254SMO, UNS S31254, W.Nr. 1.4547
Note that the designation “ 254SMO®” is registered trademarks of the Outokumpu.
254SMO is a super austenitic stainless steel with the specific levels of Cr, Ni, Mo, and N. 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. 254SMO is often referred to as a “6% Moly” grade due to the molybdenum content, the 6% Moly family has the ability to withstand high temperatures and maintain strength under volatile conditions. In some applications, 254SMO has been a more cost effective substitute for high nickel and titanium alloys.
| Material | Cr | Mo | S | Ni | Si | C | Mn | N |
|---|---|---|---|---|---|---|---|---|
| 254SMO | 19.5-20.0 | 6.00-6.50 | 1.01 max | 17.5-18.0 | 0.80 max | 0.02 max | 1.00 max | 0.18-0.20 max |
| Material | Tensile strength | Yield strength 0,2 | Elongation | Hardness HB30 |
|---|---|---|---|---|
| 254SMO | 650 N/mm² | 300 N/mm² | 35% | 2705HB |
254SMO offers excellent resistance to pitting and crevice corrosion due to its high chromium, molybdenum, and nitrogen content. The low carbon content, which means there is very little risk of carbide precipitation during heating. It also has high resistance to corrosion cracking and have excellent ductility and impact strength at both ambient and sub-zero temperatures. Also, 254SMO material possesses very good resistance in water containing chlorides, therefore, it is suitable for use in seawater as it can be exposed to it for prolonged periods without suffering from crevice corrosion.
Heat treatment of 254SMO stainless steel typically involves solution annealing, which includes heating the material to a temperature range of 1050-1150°C (1922-2102°F) to dissolve precipitates and promote a homogeneous microstructure. Rapid cooling, usually through water quenching, helps retain desired properties. Stress relieving may also be performed at a temperature of 550-650°C (1022-1202°F) to reduce residual stresses. Post-treatment passivation or surface treatment may be required.
Heating of 254SMO material is commonly performed for heat treatment purposes. The process involves heating the material to specific temperatures and holding it for a certain duration in controlled atmospheres, and air cooling or quenching, to prevent oxidation and achieve desired mechanical propertie, improve strength and hardness. Post-treatment processes may be necessary to meet the final product specifications.
Pickling is a common process used to remove surface impurities and oxide layers from 254SMO stainless steel. It involves immersing the material in a pickling solution, typically an acid-based solution, to dissolve contaminants and restore the material's corrosion resistance.
Hot Forming: 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 postprocess annealing.
Cold Forming: Cold working can be carried out using all the traditional methods; however, the process would be tough due to its high work rate. The result will provide the material with increased strength and toughness.
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 of 254SMO stainless steel requires filler material without which it results in poor strength properties. Filler metals such as AWS A5.14ERNiCrMo-3, and alloy 625 are recommende.
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