Is 318LN a type duplex stainless steel grades?
318LN is Nitrogen-enhanced stainless steel commonly used to address corrosion failures in 300 series stainless steel. The structure of 318LN stainless steel is composed of Austenite surrounded by continuous Ferrite phases. 318LN contains about 40-50% Ferrite in the annealed state and can be considered duplex stainless steel. The duplex structure combines ferrite alloys (stress corrosion cracking resistance and high strength) with the superior qualities of Austenitic alloys (ease of manufacture and corrosion resistance). The 318LN is resistant to H2S uniform corrosion, sulfide stress cracking, hydrogen embrittability and pitting, and reducing media corrosion. It is commonly used to manufacture sulfur-resistant wellheads, valves, stems, and fasteners for use in mining environments where H2S partial pressures exceed 1MPa. However, the use of 318LN duplex stainless steel should be limited to less than 600°F because prolonged high temperatures can brittle the 318LN stainless steel.
The chemical composition of 318LN steel
| Cr | Ni | Mo | C | N | Mn | Si | P | S |
| 22.0-23.0 | 4.50-6.50 | 3.00-3.50 | ≤0.030 | 0.14-0.20 | ≤2.00 | ≤1.00 | ≤0.030 | ≤0.020 |
| Mechanical Property | |||
| Ys (Mpa) | Ts (Mpa) | Elongation (%) | Hv |
| Standards | ≥ 450 | ≥ 620 | ≥ 18 |
| Physical property | |||
| Density (g/cm) | Specific heat(J/g.C) | Thermal conductivity
100C(W/m.) |
The coefficient of thermal expansion
20~100C (10/C) |
| 7.8 | 0.45 | 19.0 | 13.7 |
Features of 318LNsteel
- Excellent resistance to sulfide stress corrosion
- Good resistance to chloride stress corrosion cracking, pitting and crevice corrosion
- High strength,
- Good weldability and workability
Applications of 318LNsteel
- Chemical treatment containers, pipes and heat exchangers
- Pulp mill digesters, bleach cleaners, chip presteam containers
- Food processing equipment
- Petrochemical pipelines and heat exchangers
- Flue gas desulfurization equipment
318LN duplex stainless steel is an economical and effective solution for applications where 300 series stainless steel are susceptible to chloride stress corrosion cracking. When stainless steel is subjected to tensile stress, stress corrosion cracking will occur in contact with a solution containing chloride, and rising temperature will also increase the sensitivity of stainless steel to stress corrosion cracking. The combination of chromium, molybdenum, and nitrogen enhances the 318LN’s resistance to chloride pitting and crevice corrosion, which is critical for services such as marine environments, brackish water, bleaching operations, closed-loop water systems, and some food processing applications. In most environments, 318LN’s high chromium, molybdenum and nitrogen content provides superior corrosion resistance to ordinary stainless steels such as 316L and 317L.
