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Views: 4 Author: Rachel Publish Time: 2024-07-24 Origin: Site
UNS N08904, commonly known as 904L, is a low carbon high alloy austenitic stainless steel. The addition of copper to this grade gives it corrosion resistant properties superior to the conventional chrome nickel stainless steels, in particular to sulphuric, phosphoric and acetic acids. However, there is limited use with hydrochloric acids. It also has a high resistance to pitting in chloride solutions, a high resistance to both crevice and stress corrosion cracking. Alloy 904L performs better than other austenitic stainless steels due to the higher alloying of nickel and molybdenum.
The grade of 904L stainless steel is non-magnetic in all conditions and has excellent formability and weldability. The austenitic structure also gives this grade excellent toughness, even down to cryogenic temperatures. This high alloy stainless steel is added with copper to improve its resistance to strong reducing acids, such as sulphuric acid. The steel is also resistant to stress corrosion cracking and crevice corrosion. And offers excellent formability, toughness and weldability.
The high chromium content promotes and maintains a passive film which protects the material in many corrosive environments. 904L has a greater resistance to precipitation of ferrite and sigma phases on cooling and welding than other stainless steels containing molybdenum such as 316L and 317L. There is no risk of intercrystalline corrosion on cooling or welding due to the low carbon content. Its maximum service temperature is at 450°C.
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Grade 904L stainless steels have 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.
904L (1.4539) Chemical Composition | |||||||||
Grade | C | Mn | Si | P | S | Cr | Ni | Mo | Cu |
904L (1.4539) | %0.02 max | %2.0 max | %1.0 max | %0.045 max | %~0.01 min | %19-21 | %24-26 | %4-5 | %1.2-2.0 |
Since the carbon content of 904L is very low (0.020% maximum), there is no carbide precipitation under normal heat treatment and welding conditions. This eliminates the risk of intergranular corrosion that occurs after general heat treatment and welding. Due to its high chromium-nickel-molybdenum content and the addition of copper, 904L can be passivated even in reducing environments such as sulfuric acid and formic acid. The high nickel content gives it a lower corrosion rate even in the active state.
904L can be used at temperatures up to 40 degrees Celsius in pure sulfuric acid at concentrations ranging from 0 to 98%. The corrosion resistance is excellent in pure phosphoric acid in the concentration range of 0 to 85%. In the industrial phosphoric acid produced by the wet process, impurities have a strong influence on corrosion resistance. 904L has better corrosion resistance than ordinary stainless steel of all phosphoric acid types.
In strongly oxidizing nitric acid, Alloy 904L has lower corrosion resistance compared to highly alloyed steels that do not contain molybdenum. In hydrochloric acid, the use of 904L is limited to lower concentrations of 1-2%. In this concentration range, 904L has better corrosion resistance than conventional stainless steel, and 904L steel has a high resistance to point corrosion. The high nickel content of 904L reduces the rate of corrosion in pits and crevices. Ordinary austenitic stainless steels may be sensitive to stress corrosion in a chloride rich environment at temperatures above 60 degrees Celsius, and this sensitivity can be reduced by increasing the nickel content of the stainless steel. Due to its high nickel content, 904L has a high resistance to stress corrosion rupture in chloride solutions, concentrated hydroxide solutions and hydrogen sulfide rich environments.
904L is easy to be elongated and shaped. Compared with 304 or 304L, additional content of molybdenum and nitrogen means that it requires a more powerful processing equipment.
The temperature for hot forming is recommended to be between 1562 and 2102°F (850-1150°C). Normally, next steps of hot forming are annealing and hardening. However, for 904L, if it stops to form when the temperature goes beyond 2012°F (1100°C), the material shall be quenched directly so that the material can be used without subsequent heat treatment. More importantly, the entire workpiece can be quenched when temperature is higher than 2012°F (1100°C). When local heating or cooling is below 2012°F (1100°C), and the cooling goes too slow, hot forming shall follow the subsequent operations of annealing and quenching. 904L shall be annealed between 1940 and 2084°F (1060-1140°C).
Stainless steel 904L can be welded with most standard workplaces. The austenite plate material has uniform austenite structure and uniform distribution of alloying elements, which may cause the redistribution of certain elements such as molybdenum, chromium and nickel after welding. And the separation of these elements retained in the welded casting tissue will influence the corrosion resistance in some environments. However, element separation is less evident in 904L, and this material is usually welded with filler metal of the same composition as the base metal, or even without filler metal.
Air Pollution Control: Used in the scrubbers in coal-fired power plants.
Chemical Processing: Used in the equipment to produce sulfuric acid, phosphoric acid, inorganic and organic acids and phosphate fertilizers.
Metallurgical Processing: Used in the pickling equipment for sulfuric acid.
Oil and Gas production: Used in offshore processing equipment.
Pharmaceutical Industry: Used in processing equipment.
Pulp and Papermaking: Used in processing equipment.
Seawater and Brackish Water: Used in condensers, heat exchangers and piping systems.
