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Unveiling the mystery of ‘Rolex steel’, 904L

Rolex has been regarded as the most valuable brand identity. Owning a Rolex is the immense desire of a number of people as it is supposed to be the symbol of success and the unofficial ‘standard’ for couples of professions, for example, bankers at Wall Street, New York [2]. Rolex watches are expensive partly because of the brand and the stories told along history increasing its value. To some extent, it is disputable that the Rolex watches have superior quality, attractive design. In addition to that, Rolex are the great watchmakers as they integrated a lot of technology renovation into their watches, remarkably in Oyster Perpetual Rotor as the superlative chronometer with the outstanding of watch materials.

When it comes to the Rolex watch materials, 904L immediately hits their mind. Grade 904L, a kind of stainless steel, at first, was used in chemistry, oil refinery, submarine construction or paper industry [4]. After being used in Rolex watchmaking, Grade 904L steel became so famous that it was called ‘Rolex steel’ or ‘Rolex of stainless steel’. The names relatively indicate the quality and speciality of this material. Watch enthusiasts, obviously, are not the experts on materials, therefore, 904L is appeared to be mystery and hard to grasp. This article aims to systematically supply information about 904L and its properties, which partially help readers have the clear and close look at this material.

The first matches for 904L steel search are related to Rolex watches

The first matches for 904L steel search are related to Rolex watches

In fact, applications of 904L ranges in many industries. Source: fitwelindia.com

In fact, applications of 904L range in many industries. Source: fitwelindia.com

A brief history

904L, sometimes called SUS 904L (Society of Automotive Engineers) which means stainless steel  with low carbon content and alloys such as Chromium, Nickel, Molybdenum and copper. This distinctive chemical properties turn 904L’s content into austenite under condition of normal temperature and non-magnetic and corrosion resistance against strong reducing acid such as  sulphuric acid and pitting corrosion resistance.

With those elements, 904L at first was employed in environments which require the materials to have strong corrosion resistance, for example, pipes, joint pipes in mechanical engineering and oil refinery and paper processing industry. Up to now, the majority of stainless steel used in watchmaking industry has been 316L which is also utilized in medical devices. In 1985, Rolex started the experiments in which 904L was used [7,8] and introduced the Submariner ref.168000 models (or Sea Dweller models in some other sources)with this kind of stainless steel  in 1987-1988. However, it was until 2000 that Rolex had replaced the watches’ material from 316L to 904L [5]

Submariner ref. 168000, the first model using 904L which pushes the price high. Source: jamesedition.com.

Submariner ref. 168000, the first model using 904L which pushes the price high. Source: jamesedition.com.

Why 904L?

904L offers users with substantial benefits namely 1) excellent anti-corrosion especially pitting corrosion and 2) better polishability compared to 316L, which makes  it further fascinating. After years of observation and usage, Rolex acknowledge that the complex and varied environment, for instance, seawater and hand sweat, may cause some reactions in 316L. Consequently, the watches quickly become old and loose its inherent beauty. Hence, a kind of stainless steel which possesses the better anti-corrosion property is wanted to replace the old materials. As the result, here comes 904L with higher pitting resistance number (PREn) of 36 PREn in comparison to that of 326L that has approximately 26 PREn.

The chart presents the comparison between 904L and 316L in corrosion resistance ability in sulphuric acid. The chart shows that Biểu đồ cho thấy, với cùng lượng ăn mòn, nhiệt độ/nồng độ acid tối thiểu với 904L hầu như luôn cao hơn 316L. Ảnh: sandmeyersteel.com.

The chart presents the comparison between 904L and 316L in corrosion resistance ability in sulphuric acid. The chart shows that Biểu đồ cho thấy, với cùng lượng ăn mòn, nhiệt độ/nồng độ acid tối thiểu với 904L hầu như luôn cao hơn 316L. Ảnh: sandmeyersteel.com.

Comparison in the surface finishing between Tudor Black Bay (left, 316L) and Rolex Submariner (right, 904L). The tiny difference still can be seen. Source: ABlogToWatch.

Comparison in the surface finishing between Tudor Black Bay (left, 316L) and Rolex Submariner (right, 904L). The tiny difference still can be seen. Source: ABlogToWatch.

904L – Chemical elements and properties

904L, beside iron and a small amount of carbon (less then 0.02%), contains 19-23% Cr, 23-28% Ni, 4-5% Mo, <2.0% Mn, <1.0% Si, <0.045% P, <0.035% S and 1.0-2.0% Cu [4]. Owning these chemical elements, 904l stays in non-stabilized austenite in normal temperature. This is a high alloy stainless steel added with Cr, Ni, Mo, Cu, etc. to enhance the corrosion resistance. Notably, the stress corrosion cracking and pitting corrosion resistance was significantly improved. 904L stainless steel is non-magnetic with high formability, toughness and weldability.

It is easy to see that the high content (roughly a half, 44-56%) of expensive elements such as Cr, Ni and Mo obviously pushes the price high. Some figures point out that 904L stainless steel worth more than double the price of 316L. That is the reason why not many brands choose 904L except for the reason of high technology. In these days, 904L stainless steel has gradually been replacing by duplex 2205. 904L has normal mechanical properties in which toughness and durability are fairly similar to 316L, but the highly appreciated formability and weldability. Complementing to the corrosion resistance in normal temperature, 904L demonstrates the ability of oxidation resistance as burning below 400°C [4].

904L stainless steel production

The production process of Rolex watchcases comprises some major stages: The overall process is clarified by this following video [13].

 

To produce watchcases from 904L, Rolex did change their assembly line, equipment as well as the manufacturing process and even developed a laboratory specializing in research on this stainless steel [10]. Especially, Rolex produced watchcases by it own rather than outsourcing to reduce prices. These movements showed its determination in technology renovation and the quality preferences more than anything. It is a clear explanation why Rolex watches are so promising and expecting.

904L in Rolex watches: Misunderstandings

There are couples of misunderstandings about stainless steel in Rolex watches. It is common thought that 904L has high mechanical durability. However, it is not right.  As for durability, 904L (490MPa) is not much higher than 316L (495MPa) [14]. In term of toughness (measured in Brinell-HB),  904L with 150HB is even lower than 316L with 217HB and high carbon steels used in mechanics such as AISI 1065 with 187HB [15].

The density of 904L is 7900kg/m3 as the same as  of 316L’s [4,14] and quite similar as the other stainless steels, therefore, there is no impossible that 904L is heavier than the others. As mentioned before, 904L stainless steel is non-magnetic austenite using magnet…. This, obviously, does not mean that watches have magnetic resistance. If 904L stainless steel had been used, Rolex watches would have been somehow affected as it was placed in magnetic environment (around electric and electronic devices). To resist magnetics, Rolex employ a shield made by ferromagnetic alloys covering watches’ movements from magnetic effects. This technology was applied in renowned antimagnetic watch called Milgauss since 1956.

Conclusion

The article introduced to you the famous stainless steel, 904L, using in Rolex watches manufacturing, including its history, applications, elements and properties. Also, the stainless steel process and some misunderstandings were all clarified. Reading through the article, readers can understand why the material is expensive and the watches are extremely desirable.

We strongly hope that some practical and useful information which was provided in this article benefits you in playing and collecting watches.

Nova @ Novatime Lab.

This article was written in Novatime Lab’s Technical Report (Watch Research). It can be updated in the future for better quality.

References

[1] Clock this: Rolex watch is the most desired brand in Britain, Ruki Sayid on Mirror, URL: http://www.mirror.co.uk/news/uk-news/clock-this-rolex-watch-is-the-most-744158

[2] A Wall Street guide to watches, GSelevator on Business Insider, URL: http://www.businessinsider.com/a-wall-street-guide-to-watches-2015-8

[3] Oyster Perpetual Movement, Rolex Watch Making, URL: https://www.rolex.com/watches/rolex-watchmaking/movements.html

[4] Stainless Steel – Grade 904L (UNS N08904), AZO Materials, URL: http://www.azom.com/article.aspx?ArticleID=1022

[5] 904L – The Rolex of stainless steel, Rob Ellis on West Yorkshire Steel, URL: https://www.westyorkssteel.com/blog/904l-the-rolex-of-stainless-steel

[6] SAE 904L stainless steel, Wikipedia, URL: https://en.wikipedia.org/wiki/SAE_904L_stainless_steel

[7] Interesting read on Rolex 904L Steel, arizonapaul on Rolexforums, URL: https://www.rolexforums.com/showthread.php?t=483808

[8] Materials, Rolex Watch Making, URL: https://m.rolex.com/watches/rolex-watchmaking/materials.html

[9] 904 and 316 steel: allergy questions, mike on Rolexforums, URL: https://www.rolexforums.com/showthread.php?t=37978

[10] 10 Things To Know About How Rolex Makes Watches, Ariel Adams on ABlogToWatch, URL: http://www.ablogtowatch.com/10-things-know-rolex-makes-watches

[11] What is the big deal of Rolex’s use of 904L steel?, Matthew Becker on Beckertime, URL: https://beckertime.com/blog/what-is-the-big-deal-of-rolexs-use-of-904l-steel

[12] Relative Cost Ratio, Steeltank, URL: http://www.steeltank.com/Portals/0/Pressure%20Vessels/SSWseminarOct2012/Relative%20Cost%204%2015%202012.pdf

[13] Rolex 904L Steel, SpiritV12 on Youtube, URL: https://www.youtube.com/watch?v=9UuDJl93Z1g

[14] Stainless Steel – Grade 316L – Properties, Fabrication and Applications (UNS S31603), AZO Materials, URL: http://www.azom.com/article.aspx?ArticleID=2382

[15] AISI 1065 Carbon Steel (UNS G10650), AZO Materials, http://www.azom.com/article.aspx?ArticleID=6575

[16] Rolex Milgauss Watch 904L Steel – 116400GV, Rolex Watch Making, URL: https://www.rolex.com/watches/milgauss/m116400gv-0001.html

Author: Jennie Ng

A people person

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