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Properties of our Stainless Steel Casters
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Stainless Steel is a common name for metal alloys that consist of
10.5% or more Chromium(Cr) and more than 50% Iron(Fe). Although it is called
“Stainless”, a better term for it is “highly stain
resistant”. It is a darker metal. There are two process methods to make it
bright, surface treatments.
There are three major classes of stainless
steel,
• Austenitic: Chromium-nickel-iron alloys with 16-26% chromium
(Cr), 6-22% nickel(Ni), and low carbon content, with non-magentic properties.
Type 304(18% chromium, 8% nickel), is the most commonly used grade or
composition.
• Martensitic: Chromium-iron alloys with 10.5-17%
chromium and carefully controlled carbon content. It has magnetic properties.
Type 420 is a typical example. It is mostly used in knives and kitchen
equipment.
• Ferritic: Chromium-iron alloys with 17-27% chromium and
low carbon content, with magnetic properties. Type 430 is the most commonly
used ferritic. The chromium content in stainless steel alloys is what generally
prevents corrosion.
The chromium works by reacting with oxygen to form
a tough, adherent, invisible, passive layer of chromium oxide film on the steel
surface. If damaged mechanically or chemically, this film is self healing as
long as it has enough oxygen. Since oxygen is necessary for reaction, liquids
and other things stored for a prolonged time in stainless can prevent oxygen
contact and thus promote corrosion. If you remove rust with a cloth the
chromium will chemically react with the oxygen in the air and produces a new
protective layer.
An increase of chromium content improves the
corrosion resistance of stainless steel. The addition of nickel is used to
raise the general corrosion resistance required in more aggressive usage or
conditions. The presence of molybdenum(Mo) improves the localized corrosion
resistance. Other alloy metals are also used to improve the structure and
properties of stainless steel, such as Titanium, Vanadium, and Copper. Non
metal additions typically include natural elements such as Carbon and Nitrogen,
as well as Silicon. The S304 we use to make our casters has 8.07% Ni, and
18.2277% Cr.
The process of stamping and surface polishing can change
the non-magentic property of S304. The pressure used in stamping and forming
changes the distribution of chemicals in the alloy. The stamping die used can
also chemically leave iron ions on the surface of the stainless steel. Both
factors can cause change to the non-magentic property of the stainless, and
thus create magnetism. Foreign residue can also be corrosive.
We apply
an electrocleaning surface treatment to our stainless Models K3, 3, 3A, and 9.
This does not necessarily prevent the stainless steel from picking up iron
particles as the result of how it is handled or stored.
We apply an
additional passivation treatment only to our stainless Model G15 using nitric
or mild organic acids to enhance the protective nature of the natural,
air-formed film. Nitric acid treatment enhances the level of chromium in the
protective film on stainless steel. This passivation treatment also makes it
easier to remove rust if it were to occur.
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