Hello. Nedzigon here!
vol.5 "Varieties of SUS Screws"introduced stainless steel screws with various different features. However, iron and stainless steel are not the only metals used in screws. There are other metals with interesting characteristics, each with a particular role to play. Today, I would like to introduce you to screws made of these special materials.
Lightweight, non-magnetic titanium screws with excellent corrosion resistance
Titanium is known as a lightweight, non-magnetic, and corrosion resistant metal.
Its specific gravity is approximately 4.5, which is extremely lightweight, 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 the generation of an oxide film (passive film) on the surface, similar to stainless steel; but in contrast to the passive film on stainless steel, it is especially resistant to corrosion from seawater, as well as chemicals.
Incidentally, the strength of pure titanium is roughly equal to that of SUSXM7, but titanium alloy has a strength that can rival that of 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.
Featuring light weight with corrosion resistance and high strength, it is used in bicycles, automobiles, and aircraft. Its non-magnetic properties also make it attractive for semiconductor manufacturing equipment using plasma.
＞＞Summary of physical properties, mechanical properties, chemical resistance, etc. of titanium
Highly chemical-resistant inconel* and hastelloy*
Titanium is not the only metal resistant to chemicals: inconel and hastelloy are also metals with excellent chemical resistance.
However, there are various types of chemical resistance too, 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 which can withstand hypochlorous acid and chlorine dioxide. Please refer to the list summarizing this content for more information.
＞＞Chemical resistance of inconel, hastelloy, and nickel
＞＞Chemical resistance of titanium
A 2600°C melting point! Highly heat-resistant molybdenum screws
The melting point of molybdenum is an incredible 2623°C!! 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, introduced above, 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 both heat and electricity, as well as yet another important advantage.
That is, electrical corrosion does not occur with 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, if both are aluminum, then clearly no electrical corrosion will occur.
So we can't fasten different kinds of metals using aluminum screws, or fasten aluminum structures using different kinds of metals such as stainless steel? No, that's not true. Electrical insulation can be provided by anodizing treatment, which is a countermeasure against electric corrosion.
Note that care is needed with contact 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
A metal whose principal components are iron, nickel, and cobalt, featuring extremely low dimensional variation due to heat, 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).
This feature can be used to the full in measuring devices and precision instruments which cannot tolerate subtle dimensional variations from screws expanding.
It can also prevent screw loosening due to repeated expansion and contraction in equipment where the temperature frequently changes.
I'll be sure to tell you if I come across any other interesting materials.
But that is all for today.