904L is a high-alloy austenitic stainless steel with low carbon content. This kind of steel is designed for harsh corrosive environment. Initially, this alloy was developed to resist corrosion in dilute sulfuric acid. After years of practical application, this function has been proved to be very successful. Now 904L has been standardized in many countries, and has been approved for manufacturing pressure vessels. Like other commonly used CrNi austenitic steels, 904L alloy has good pitting and crevice corrosion resistance, high stress corrosion cracking resistance, good intergranular corrosion resistance, good machinability and weldability. During hot forging, the maximum heating temperature can reach 1 180℃, and the minimum stop forging temperature is not lower than 900℃. The hot forming of this steel can be carried out at 1000- 1 150 degrees Celsius. The heat treatment process of this steel is1100-1150℃, and it is cooled rapidly after heating. Although this kind of steel can be welded by general welding technology, the most suitable welding methods are manual arc welding and argon tungsten arc welding. When manual arc welding is used to weld plates not greater than 6 mm, the diameter of covered electrode is not greater than 2.5mm；; When the plate thickness is greater than 6 mm, the covered electrode diameter is less than 3.2 mm. When heat treatment is needed after welding, 1075- 1 125℃ can be used for heat treatment, and then rapid cooling can be performed. The filler metal used in TIG welding can be covered electrode, and the weld must be passivated by pickling after welding.

Physical and mechanical properties:

Density: 8.24 g/cm3. Tensile strength: σb≥520Mpa. Elongation: δ≥35%.

Chemical composition:

C: ≤ 0.02

Manganese Mn: ≤ 2.00

Silicon Si: ≤ 1.00

Chromium Cr: 19.0 ~ 23.0

Nickel: 23.0 ~ 28.0

Phosphorus P: ≤ 0.045

Sulfur content: ≤ 0.035

Molybdenum Mo: 4.0 ~ 5.0

Copper Cu: 1.0 ~ 2.0

Metallographic organization:?

904L has a completely austenitic structure. Compared with ordinary high molybdenum austenitic stainless steel, 904L is insensitive to the precipitation of ferrite and α phase.

Corrosion resistance:?

Because the carbon content of 904L is very low (the maximum is 0.020%), there will be no carbide precipitation under general heat treatment and welding. This eliminates the danger of intergranular corrosion after general heat treatment and welding. Because of the high content of Cr, Ni and Mo and the addition of Cu, 904L can be passivated even in reducing environment such as sulfuric acid and formic acid. High nickel content makes it have low corrosion rate in active state. In the concentration range of 0~98%, the service temperature of 904L can reach 40℃. Pure phosphoric acid has good corrosion resistance in the concentration range of 0~85%. In industrial phosphoric acid produced by wet process, impurities have great influence on corrosion resistance. Among all kinds of phosphoric acid, the corrosion resistance of 904L is better than that of ordinary stainless steel. In strongly oxidizing nitric acid, the corrosion resistance of 904L is lower than that of high alloy steel without molybdenum. In hydrochloric acid, the use of 904L is limited to the lower concentration of 1-2%. In this concentration range. The corrosion resistance of 904L is better than that of conventional stainless steel. 904L steel has high pitting resistance. It also has good crevice corrosion resistance in chloride solution. The high nickel content of 904L reduces the corrosion rate in pits and cracks. When the temperature is higher than 60 degrees Celsius, ordinary austenitic stainless steel may be sensitive to stress corrosion in chloride-rich environment. This sensitivity can be reduced by increasing the nickel content in stainless steel. Due to the high nickel content, 904L has high stress corrosion cracking resistance in chloride solution, concentrated hydroxide solution and environment rich in hydrogen sulfide.

Scope of application of 904L:?

904L alloy is a versatile material, which can be used in many industrial fields:

1. Petroleum and petrochemical equipment, such as reactors in petrochemical equipment.

2. Sulfuric acid storage and transportation equipment, such as heat exchanger.

3. The power plant flue gas desulfurization device is mainly used for the tower body, flue, baffle, internals and spray system of the absorption tower.

4. Scrubbers and fans in the organic acid treatment system.