What Is 904L Stainless Steel?
When to Specify 904L
904L (UNS N08904) is a high-alloy austenitic stainless steel with 4.5% molybdenum and copper additions for resistance to sulfuric acid, phosphoric acid, and moderate chloride environments. It occupies a cost-performance position between standard 316L and Alloy 20, offering better acid resistance than 316L at a lower nickel content (and cost) than Alloy 20.
| Property | Value |
|---|---|
| UNS designation | N08904 |
| EN designation | 1.4539 (X1NiCrMoCu25-20-5) |
| Common names | 904L, Alloy 904L |
| Chromium | 19.0-23.0% |
| Nickel | 23.0-28.0% |
| Molybdenum | 4.0-5.0% |
| Copper | 1.0-2.0% |
| Carbon | 0.020% max |
| Manganese | 2.00% max |
| Silicon | 1.00% max |
| Tensile strength (min) | 490 MPa (71 ksi) |
| Yield strength (min) | 220 MPa (32 ksi) |
| Elongation (min) | 35% |
| PREN | ~34-36 |
| Density | 7.95 g/cm3 |
| Heat treatment | Solution anneal 1090-1175°C, water quench |
| Max service temp | 400°C (752°F); continuous |
904L vs Related Alloys
| Property | 316L | 904L | Alloy 20 | 6Mo (254 SMO) |
|---|---|---|---|---|
| Ni | 10-14% | 23-28% | 32-38% | 17.5-18.5% |
| Mo | 2-3% | 4-5% | 2-3% | 6-6.5% |
| Cu | - | 1-2% | 3-4% | 0.5-1% |
| PREN | ~26 | ~35 | ~28 | ~43 |
| H2SO4 (40%, 80°C) | Fails | Good | Excellent | Moderate |
| Seawater pitting | Vulnerable | Moderate | Moderate | Resistant |
| Relative cost | 1x | 2x | 2.5x | 2.5x |
904L is preferred over Alloy 20 when sulfuric acid concentrations are below 40% and chloride pitting resistance is more important than hot sulfuric acid resistance. For seawater and aggressive chloride service, 6Mo grades (PREN >40) outperform 904L.
Applications
- Sulfuric acid plants: dilute to intermediate concentration acid piping and equipment
- Phosphoric acid plants: evaporator circuits and wet-process phosphoric acid handling
- Chemical processing: organic acid handling, bleaching circuits
- Petrochemical: hydrocarbon streams contaminated with sulfuric or phosphoric acid
- Cooling water systems: mildly chlorinated water at moderate temperatures
- Pharmaceutical: process piping requiring superior acid resistance over 316L
ASTM Specifications
| Product Form | ASTM Specification |
|---|---|
| Seamless pipe | A312 (UNS N08904) |
| Welded pipe | A358 (UNS N08904) |
| Forgings | A182 (UNS N08904) |
| Plate | A240 (UNS N08904) |
Testing
Solution annealing (1090-1175°C, water quench) is mandatory. Mill test certificates per EN 10204 3.1 must document full chemistry (Mo and Cu are critical), mechanical properties, and heat treatment condition. Intergranular corrosion testing per ASTM A262 Practice E is commonly required. NDT and hydrostatic testing follow the applicable piping standard. The low carbon content (0.020% max) resists sensitization during welding, similar to “L” grade stainless steels.
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