Corrosion Resistant Alloys
NAS 255NM (UNS N08926) - Super Austenitic Stainless Steel
With its high chromium and high molybdenum content, NAS 255NM (equivalent to UNS N08926) provides strong protection against corrosion. Durable even in harsh environments such as high-temperature seawater and flue gas desulfurization plants, this is an economical alloy as powerful as Hastelloy® alloys and pure titanium in many environments. Nippon Yakin supplies this product in plate, sheet and strip forms.
Coil
Sheet
Plate
Chemical Composition
UNS N08926
| C | Si | Mn | P | S | Ni | Cr | Mo | Cu | N | |
|---|---|---|---|---|---|---|---|---|---|---|
| Min | ― | ― | ― | ― | ― | 24.0 | 19.0 | 6.0 | 0.5 | 0.15 |
| Max | 0.020 | 0.50 | 2.00 | 0.030 | 0.010 | 26.0 | 21.0 | 7.0 | 1.5 | 0.25 |
Physical Properties
| Specific heat (J/kg･K) | 415 | |
| Specific resistivity (μΩ･cm) | 96 | |
| Thermal conductivity (W/m･K) | 12.0 | |
| Coefficient of thermal expansion (10-6/℃) | 20–200℃ | 16.6 |
| 30–300℃ | 16.6 | |
| 30–400℃ | 16.6 | |
| Young's modulus (MPa) | 19.3 x 104 | |
| Ferromagnetism | None | |
| Melting point (℃) | 1320-1390 | |
Mechanical Properties
Mechanical Properties at Room Temperature
UNS N08926
| 0.2% proof stress (N/mm2) |
Tensile strength (N/mm2) |
Elongation (%) |
Hardness (HRB) |
|---|---|---|---|
| ≥ 295 | ≥ 650 | ≥ 35 | ― |
Examples
| 0.2% proof stress (N/mm2) |
Tensile strength (N/mm2) |
Elongation (%) |
Hardness (HRB) |
|
|---|---|---|---|---|
| Cold-rolled sheet, 3-mm thick |
356 | 731 | 48 | 84 |
Corrosion Resistance
Because of the high concentrations of chromium, molybdenum and nitrogen, NAS 255NM exhibits superior resistance to pitting and crevice corrosion in chloride environments. NAS 255NM shows powerful resistance even in such harsh corrosion environments that conventional duplex stainless steels cannot handle.
Pitting and Crevice Corrosion Resistance
Workability
Although NAS 255NM can be hot- and cold-worked much like standard austenitic stainless steels such as SUS 304 and SUS 316, the high strength of the alloy must be kept in mind when doing so.
Weldability
NAS 255NM can be welded using such techniques as shield metal arc welding, TIG welding and plasma welding as with any standard austenitic stainless steel. We recommend using Hastelloy® C-type welding consumables with this product. Preheating and post-heating are not required.
Machinability
As with any stainless steel endowed with a high nickel content, machining is more challenging than standard austenitic stainless steels, though NAS 255NM is more easily handled than nickel-based alloys. We recommend the use of an cemented carbide tool, and suggest combining a slower feed rate with a greater cutting depth.
Heat Treatment
As NAS 255NM is an austenitic stainless steel, the same sort of heat treatment may be used. The typical heat treatment is:
- Solution treatment 1125–1175℃ Water cooling
Pickling
A mixture of nitric and hydrofluoric acids is used in pickling. To the extent that NAS 255NM is more resistant to corrosion than SUS 304, descaling is more difficult. Alkaline immersion for a short time before acid pickling and shot blasting if possible are effective at descaling.
Applications
- Seawater environments: desalination systems, seawater heat exchangers, condenser tubes
- High-chlorine environments: flue gas desulfurization equipment, pulp and paper mills, bleaching equipment
- Environments containing concentrated sodium chloride: plastic manufacturing equipment, chemical reactor vessels and tubing
* Hastelloy is a registered trademark of Haynes International, Inc.
