904L stainless steel is a low-carbon, high-alloy austenitic stainless steel. Added nickel, chromium, copper and molybdenum help it resist corrosion better than more common grades. In watches, it is the steel Rolex has used for all its steel lines since 2003. It also polishes to a brighter finish than 316L.
What is in 904L steel?
The base is iron. To this are added nickel, chromium, copper and molybdenum, with very little carbon. Each addition has a job. Molybdenum helps the steel resist localised corrosion. That means pitting, where small holes eat into the surface, and crevice attack, where corrosion starts in tight gaps. Copper helps it resist sulfuric acid at any strength. Industry uses 904L for piping, pollution control equipment, heat exchangers and bleaching systems. Engineers choose it when the more common 316L and 317L grades do not resist corrosion well enough.
Where it comes from
The diving firm COMEX used this steel for its diving bells, because it stood up well to seawater. COMEX suggested the idea to Omega. Omega tried the steel, which it called Uranus Steel, in its PloProf dive watches in 1971 and 1972. Rolex started making watches from 904L in 1985. At first it kept the steel for its Sea-Dweller deep-sea diving watches. In 2003, Rolex moved every one of its steel watch lines over to 904L. At the Baselworld fair in 2018, Rolex began calling its 904L steel Oystersteel.
Why do watchmakers use 904L?
Watchmakers like 904L for its finish. It can be polished to a brighter shine than 316L, the steel most other brands use. Put side by side, a polished 904L surface can look brighter than a polished 316L one. There is a cost. The alloy is hard to shape. Cutting and machining it usually needs special equipment.
Checked