I was having a discussion with Daniel about stainless steel. It was not terribly exciting, it was to make sure we had the correct references on our new website. We use 316L stainless steel in our watch cases, but then Daniel came back and said he had checked and we actually used 1.4404 stainless steel in our watch cases.
This was making me rather frustrated for a number of reasons. First, I had been telling everyone up to now we had used 316L stainless steel. The second reason was that I had spent the best part of three years of my life studying steel at university and I should know the answer to this. I found it rather embarrassing that we did not have the answer at our fingertips.
Not content with the confirmation off the purchase order I decided that this needed a more in depth review. I recalled quite a detailed conversation when we were designing SNGLRTY. That conversation was filled with all the usual buzz words – 316L stainless steel, medical grade stainless steel, 1.4404 stainless steel, 1.4401 stainless steel. Just for good measure I am sure 304 stainless steel popped into the conversation too. I wanted to find the answer, the answer that was bullet proof, the definitive answer. It had to be correct on our new website.
Stainless Steel – We All Clearly Care
We all see many references to stainless steel in the marketing materials for watches. Watch owners appear to put a great deal of weight on the specific material that their watch case is made of. It struck me that these terms are bandied around but I how many people really understand the implications for a watch owner? How many people out there can really understand the difference between 316L and 1.4401 stainless steel? The difference between 304 and 316 stainless steel? And why did Rolex create its own stainless steel, Oystersteel, a variation of 904L stainless steel?
Back to The Books
On my quest to get to the bottom of the SNGLRTY steel conundrum I thought I would share my research and try, as best I can, to make it relevant to watches. It may come as a bit of a surprise but I actually understand the difference between all these steels, intimately. Well not the exact differences. I do not hold the specification sheets for each of these steels in my head for entertainment. I do, though, have a degree in materials engineering. Years of my life were spent getting into the details of steels. So here we go, let me be your tour guide through the stainless steels of watch making – and I have tried to make it as jargon free as possible.
The Stainless Steel Reduction Plan
Before we get into the guts, be warned, this will be a long journey. If you do not want to miss any of of it I suggest you sign up to our newsletter – you will want to get to the end!
I had to choose a starting point. The obvious place is the most popular stainless steel for making a quality timepiece – 316L Stainless Steel. Another reasonable starting point would have been 304 Stainless Steel as it is the most commercially important stainless steel on the market. We are watch people so I decided to go with what is important to us.
Let’s Break It Down
My next step was to search for the technical datasheet for 316 Stainless Steel. How the manufacturer describe their steel would be the starting point for the further investigation. This is what they say about their stainless steel, “316 stainless steels are are molybdenum-bearing austenitic stainless steel which are more resistant to general corrosion and pitting/crevice corrosion than the conventional chromium nickel austenitic stainless steel”
Does that help anyone? It is a lot of technical speak but for most watch enthusiasts it really does not help understand the benefits or the implications of using this stainless steel over any other stainless steel. The next step is to break down this description to its key constituents and then understand what each of these key concepts means.
Dissecting the Description
So what are the important words in that description? For the moment you are going to have to trust me when I tell you the four key concepts that we need to address are:
Resistant to general corrosion
My hope is that at the end of this series of posts you will understand why these are the key concepts to understand for watchmaker’s stainless steel. For the moment please just trust me, and then you can be the judge at the end.
What is a Steel?
I wonder how many people can actually clearly articulate what a “steel” is? One of my tutors from university used to oft repeat, “if you don’t understand it from first principles, then you do not understand it.” It may seem obvious but a lot of detail and nuance can be glossed over without getting to the fundamentals. So I think it is worthwhile taking some time here to ensure a full understanding of steel because it is the primary constituent of all stainless steels.
Steel is an alloy of iron. What does this mean? It means that pure iron (its elemental state) is blended with another element to alter its physical properties. Steel is created when iron is blended, or alloyed, with carbon. Carbon impacts elemental iron in two distinct ways. The first is that it reduces the temperature at which iron will melt. Pure elemental iron will melt at approximately 1,500oC but an iron alloy with 4.3% dissolved carbon will melt at 1130oC. Thus when iron ores are smelted with carbon in a blast furnace the first crude steel to be created will have a carbon composition of 4.3%. This steel will be the first to melt and run to the bottom of the blast furnace and is known as pig iron.
More Carbon Equals Harder Steel
The second impact that carbon has on iron is that it will make the iron harder. The problem is that it also makes the iron brittle so it can crack or shatter easily. This is evident with anything that is made from pig iron (sometimes referred to as cast iron). These were the very first steel alloys to be discovered but the brittleness of the material ensured it had limited applications.
The brittleness of the pig iron is caused by polluting compounds forming, or precipitating, as the iron cools. These polluting compounds cause weaknesses in the crystal structure of the steel alloy. The result is that if a large force is applied to the material it can fail easily, and shatter like glass. This is not an ideal material to house your delicate watch movement! The key is to reduce the carbon in the steel so that on cooling the crystal structure remains consistent (or homogenous) and no polluting compounds form. This results in a steel that is more ductile and can be shaped and machined more easily.
Iron, Carbon and Temperature
As you may imagine the interactions between iron and carbon become quite complex as the amount of carbon changes and the temperature changes. Thankfully all the key data can be summarized on a chart called a phase diagram for the iron-carbon system (sometimes also called an equilibrium diagram). In the diagram you can see that the maximum solubility of carbon in iron is 1.7% at 1130oC, this means that there is a solid solution of iron and carbon. This is perhaps a counter intuitive term, “How can a solid be a solution?” is the usual refrain. What it means is that all the carbon atoms are dissolved within the matrix of iron atoms – there is a homogeneous structure throughout the material.
A Bit More Complexity
A problem that you may have noticed is that this structure occurs at temperatures that are greater than 723oC. This is not terribly useful for everyday applications. Another issue is that as the temperature declines the solubility of carbon in the iron matrix reduces to a point where at 723oC the solubility of carbon in iron is 0.89%. Any temperature below this will cause polluting compounds to form, or precipitate, in the steel and will reduce its ductility and increase its brittleness.
The key area to identify is where all the carbon is held in a solid solution. This area is called the austenitic area and in the phase diagram above is indicated by the Greek letter gamma. We will return to this in more detail as we have identified “austenitic” as one of our key concepts. But to summarize an austenitic steel can be most easily thought of as a solid solution of iron-carbon or that its crystal structure is homogenous, or that no polluting compounds or precipitations were created during the cooling process.
What Happens At Room Temperature?
That cooling process from 723oC to room temperature is critical to ensure that the structure is consistent and no pollutants precipitate out of the austenitic steel. If the steel is cooled quickly the time available for the pollutants to precipitate out is reduced. This results in the the solid solution effectively being frozen in place. Unfortunately this impacts the way these simple steels can be worked and formed, but for our purposes we can ignore these issues for the moment.
Going back to our original question, what is a “steel”? It is an alloy of iron and carbon. It may be a very simple statement but as you can already begin to appreciate as you dive into the details it reveals a great deal of complexity.
I hope that this has increased your understanding of steel a little bit. I will continue to unpick the rest of this puzzle, but this is the foundation.
It Is All About Stainless
Next time, as we dive into the “stainless” aspect of this puzzle. Make sure you do not miss this and sign up to our newsletter. You will receive more like this and have a seat beside two entrepreneurial watch innovators on their journey not to mention the latest developments at SNGLRTY. Join us and #seetimedifferently.