Corrosion Resistant Alloys
NAS 155N (UNS S31727) - Super Austenitic Stainless Steel
With excellent resistance to sulfuric acid dew point corrosion, NAS 155N (UNS S31727) is a stainless steel custom-made for the heat exchangers, flues and chimneys of heavy oil-fired boilers. This alloy is suitable in a wide range of material applications for anti-pollution equipment and other devices exposed to a sulfuric acid environment. Nippon Yakin supplies this product in plate, sheet and strip forms.
Coil
Sheet
Plate
Chemical Composition
UNS S31727
| C | Si | Mn | P | S | Ni | Cr | Mo | Cu | N | |
|---|---|---|---|---|---|---|---|---|---|---|
| Min | ― | ― | ― | ― | ― | 14.5 | 17.5 | 3.8 | 2.8 | 0.15 |
| Max | 0.030 | 1.00 | 1.00 | 0.030 | 0.030 | 16.5 | 19.0 | 4.5 | 4.0 | 0.21 |
Physical Properties
| Specific heat (J/kg･K) | 25℃ | 460 |
| Specific resistivity (μΩ･cm) | 86.0 | |
| Thermal conductivity (W/m･K) | 25℃ | 21.8 |
| Coefficient of thermal expansion (10-6/℃) | 30–200℃ | 16.4 |
| 30–300℃ | 17.0 | |
| 30–400℃ | 17.4 | |
| Young's modulus (MPa) | 20.3 x 104 | |
| Ferromagnetism | None | |
| Melting point (℃) | 1364-1424 | |
Mechanical Properties
Mechanical Properties at Room Temperature
UNS S31727
| 0.2% proof stress (N/mm2) |
Tensile strength (N/mm2) |
Elongation (%) |
Hardness (HB) |
|---|---|---|---|
| ≥ 245 | ≥ 550 | ≥ 35 | ≤ 217 |
Examples
| 0.2% proof stress (N/mm2) |
Tensile strength (N/mm2) |
Elongation (%) |
Hardness (HB) |
|
|---|---|---|---|---|
| Cold-rolled sheet, 2-mm thick |
322 | 690 | 41 | 167 |
| Hot-rolled plate sheet, 4-mm thick |
315 | 681 | 45 | 162 |
| Hot-rolled plate sheet, 10-mm thick |
341 | 662 | 49 | 178 |
High-Temperature Strength
Corrosion Resistance
NAS 155N provides excellent resistance to sulfuric acid dew point corrosion approximately equivalent to Carpenter 20CbR®.
Sulfuric Acid Dew Point Corrosion Resistance
Sulfuric acid corrosion resistance – comparison to other stainless steels
Pitting Corrosion Resistance
Pitting corrosion resistance (V'c10 vs. SCE) in 3.5% NaCl during argon degassing
| Material/Dimensions | Temperature | |
|---|---|---|
| 30℃ | 50℃ | |
| NAS 155N, Cold-rolled sheet 3.0-mm thick | > 0.90 | > 0.53 |
| NAS 64, Cold-rolled sheet 1.6-mm thick | > 0.90 | 0.85 |
| Carpenter 20Cb equivalent, Cold-rolled sheet 2.0-mm thick | 0.30 | — |
Stress Corrosion Cracking Resistance
| Material | Test conditions: Boiling MgCl2 solution, 100h | |||||
|---|---|---|---|---|---|---|
| 30% (117℃) | 35% (126℃) | 42% (142℃) | ||||
| NAS 155N | ○ | ○ | × | |||
| SUS 316L | × | × | × | |||
| NAS 64 | × | × | × | |||
(U-bending test)
○: No cracks
×: Cracks present
Workability
NAS 155N can be worked as any ordinary austenitic stainless steel, though the deformation resistance of this product is slightly higher when working at room temperature.
Weldability
The weldability is similar to any ordinary austenitic stainless steel. Preheating and post-heating are not necessary, and we recommend using Carpenter 20Cb-3® or Hastelloy® C-type welding rods.
Heat Treatment
As NAS 155N is an austenitic stainless steel, the same sort of heat treatment may be used. We suggest water-cooled solution treatments in a range between 1125 and 1175℃.
Pickling
A mixture of nitric and hydrofluoric acids is used in pickling. To the extent that NAS 155N 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
NAS 155N is excellent for use in the chimneys and dampers of heavy oil boilers, exhaust gas heat exchangers and scrubbers, dilute sulfuric acid tanks and carbon black driers as well as any equipment that is routinely subjected to a sulfuric acid dew point.
* Hastelloy is a registered trademark of Haynes International, Inc.
