904L, UNS N08904, W.Nr. 1.4539
904L is a non-stabilised low carbon high alloy austenitic stainless steel, which designed for moderate to high corrosion resistance in a wide range of process environments. The addition of copper to this grade gives it greatly improved resistance to strong reducing acids, particularly sulphuric acid. The combination of high nickel and molybdenum content, assure good chloride stress corrosion cracking resistance, including pitting, crevice corrosion and stress corrosion cracking. Even down to cryogenic temperatures, the austenitic structure also gives 904L stainless steel excellent toughness, easily welded and processed by standard shop fabrication practices.
| Material | Cr | Mo | S | Ni | Si | C | Mn | N |
|---|---|---|---|---|---|---|---|---|
| 904L | 19.0-23.0 | 4.00-5.00 | 0.035 max | 23.0-28.0 | 1.00 max | 0.02 max | 2.00 max | 0.01 max |
| Material | Tensile strength | Yield strength 0,2 | Elongation | Hardness HB30 |
|---|---|---|---|---|
| 904L | 490 N/mm² | 220 N/mm² | 36% | 70-90HB |
904L was originally developed for its resistance to sulphuric acid, its offers exceptionally good resistance to uniform corrosion due to the high content of alloying elements, and also has a very high resistance to a wide range of environments such as warm sea water and other high chloride. In most environments, 904L material has a corrosion performance intermediate between the standard austenitic grade 316L and the very highly alloyed 6% molybdenum and similar "super austenitic" grades, which has proven to be a good alternative to most conventional stainless steels. Its important to note that 904L should be solution treated after cold work to maximum stress corrosion cracking resistance in critical environments.
Solution Treatment (Annealing): Heat to 1090-1175°C and cool rapidly. 904L cannot be hardened by thermal treatment.
Heating of 904L stainless steel is commonly performed in heat treatment. The process involves heating the material to specific temperatures and holding it for a certain duration in controlled atmospheres, and air cooling or quenching. Heating treatment of 904L material can be performed for various purposes such as stress relieving, solution annealing, or precipitation hardening.
Pickling is a common process used to remove surface impurities and oxide layers from 904L. It involves immersing the material in a pickling solution, typically an acid-based solution, to dissolve contaminants and restore the material's corrosion resistance. During the pickling process, it is crucial to control the concentration and temperature of the pickling solution to prevent excessive etching or corrosion of the stainless steel surface.
Hot Forming: The recommended temperatures range for 904L hot working is from 1562 to 2102°F (850 to1150°C), normally hot working should be followed by a solution anneal and quench. If hot forming is discontinued at a temperature above 2012°F (1100°C) and the material is quenched directly thereafter, the material may be used without subsequent heat treatment. If partial heating or cooling below 2012°F (1100°C), the material should be solution annealed and quenched at after hot working. 904L should be solution annealed at 1940–2084°F (1060–1140°C).
Cold Forming: 904L stainless steel is quite ductile and forms easily. The addition of molybdenum and nitrogen implies more powerful processing equipment may be necessary when compared with the standard 304/304L grades.
904L material is a high purity, low sulphur grade, can be machined using standard techniques, but the machinability is not as well as other grades optimised for machinability. The cold work hardening rate of 904L makes it less machinable than 410 and 304 stainless steels.
904L can be welded using a common welding process, the most appropriate weldingmethods are manual arc welding and tungsten-argon arc welding. When welding, a filler metal of the same composition as the base metal is suggested, such as 20 25 CuL covered electrode and 20 25 CuL wire, for the best corrosion properties in welds. Care needs to be taken as this grade solidifies fully austenitic, so is susceptible hot cracking, particularly in constrained weldments. No pre-heat should be used and in most cases post weld heat treatment is also not required.
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