904L stainless steel is a superaustenitic stainless steel grade, Alloy 904L (UNS N08904) contains high levels of chromium and nickel with additions of molybdenum and copper to provide superior corrosion resistance.

The combination of high chromium and nickel content, coupled with additions of molybdenum and copper, assures good to excellent corrosion resistance, and moderate to high corrosion resistance in a wide range of process environments, containing dilute sulfuric acid. It also offers good resistance to other inorganic acids such as hot phosphoric acid as well as most organic acids.

With high-level alloys of 25% nickel and 4.5% molybdenum, 904L stainless steel provides good chloride stress corrosion cracking resistance, pitting and general corrosion resistance superior to 316L and 317L molybdenum enhanced stainless steels, The copper addition provides added resistance to hot phosphoric acid and dilute sulfuric acid.

904L stainless steel is non-magnetic and offers excellent formability, toughness, and weldability.

Grade 904L contains high amounts of expensive ingredients, such as molybdenum and nickel. Today, most of the applications that employ grade 904L are replaced by low-cost duplex stainless steel 2205.

904L Stainless Steel Chemical Composition (Max %)

The chemical composition of stainless steel 904L contains roughly 19 to 23% chromium, 23 to 28% nickel, 4 to 5% molybdenum, and 1.0-2.0% Cu, which gives the alloy greater corrosion resistance than 316L.

904L Stainless Steel Mechanical Properties

904L Stainless Steel Physical Properties

904L Stainless Steel Equivalents

904L, 1.4539 Comparison

Corrosion Resistance

The high content of alloying elements in 904L gives the alloy exceptionally good resistance to uniform corrosion.

904L was originally developed to withstand environments containing dilute sulfuric acid and is one of the few stainless steels that, at temperatures up to 95°F (35°C), provide full resistance in such environments within the entire concentration range of 0 to 100%. 904L also offers good resistance to a number of other inorganic acids, such as phosphoric acid and most organic acids. However, acids and acid solutions containing halide ions can be very aggressive, and the corrosion resistance of 317L, 317LMN and 904L may be insufficient.

Fractional distillation of tall oil often needs better material than 316L or even the more frequently used 317LMN. In these hot concentrated caustic solutions, the corrosion resistance is mainly determined by the nickel content of the material. With a nickel content of 25%, 904L has proven to be a good alternative to most conventional stainless steels.

Conventional stainless steels such as 304L and 316L are susceptible to chloride stress corrosion cracking (SSC) under certain conditions. Resistance to SSC increases with the increased content of nickel and molybdenum. Therefore, the higher performance austenitic stainless steels such as 904L have very good resistance to SSC. The table in the upper right corner shows resistance to SSC in a chloride solution under evaporative conditions. High-performance austenitic steels and duplex stainless steels clearly outperform 316L.

Grade 904L stainless steel has excellent resistance to warm seawater and chloride attack. The high resistance of grade 904L against stress corrosion cracking is due to the presence of high amounts of nickel in its composition. Moreover, the addition of copper to these grades develops resistance to sulphuric acid and other reducing agents in both aggressive and mild conditions.

The corrosion resistance of grade 904L is intermediate between super austenitic grades, with 6% molybdenum content, and standard 316L austenitic grades. Grade 904L is less resistant to nitric acid than grades 304L and 310L, which are free of molybdenum. This steel grade needs to be solution-treated following cold working, to achieve maximum stress corrosion cracking resistance under critical environments.

Cold Forming

904L 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.

Hot Forming

Working temperatures of 1562 – 2102°F (850 –1150°C) are recommended for hot working processes. Normally hot working should be followed by a solution anneal and quench, but for 904L, 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. The entire workpiece must be quenched from temperatures above 2012°F (1100°C). In the event of partial heating or cooling below 2012°F (1100°C), or if the cooling has been too slow, hot working should always be followed by a solution anneal and quenching. 904L should be solution annealed at 1940 – 2084°F (1060 –1140°C).

Heat Resistance

Grade 904L stainless steels offer good oxidation resistance. However, the structural stability of this grade collapses at high temperatures, particularly above 400°C.

Fabrication

Grade 904L stainless steels are high-purity steels with low sulfur content. They can be machined using any standard methods. These grades can be readily bent to a small radius under cold conditions. Although subsequent annealing is not required in most cases, it should be carried out when the fabrication is performed under severe stress corrosion cracking conditions.

Heat Treatment

Grade 904L stainless steels can be solution heat-treated at 1090 to 1175°C, followed by rapid cooling. Thermal treatment is suitable for hardening these grades.

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