditions, and as ASTM A193 B8C or A320 B8C for pressure bolting.
Chemical composition.
| Element | Symbol | Min % | Max % |
|---|---|---|---|
| Carbon | C | — | 0.08 |
| Manganese | Mn | — | 2.00 |
| Phosphorus | P | — | 0.045 |
| Sulfur | S | — | 0.030 |
| Silicon | Si | — | 1.00 |
| Chromium | Cr | 17.0 | 19.0 |
| Nickel | Ni | 9.0 | 12.0 |
| Niobium (columbium) | Nb | 10 × C | 1.10 |
Values per ASTM A276/A276M-17 Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team.
Mechanical properties.
| Condition and nominal diameter (in.) | Tensile strength, min (ksi) | Yield strength 0.2% offset, min (ksi) | Elongation, min (%) | Reduction of area, min (%) | Hardness |
|---|---|---|---|---|---|
| Condition A (annealed), hot finished — all | 75 | 30 | 40 | 50 | — |
| Condition A, cold finished — ½ and under | 90 | 45 | 30 | 40 | — |
| Condition A, cold finished — over ½ | 75 | 30 | 30 | 40 | — |
Values per ASTM A276/A276M-17 Tables 1 and 2 (verified against the specification). Confirm the edition called out on your purchase order with our estimating team. A276 lists 347 in Condition A only; cold-worked fastener strengths are per F593.
| F593 condition · marking | Nominal diameter (in.) | Tensile strength (ksi) | Yield strength, min (ksi) | Rockwell hardness |
|---|---|---|---|---|
| AF (annealed after forming) · F593J | ¼ to 1½ | 65 to 85 | 20 | B85 max |
| A (annealed) · F593K | ¼ to 1½ | 75 to 100 | 30 | B65 to B95 |
| CW1 (cold worked) · F593L | ¼ to ⅝ | 100 to 150 | 65 | B95 to C32 |
| CW2 (cold worked) · F593M | ¾ to 1½ | 85 to 140 | 45 | B80 to C32 |
| SH1 (strain hardened) · F593J | ¼ to ⅝ | 120 to 160 | 95 | C24 to C36 |
| SH2 (strain hardened) · F593K | ¾ to 1 | 110 to 150 | 75 | C20 to C32 |
| SH3 (strain hardened) · F593L | 1⅛ to 1¼ | 100 to 140 | 60 | B95 to C30 |
| SH4 (strain hardened) · F593M | 1⅜ to 1½ | 95 to 130 | 45 | B90 to C28 |
Values per ASTM F593-24 Table 2, full-size tests (verified against the specification). Yield is 0.2 % offset; hardness is Rockwell. Machined-specimen limits differ slightly.
Specifications that apply.
How 347 Stainless ships from Houston.
- Hex and heavy hex bolts and cap screws
- Full-thread studs and threaded rod
- Socket head cap and set screws
- Hex, jam and lock nuts
- Washers and spacers
- Machined components from certified bar
347 Stainless fasteners we make.
Every item is manufactured to your drawing or standard in 347 Stainless, imperial or metric, with an EN 10204 3.1 certificate.
What is 347 stainless steel?
347 is the niobium-stabilized 18-8. Niobium forms stable carbides that keep chromium in solution, so the alloy can be welded without a post-weld anneal and can run for years in the 800 to 1500 °F range without intergranular attack. Compared with 321 it stabilizes more completely, loses less stabilizer in the weld arc and has marginally better creep strength, which is why refinery and power specifications tend to prefer it.
Where 347 is specified
- Piping, exchangers and vessels in hydrogen and hydrocarbon service at temperature.
- Any welded stainless equipment where a post-weld anneal is impossible.
- Fasteners on hot flanges where 304 would sensitize over the life of the joint.
- As A193 B8C studs with A194 8C nuts on the pressure boundary.
Testing, certification and marking
- Tensile and hardness tested per lot on the finished product
- Grade or marking symbol F593J to F593M and manufacturer's identification on every piece
- EN 10204 3.1 material test report with heat number and chemistry; 3.2 witness available
- PMI verification and intergranular corrosion testing (ASTM A262) arranged on request
321 Stainless Steel (UNS S32100)
18-8 stabilized with titanium so it can be welded and run hot without sensitizing — the stainless for exhaust, expansion joints and furnace hardware.
ASTM A276 bar · ASTM F593 bolts304 Stainless Steel (UNS S30400)
The general-purpose 18-8 stainless: 75 ksi annealed bar, cold-worked bolts to 160 ksi, non-magnetic, and resistant to water, food and most mild chemicals.
ASTM A193 / ASME SA-193ASTM A193 Grade B8C Class 1 (347 Stainless)
347 stainless bolting stabilised with niobium against sensitisation — for hot hydrogen, refinery and welded stainless service between 800 and 1500 °F.