254SMO (UNS S31254) is a super austenitic stainless steel with a high molybdenum and nitrogen content, designed to provide exceptional corrosion resistance in aggressive environments, particularly those containing chlorides. With a Pitting Resistance Equivalent (PRE) number of approximately 43, 254SMO significantly outperforms standard austenitic grades like 316L (PRE ~25) and even duplex grades like 2205 (PRE ~35) in pitting and crevice corrosion resistance. Its high nickel and molybdenum content also provides excellent resistance to stress corrosion cracking (SCC) in chloride-bearing environments.
1. 254SMO Basic Information
254SMO is a fully austenitic stainless steel with a face-centered cubic (FCC) structure, which gives it excellent toughness and ductility even at cryogenic temperatures. The alloy is non-magnetic in the annealed condition. Its high nitrogen content (up to 0.22%) enhances both pitting resistance and mechanical strength through solid-solution strengthening. 254SMO is typically supplied in the solution-annealed condition (1150-1200°C) followed by rapid quenching to prevent the precipitation of intermetallic phases such as sigma and chi phases.
2. Chemical Composition
The composition of 254SMO is specifically formulated to maximize resistance to localized corrosion. The high molybdenum (6.0-6.5%) and nitrogen (0.18-0.22%) content are the primary contributors to its high PRE value (PRE = %Cr + 3.3%Mo + 16%N).
| Element | Composition Range (wt%) |
|---|---|
| Carbon (C) | ≤ 0.020 |
| Silicon (Si) | ≤ 0.80 |
| Manganese (Mn) | ≤ 1.00 |
| Phosphorus (P) | ≤ 0.030 |
| Sulfur (S) | ≤ 0.010 |
| Chromium (Cr) | 19.50 – 20.50 |
| Nickel (Ni) | 17.50 – 18.50 |
| Molybdenum (Mo) | 6.00 – 6.50 |
| Nitrogen (N) | 0.18 – 0.22 |
| Copper (Cu) | 0.50 – 1.00 |
| Iron (Fe) | Balance |
3. Mechanical Properties
254SMO offers higher strength than conventional austenitic stainless steels due to its nitrogen content. The mechanical properties are specified per ASTM A240 (plate) and ASTM A276 (bar).
| Property | Minimum Value (Annealed) | Typical Value |
|---|---|---|
| Tensile Strength (MPa) | 650 | 700 – 800 |
| Yield Strength 0.2% (MPa) | 300 | 330 – 380 |
| Elongation (%) | 35 | 40 – 50 |
| Hardness (HB) | ≤ 220 | 180 – 210 |
| Impact Toughness (J, -196°C) | ≥ 60 | 80 – 120 |
4. Corrosion Resistance
254SMO performs excellently in the following environments, making it the material of choice for the most demanding corrosive conditions:
- Seawater and Brackish Water: Excellent resistance to pitting and crevice corrosion in natural and chlorinated seawater. Critical pitting temperature (CPT) in 1M NaCl is > 85°C, compared to < 30°C for 316L.
- Chloride Environments: Resistant to stress corrosion cracking (SCC) in chloride solutions up to 120°C, unlike 304 and 316 which are susceptible.
- Acidic Environments: Good resistance to sulfuric acid (H₂SO₄) up to 50% concentration at 60°C, and to phosphoric acid (H₃PO₄) containing chlorides.
- Alkaline Environments: Suitable for sodium hydroxide (NaOH) solutions up to 50% concentration at moderate temperatures.
- Oxidizing Media: Excellent resistance to nitric acid (HNO₃) and other oxidizing acids due to high chromium content.
5. Typical Applications
Due to its exceptional corrosion resistance, 254SMO is used in industries where downtime due to corrosion failure is unacceptable.
- Chemical Processing: Heat exchangers, reactors, and piping systems handling chlorides, sulfuric acid, and organic acids.
- Oil & Gas: Offshore platform equipment, seawater injection systems, and subsea control lines.
- Pulp & Paper: Bleach plant washers, towers, and piping in chlorine dioxide (ClO₂) environments.
- Desalination: Reverse osmosis (RO) plant components, evaporators, and brine heaters.
- Marine Engineering: Propeller shafts, pump impellers, and seawater cooling systems.
- CNC Machining Parts: Precision-machined valve bodies, pump housings, and fittings for corrosive service.
6. CNC Machining Performance
254SMO is classified as a difficult-to-machine material due to its high work-hardening rate, low thermal conductivity (approximately 14 W/m·K), and high toughness. The alloy tends to form long, stringy chips and can cause built-up edge (BUE) if cutting parameters are not optimized.
| Operation | Cutting Speed (m/min) | Feed (mm/rev) | Depth of Cut (mm) | Tool Material |
|---|---|---|---|---|
| Turning | 80 – 120 | 0.15 – 0.30 | 1.0 – 3.0 | Carbide (M15/M25, TiAlN coated) |
| Milling | 60 – 100 | 0.08 – 0.15 (per tooth) | 0.5 – 2.0 | Carbide (AlTiN coated, variable helix) |
| Drilling | 20 – 40 | 0.05 – 0.12 | N/A | HSS-Co (M42) or Carbide (TiAlN coated) |
| Tapping | 5 – 10 | N/A (pitch) | N/A | HSS-Co (TiCN coated, spiral flute) |
Key Machining Tips:
- Use high-pressure coolant (minimum 50 bar) to break chips and reduce heat at the cutting edge.
- Maintain a constant feed rate; avoid dwell or light cuts that cause work hardening.
- Use chip breakers on inserts to manage long, stringy chips.
- For tapping, use roll-form taps in pre-drilled holes (65% thread depth) to avoid chip packing.
- Surface finish can achieve Ra ≤ 0.8 µm with proper finishing passes and sharp tools.
7. Procurement Guide
Market Price Reference (as of 2025):
- Sheet/Plate (2-10 mm): 45-65 RMB/kg
- Bar (Round, 10-100 mm): 55-80 RMB/kg
- Tube (Seamless): 70-100 RMB/kg
Quality Identification:
- Check the Mill Test Certificate (MTC) per EN 10204 3.1 or ASTM A240.
- Perform spectrographic analysis to confirm composition, especially Mo and N content.
- Perform a ferrite test (should be < 0.5% ferrite in the annealed condition).
- Surface inspection for defects such as cracks, laps, or slag inclusions per ASTM A480.
8. Our Services
Dongguan Stirling Metal Products Co., Ltd. specializes in providing 254SMO material + CNC Machining One-Stop Service:
- ✅ Material Procurement: Genuine product guarantee, complete MTC with full traceability
- ✅ CNC Machining: Accuracy ±0.01mm, surface finish down to Ra 0.2 µm
- ✅ Sample Making: Fast delivery in 3-5 days for material and process validation
- ✅ Batch Production: Delivery in 7-15 days with ISO 9001 quality control and NDT reports
- ✅ Surface Treatment: Electropolishing, passivation, and pickling for enhanced corrosion resistance
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