This is the second part of a multi-part investigation into stainless steel and how the cast array of steels apply to the world of watches and in particular to stainless steel watch cases. Last time out, we identified the key aspects of the most common stainless steel used in a watch that we need to understand. These are 1) steel, 2) stainless, 3)austenitic, and 4) corrosion resistance. Hopefully, after the first piece introduces us to steel, we are all clear that steel is an alloy of iron and carbon. The next challenge is to make the steel stainless, so it does not dirty our shirts.
Why We Need Stainless Steel Watches?
As most people are aware, steel corrodes or rusts when it is left exposed to the elements. That muddy brown surface develops when steel is left exposed to the elements created by an oxidation process. More specifically, oxygen reacts with the iron in the steel to create iron oxide. Iron oxide is also referred to as iron ore and is one ingredient for making steel in a blast furnace. If you have ever touched the rusty surface of a steel piece, you will understand that iron oxide will stain most things very quickly.
This is certainly not what you want to happen to your watch. Not only would it make washing your white shirt pretty tricky, but over time your watch would decay to being unusable. The challenge is to make steel environmentally stable to perform its function as a watch case without corroding. Steel that can withstand the physical environment of its primary point of use without rusting or corroding is stainless steel.
It Is About Alloying
As mentioned before, steel is an iron alloy where the alloying element is carbon. Carbon steels have various physical properties depending on the quantity of carbon in the steel (generally, more carbon makes the steel harder and brittle) and how it is cooled. The exact details of this are not necessary for our purposes. What is essential to understand is that carbon and iron make steel. By further adding elements to the steel, its physical properties can be altered in many ways. The most crucial transformation from our perspective is to stop the corrosion process.
% by Weight
16.5 ~ 18.5
2 ~ 2.5
10 ~ 13
What Makes It Stainless
The table above shows that the nickel and chromium content in a 316L stainless steel is between 25 and 30% of finished stainless steel. A high proportion of chromium in any steel will confer corrosion resistance as the chromium reacts with oxygen in the air and forms an oxide film across the steel’s surface. This oxide film is impervious to oxygen, so once this oxide layer has developed, further corrosion is prevented. There is a direct correlation between the chromium content and the alloy’s corrosion resistance; the more chromium, the more corrosion resistance. For a highly corrosion-resistant stainless steel, nickel is added as its presence improves its corrosion resistance.
That leaves us with Molybdenum, Nitrogen, Sulfur, Phosphorus, and Silicon in stainless steel. What are the purposes of these elements in the alloy? Let’s start with the ones that should not be there.
Elements That Should Not Be There
Commercial steels are made from ores that are not pure in their oxidation state and contain contaminants. The most prevalent and detrimental of these are sulfur and phosphorus; if either is present in the steel in an amount greater than 0.05%, then the steel will become brittle, hard to shape, machine, and finish. The information on any material data sheet ensures that phosphorus and sulfur are below these critical thresholds. The stainless steel can then be worked, machined, and shaped as the engineer would expect. We will come back to this point in detail in the next part of this series when discussing austenitic stainless steel.
Another Contaminant By Design
Silicon is another contaminant. The difference here is that higher amounts of Silicon can be tolerated in the finished stainless steel. In amounts of less than 0.2%, silicon does not have any material effect on the finished stainless steel’s strength or ductility. If silicon is present in quantities above 0.4%, it will make the steel less ductile (more brittle) and make the stainless steel hard to shape. Silicon is different from sulfur and phosphorus as it is deliberately introduced in the steel manufacturing process to remove oxygen. Not all of the silicon is consumed in this de-oxygenation process, so small amounts will remain in the steel. It remains imperative that the silicon’s final concentration must be carefully controlled to ensure the best physical properties.
That leaves molybdenum and nitrogen that are both added in small quantities. These two elements improve steel’s resistance to pitting and crevice corrosion, especially in chlorides. This is particularly important because salt is chloride and particularly corrosive to steel. Both body sweat and seawater have a high salt concentration (chloride), so these small additions are significant for any steel used in watchmaking.
The Journey So Far
Steel is an alloy of iron and carbon. Stainless steel is an alloy of steel where the alloying elements reduce, or more ideally wholly remove, any corrosive reaction. The introduction of chromium and nickel achieves this, but the addition of other elements in small quantities can improve this further.
Here we have focused on the observable physical properties, which a user would be most interested in. The alloying of steel also change the structural properties of steel. These cause significant changes to the stainless steel at the atomic level and are most important to the engineer designing the watch manufacturing process.
Next time we will look into this strange term “austenitic,” which is most important in the manufacture of a watch case. This will take us into the various processes that a watch case goes through on its journey from a piece of sheet steel to a finished and polished watch case. The structural properties of the steel will be revealed to be critical to making a high-quality watch case.
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