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.

Welding

Alloy 904L can be readily welded by most standard processes. Solidification after welding causes the redistribution of certain elements such as molybdenum, chromium and nickel. These segregations remain in the cast structure of the weld and can impair the corrosion resistance in certain environments. Segregation is less evident in 904L, and this material is normally welded using a filler metal of the same composition as the base metal and can even be welded without filler metal.

Welding of grade 904L stainless steels can be performed using all conventional methods. This grade does not require pre-heat and post-weld heat treatments. Grade 904L can be subjected to hot cracking in constrained weldment.

904L Stainless Steel Specifications

• B625/SB625, Standard Specification for UNS N08925, UNS N08031, UNS N08034, UNS N08932, UNS N08926, UNS N08354, UNS N08830, and UNS R20033 Plate, Sheet, and Strip
• B673/SB673, Standard Specification for UNS N08904, UNS N08925, and UNS N08926 Welded Pipe
• B674/SB674, Standard Specification for Nickel-Iron-Chromium-Molybdenum and Iron-Nickel-Chromium-Molybdenum-Copper Welded Tube
• B677/SB677, Standard Specification for Nickel-Iron-Chromium-Molybdenum and Iron-Nickel-Chromium-Molybdenum-Copper Seamless Pipe and Tube
• B649/SB649, Standard Specification for Ni-Fe-Cr-Mo-Cu-N Low-Carbon Alloys (UNS N08925, UNS N08031, UNS N08034, UNS N08354, and UNS N08926), and Cr-Ni-Fe-N Low-Carbon Alloy (UNS R20033) Bar and Wire, and Ni-Cr-Fe-Mo-N Alloy (UNS N08936) Wire

Forms

• Bar
• Plate
• Pipe
• Tubing
• Sheet
• Coil

Disadvantages of 904L Stainless Steel

• Oxidation at high temperature: 904L stainless steel is easy to oxidize at high temperatures, especially at more than 600°C, the surface is easy to form an oxide film, which will affect its corrosion resistance.
• Processing difficulty: Because 904L stainless steel contains high chromium, nickel, copper, and other elements, its processing difficulty is greater than ordinary stainless steel. Higher technical and operational skills are required in welding, bending, cutting and other processing.
• High cost: The price of 904L stainless steel is much higher than that of ordinary stainless steel, which makes its application range limited. In addition, due to its processing difficulty, the processing cost is also relatively high.
• Magnetism: Compared with ordinary stainless steel, 904L stainless steel has weak magnetism. This may affect the performance of certain applications, such as parts or equipment used in magnetic fields.
• Brittleness: 904L stainless steel may exhibit brittle behavior under certain conditions, particularly at low temperatures. This may reduce its toughness and make it more susceptible to rupture.

Applications Of 904L Stainless Steel

• Process equipment in the 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