Q. What is stainless
steel?
Stainless steel is the generic name for a number of different
steels used primarily because of their corrosion resistance.
All stainless steels share a minimum percentage of 10.5% chromium.
It is this element that reacts with the oxygen in the air
to form a complex chrome-oxide surface layer that is invisible
but strong enough to prevent further oxygen from "staining"
(rusting) the surface.
The film itself is extremely thin, about 130 Angstroms and
one Angstrom is one millionth of a centimeter. This layer
is described as passive (does not react or influence other
materials), tenacious (clings to the layer of steel and is
not transferred elsewhere) and self-renewing (if damaged,
more chromium from the steel will be exposed to the air and
form more chromium oxide).
This means that over a period of years a stainless steel knife
can literally be worn away by daily use and will still remain
stainless. Higher levels of chromium and the addition of other
alloying elements such as nickel and molybdenum enhance this
surface layer and improve the corrosion resistance of the
stainless material. Chromium is always the deciding factor,
although other elements, particularly nickel and molybdenum,
are added to improve corrosion resistance.
Furthermore, stainless steel is fully recyclable.
The three
major classes of stainless steel are:
Austenitic: Chromium-nickel-iron alloys with 16-26% chromium,
6-22% nickel (Ni), and low carbon content, with non-magnetic
properties (if annealed - working it at low temperatures,
then heated and cooled). Nickel increases corrosion resistance.
Harden able by cold-working (worked at low temperatures) as
well as tempering (heated then cooled). Type 304 (S30400)
or "18/8" (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, harden able by quenching
(quickly cooled in water or oil) and tempering (heated then
cooled). It has magnetic properties. Commonly used in knives.
Martensitic grades are strong and hard, but are brittle and
difficult to form and weld. Type 420 (S42000) is a typical
example.
Ferritic: Chromium-iron alloys with 17-27% chromium and low
carbon content, with magnetic properties. Cooking utensils
made of this type contain the higher chromium levels. Type
430 is the most commonly used ferritic. Two additional classes
worth mentioning include Duplex (with austenitic and ferritic
structures), and Precipitation Hardening stainless steel,
used in certain extreme conditions.
Q. What
does chrome-nickel content mean?
Chrome-Nickel content refers to the percentage of chrome and
nickel in the stainless steel. Chrome gives the stainless
steel its luster & durability, while the nickel is for
the hardness & strength. 18/10 Stainless Steel means it
contains 18% Chromium or Chrome and 10% Nickel. A sink with
an 18/10 chrome-nickel content is considered to be an excellent
grade, due to its corrosive resistance and greater durability.
Q. What
is the difference between 18/8 and 18/10 stainless steel?
The first number is the amount of chromium that is contained
in the stainless, i.e., 18 is 18% chromium. The second number
is the amount of nickel, i.e., 8 stands for 8% nickel. So
18/8 means that this stainless steel contains 18% chromium
and 8% nickel. 18/10 is 18% chromium and 10% nickel.
The higher the numbers the more corrosion resistant the material.
Both 18/8 and 18/10 contain nickel and are part of the grade
family "300 series" stainless. 18/0 means that there
is 18% chromium but zero nickel. When there is no nickel the
stainless grade family is the "400 series".
400 series are not as corrosion resistant as the 300 series
and are magnetic, where the 300 series are non-magnetic and
is the grade mainly used for sinks, Food processing equipment,
Restaurant food preparation areas etc.
Q. What is the
difference between 304 and 316 stainless steel?
304 contains 18% chromium and 8% nickel. 316 contains 16%
chromium, 10% nickel and 2% molybdenum. The "moly"
is added to help resist corrosion to chlorides (like sea water
and de-icing salts.
Q. Can stainless
steel be "welded"?
YES. Stainless steel is easily welded, but the welding procedure
is different than that used with carbon steel. The "filler"
rod or electrode must be stainless steel.
Q. What temperatures can stainless steel withstand?
Stainless steel has excellent properties at both extremes
of the temperature scale. Some stainless steel can be used
down to liquid nitrogen temperatures and some up to about
1800° F.
Q. What is the
recycle content of stainless steel?
Stainless steel can be recycled 100%. That is all stainless
steel can be re-melted to make new stainless steel. The typical
amount of recycled stainless steel "scrap" that
is used to make new stainless steel is between 65 & 80%.
Q. Can stainless
steel rust? Why? (I thought stainless did not rust!)
Stainless does not "rust" as you think of regular
steel rusting with a red oxide on the surface that flakes
off. Corrosion is generally caused by contaminants settling
on the surface of the stainless steel.
Q. Is stainless
steel magnetic?
There are several "types" of stainless steel. The
300 series (which contains nickel) is NOT magnetic. The 400
series (which just contains chromium and no nickel) ARE magnetic.
Q. How
are different classes of stainless steels used?
The austenitic microstructure is most commonly used for knives
and cooking utensils. It is very tough, hardened through a
process that consists of heating, cooling and heating. It
resists scaling and retains strength at high temperatures.
Both ferritics and austenitics are used in kitchenware and
household appliances. Austenitics are preferred in the food
industry and beverage equipment due to the superior corrosion
resistance and ease of cleaning. Type 301, for example, is
an austenitic stainless steel, with 17% chromium, 7% nickel,
and .05% carbon, and is widely used for institutional food
preparation utensils.
You can easily make do with the lesser quality cookware for
most oven use. For stovetop cooking, however, don't skimp
on quality; buy only the better ones. Most manufacturers of
high quality cookware use stainless steel similar to the Type
304 grade, with thick heat diffusing bottoms. Metals that
provide better diffusion of heat, such as copper and aluminum,
are attached to the bottom for heat diffusion, to prevent
hot spots and uneven cooking.
Low quality cutlery is generally made out of grades like 409
and 430 (ferritic), while the finest Sheffield cutlery uses
specially produced 410 and 420 (martensitic) for the knives,
and 304 (austenitic) for the spoons and forks. Grades like
the 410/420 can be hardened and tempered so that the knife
blades will take a sharp edge, whereas the more ductile 304
stainless is easier to work and therefore more suitable for
objects that have to undergo numerous shaping, buffing and
grinding processes.
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