The U-boat steel / Das U-Boot Stahl of Sinn watches
Well, I can see that it is time to post this. l wanted to wait until I had even more information about the U-boat steel used in the Sinn U-series watches, namely the U1, the U2 (EZM 5), the UX (EZM 2B) and the U1000 (EZM 6) and some additional detail about the process making and finishing steel...but, there seems to be enough argument and misguided thought expressed on the watch forums to hasten this post.
First, Sinn U-series watches use High Yield (HY) strength steel alloy forgings, which are based upon high-strength low-alloy steels; HY-80 and HY-100 Ni-Cr-Mo steels. These are NOT 316L, 316L VM and 904L steels typically used in watch industry. Most divers’ watches are made of either 316L or 316L VM stainless steel; only Rolex uses 904L stainless steel (Submariner and Sea Dweller). The HY-80 and HY-100 steels are characterized by a high carbon content ranging from about 0.12 to 0.20 wt% (percent by weight). There are also other high-strength low-alloy (HSLA) steels; HSLA-80 and HSLA-100 steels. The HSLA-80 and HSLA-100 steels have low carbon content (0.07 wt% maximum) and exhibit a ferritic and bainitic microstructure respectively. The difference btw HY and HSLA alloy steels is minor; HY alloy steels are used in submarines (a bit “stronger” than HSLA) and HSLA alloy steels are used in surface ships (easier to weld than HY). For example: U.S. Navy surface ships and submarines are presently constructed of structural steel plates that are rolled to four basic yield strength (kips per square inch or ksi) levels: ordinary or medium (aka mild) strength (32-34 ksi); higher-strength or high-tensile strength (HTS) (45.5-51 ksi); high-yield strength Grade HY-80 or HSLA-80 (80 ksi); high-yield strength HY-100 or HSLA-100 (100 ksi).
Special High Yield (HY) steel alloys have been developed to increase the diving depth of submarines, although the improved depth performance of these alloys imposes a price of increased fabrication challenges. These special steels are denominated by their yield stress in thousands of pounds per square inch. The MIL-S-16216K and MIL-S-16216 specifications for high-pressure HY-100 steel alloy and HY-80 steel alloy is 350 Vickers hardness. HY-80 steel alloy has a yield stress of 80,000 pounds per square inch (psi); corresponding to a theoretical depth of 5,443.677 ATM ≈ 54,436 meters ≈ 178,597 feet. HY-100 steel alloy has a yield stress of 100,000 pounds per square inch (psi); corresponding to a theoretical depth of 6,804.5961 ATM ≈ 68,046 meters ≈ 223,247 feet.
1. Pound per square inch (psi) ≡ 1 lbf/in² ≈ 6.894757 kPa ≈ 6,894.757 Pa
2. Atmosphere (standard) (ATM) ≡ 101,325 Pa
3. 80,000 pounds per square inch ≡ 56.2455663711 kgf per square mm
4. 100,000 pounds per square inch ≡ 70.3069579639 kgf per square mm
5. kilogram-force per square millimeter (kgf/mm²) ≡ 1 kgf/mm² = 9.80665 MPa ≈ 9,806,650 Pa
Theoretical calculations and values:
1. HY-80 steel alloy:
(6,894.757 Pa/101,325 Pa) * 80,000 = 5,443.6769 ATM ≈ 54,436 m ≈ 178,597 feet
9,806,650 Pa * 56.2455663711 = 551,580,583.4534689069 Pa
551,580,583.4534689069 Pa/101,325 Pa = 5,443.6771 ATM ≈ 54,436 m ≈ 178,597 feet.
2. HY-100 steel alloy:
(6,894.757 Pa/101,325 Pa) * 100,000 = 6,804.5961 ATM ≈ 68,046 m ≈ 223,247 feet
9,806,650 Pa * 70.3069579639 = 689,475,729.316679935 Pa
689,475,729.316679935 Pa /101,325 Pa = 6,804.5964 ATM ≈ 68,046 m ≈ 223,247 feet
Secondly, the case of the following U-series watches is untegimented: The U1, the U2 (EZM 5) and the UX (EZM 2B). The HY-100 U-boat steel case of the U1, the U2 (EZM 5) and the UX (EZM 2B) has circa 350 HV on a Vickers Hardness scale and the tegimented bezel of these watches has circa 1,500 HV on a Vickers Hardness scale. The sapphire crystal of the U1, the U2 (EZM 5) and the UX (EZM 2B) has circa 1,800 HV on a Vickers Hardness scale. The HY-100/HY-80 U-boat steel is also anti-magnetic. This is very important in the sub industry, as subs need to avoid magnetic contact as well as contact with magnetic mines. And we all know the advantages of having anti-magnetic qualities in watches. The HY-100/HY-80 U-boat steel is highly resistant to corrosion caused by seawater. The HY-100/HY-80 U-boat steel has seawater pitting resistant at 38 PRE. PRE = % Cr (chromium) + 3.3 x % Mo (molybdenum) + 30 x % N (nitrogen). Since the HY-100/HY-80 U-boat steel is very difficult material to work with, it takes close to a month to make this steel.
As for the comparison; the annealed austenitic stainless steels, 316L and 316L VM, have 150 - 190 HV on a Vickers Hardness scale. If these austenitic stainless steels, 316L and 316L VM, are cold hardened, they can achieve 250 - 300 HV on a Vickers Hardness scale. Normal hardened austenitic stainless steel used in watches (usually either 316L or 316L VM) has a hardness of between 200 and 240 HV on a Vickers Hardness scale. The austenitic stainless steels, 316L and 316L VM, have sea water pitting resistant at 26 – 30 PRE; on average 28 or less. With these normal austenitic stainless steel watches, one must wash the watch carefully with fresh water to get the seawater off, since it will pit over time.
The super austenitic stainless steel 904L (annealed) has circa 150 - 190 HV on a Vickers Hardness scale and cold hardened super austenitic stainless steel 904L can achieve 250 - 300 HV on a Vickers Hardness scale. Normal hardened super austenitic stainless steel used in watches has a hardness of between 200 and 240 HV on a Vickers Hardness scale. Therefore, there are no differences in hardness between 316L, 316L VM and 904L stainless steels used in watches. The super austenitic stainless steel 904L has seawater pitting resistant at 35 PRE; on average 32 PRE (better than with 316L, 316L VM steels and worse than with the HY-100/HY-80 U-boat steel). The yield strength and tensile strength of the super austenitic stainless steel 904L are a little bit better (not notable) than with the austenitic stainless steels, 316L and 316L VM, but fall far behind of the strengths of the HY-100/HY-80 U-boat steel.
Thirdly, the tegiment (Kolsterizing) technology; a special hardening process that creates a hardened barrier about 25 microns deep on the upper layer, thus the upper layer of the steel becomes as hard as ceramics. The Tegiment layer is not a coating consisting of a foreign material, instead the HY-100/HY-80 U-boat steel itself that has been hardened as much as 1,500 HV on a Vickers Hardness scale. This tegimenting process provides extreme protection from scratches and damage to the case. If the case has the black PVD treatment, this process also acts to harden the black PVD treatment on the case. The black color is not necessarily a result of the tegimenting (Kolsterizing) process. The blackening comes from the surface activation, which is done prior to carburizing. We all know the advantages of having the tegimented or Kolsterized watch; we won’t be able to scratch it as long as we don’t use much pressure. However, if we hit something hard, e.g. steel door handle, use scissors, a tegimented watch will get dents and scratches and there’s no way to refinish or restore the tegimented case or bracelet. If we bead blast a tegimented watch case, the tegimented layer will be destroyed within seconds.
There exists a handful of other low-temperature, between 400 and 600 Celsius, carburizing processes besides tegimenting (Kolsterizing), these processes are usually very well guarded and patented. The downside of these processes is that the corrosion resistance is severely affected. The process imparts nitrogen and carbon into the surface and carbon ties up the chromium such that it cannot form a protective chromium oxide surface layer. The bulk corrosion resistance of stainless is not much better than carbon steel. One has to rely almost exclusively on the corrosion resistance of the resulting case, which will be better than carbon steel but not as good as stainless. This may (will?) weaken partly (totally?) the excellent corrosion resistant of the HY-100/HY-80 U-boat steel. The features of the tegimented or Kolsterized watch: Distortion free, high hardness and exceptional wear resistance, prevention of “galling”, improvement of fatigue strength full retention of corrosion resistance on certain base materials and retention of non-magnetic properties.
The only totally tegimented U-series watch is the U1000. The U1000 is made of the HY-100 U-boat steel and has circa 1,500 HV case, bezel and bracelet on a Vickers Hardness scale due to a tegimenting process. Underneath the tegimented upper layer, the HY-100 U-boat steel has circa 350 HV on a Vickers Hardness scale. The sapphire crystal of the U1000 has circa 1,800 HV on a Vickers Hardness scale. The HY-100/HY-80 U-boat steel of the U1000 is also anti-magnetic. The U1000 should also have seawater pitting resistant at 38 PRE, like the other U-series watches, but as the tegimenting (Kolsterizing) process may weaken partly (or totally) the very high corrosion resistance of the HY-100/HY-80 U-boat steel, this PRE value is questionable. Furthermore, it’s not commonly known what the tegimenting (Kolsterizing) process may do to the excellent yield stress of the HY-100/HY-80 U-boat steel.
Fourtly, already 30 years ago IWC pioneered the use of titanium in watches. In the late 1990’s (1997/1998) IWC achieved a very high level of Vickers Hardness of 2,400 HV for the bezel of titanium GST Aquatimer (3536-01) using nitriding process for hardening titanium. In addition to that IWC was able to harden the case of the titanium GST Aquatimer up to 360 HV on a Vickers Hardness scale. However, the bracelet of the titanium GST Aquatimer has only 180 HV on a Vickers Hardness scale (180 HV is a typical Vickers Hardness for titanium watches even currently). IWC’s titanium alloys contained aluminium and vanadium with a tensile strength of 1,000 MPa and extreme high temperature stability. At the same time IWC developed techniques for polishing and abrasive blasting of titanium with sapphire beads. IWC used grade 5 titanium alloy for the GST Aquatimer’s case and grade 1 titanium alloy for the GST Aquatimer’s bracelet. Hence, titanium IWC GST Aquatimer’s values (HV, tensile strength and yield strength) closely match or may even exceed Sinn U-series values.
Fifthly, let’ take a closer look at the already discussed submarine applications of the HY-100/HY-80 U-boat steel. A submarine's hull is normally constructed of steel, or exceptionally of titanium (Russian subs). Special High Yield (HY) steel alloys have been developed to increase the diving depth of submarines, although the improved depth performance of these alloys imposes a price of increased fabrication challenges. These special steels are denominated by their yield stress in thousands of pounds per square inch; therefore the HY-80 steel has a yield stress of 80,000 pounds per square inch and the HY-100 has a yield stress of 100,000 pounds per square inch. There aren't any known submarine applications of the newest HY-130 U-boat steel, which has a yield stress of 130,000 pounds per square inch.
During World War II, American fleet submarines normally operated at a depth of 200 feet, though in emergencies they would dive to a depth of 400 feet. Post-War American submarines, both conventional and nuclear, had improved designs and were constructed of improved materials (the equivalent of HY-42). These boats had normal operating depths of some 700 feet, and a crush depth of 1,100 feet. The Thresher, the first American submarine constructed of HY-80, reportedly had a normal operating depth of 1,300 feet. The US SSN-21 Seawolf submarine –class; the first American submarine constructed of HY-100; has a max operating depth of 2,000 feet (610 m). The US SSN-774 Virginia submarine –class, also constructed of HY-100 steel, has also a max operating depth of 2,000 feet (610 m). The German Type 212A submarine –class, constructed of HY-100 steel as well as Sinn U-series watches, has a max operating depth of over 2,296 feet (700 m). The deepest diver is the Russian Alfa submarine -class, constructed of titanium, reportedly has a max operating depth of over 4,265 feet (1,300 m). Submarine designers normally intend their creations to operate well away from the hull's physical limits, imposing a safety margin that varies from country to country (1.5 in the USA, 1.75 in the UK, and 2.0 in Germany). Typically a submarine will have three diving depths: 1) a normal operating or test depth, 2) a safe excursion depth and 3) a crush or collapse depth.
Finally, Sinn U-series offer high class divers’ watches for professional and recreational users made of the very best steel available at the moment; the HY-100/HY-80 U-boat steel. UX watches are currently used by the professional elite law enforcement and military divers, that’s very well known fact. There’s no question about it that also U1000 watches will be also used by the professional elite law enforcement and military divers in the very near future. Quite likely U1 and U2 watches have already seen similar use. At the present time, it seems that Sinn U-series watches are in the league of their own in the terms technological innovations and solutions, particularly in the material applications with the exception of the already old, but still technically advanced titanium IWC GST Aquatimer.
Use all of this information and then you can decide whether Sinn U-series watches meets your needs and requirements.
I hope this is of help.
Copyright, 2008, Ice42
Some additional reading (of course there's more available on the Net):
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