UNS NO8904, commonly known as 904L, is a low carbon, high alloy austenitic stainless steel which is broadly utilized in applications where the corrosion properties of AISI 316L and AISI 317L are not sufficient.
The presence of copper in this grade gives its corrosion resistant properties higher than the conventional chrome, nickel stainless steels, in particular to sulphuric, phosphoric and acetic acids. However, there is restricted use with hydrochloric acids. It also possesses a great resistance to pitting in chloride solutions, a great resistance to crevice and stress corrosion cracking.
Alloy 904L displays better performance compared to other austenitic stainless steels due to the higher alloying of nickel and molybdenum. The grade is non-magnetic in every condition and possesses exceptional formability and weldability. The austenitic structure of this grade gives it excellent toughness, even at cryogenic temperatures.
The high chromium content helps maintain a passive film, which shields the material in several corrosive environments. 904L possesses a higher resistance to precipitation of ferrite and sigma phases on cooling and welding than other stainless steels containing molybdenum like 316L and 317L. There is absolutely no risk of intercrystalline corrosion on cooling or welding due to the low carbon content. Its maximum service temperature is at 450°C.
This grade is mostly useful in instrumentation and control tubing applications where 316 and 317L are not suitable.