Deceiving the anticipation for Rolex's news at Basel 2007, I was distracted by watching the beautiful films on the website Rolex.ch (Pictured are four stills from the Rolex website, © Rolex). One of the films reminds me (if there was any need) of a piece of information that is even mentioned in Wikipedia, under the heading Stainless Steel: "Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L". That is: 'Most watches that are made of stainless steel are made of Type 316L; Rolex is an exception in that they use Type 904L'.
So I decide to take a break on this day dedicated to packing for Switzerland, and to go into more detail. Here, then, are the most important differences between the two steel alloys used in watchmaking: 316 L and 904 L, used by Rolex.
I anticipate the conclusions. As you will see from the comparison, 904 L steel is much more resistant to all types of corrosion. Add to this, as revealed by several sources, that the cost of 904 L is much higher and its machining more difficult, although it can still be done with the same machine tools as 316 L steel.
The higher corrosion resistance is due to the higher copper (Cu) content, while the lower workability is due to the lower carbon (C) content.
Below are all the details...
DESCRIPTION
Steel 316 L
Cr Ni Mo stainless steel, austenitic, non-hardenable, hardenable by cold deformation. Resistance to intercrystalline corrosion and chloride pitting.
Steel 904 L
Austenitic Ni-Cr-Mo-Cu super stainless steel, resistant to pitting, stress corrosion and crevice corrosion.
EMPLOYMENTS
Steel 316 L
Like 316 (equipment for ships, for the chemical, pharmaceutical, photographic, food, textile, paper industry, oven parts, heat exchangers, orthopaedics) but more suitable for severe cold deformation and for parts to be welded due to its resistance to intercrystalline corrosion.
Steel 904 L
Reactors, distillation equipment. Hoses for treating highly aggressive substances: sulphuric acid, phosphoric acid, nitrohydrochloric acid, acetic acid. High resistance in the presence of chlorine ions.
CORROSION RESISTANCE
Steel 316 L
Excellent in the atmosphere and in a wide variety of salts, organic acids and food substances, discrete against weak solutions of reducing acids, better than other non-Mo-containing austenitic steels, towards halides and seawater. Type 316L
Due to its very low carbon content, it is virtually insensitive to intercrystalline corrosion.
Steel 904 L
Superior to the 316L series with regard to all types of corrosion (pitting, crevice, intergranular, live).
INDICATIVE CHEMICAL COMPOSITION
Steel 316 L
C 0.02
S 0.001
P 0.025
Yes 0.30
Mn 1.8
Cr 17
Ni 13.00
Mo 2.70
Cu 0.30
Steel 904 L
C 0.016
P 0.015
Cu 1.4
Yes 0.35
Mn 1.2
Cr 20
Ni 25
Mo 4.3
N 0.050
