What is Alloy 904L?

Austenitic stainless steels are widely used for their availability, lower cost, reasonable corrosion resistance and good impact toughness. Increasing their chromium and molybdenum content significantly improves resistance to pitting corrosion. In the case of Alloy 904L (UNS N08904, 1.4539) there is also an addition of 1-2% copper, which greatly improves resistance to strong reducing acids such as sulphuric acid and phosphoric acid. It has therefore been widely used across the chemical process industry, in both plate and bar.

904L compared with super duplex

The substantial nickel content makes 904L a lot more expensive than regular stainless steels, so it has seen some substitution by more cost-effective super duplex stainless steels (SDSS). A super duplex offers two to three times the yield strength of Alloy 904L, so it can be used in lesser thicknesses for the same functional performance. The PREN of a super duplex is greater than 40, compared with 43 for Alloy 904L, so the two are roughly comparable. Ferralium^® 255 (UNS S32550, F61, 1.4507) also exploits the same increased copper content as Alloy 904L, and so achieves excellent corrosion resistance in various reducing acids. Its increased strength also gives it better wear and abrasion resistance, making it well suited to mixers and agitators.

The main reason to continue using Alloy 904L rather than a super duplex stainless steel is when operating temperatures fall below or above the sensible working range for SDSS. Being austenitic, 904L retains its impact toughness down to cryogenic temperatures (-196degC).

An interesting side-line is the use of Alloy 904L in the construction of Rolex watches. For this application the metal of choice must be non-magnetic, which excludes duplex and super duplex stainless steels.

We stock a wide range of duplex and super duplex stainless steels, austenitic stainless steels and nickel alloys, from 1/2" (12.7mm) to 16" (406.2mm) diameter, and plate up to 3" thick.