The main elements in
Chromium and Nickel:
Chromium is the element that makes stainless steel
stainless. It is essential in forming the passive film. Other
elements can influence the effectiveness of chromium in forming
or maintaining the film, but no other element by itself can
create the properties of stainless steel.
At about 10.5% chromium, a weak film is formed and will provide
mild atmospheric protection. By increasing the chromium to
17-20%, which is typical in the type-300 series of austenitic
stainless steels, the stability of the passive film is
increased. Further increases in the chromium content will
provide additional protection.
Nickel will stabilize the austenitic structure (the grain or
crystal structure) of the stainless steel and enhance the
mechanical properties and fabrication characteristics. A nickel
content of 8-10% and above will decrease the tendency of the
metal to crack due to stress corrosion. Nickel also promotes
repassivation in case the film is damaged.
Manganese, in association with nickel, performs many of
the functions attributed to nickel. It will also interact with
the sulfur in stainless steel to form manganese sulfites, which
increases the resistance to pitting corrosion. By substituting
manganese for nickel, and then combining it with nitrogen,
strength is also increased.
Molybdenum, in combination with chromium, is very effective in
stabilizing the passive film in the presence of chlorides. It is
effective in preventing crevice or pitting corrosion.
Molybdenum, next to chromium, provides the largest increase in
corrosion resistance in stainless steel. Edstrom Industries uses
316 stainless because it contains 2-3% molybdenum, which gives
protection when chlorine is added to the water.
Carbon is used to increase strength. In the martensitic grade,
the addition of carbon facilitates hardening through
Nitrogen is used to stabilize the austenitic structure of
stainless steel, which enhances its resistance to pitting
corrosion and strengthens the steel. Using nitrogen makes it
possible to increase the molybdenum content up to 6%, which
improves corrosion resistance in chloride environments.
Titanium and Miobium:
Titanium and Miobium are used to reduce the sensitization of
stainless steel. When stainless steel is sensitized,
intergranular corrosion can occur. This is caused by the
precipitation of chrome carbides during the cooling phase when
parts are welded. This depletes the weld area of chromium.
Without the chromium, the passive film cannot form. Titanium and
Niobium interact with carbon to form carbides, leaving the
chromium in solution so a passive film can form.
Copper and Aluminum:
Copper and Aluminum, along with Titanium, can be added to
stainless steel to precipitate its hardening. Hardening is
achieved by soaking at a temperature of 900¢X to 1150¢XF. These
elements form a hard intermetallic microstructure during the
soaking process at the elevated temperature.
Sulfur and Selenium:
Sulfur and Selenium are added to 304 stainless to make it
machine freely. This becomes 303 or 303SE stainless steel, which
is used by Edstrom Industries to make hog valves, nuts, and
parts that are not exposed to drinking water.
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