Ultra 654 SMO
EN 1.4652, ASTM UNS S32654

General characteristics

7% Mo, very high N alloyed austenitic grade. The most corrosion resistant stainless steel in the world. High mechanical strength. Useful for e.g. pressurized and erosive systems handling chlorinated sea water at higher temperatures, plate heat exchangers and flue gas cleaning applications.

Typical applications

  • Pressurized and erosive systems handling chlorinated sea water at higher temperatures
  • Plate heat exchangers
  • Flue gas cleaning
  • Process equipment in chemical industry
  • Bleaching equipment in the pulp and paper industry
  • Desalination
  • Hydrometallurgy
  • Food and beverage
  • Pharmaceuticals


Product forms, available sizes and finishes
Chemical composition

The typical Outokumpu composition as well as the demands in some standards are given below

The chemical composition is given as % by weight.

TypicalUltra 654 SMO0.013.524.
ASTM A240/A240MUNS S32654<0.0202-4.024.0-25.021.0-23.07.0-8.00.45-0.55Si:<0.50 P:<0.030 S:<0.005 Cu:0.30-0.60
EN 10088-21.4652<0.0202-4.023.0-25.021.0-23.07.0-8.00.45-0.55Si:<0.50 P:<0.030 S:<0.005 Cu:0.30-0.60
Mechanical properties

The strength and elongation of 904L are similar to those for conventional austenitic stainless steels. The addition of nitrogen in 254 SMO®, 654 SMO® and 4565 gives higher proof strength and tensile strength.Despite the greater strength of these steels, the possibilities
for cold as well as hot forming are very good.


StandardGradeRp0.2Rp1.0RmElongationImpact strengthRockwellHBHV
Product type: Hot rolled quarto plate
Typical (thickness 15 mm)Ultra 654 SMO46049086060200

1)Elongation according to EN standard:
A80 for thickness below 3 mm.
A for thickness = 3 mm.
Elongation according to ASTM standard A2” or A50.

Corrosion resistance

Uniform corrosion
The high content of alloying elements gives the steels 904L, 254 SMO® and 4565 exceptionally good resistance to uniform
904L was originally developed to withstand environments involving dilute sulphuric acid and it is one of the
few stainless steels that at temperatures of up to 35°C provides full resistance in such environments within the entire
range of concentration, from 0 to 100%. 904L also offers good resistance to a number of other inorganic acids,
e.g., phosphoric acid, as well as most organic acids. Acids and acid solutions containing halide ions can, however,
be very aggressive and the corrosion resistance of 904L may be insufficient. Examples of such acids are hydrochloric
acid, hydrofluoric acid, chloride contaminated sulphuric acid, phosphoric acid produced according to the wet process (WPA) at elevated temperatures, and also pickling acid based on nitric acid and hydrofluoric acid mixtures. In these cases
254 SMO® and 4565 are preferable and in certain cases they can be an alternative to other considerably more expensive
Pitting and Crevice corrosion Resistance to pitting corrosion (and also crevice corrosion) is determined mainly by the content of chromium, molybdenum and nitrogen in the material. This is often illustrated using the pitting resistance equivalent (PRE) for the material, which can be calculated usingusing the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N. The PRE value can be used for rough comparisons of different materials.
A much more reliable means, however, is to rank the steel according to the critical pitting temperature of the material (CPT). There are several different methods available to measure the CPT and ASTM G 150  is one method that uses the Avesta Cell with a 1M NaCl solution (35,000 ppm or mg/l chloride ions). CPT-values are shown in the table below. Grades 4565 and 654 SMO® have such a good resistance to pitting that common test methods are not sufficiently ggressive to initiate any corrosion. A better measure of resistance is given by evaluating the results of various crevice corrosion tests.
In narrow crevices the passive film may more easily be damaged and in unfavourable circumstances stainless steel can be subjected to crevice corrosion. Examples of such narrow crevices may be under gaskets in flange fittings, under seals in certain types of plate heat exchangers, or under hard adherent deposits. Crevice corrosion occurs in the same nvironments as pitting. Higher contents of chromium, molybdenum or nitrogen enhance the corrosion resistance of the steel. Typical critical crevice corrosion temperature (CCT) according to ASTM G48 Method F are shown in the table below. (Test surfaces dry ground to 120 mesh.) CCT varies with product form and surface finish.The actual value of mill finish surface may differ between product forms. 

Stress Corrosion Cracking
Conventional stainless steels such as 4307 and 4404 are sensitive to stress corrosion cracking (SCC) under certain onditions, i.e. a special environment in combination with tensile stress in the material and often also an elevated temperature. Resistance to SCC increases with the increased content of above all nickel and molybdenum. This implies that the high performance austenitic steels 904L, 254 SMO®, 654 SMO®and 4565 have very good resistance to SCC.

Pitting corrosion resistanceCrevice corrosion resistance

PRE Pitting Resistant Equivalent calculated using the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N
CPT Corrosion Pitting Temperature as measured in the Avesta Cell (ASTM G 150), in a 1M NaCl solution (35,000 ppm or mg/l chloride ions).
CCT Critical Crevice Corrosion Temperature is the critical crevice corrosion temperature which is obtained by laboratory tests according to ASTM G 48 Method F



Physical properties

The typical values of some physical properties are given in the table below.


DensityModulus of elasticityThermal exp. at 100 °CThermal conductivityThermal capacityElectrical resistanceMagnetizable

*) Austenitic stainless steel grades may be magnetizable to a certain degree after cold deformation, e.g. in temper rolled condition.


The high performance austenitic stainless steels cold-harden considerably faster than conventional austenitic grades. This, together with the initial high strength, makes it necessary to apply high forming forces. The spring back for grades 254 SMO® and 4565 is also greater than for conventional austenitic steels.In complicated cold-forming operations, intermediate nnealing of the material may sometimes be necessary, especially if the workpiece is welded. 654 SMO® has a better performance than the other superaustenitic grades.
Austenitic stainless steels work harden quickly and this, together with their toughness, means that they are often perceived as problematic from a machining perspective, e.g. n operations such as turning, milling and drilling. This applies to an even greater extent to most highly alloyed steels and especially those that have a high nitrogen content, i.e. 254 SMO®, 4565 and 654 SMO®. However, with the right choice of tools, tool settings and cutting speeds, these materials can be sucessfully machined. For further information contact Outokumpu.
654 SMO® should be welded with over-alloyed Ni-base filler to overcome molybdenum segregation. If autogenous welding is performed and no post weld heat treatment is used, the weld will have a reduced corrosion resistance. High-energy methods (>1.5kJ/mm) such as submerged arc welding should be used with some care. MAG welding may require modern pulse equipment and the use of special shielding gases containing AR, He and O2/CO2.


Standards & approvals

Some standards are given in the table below.


ASTM A240/A240MUNS S32654
EN 10088-21.4652