Austenitic stainless steels are widely used due to their availability, lower cost, reasonable corrosion resistance and good impact toughness. Increasing their chromium and molybdenum content will significantly improve their resistance to pitting corrosion. In the case of Alloy 904L (UNS N08904, 1.4539) there is also an addition of 1-2% copper, which is known to greatly resistance to strong reducing acids such as sulphuric acid and phosphoric acid. Therefore, it has been widely used in numerous chemical process industry applications, from both plate and bar.
The substantial nickel content means that it is a lot more expensive than regular stainless steels. Therefore, it has seen some substitution from more cost-effective super duplex stainless steels (SDSS). A super duplex stainless steel offers 2-3 times the yield strength of Alloy 904L, meaning that 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 roughly comparable. However, 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. The increased strength of Ferralium 255 compared also gives it better wear and abrasion resistance, making it well suited for use in mixers and agitators.
The main reason for continuing to use Alloy 904L rather than a super duplex stainless steel is if operating temperatures are below or above the sensible working range for SDSS. As it is austenitic, it retains its impact toughness down to cryogenic temperatures (-196degC).
An interesting side-line of Alloy 904L is its use in the construction of Rolex watches. For this application, the metal of choice needs to be non-magnetic, therefore excluding duplex and super duplex stainless steels.
If you have any questions about our range of stainless steel alloys, please contact Langley Alloys today.