ASTM A790 254SMO Stainless Steel Seamless Tubes S31254 Pipe Fittings 254 SMO Tubing
254 SMO (UNS S31254) is an austenitic stainless steel, compatible with the common austenitic stainless steels, designed with high levels of chromium, molybdenum, and nitrogen for maximum resistance to pitting and crevice corrosion. 254 SMO is substantially stronger than the common austenitic grades, but is also characterized by high ductility and impact strength. Often referred to as a “6 Moly” grade due to the molybdenum content, 254 SMO Tubing has the ability to withstand high temperatures and maintain strength under volatile conditions.
Commonly used as a replacement in critical components of larger constructions where Type 316L or 317L has failed by pitting or chloride stress corrosion cracking, 254 SMO Tubing withstands the same corrosive environments. In new construction, 254 SMO has been found in many cases to be a technically adequate and more cost efficient substitute for other nickel based alloys and titanium; the allowable design values may be about 50% higher than those for standard austenitic steels. This means that the possibility of designing thinner walls can save cost. It is often considered a “high end” austenitic stainless. 254 SMO Tubing is readily fabricated and welded.
254 SMO Tubing offers chloride resistance superior to that of other alloys like Alloy 904L, Alloy 20, Alloy 825 and Alloy G and is specially suited for high chloride environments such as brackish water or seawater. Applications for 254 SMO Tubing include petroleum production, saltwater handling, food processing and chemical processing equipment, pulp mill bleach systems, flue gas desulfurization scrubbers and tall oil distillation columns.
Introduction
Stainless steel grade 254 SMO™ is a very high end austenitic stainless steel. It is designed with a combination of impact toughness resistance to chloride stress corrosion cracking, and pitting and crevice corrosion with strength that is twice that of the stainless steel 300 series.
For certain applications, grade 254 SMO™ has been reported to be a cost-effective substitute for high nickel and titanium alloys. It is known to possess excellent workability as well.
The following sections will discuss in detail about stainless steel grade 254 SMO™.
Chemical Composition
The chemical composition of stainless steel grade 254 SMO™ is outlined in the following table.
| Element | Content (%) |
| Iron, Fe | 55.69 |
| Chromium, Cr | 20 |
| Nickel, Ni | 18 |
| Molybdenum, Mo | 6.1 |
| Nitrogen, N | 0.20 |
| Carbon, C | 0.010 |
Physical Properties
The physical properties of stainless steel grade 254 SMO™ are tabulated below.
| Properties | Metric | Imperial |
| Density | 8 g/cm3 | 0.289 lb/in3 |
Mechanical Properties
The following table shows mechanical properties of stainless steel grade 254 SMO™.
| Properties | Metric | Imperial |
| Tensile strength | 680 MPa | 98600 psi |
| Yield strength | 300 MPa | 43500 psi |
| Modulus of elasticity | 195 GPa | 28300 ksi |
| Shear modulus | 75 GPa | 10900 ksi |
| Poisson’s ratio | 0.30 | 0.30 |
| Elongation at break | 50 % | 50 % |
| Hardness, Brinell | 210 | 210 |
Thermal Properties
The thermal properties of stainless steel grade 254 SMO™ are tabulated below.
| Properties | Metric | Imperial |
| Thermal expansion co-efficient (@20-100°C/68-212°F) | 16.5 µm/m°C | 9.17 µin/in°F |
| Thermal conductivity | 14 W/mK | 97.2 BTU in/hr.ft².°F |
Other Designations
Other designations that are equivalent to stainless steel grade 254 SMO™ include the following.
| ASTM A813 | ASTM A469 | ASTM A269 | ASTM A240 | ASTM A182 (F44) |
ASTM A814
| ASTM A312
| ASTM A249
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Fabrication and Heat Treatment
Machinability
Stainless steel grade 254 SMO™ is quite tough to machine due to the extremely high work hardening rate and lack of sulfur content; however using sharp tools, overpowered machine tools, positive feeds, good amount of lubrication, and slow speeds tend to provide good machining results.
Welding
Welding of stainless steel grade 254 SMO™ requires filler material without which it results in poor strength properties. Filler metals such as AWS A5.14 ERNiCrMo-3, and alloy 625 are recommended. Electrodes used in the process, have to match with AWS A5.11 ENiCrMo-12.
Annealing
Annealing of this material should be performed at 1149-1204°C (2100-2200°F), which should be followed by a water quench.
Hot Working
Forging, upsetting and other operations relating to this material can be performed at 982 - 1149°C (1800 - 2100°F). It is recommended that temperatures do not exceed this range as it would result in scaling and reduction in the workability of the material. To re-attain maximum corrosion resistant properties, it is advisable to perform post-process annealing.
Cold Working
Cold working can be carried out using all the traditional methods; however the process would be tough due to its high work hardening rate. The result will provide the material with increased strength and toughness.
Hardening
Stainless steel grade 254 SMO™ does not respond to heat treatment. Hardening is possible only through cold reduction.
Applications
Stainless steel grade 254 SMO™ is used in the following areas:
- Saltwater handling
- Tall oil distillation columns
- Flue gas desulfurization scrubbers
- Components used in petroleum production
- Food processing equipment
- Process equipment in chemical industry
- Bleaching equipment in the pulp and paper industry
- Flue-gas cleaning
- Desalination
- Heat exchangers
