The mechanical strength of the case with regards to wear and signs of usage and thus preserving the value of a wristwatch is a central consideration in watchmaking from our perspective.
Who has not experienced accidentally scraping the elevator edge or the wall in the stairwell with his expensive, highly polished piece for the first time and having to lament up to 1/10 mm deep scratches or scrapes before you know it. Unfortunately the austenitic stainless steels used in the manufacture of watch cases, such as 1.4435 (316L) or 1.4301, do not harden due to their austenitic structure. Although there is a method wherein the surface can be hardened to 1200 HV using a diffusing carbon, this hard layer or “protective coating” is only a few hundredths of a millimeter thick and tends to already break at low mechanical stress. In our experiments cases with such a surface treatment could be scratched with as little as a normal watchmaker’s tweezers. For this reason, this case and this procedure did not qualify for our watch cases. Rather, this method is industrially applied to or used for components prone to surface pressure, such as balls for ball valves or pistons for piston pumps.
Thus it is easy to understand why we already applied for a patent for the first temperable martensitic stainless steels for use in watch case construction back in 1994. These steels are nickel-free, can be hardened up to 64 HRC or 800 HV and are therefore ideal for watch cases. After 5 years of research and development work, about 20 different martensitic stainless steels were filed for patent. In our company it was decided to to use nitrided, martensitic stainless steel in the future because of its superior properties. In the manufacture of this stainless steel the molten steel is enriched with nitrogen under pressure. The addition of nitrogen reduces the required percentage of the alloy constituent carbon.
While carbon is required for high hardness in the alloy, it also lowers the corrosion resistance. About 1% carbon is required as an alloy component in order to harden the martensitic stainless steel to about 60 HRC. It has now been shown that the required amount of carbon is also reduced to 0.35 % by the addition of 0.35% nitrogen, and a hardness of 60 HRC is achieved by this favorable combination of alloy constituents. By lowering the carbon content, however, the corrosion resistance is increased many times over. This stainless steel, which is also called “ice tempered, nickel-free stainless steel” due to the special heat treatment, is patent protected by our company for use in the watch case production, and has been delivered until the end of 2002 as a complete case to a German watch manufacturer, inclusive of crown and pushers produced and hardened by us.
From now on this case made of hardened stainless steel is exclusively available at Damasko watchmaking. After the heat treatment this stainless steel reaches a hardness of up to 62 HRC or 760 HV. This corresponds to approximately four times the hardness of austenitic stainless steel normally used in watch case production. Our cases are fully hardened, which renders scratching or breaking with steel tweezers or similar implements impossible. Owing to its enormous resilience, this very expensive stainless steel is mainly used in the aviation and aerospace industry, such as in:
- Turbine bearings of jet engines
- Rotor bearings for helicopters
- Fuel pump in the Space Shuttle
- High speed bearings in machine tools
- Dental instruments
- Surgical instruments
- Medical technology
- And last but not least in Damasko watchmaking