904L is a highly alloyed austenitic stainless steel with very low carbon content. The steel is designed for environments with harsh corrosive conditions.

Alloy steel 904L has the following characteristics:
904L is a highly alloyed austenitic stainless steel with very low carbon content. The steel is designed for environments with harsh corrosive conditions. Originally the alloy was developed for corrosion resistance in dilute sulphuric acid. This property has proven successful in practical applications over the years. Now standardised in many countries and validated for the manufacture of pressure vessels, 904L alloy has good resistance to pitting and crevice corrosion, high resistance to stress corrosion rupture, good resistance to intergranular corrosion, good machinability and weldability, as do other commonly used CrNi austenitic steels. The maximum heating temperature during hot forging is up to 1180 degrees Celsius, and the minimum stop forging temperature is not less than 900 degrees Celsius. Hot forming of this steel can be carried out at 1000 - 1150 degrees Celsius. The heat treatment process for this steel is 1100 - 1150 degrees Celsius, fast cooling after heating. The steel can be welded by common welding process, but the most appropriate welding method is hand arc welding and tungsten arc welding. When the use of manual arc welding welding is not greater than 6 mm plate when the electrode diameter is not greater than 2.5 mm; when the plate thickness is greater than 6 mm when the electrode diameter is less than 3.2 mm. When the welding needs heat treatment, can be heated at 1075-1125 degrees Celsius after fast cooling for processing. With tungsten argon arc welding filler metal can be used with the same material welding rod, after welding the weld shall be pickling, passivation treatment.
904L metallographic structure
904L is completely austenitic organisation, compared with general austenitic stainless steel with high molybdenum content, 904L is insensitive to precipitation of ferrite and alpha phase.
Corrosion resistance of 904L
Since the carbon content of 904L is very low (maximum 0.020%), 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 the high chromium-nickel-molybdenum content and the addition of copper, 904L can be passivated even in reducing environments such as sulphuric acid and formic acid. The high nickel content results in a low corrosion rate even in the active state. In pure sulphuric acid in the concentration range of 0~98%, 904L can be used up to 40 degrees Celsius. In pure phosphoric acid in the 0~85% concentration range, its corrosion resistance is excellent. In industrial phosphoric acid produced by wet process, impurities have a strong influence on corrosion resistance. In all kinds of phosphoric acid, 904L corrosion resistance is better than ordinary stainless steel. In strongly oxidising nitric acid, 904L has a lower corrosion resistance than highly alloyed steels without molybdenum. In hydrochloric acid, the use of 904L is limited to lower concentrations of 1-2%. In this concentration range, the corrosion resistance of 904L is better than that of conventional stainless steels. 904L steel has a high resistance to pitting corrosion. Its resistance to crevice corrosion in chloride solutions is also excellent. 904L's high nickel content reduces the rate of corrosion at pockmarks and crevices. Ordinary austenitic stainless steels can be sensitive to stress corrosion in a chloride-rich environment at temperatures above 60 degrees C. This sensitisation can be reduced by increasing the nickel content of the stainless steel. Due to the high nickel content, 904L has a high resistance to stress corrosion rupture in chloride solutions, concentrated hydroxide solutions and hydrogen sulphide rich environments.
904L Application Areas:
Alloy 904L is a versatile material that can be used in many industrial applications:
1. Petroleum and petrochemical equipment, such as reactors in petrochemical equipment.
2. Sulfuric acid storage and transport equipment, such as heat exchangers.
3. Flue gas desulphurisation devices in power plants, the main parts used are: the tower of the absorption tower, the flue, the stall door plate, the inner parts, the spraying system and so on.
4. Scrubber and fan in organic acid treatment system.