Why Titanium Is Used In Watches
General Info on Titanium
Base: TimeZone Forum
Date: Mon, 21 Jul 1997 22:46:46 GMT
From: Justin Time <email@example.com>
It looked like we opened another can of worms with titanium on the Open Forum! Since this topic comes back with regularity, I thought I would post the same info here for a longer-term access.
I am a strong proponent of titanium as a watch material, especially to make cases and bracelets for sports watches. I own several titanium watches. I like them. Much of what has been said about titanium is factual, though some advantages of titanium may have been overstated. It is quite possible that many of the arguments in the Open Forum could have been avoided with a less confrontational tone. Here are some information about titanium that you may find useful as basis of further discussion:
Titanium is a light metal, relatively scratch-resistant, and non-magnetic. Overall, titanium is superior to stainless steel (SS) in terms of density and corrosion resistance, especially in salt water. Its yield strength, tensile strength, and thermal conductivity are better than those of many SS, but not all. Pure titanium is difficult and expensive to produce. This difficult production may also explain why titanium has failed unpredictably and is no longer use in some aerospace applications. Titanium is also harder to work with than SS, thus adding to the overall cost in a watch.
SPECIFIC GRAVITY OF TITANIUM
As a pure metal, titanium weighs more than aluminum, but less than steel. The specific gravity (density relative to water) of common watch metals are (in increasing order): Aluminum (Al) = 2.70; Titanium (Ti) = 4.54; Iron (Fe) = 7.86; SS = 7.7 to 8.0; Silver (Ag) = 10.5; Tantalum (Ta) = 16.6; Gold (Au) = 18.88; Platinum (Pt) = 21.45.
The surface hardness of gold, platinum, and steel varies greatly depending on how these metals were treated (annealed, rolled, etc.) and on other metal(s) used in the alloys. Commercially pure titanium (some impurities) is harder than some SS, but softer than others. The nitrogen-hardened surface of titanium (titanium nitride) is one of the hardest surfaces used in watches (Ventura), in addition to ceramics (Rado).
The hardness of metals surfaces in the arbitrary MOHS scale (1 = hardness of Talc surface; 10 = hardness of Diamond surface) are, in increasing order: Aluminum = 2 to 2.9; Gold (pure) = 2.5 to 3 (typically 2.75); Silver (pure) = 2.5 to 4 (typically 3.25); Platinum (pure) = 4.3; Glass = 5 to 6; Titanium = 5 to 6 (strongly impurity dependent); SS = 5 to 8.5; Platinum-Iridium (950) = 6.5; Quartz = 7; Titanium nitride (TiN) = 9; Diamond (C) = 10
In Brinell Hardness scale: Aluminum (99+) = 23-44; Gold (pure) = 28-48; Silver (99.9%+) = 30-90; Tantalum = 55-123; Platinum (commercial) = 65-101; Silver (sterling, 92.5%) = 65-125; Platinum (10% rhodium) = 79-169; Platinum (10% Iridium) = 104-169; Gold (white 18K) = 211-323; Titanium (commercial) = 200; Titanium (5 Al-2.5Sn) = 34 (RockwellC); SS = 160-400.
Thermal conductivity (sq. ft x hr x ¡F/in.) A low number indicates slow heat transfer and a comfortable feel to the skin: Titanium (commercial) = 114; Titanium (5 Al-2.5Sn) = 54; SS = 96-400.
Yield Strength (0.2% offset; 1000 psi) A high number indicates a strong metal: Titanium (commercial) = 75; Titanium (5 Al-2.5Sn) =120; SS = 30-165.
Tensile Strength (1000 psi) A high number indicates a strong metal: Titanium (commercial) = 85; Titanium (5 Al-2.5Sn) =127; SS = 70-220.
Pure titanium is expensive to produce. Although titanium is relatively abundant–it makes up 0.6% of earth crust from two main ores, ilmenite (FeTiO3) and rutile (TiO2)–pure titanium metal is expensive to produce because of the tortuous Kroll process used. The ores are treated at red heat with carbon and chlorine to yield TiCl4, which is then fractionated from impurities, such as FeCl3. The titanium chloride is then reduced with molten magnesium at about 800¡C in argon atmosphere. The excess Mg and MgCl2 are removed from the titanium metal by volatilization at about 1,000¡C. The pure but spongy titanium is then fused in argon or helium atmosphere, using an electric arc (!) and then cast into ingots. This difficult process may explain why titanium can behave unpredictably in some aerospace usage. Extremely pure titanium made in small scale from the van Arkel-de Boer method cost even more.