6% Molybdenum Super-Austenitic Stainless for Extreme Chloride Service
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UNS NUMBER
S31254
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ALLOY FAMILY
Super Austenitic (6Mo)
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STRUCTURE
100% Austenite
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UTS (MIN)
≥655 MPa
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MAX TEMP
~400°C
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DENSITY
8.00 g/cm³
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Stainless Steel 254SMO (UNS S31254, F44) is a 6% molybdenum super-austenitic stainless with copper and high nitrogen, reaching PREN 42–45 and yield strength near 310 MPa. It is the standard for extreme chloride service — seawater, desalination, FGD, and pulp bleaching — where 316L and 904L fall short.
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Cr
19.5–20.5%
Oxidation + pitting
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Ni
17.5–18.5%
Austenite; SCC resistance
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Mo
6–6.5%
Pitting / crevice (6Mo)
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N
0.18–0.22%
Strength + PREN
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UTS
≥655 MPa
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YS 0.2%
≥310 MPa
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Elong.
≥35%
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Service
-196~400°C
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What is Stainless Steel 254SMO?
254SMO (also known as 6Mo or F44) is a super-austenitic built around 6–6.5% molybdenum, ~20% chromium, 17.5–18.5% nickel, plus copper and 0.18–0.22% nitrogen. The 6Mo plus nitrogen pushes PREN to 42–45 and roughly doubles the yield strength of 316L, while copper broadens acid resistance. Carbon is ultra-low (≤0.020%), so it is immune to sensitization.
It is the material of choice for the harshest chloride environments: warm seawater, RO desalination, FGD scrubbers, and pulp-bleaching washers. Its cost sits between 904L and nickel alloys like C-276 — typically 40–50% of a C-276 equivalent. For most seawater and chloride-acid duties it is the most cost-effective high-performance choice; AL6XN is the near-equal alternative.
Chemical Composition (Weight %)
| Element |
Min (%) |
Max (%) |
Role in Alloy |
| Chromium (Cr) |
19.5 |
20.5 |
Oxidation + pitting resistance |
| Nickel (Ni) |
17.5 |
18.5 |
Austenite; SCC resistance |
| Molybdenum (Mo) |
6.0 |
6.5 |
Pitting / crevice (6Mo) |
| Nitrogen (N) |
0.18 |
0.22 |
Strength + PREN boost |
| Copper (Cu) |
0.5 |
1.0 |
Acid resistance |
| Carbon (C) |
— |
0.020 |
Ultra-low; no sensitization |
| Manganese (Mn) |
— |
1.00 |
Deoxidizer |
| Silicon (Si) |
— |
0.80 |
Deoxidizer |
| Phosphorus (P) |
— |
0.030 |
Impurity |
| Sulfur (S) |
— |
0.010 |
Impurity |
| Iron (Fe) |
Balance |
— |
Matrix former |
★ 6–6.5% molybdenum + 0.18–0.22% nitrogen + copper give PREN 42–45 — the defining 6Mo super-austenitic profile. Yield (~310 MPa) is about double 316L. Carbon ≤0.020% avoids sensitization.
Mechanical Properties
Values are for the solution-annealed condition per ASTM A240, A312 (TP254SMO), A182 (F44), A403 (WP254SMO), and A276/A479.
| Condition / Form |
UTS (MPa) |
YS 0.2% (MPa) |
Elong. % (min) |
Hardness (max) |
| Minimum (ASTM A240 / A312) |
≥655 |
≥310 |
35 |
223 HBW (100 HRB) |
| Typical mill-annealed |
690–820 |
320–420 |
40–50 |
95–100 HRB |
Modulus of Elasticity 195 GPa. ASME allowable stresses run up to 40% higher than 316L. Not hardenable; strength from alloy + nitrogen. Avoid exposure 600–900°C to prevent sigma-phase embrittlement.
Physical Properties
| Property |
Value |
Note |
| Density |
8.00 g/cm³ |
Above 316L |
| Melting Range |
1320–1390°C |
Solidus ~1320°C |
| Thermal Conductivity (100°C) |
~14 W/m·K |
Lower than 316 |
| Thermal Expansion (20–100°C) |
16.5 ×10⁻⁶ /K |
Similar to 316 |
| Specific Heat |
500 J/kg·K |
At 21°C |
| Electrical Resistivity |
0.85 µΩ·m |
At 21°C |
| Magnetic Property |
Non-magnetic |
Fully austenitic |
| PREN (Pitting Resistance) |
42–45 |
PREN = %Cr + 3.3×%Mo + 16×%N |
Corrosion Resistance
Seawater & Desalination
PREN 42–45 gives 254SMO practical immunity to chloride pitting and crevice corrosion in seawater, RO desalination, and ballast systems — the main reason to select it over 316L/904L.
Stress Corrosion Cracking
High nickel (17.5–18.5%) makes 254SMO highly resistant to chloride stress-corrosion cracking, far better than the 300-series austenitics.
FGD, Pulp & Chemical
Used in flue-gas-desulfurization, pulp-bleaching washers, and aggressive chemical streams where 316L/904L would fail. Sulfuric/phosphoric resistance is good but below specialized nickel alloys.
Weld Sensitization
Ultra-low carbon (≤0.020%) means 254SMO does not sensitize after welding; post-weld annealing is not required.
Stainless Steel 254SMO vs Similar Grades — Selection Guide
| If you need... |
Choose... |
Why |
| Extreme seawater / desalination |
254SMO |
PREN 42–45, SCC immune |
| Above 904L chloride need |
254SMO / AL6XN |
PREN 42–48 |
| General chloride, cheaper |
316L / 904L |
Lower cost |
| Max acid + chloride |
Nickel alloy (C276) |
Higher capability |
| Pulp bleach / FGD |
254SMO |
Acid + chloride |
Applicable Standards
| Standard |
Product Form |
| ASTM A240 |
Plate, sheet, strip |
| ASTM A312 TP254SMO |
Seamless & welded pipe |
| ASTM A182 F44 |
Forged flanges & fittings |
| ASTM A403 WP254SMO |
Butt-weld fittings |
| ASTM A276 / A479 |
Bar, rod, forging stock |
| EN 1.4547 / X1CrNiMoCuN20-18-7 |
European equiv. |
HT PIPE Supply — Stainless Steel 254SMO Product Forms
| Product Form |
Specifications |
HT PIPE Supply |
| Seamless Pipe |
ASTM A312 TP254SMO |
✅ Available |
| Welded Pipe |
ASTM A312 TP254SMO |
✅ Available |
| Forged Flanges |
ASTM A182 F44, ASME B16.5 |
✅ In stock |
| Butt-Weld Fittings |
ASTM A403 WP254SMO |
✅ Available |
| Plate / Sheet |
ASTM A240 |
✅ Available |
| Bar / Rod |
ASTM A276 / A479 |
✅ Available |
HT PIPE supplies 254SMO (UNS S31254 / F44, EN 1.4547) in pipe, flanges, fittings, plate, and bar for seawater, desalination, FGD, and pulp service, with full MTRs and matching 6Mo welding consumables.
Project Experience — Stainless Steel 254SMO
HT PIPE supplies 254SMO for seawater, reverse-osmosis desalination, FGD, and pulp-bleaching systems worldwide.
| Project |
Product |
Specification |
| SWRO Desalination Train (Middle East) |
254SMO Pipe + Fittings |
A312, A403 |
| Seawater Cooling System (Asia) |
254SMO Flanges + Plate |
A182 F44, A240 |
| Pulp Bleaching Washer (Europe) |
254SMO Plate + Forgings |
A240, A182 |
We advise 254SMO vs AL6XN and supply matching 625 (ERNiCrMo-3) or 254SMO filler.
| FREQUENTLY ASKED QUESTIONS |
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FAQ — Stainless Steel 254SMO
| What is 254SMO used for? |
| The harshest chloride environments: seawater, RO desalination, FGD scrubbers, and pulp-bleaching washers where 316L and 904L pit or crevice-corrode. PREN 42–45 and high SCC resistance are the drivers. |
| How does 254SMO compare to 904L? |
| 254SMO has 6% molybdenum (vs 4–5% in 904L) and nitrogen, raising PREN from 36–38 to 42–45 and roughly doubling yield strength (310 vs 220 MPa). It is the stronger, more chloride-resistant upgrade. |
| Does 254SMO need post-weld heat treatment? |
| No. Ultra-low carbon (≤0.020%) prevents sensitization; welded fabrications stay corrosion-resistant as-welded. |
| What filler is used for 254SMO? |
| ERNiCrMo-3 (Inconel 625 type) or a matching 6Mo wire to preserve the high molybdenum/nitrogen and PREN. Plain 316/904L filler would undermatch badly. |
| 254SMO or AL6XN? |
| Very similar (both 6Mo, PREN 42–48). 254SMO adds copper for some acid services; AL6XN has slightly higher nickel. Either outlasts 904L in aggressive chloride; choice is often form availability. |
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Need Stainless Steel 254SMO for Extreme Chloride Service?
HT PIPE supplies 254SMO (UNS S31254 / F44) pipe, flanges, fittings, plate, and bar for seawater, desalination, FGD, and pulp applications with full MTR documentation. Contact our technical team for material selection and welding advice.
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