Date Published:Dec. 21. 2016
Vol.6 Varieties of Screws "Beyond" SUS
Hello. Nedzigon here!
In vol.5 "Varieties of SUS Screws," I introduced stainless steel screws with various distinctive features. It was noted that iron and stainless steel are not the only metals used in screws. There are other metals with interesting characteristics, each with a specialized function. Today, I would like to introduce you to screws made of these alternative materials.
Light-Weight, Corrosion-Resistant, Non-Magnetic Titanium Screws
Titanium is known as a light-weight, non-magnetic, and corrosion resistant metal.
Its specific gravity is approximately 4.5, which is extremely light-weight - about 60% of the value of iron. The magnetic permeability (the ratio of the magnetic flux density and magnetic field strength) is as low as 1.0001, showing us just how non-magnetic it is when compared with the 1.4 magnetic permeability of SUSXM7.
Also, it has excellent corrosion resistance due to an oxide film (passive film) on the surface, similar to stainless steel. But in contrast to the passive film on stainless steel, it is particularly resistant to corrosion from seawater and chemicals.
Incidentally, the strength of pure titanium is roughly equal to that of SUSXM7, but titanium alloy has the strength that rivals steel screws. Titanium 64 (Ti-6Al-4V), for instance, and beta titanium (Ti-15-3-3-3) are among the titanium alloys mainly used in screws.
Valued for being a light-weight, corrosion resistant and high-strength material, it is used in bicycles, automobiles, and aircraft. Its non-magnetic properties also make it attractive for semiconductor manufacturing equipment that uses plasma.
＞＞Summary of physical properties, mechanical properties, chemical resistance, etc. of titanium
Highly Chemical-Resistant Inconel* and Hastelloy*
Titanium is not the only chemical-resilient metal: inconel and hastelloy are also metals with excellent chemical resistance.
However, there are various types of chemical resistance and the resistance will differ according to the type of metal. For example, inconel demonstrates almost total corrosion resistance to ammonia. Hastelloy C-22 is one of the few metals that can withstand hypochlorous acid and chlorine dioxide. Please refer to the summary list for more information.
＞＞Chemical resistance of inconel, hastelloy, and nickel
＞＞Chemical resistance of titanium
A 2600°C Melting Point! High Heat-Resistant Molybdenum Screws
The melting point of molybdenum is an incredible 2623°C!! Needless to say, the metal in molybdenum screws has excellent heat resistance, with a tensile strength of at least 200 N/mm2 even at 1000°C and above. Since inconel is also a metal with excellent heat resistance, we recommend the use of inconel screws in environments with high ambient temperatures where chemicals are handled.
＞＞Tensile strength at all temperatures
Super Light! Aluminum Screws
Everyone knows that aluminum is famous for its lightness. Its specific gravity is approximately 2.7, about 1/3 that of iron.
Aluminum also features excellent conduction of heat and electricity, as well as yet another important advantage – that is, electrical corrosion does not occur with an aluminum alloy.
Electrical corrosion is a phenomenon in which two metals with different electrical potentials are in contact, and moisture between them acts as a kind of battery to cause corrosion. But since the same metals have the same electrical potential - and if both are aluminum - then no electrical corrosion will occur.
So, can we fasten different kinds of metals using aluminum screws, or fasten aluminum structures using different kinds of metals such as stainless steel? Yes, we can. Electrical insulation can be provided by an anodizing treatment, which is a countermeasure against electric corrosion.
Note that care is needed when contact is made between any metals with different potentials as electrical corrosion is not limited to aluminum.
What About Precision Devices? Super Invar With Extremely Low Thermal Expansion
Super invar is a metal whose principal components are iron, nickel, and cobalt, featuring extremely low dimensional variation due to heat, and with a coefficient of thermal expansion of 0.69×10-6 (K-1). Incidentally, the thermal expansion coefficient of SUS304 is 17.3×10-6 (K-1).
These features can be used to maximum effect with measuring devices and precision instruments that cannot tolerate subtle dimensional variations from screws that expand.
It prevents screw loosening due to repeated expansion and contraction in equipment where temperatures often fluctuate.
I'll be sure to tell you if I come across any other interesting materials.
But that is all for today.