904L super austenitic stainless steel is a low-carbon high-nickel, molybdenum austenitic stainless acid-resistant steel. 

Grades and standards: UNS N08904, DIN1.4539, ASTM A240 (new standard is classified as stainless steel series, the original standard ASME SB-625 is classified as nickel-based alloy series), SUS890L.

Corrosion resistance of 904L stainless steel: Since the 904L carbon content is very low (maximum 0.020%), in the case of general heat treatment and welding, no carbide precipitation occurs. This eliminates the risk of intergranular corrosion that occurs after general heat treatment and soldering. Due to the 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 in the active state. In pure sulfuric acid in the concentration range of 0~98%, 904L can be used up to 40 degrees Celsius. The corrosion resistance is very good in pure phosphoric acid in the range of 0-85%. In industrial phosphoric acid produced by the wet process, impurities have a strong influence on corrosion resistance. Among all the various phosphoric acids, 904L is superior in corrosion resistance to ordinary stainless steel. In the highly oxidizing nitric acid, 904L has lower corrosion resistance than the highly alloyed steel without molybdenum. In hydrochloric acid, the use of 904L is limited to a lower concentration of 1-2%. In this concentration range. 904L has better corrosion resistance than conventional stainless steel. 904L steel has a high resistance to pitting corrosion. Its resistance to crevice corrosion in chloride solutions is also good. The high nickel content of the 904L reduces the rate of corrosion at the pits and gaps. Ordinary austenitic stainless steels may be sensitive to stress corrosion in a chloride-rich environment at temperatures above 60 . This sensitization can be reduced by increasing the nickel content of the stainless steel. Due to its high nickel content, 904L has high resistance to stress corrosion cracking in chloride solutions, concentrated hydroxide solutions and hydrogen sulfide-rich environments.

Application range

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

Storage and transportation equipment for sulfuric acid, such as heat exchangers.

Power plant flue gas desulfurization device, the main use parts are: tower body of the absorption tower, flue, door panel, internal parts, sprinkler system, etc.

Washers and fans in organic acid treatment systems.

Seawater treatment equipment, seawater heat exchangers, paper industry equipment, sulfuric acid, nitric acid equipment, acid production, pharmaceutical industry and other chemical equipment, pressure vessels, food equipment.

Pharmaceutical plants: centrifuges, reactors, etc.

Plant food: soy sauce cans, cooking wine, salt shakers, equipment and dressings.

Strongly corrosive medium 904L for dilute sulfuric acid is a matching steel grade.

Welding performance

Like ordinary stainless steel, the 904 L can be welded in a variety of welding methods. The most common welding method is manual arc welding or inert gas welding. The welding rod or wire metal is based on the composition of the base metal and has higher purity. The molybdenum content is higher than that of the base metal. Preheating is generally not required before welding, but in cold outdoor operations, in order to avoid condensation of water vapor, joints or adjacent areas can be uniformly heated. Note that the local temperature should not exceed 100  to avoid carbon accumulation and cause intergranular corrosion. Small line energy, continuous and fast welding rates should be used for welding. Generally, no heat treatment is required after welding. If heat treatment is required, it must be heated to 110 0 to 1150  and then rapidly cooled.

Machining performance

The machining characteristics of 904 L are similar to those of other austenitic stainless steels, which have a tendency to stick to the knife and work harden during processing. The positive rake cemented carbide tool shall be used, and the vulcanized and chlorinated oil shall be used as the cutting coolant. The equipment and process shall be based on the reduction of work hardening. Slow cutting speeds and feed rates should be avoided during the cutting process.