904L, Oystersteel, 316L, or 304L – Here is Meaning In The Codes

904L, Oystersteel, 316L, or 304L – Here Is The Meaning

904L, Oystersteel, 316L, or even 304L are all the numbers that have been bandied about, and now we arrive at the business end of exploring stainless steel. This article will sort out the differences between 316L, 304L, 904L, Oystesteel, and 1.4404 stainless steel peer behind the curtain of Oystersteel.  This will be a look from the ground up; I will remove all the marketing speak and look behind the curtain so that at the end of this, you will have the knowledge you need to make and understand where fact ends, and marketing has taken over. 

Essential Background

Everything we have covered until now will be necessary background for the following discussion of the grades of steel used most commonly to make watches.  If you have not read the previous articles on steel, I recommend taking a few minutes to review them. In those articles, I take you through what steel is and how it’s created, then I give you a look at the process of alloying and making it “stainless.” Finally, we explore a key but often overlooked property of stainless steel—the fact that it is austenitic steel—and why that is important. 

There Are Only Three Stainless Steels To Consider

From the very beginning, we have been referring to these steels by various codes such as 316L, 304L, and 904L.  Sometimes I have dropped in a marketing term such as Oystersteel and mixing them all up with 1.4404, 1.4307, and maybe even N08904. What do all these numbers mean?

Easiest First  – 904L & Oystersteel

Let me start by simplifying the situation for you. If you have a stainless steel watch, it is overwhelmingly likely to be made of one of three specific stainless steel. The easiest to identify is for all Rolex watches; these are made from 904L stainless steel. Rolex has created a marketing or tradename for this steel, Oystersteel. I am aware of several small brands that are now experimenting with 904L steel. Still, they will clearly label these watches as manufactured from 904L stainless steel as it will be a crucial point of differentiation for their watches. Every other stainless steel wristwatch in the market will be manufactured from either 304L or 316L stainless steel. The more expensive your watch, the more likely it is made from 316L stainless steel.   

What Do All The Other Number Mean?

So why are there so many ways to refer to these stainless steels? That’s simple: numerous standards organizations worldwide refer to the same steel with different reference numbers. For instance, 316L stainless steel is a reference from the American Society for Testing Materials (ASTM), but the European Standard for the same steel is EN:1.4404.  Yes, 1.4404 steel is exactly the same as 316L stainless steel. The chart below provides the reference codes for ASTM, EN, and Unified Numbering System (UNS) references for the three stainless steel used in watches.

The Differences Between the Stainless Steels

Now that the difference between all the reference numbers has been cleared up, we need to look into the differences between these steels and why they are chosen for certain watches. For this explanation, we will need to look at the alloying elements in more detail. 

316L Versus 904L Oystersteel

In reviewing the differences between 316L stainless steel and 904L stainless steel, let me make four key observations:  

• Impurity components (silicon, phosphorus, and sulfur) have been reduced below 316L stainless steel.  
• Key alloying elements (chromium, nickel, and molybdenum) are present in much higher proportions.
• The total of all alloying elements is more than 50% of the material by weight. Also, an additional alloying element has been introduced: copper. 
• Carbon content has been reduced by 30%, and the nitrogen content has been increased by 50%.  

What does all this mean?

Why Was 904L Stainless Steel Created?

NASA initially developed 904L as aerospace steel, so it needed to withstand fatigue loading and have highly predictive corrosion resistance with large safety margins. To achieve this, the purity of the final alloy needs to be increased, reducing impurities.  This ensures not only higher performance in service but also more consistent performance during manufacture. The substantial increase in the critical alloying elements increases the corrosion resistance of the 904L stainless steel, and doubling the molybdenum makes it significantly more expensive.  

What Does The Copper Do?

Adding copper to the 904L stainless steel further increases its corrosion resistance but at the expense of the alloy’s final hardness. I speculate that the increase in the nitrogen content boosts the hardness of 904L stainless steel to mitigate the addition of copper and the reduction of carbon. This composition is not ideal for a watch case because although it will make the final alloy more ductile and easier to shape and machine, it will also make it more susceptible to scratching and damage. To illustrate this point the Rockwell B hardness measurement of the three stainless steel is presented below (the higher the number, the harder the materials).

I am not saying that Oystersteel is exactly 904L. Rolex always takes pains to stress that their Oystersteel is “based on” 904L stainless steel, and there are plenty of options that can change the final physical properties and make it harder or change the surface properties of the final product.

When I see a Rolex watch and set it beside a watch made from 316L stainless steel, I notice the slightly different sheen the Rolex watch has.  This is most likely due to the significant increase in the critical alloying elements and the addition of copper. 

316L versus 304L Stainless Steel

316L and 304L have many more similarities than 316L and 904L do. As you can see, the impurities are identical, and the carbon, manganese, and nitrogen are also the same. Chromium has been reduced slightly, nickel has increased marginally, and molybdenum has been introduced as an alloying element in the 316L stainless steel. All of these tweaks increase the corrosion resistance of the final alloy. In particular, the addition of molybdenum increases the corrosion resistance as well as the price.  

The difference between 304L and 316L stainless steel is that 316L is more corrosion resistant by the addition of molybdenum. Molybdenum makes 316L more expensive and increases its hardness, with a Rockwell B hardness of 95, which is equivalent to a Vickers hardness of 230. 

How To Identify Which Stainless Steel?

If you are curious about which stainless steel your watch is made out of and it is not clearly engraved on the case back, then it is safe to assume that, if you spent less than US$200, it is most likely made from 304L stainless steel. The more expensive your watch, the more likely it is to be made from 316L stainless steel. It was in the 1990s that the trend for labeling and differentiation of the watch case material started in Switzerland as they were seeking competitive differentiation from the Japanese watch manufacturers who had decimated the Swiss industry over the previous two decades.

Is 304L Stainless Steel Bad for Watches?

Certainly not! Globally, 304L stainless steel is the most commercially important stainless steel for all industries globally.  With such demand, it is made in huge quantities and is, therefore, cheaper to manufacture, not to mention that its constituents are cheaper. The key consideration for a watch owner is whether 304L stainless steel performs as well as 316L stainless steel; for example, will it corrode in any environment that your wrist can encounter? Here the answer is no.  Or will it be more or less resistant to scratching than the alternative of 316L stainless steel?  The answer here is that it is less resistant to scratching. So the choice of 316L stainless steel over 304L is that it will maintain its aesthetics longer than 304L stainless steel longer because it is harder. 

What is in the “L”?

A common theme in all our discussions so far is that every stainless steel reference number is followed by the letter “L.” What does that mean? The “L” simply stands for “low carbon.” This distinction is essential in the manufacture of the watch case because, if you recall, in my first steel blog post, I went into detail about the impact carbon has on the properties of steel. Specifically, the higher the carbon content, the harder and more brittle the final steel will be. This knowledge is vital in the manufacturing process, as the manufacture of a watch case is an exact piece of engineering. Therefore, the machining and finishing need to be done to extremely fine tolerances. A ductile and easily machinable steel is necessary to accomplish this goal, requiring low carbon content. If higher carbon steel were used, the machining may cause chipping and uneven surface finishes—not what a keen watch enthusiast would want.

Scratch Resistant Steel?

Scratches are the bane of all watch owners. They catch your eye when you look at your timepiece closely, or sometimes even when you just check the time if it is a deep scratch. Indeed, that has been my experience, and it is one I am keen to improve as we move forward at SNGLRTY.  I have made constant reference to the effect that both nitrogen and carbon have on stainless steel; they make the steel harder. By making the stainless steel harder, it will make it more scratch-resistant too. This fact can be used to harden just s thin top layer of the finished steel product – this is referred to as case-hardened stainless steel.  

To ‘case-harden’ a watch case, the finished product is placed into a suitable atmosphere of nitrogen or carbon, which over time will diffuse into the first few atomic layers of the stainless steel item and increase the hardness of the surface. This process must complete the manufacture, as once it is performed, it will be difficult to change the shape or finish of the piece because it will be too hard. 

PVD Coating

The case hardening process is similar to adding a physical vapor deposition (PVD) coating to a watch case. From a physical performance perspective, both processes have a very similar result for the user. The key difference between the two is that the PVD coating adds a microscopic layer of material on top of the stainless steel. In contrast, with case hardening, the atoms diffuse into the atomic structure of the steel. This is why PVD coatings can be used to change the final color of a watch case.   

The Exploration Goes On

I had planned this to be the end of our discussion on steel watch cases, but I have received several questions about the use of steel in watchmaking, so over the next few months, I will be exploring these too.  These will cover areas from using historic steel used for knives and swords to make watch cases to how steel is used to make the mainspring and store energy. 

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We look forward to seeing you the next time.