All stainless steel is NOT necessarily
|The magnet test is NOT a correct way to verify 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, both are surface treatments.
are three major classes of stainless steel.
Chromium-nickel-iron alloys with 16%-26% chromium (Cr), 6%-22% nickel (Ni), and
low carbon content, with non-magnetic properties. Type 304 ( 18% chromium, 8%
nickel ) is the most commonly used grade or composition.
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.
3. 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 prolong 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 produce 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 stainless casters has 8.07%
nickel (Ni) and 18.23% chromium (Cr).
The process of stamping and
surface polishing can change the non-magnetic 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-magnetic
property of the stainless and thus create magnetism! Foreign residue can also
We apply an electrocleaning surface treatment to our
stainless caster models K3, 3, 3A, and 9. It 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 caster 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.
The stainless steel we commonly use in our stainless casters is 304.
After cold work (the process of stamping, forming, surface polishing, etc) a
stainless caster usually becomes magnetic in the worked areas. These areas are
relatively prone to rust in a corrosive environment. If it is necessary,
annealing is the most effective way to restore non-magnetic property and
enhance corrosion resistance. Actually it changes martensite back to austenite.
In this process the stainless product is heated to 1800F - 2100F and cooled
down slowly. If the temperature is not high enough the corrosion resistance of
the stainless will be reduced. This process increases cost a lot. The
passivation process washes away free particles and forms a passive coating on
the stainless surface. This process is what we do for only stainless caster
model G15. It is a relatively economic way to enhance corrosion resistance, but
it is not meant to completely restore non-magnetic property.
magnet test is NOT a correct way to verify stainless steel. Stainless steel
is graded by the ingredients and percentages. Stainless is a man made alloy.
The nickel content determines the grade of stainless. The chromium content must
be 18% or more to be 304 stainless. It starts out as non-magnetic. After a 500
ton press squeezes the nickel it changes the distribution of the nickel. The
same is true where the die cuts the stainless increasing the possibility that
rust will eventually occur there. All stainless steel is magnetic except
austenitic stainless steel which is actually 300 series stainless such as 304
and 316. However, 300 series stainless is non-magnetic only after it is freshly
formed. 304 is almost for sure to become magnetic after cold work such as
pressing, blasting, cutting, etc. Initially the cold work causes the stainless
to pick up foreign particles such as free iron. Then at some spots the metallic
crystal structure changes from austenite to martensite. 400 series stainless
(ie. martensitic stainless steel) is magnetic. Stainless steel containing more
nickel (310 and 316 grades) is more likely to remain non-magnetic after cold
All stainless steel is NOT necessarily non-magnetic. It is
common for stainless to pick up iron ions from the die and tool used during the
stamping process. Iron ions cause magnetism and later may cause minor rust. The
casters you received are stainless steel even though your magnet may stick to
them. We use 304 stainless to make our stainless casters. We do not use 316
stainless because it would ruin our tooling. Our customers use our stainless
casters and it is very rare that there is a problem with rust unless their
application is extremely caustic. If your application is extremely caustic you
should require stainless steel that has gone through both the passivation and
annealing processes. Passivation enhances the rust resistance of the stainless
surface. The passivation process is not meant to completely restore
non-magnetic property. It is only a relatively economic way to enhance
corrosion resistance. It is available on special order for very large volume at
substantial extra cost on some of our stainless casters. We use the passivation
process standard only on stainless caster model G15. It removes all ions.
Annealing is the most effective way to restore non-magnetic property and
enhance corrosion resistance. However, in this process if the stainless is not
heat treated high enough and then cooled down slowly the corrosion resistance
of the stainless steel will be reduced. Annealing stainless is almost cost
prohibitive. When both processes are applied, passivation should be done after