Ultra 904L
EN 1.4539, ASTM TYPE 904L / UNS N08904

General characteristics

High Ni and Mo austenitic grade with very high resistance to corrosion. Commonly used in chemical and petrochemical industry for handling medium concentrated sulphuric acid.

Typical applications

  • Process equipment in chemical industry
  • Petrochemical industry
  • Bleaching equipment in the pulp and paper industry
  • Flue gas cleaning
  • Desalination
  • Seawater handling
  • Hydrometallurgy
  • Food and beverage
  • Pharmaceuticals
  • Heat exchangers


Product forms, available sizes and finishes


Product typeFinishesThicknessWidth
Cold rolled coil and sheet2E, Bright annealed0,40-7,00≤ 2000
Hot rolled coil and plateHot rolled white4,00-10,00≤ 2000
Quarto plateHot rolled white6,00-50,00≤ 3000
Chemical composition

The typical chemical composition for this grade is given in the table below, together with composition limits given for this grade according to different standards. The required standard will be fully met as specified on the order.

The chemical composition is given as % by weight.

TypicalUltra 904L0.0119.824.24.3Cu:1.4
ASTM A240/A240MTYPE 904L / UNS N08904<0.020<219.0-23.023.0-28.04.0-5.0<0.10Si:<1 P:<0.045 S:<0.035 Cu:1-2
EN 10088-21.4539<0.020<219.0-21.024.0-26.04.0-5.00.040-0.15Si:<0.70 P:<0.030 S:<0.010 Cu:1.20-2
Mechanical properties

The mechanical properties of the available products are given in the table below.


StandardGradeRp0.2Rp1.0RmElongationImpact strengthRockwellHBHV
Product type: Cold rolled coil and sheet
Typical (thickness 1 mm)Ultra 904L3403756555582HRB
Product type: Hot rolled quarto plate
Typical (thickness 15 mm)Ultra 904L26028560050155

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 corrosionUniform corrosion occurs when all, or at least a large section, of the passive layer is destroyed. This typically occurs in acids or in hot alkaline solutions. The influence of the alloy composition on the resistance to uniform corrosion may vary significantly between different environments; chromium is essential for ensuring the passivity of stainless steels, nickel helps reduce the corrosion rate of depassivated steel, molybdenum enhances passivity (except for strongly oxidising environments, such as warm concentrated nitric acid) and copper has a positive effect in the presence of reducing acids such as dilute sulphuric acid. In an environment with constant temperature and chemical composition, uniform corrosion occurs at a steady rate. This rate is often expressed as a loss of thickness per unit time, e.g. mm/y. Stainless steels are normally considered to be resistant to uniform corrosion in environments in which the corrosion rate does not exceed 0.1 mm/y. Impurities may drastically affect the corrosivity of acid solutions. For guidance on materials selection in a large number of environments capable of causing uniform corrosion, the tables and iso-corrosion diagrams in Outokumpu Corrosion Handbook may be consulted.

Pitting and Crevice corrosion Chloride ions in a neutral or acidic environment facilitate local breakdown of the passive layer. As a result, pitting and crevice corrosion can propagate at a high rate, causing corrosion failure in a short time. Since the attack is small and may be covered by corrosion products or hidden in a crevice, it often remains undiscovered until perforation or leakage occurs.
Resistance to pitting corrosion is determined mainly by the content of chromium, molybdenum and nitrogen in the stainless steel. This is often illustrated using the pitting resistance equivalent (PRE) for the material, which can be calculated using the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N. The PRE value can be used for rough comparisons of different materials. A more reliable means, however, is to rank the steel according to the critical pitting temperature (CPT) of the material. There are several different methods available, for example ASTM G 150 that uses the Avesta Cell with a 1M NaCl solution (35 000 ppm or mg/l chloride ions). The CPT-values are shown in the table below. Higher contents of chromium, molybdenum and nitrogen also enhance the crevice corrosion resistance of the stainless steel. Typical values of the critical crevice corrosion temperature (CCT) in 6% FeCl3 + 1% HCl according to ASTM G48 Method F are included in the table below. The CPT and CCT values vary with product form and surface finish, the values given are for ground surfaces. Both ASTM G150 and G48 are methods for ranking the relative pitting or crevice corrosion resistance for the different stainless steels but they do not give the maximum temperature for using these alloys in real service environments.

 CPT according to ASTM G150. Wet ground to 120 grit
 CPT according to ASTM G48E. Dry ground to 120 grit
 CCT according to ASTM G48F. Dry ground to 120 grit

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 physical properties at room temperature are shown in the table below. Data according to EN10088 or EN10095.


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.


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.  However, with the right choice of tools, tool settings and cutting speeds, these materials can be sucessfully machined. For further information contact Outokumpu.
All the highly alloyed austenitic steels are well suited for welding and the methods used for welding conventional austenitic steels can also be used on 904L. However, due to the stable austenitic structure, it is somewhat more sensitive to hot cracking in connection with welding and generally welding should be performed using a low heat input.On delivery, sheet, plate and other processed products
have a homogeneous austenitic structure with an even distribution of alloying elements in the material. Solidification after partial remelting, e.g. by welding, causes redistribution of certain elements such as molybdenum, chromium and nickel. These variations, segregation, remain in the cast structure of the weld and can impair the material’s corrosion resistance in certain environments.


Standards & approvals

Outokumpu produce and certify materials to most international and national standards. Work is continuously on-going to get the different grades approved for relevant standards. The most commonly used international product standards are given in the table below.


ASTM A240/A240MTYPE 904L / UNS N08904
EN 10088-21.4539