Horology 101: Stainless Steel

Stainless steel (SS) is an alloy made primarily of iron, nickel, and a minimum of 10% chromium. There are 15 commonly used grades of SS out of 150+ varieties. Watch cases use less than a handful of these with the necessary combination of tensile strength, hardness, formability, corrosion resistance, and cosmetic appearance. 300 series alloys have the best mix of these traits. Other series alloys may excel in one area but are deficient in others. For instance, 400 series alloys are more corrosion resistant but sacrifice durability and finish.

A polished SS Breitling on the left and a brushed SS Daniel JeanRichard on the right:




316L is non-magnetic and the most widely used SS grade in the watchmaking industry. It can be hardened by annealing, meaning the product is heated to 1010 - 1120˚C and then cooled rapidly in a liquid solution. 316L differs from most other SS because molybdenum is added to improve resistance to pitting and corrosion. The L stands for low carbon - less than half of what regular 316 has. The benefit is resistance to cracking, and the drawback is a nearly insignificant lower minimum strength. Here's a breakdown of typical elements that may be used in 316L alloys:
Iron 72%
chromium 10 - 18%
nickel 10 - 14%
molybdenum 2 - 3%
manganese up to 2%
carbon MAXIMUM of .03%
sulphur up to .03%
nitrogen up to .1%
phosphorus up to .045%
silicon up to .75%

Just as nickel can bleed from any alloy, so can molybdenum. It's not in sufficient quantities to be toxic, and is a natural trace element found in plants & animals.

While 316 alloys are considered marine grade SS they are not immune to warm sea water. Rough surfaces & crevices can show brown staining and pitting if not cleaned properly with fresh water. It is accepted that less than .1 mm of corrosion annually meets durability standards. The chromium in SS is the prime factor in preventing rust. Similar to titanium, chromium creates a microscopic film when it oxidizes that seals out air & water. If scratched it quickly regenerates it's shield. 904L has the same hardness as 316L but has a slightly higher discharge of nickel. While this makes 904L more corrosion resistant this fact can make it more prone to allergic reactions as mentioned in the 'white gold' installment. 904L also has copper added to aid in it's corrosion resistance, particularly to sulphuric acid.

Damasko has patented several alloys and a process they call "ice hardening". They manufacture SS similar to 400 series alloys without adding any nickel. The metal is infused with nitrogen while in a molten state. It also has a much higher carbon count - .35% compared to the .03% of 316L and the .08% of normal 316. The end result is a product almost 4X harder than any other material used as a watch case. It also improves corrosion resistance, and since it contains no nickel allergic reactions are nullified. The main drawback is ice hardened SS can be easily magnetized so an anti-magnetic case must surround the movement.

Here's a picture of a plain Damasko case:



and one of a finished product:




Sinn uses tegimented 316 SS to increase scratch resistance. It forms a shell that is 4 - 5 times harder than the underlying SS. The process is called SAT12 and was patented by an Ohio company called Swagelok in August of 1998. In typical case hardening, SS is heated to near it's melting point, but with tegimentation the temperature is kept much lower. This keeps carbides (carbon bonded with other elements - particularly chromium in this case) from forming. Then they carburize (infuse carbon onto) the surface layer to increase the strength of the steel. The finished product can be dented under extreme cases, but resists scratches better than even ice-hardened steel.

Sinn 857:




On the eco-friendly green side, SS is completely recyclable. About half of new material is made from scrap. Even the process used to create new SS is more energy efficient than in the past due to technological improvements.

You forgot to discus Rolex SS.

Rolex uses 904L and 316L - both discussed. 😉

It's my understanding that annealing doesn't harden, it increases ductility and reduces hardness, to relieve stresses and make the steel easier to work. Further, annealing utilizes a slow cooling, not a rapid quench in liquid.

Wow, had no idea there were so many variations of SS.

Interesting! Thanks 😀

A bit of a bump but it is relevant to this topic. Can modern Omega stainless steel cases crack when dropped on tile, marble or hardwood from waist height? Even hairline cracking? Or would the case dent/ding first before it ever cracked?

I guess in theory anything is possible so I’m talking about in practice. Would a stainless steel case dent/ding before it would ever crack? Talking about real life situations here like dropping it on tile floor, etc

Just curious more than anything else, mainly because I don’t recall ever seeing reports of a modern SS case crack whereas you see it with Ceramics. I guess this is why SS is the chosen material for most watches. I know it’s a whole materials science discussion though.

And I was asking specifically about Omega cases because I believe they use a different grade of SS than Rolex, etc. Really, just wanted to know what typically happens first when there’s impact to a SS case —> scratch, dent and then crack? Whereas for ceramics upon impact there is no scratching and it’ll just go straight to cracking.