The industry standard stainless steel is 316L. In addition to watches, this alloy is also commonly used in the food industry, the medical field, and a wide variety of water and marine applications. You may see some watch brands refer to 316L as surgical grade steel or marine grade steel. Its popularity in these areas comes from the properties which make it resistant to rust and corrosion. 904L steel takes the resistance to corrosion up a notch.
In the watch community, 904L is largely regarded as being a Rolex thing. Even the Wikipedia entry for 904L stainless steel has been edited to include a Rolex mention (well done, Rolex!):
"In 1985 Rolex became the first wristwatch manufacturer to utilize 904L grade steel in its watches. Rolex chose to use this variety of steel because it takes a higher polish than other grades of steel and provides greater corrosion resistance, though it does not machine as well and requires specialized equipment to be properly modified into the required shapes."
Perhaps Rolex deserves credit for being recognized as the first brand to sell watches with cases made out of 904L steel, but this has led many to think that 904L was a Rolex invention or a material exclusive to Rolex.
It could also be due to the verbiage around the term Oystersteel, which Rolex defines as:
"…a steel alloy specific to the brand. Oystersteel belongs to the 904L steel family, which is particularly resistant to corrosion and acquires an exceptional sheen when polished." –source
Is 904L steel a precious metal?
To say Rolex is serious about their 904L steel is an understatement. Their advertisements have gone as far as calling it a precious metal (gold and platinum are precious metals).
316L VS 904L Steel Composition:
We're watch enthusiasts, not metallurgists, but the chart below sums up the main property differences between 316L and 904L steel:
| 316L Steel | 904L Steel | |
| Carbon | 0.035% max | 0.2% max |
| Chromium | 16-18% | 19-23% |
| Copper | 0 | 1-2% |
| Iron | Balance | Balance |
| Molybdenum | 2-3% | 4-5% |
| Manganese | 2% max | 2% max |
| Nickel | 10-12% | 23-28% |
| Phosphorus | 0.45% | 0.03% max |
| Silicon | .75% max | 1% max |
| Hardness (Rockwell) | 95 | 70-90 |
| Hardness (Brinell) | 217 | 150 |
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