Jean-François Dionne en Directors and Executives, beBee in English, Engineers and Technicians Metallurgical Engineer • Laboratoire Dubois SA Hace 6 d · 2 min de lectura · +400

A little twist to tradition

A little twist to tradition

The watch industry doesn't only make intricate and complicated assemblies, it is also amongst the most prolific patent producers in the world. As many as 1032 patents were published in 2016 on mechanical and electronic watches (excluding connected devices). That was an increase of 16% compared to the previous year

With so much innovation going on, the watch industry must be using some awesome materials! Well yes... and no. Some exotic materials are used for watch cases: ceramics, amorphous metals and titanium just to name a few. 

One recent innovation that is spreading inside the watch is the use of silicon. The same wafers used to make microprocessors are now etched to make for example spirals and other high performance escapement components.

In other instances however the materials are less prestigious: plain steel and brass. None the less these materials can have excellent properties and are quite convenient. Moreover, they can be nicely decorated with for example luxurious platings, CVD or PVD layers.

One of these ordinary materials is known in the watch industry as "20AP". This name is registered to Sandvik but is commonly used  in the industry just like Kleenex for nose tissue. In simple terms, it is a 1% carbon steel with lead (Pb) to facilitate machining. This steel is used to make pinons, pins and axles working inside the watch case. These very small components usually require a very high yield stress or hardness. Unfortunately, this alloy is very brittle.

Many problems my clients face are related to this particular steel. Heat treating "20AP" is not that difficult to do but the quality of the wire stock can be variable depending on the supplier and is not always properly characterised before use (this would be a good start for you quality professionals reading this).

I intentionally avoid the machining considerations and concentrate on the metallurgical aspect. In my opinion, there is more to learn about machining by optimising the real clean stuff than using Pb or sulphured steel (sulphur is a bad idea... keep the Mn:S ratio above 8... you've been warned). I'm aware that's easy for me to say, I'm not the one making chips.

Ideas for improvement

Here are some proposals to solve the brittleness problem. Most of them may not be "patentable" per say but alloys can still be tailor made/invented for such purposes.

Let us start by imagining a (very) small axle pivoting on a corundum ruby. To avoid wear, the axle has to be very smooth and have a hard surface. The typical hardness of these steel components is around 700HV (~60HRC).

The first idea for improvement would be to use a low alloyed steel with a similar carbon content. Say you add a little nickel, or manganese, or chromium, or molybdenum, you could get a much tougher steel. Without going into unnecessary details, alloys like 100Cr2, 100CrMo5 and 100CrMn6 or similar would bring a significant increase in toughness to the components.

Adding elements like nickel, chromium, molybdenum, manganese, etc. will increase the quenchability of the steel but promote residual austenite at high carbon levels (above ~0.8%C). Such a problem could be mitigated by lowering the carbon content, my next proposition.

Alloys with around 0.6-0.8%C will still make it possible to get to 700HV with relative ease after proper quench and temper. Lower carbon also means increased toughness. Alloys 60Cr3, 70Cr2 or 80CrV2 are good examples of such alloys. Other similar alloys with manganese, nickel or any combination of these elements could also be very satisfactory.

My third proposition is perhaps a stretch but would give amazing results. The idea is to use a lower carbon alloy and then increase the surface hardness. Imagine starting with a 0.2-0.4%C alloy and carburising the surface (or nitriding, carbo-nitriding, etc.). This way you can imagine having a VERY hard surface and a VERY tough core. An additional advantage is that machining 0.2-0.4%C is easier than 1.0%C.

Every thing is possible!

Other exotic possibilities could be the use of stainless steels, titanium, composites, etc.  Keep in mind though that radically changing the material will change the manufacturing process. No worries, I can guide you into that kind of adventure.

At first, changing the paradigm can be seen as a costly adventure. Research and development has always been in the cost category. But what happens in the long run if this improvement simplifies your process/product, lowers scrap, has less steps or gives better properties?  

What is YOUR wish to make your product new again?

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