Rabu, 23 Maret 2011

A BRIEF OVERVIEW OF STAINLESS STEEL


Stainless steel is primarily when corrosion or oxidation are a problem. The function that
they perform cannot be duplicated by other materials for their cost. Over 50 years ago, it
was discovered that a minimum of 12% chromium would impart corrosion and oxidation
resistance to steel. Hence the definition “Stainless Steels”, are those ferrous alloys that
contain a minimum of 12% chromium for corrosion resistance. This development was the
start of a family of alloys which has enabled the advancement and growth of chemical
processing and power generating systems upon which our technological society is based.
Subsequently several important sub-categories of stainless steels have been developed.
The sub-categories are austenitic, martensitic, ferritic, duplex, precipitation hardening and
super alloys.

Austenitic Grades
Austenitic grades are those alloys which are commonly in use for stainless applications.
The austenitic grades are not magnetic. The most common austenitic alloys are ironchromium-
nickel steels and are widely known as the 300 series. The austenitic stainless
steels, because of their high chromium and nickel content, are the most corrosion resistant
of the stainless group providing unusually fine mechanical properties. They cannot be
hardened by heat treatment, but can be hardened significantly by cold-working.
 
 
 
 
Straight Grades

The straight grades of austenitic stainless steel contain a maximum of .08% carbon. There
is a misconception that straight grades contain a minimum of .03% carbon, but the spec
Stainless Plate Products
does not require this. As long as the material meets the physical requirements of straight
grade, there is no minimum carbon requirement.

“L” Grades

The “L” grades are used to provide extra corrosion resistance after welding. The letter

“L” after a stainless steel type indicates low carbon (as in 304L). The carbon is kept to
.03% or under to avoid carbide precipitation. Carbon in steel when heated to temperatures
in what is called the critical range (800 degrees F to 1600 degrees F) precipitates out,
combines with the chromium and gathers on the grain boundaries. This deprives the steel
of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By
controlling the amount of carbon, this is minimized. For weldability, the “L” grades are
used. You may ask why all stainless steels are not produced as “L” grades. There are a
couple of reasons. First, the “L” grades are more expensive. In addition, carbon, at high
temperatures imparts great physical strength
Frequently the mills are buying their raw material in “L” grades, but specifying the
physical properties of the straight grade to retain straight grade strength. A case of having
your cake and heating it too. This results in the material being dual certified 304/304L;
316/316L, etc.

“H” Grades

The “H” grades contain a minimum of .04% carbon and a maximum of .10% carbon and
are designated by the letter “H” after the alloy. People ask for “H” grades primarily when
the material will be used at extreme temperatures as the higher carbon helps the material
retain strength at extreme temperatures.

You may hear the phrase “solution annealing”. This means only that the carbides which
may have precipitated (or moved) to the grain boundaries are put back into solution
(dispersed) into the matrix of the metal by the annealing process. “L” grades are used
where annealing after welding is impractical, such as in the field where pipe and fittings
are being welded.

Type 304
The most common of austenitic grades, containing approximately 18% chromium and 8%
nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries,
for heat exchangers, and for the milder chemicals.

Type 316
Contains 16% to 18% chromium and 11% to 14% nickel. It also has molybdenum added
to the nickel and chrome of the 304. The molybdenum is used to control pit type attack.
Type 316 is used in chemical processing, the pulp and paper industry, for food and
beverage processing and dispensing and in the more corrosive environments. The molybdenum
must be a minimum of 2%.

Type 317
Contains a higher percentage of molybdenum than 316 for highly corrosive environments.
It must have a minimum of 3% “moly”. It is often used in stacks which contain
scrubbers.


Type 317L
Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra
corrosion resistance.

Type 317LM
Requires molybdenum content of 4.00% min.

Type 317LMN
Requires molybdenum content of 4.00% min. and nitrogen of .15% min.

Type 321, Type 347
These types have been developed for corrosive resistance for repeated intermittent exposure
to temperature above 800 degrees F. Type 321 is made by the addition of titanium
and Type 347 is made by the addition of tantalum/columbium. These grades are primarily
used in the aircraft industry.

Martensitic Grades
Martensitic grades were developed in order to provide a group of stainless alloys that
would be corrosion resistant and hardenable by heat treating. The martensitic grades are
straight chromium steels containing no nickel. They are magnetic and can be hardened by
heat treating. The martensitic grades are mainly used where hardness, strength, and wear
resistance are required
 
 
 
 
Type 410
Basic martensitic grade, containing the lowest alloy content of the three basic stainless
steels (304, 430, and 410). Low cost, general purpose, heat treatable stainless steel. Used
widely where corrosion is not severe (air, water, some chemicals, and food acids. Typical
applications include highly stressed parts needing the combination of strength and corrosion
resistance such as fasteners.

Type 410S

Contains lower carbon than Type 410, offers improved weldability but lower
hardenability. Type 410S is a general purpose corrosion and heat resisting chromium
steel recommended for corrosion resisting applications.


Type 414
Has nickel added (2%) for improved corrosion resistance. Typical applications include
springs and cuttlery.

Type 416
Contains added phosphorus and sulfer for improved machinability. Typical applications
include screw machine parts.

Type 420
Contains increased carbon to improve mechanical properties. Typical applications include
surgical instruments.

Type 431
Contains increased chromium for greater corrosion resistance and good mechanical
properties. Typical applications include high strength parts such as valves and pumps.

Type 440
Further increases chromium and carbon to improve toughness and corrosion resistance.
Typical applications include instruments.

Ferritic Grades
Ferritic grades have been developed to provide a group of stainless steel to resist corrosion
and oxidation, while being highly resistant to stress corrosion cracking. These steels
are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold
worked and softened by annealing. As a group, they are more corrosive resistant than the
martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades,
these are straight chromium steels with no nickel. They are used for decorative trim,
sinks, and automotive applications, particularly exhaust systems.
 
 
 

 
Type 430

The basic ferritic grade, with a little less corrosion resistance than Type 304. This type
combines high resistance to such corrosives as nitric acid, sulfur gases, and many organic
and food acids.

Type 405

Has lower chromium and added aluminum to prevent hardening when cooled from high
temperatures. Typical applications include heat exchangers.

Type 409

Contains the lowest chromium content of all stainless steels and is also the least expensive.
Originally designed for muffler stock and also used for exterior parts in non-critical
corrosive enviornments.

Type 434

Has molybdenum added for improved corrosion resistance. Typical applications include
automotive trim and fasteners.

Type 436
Type 436 has columbium added for corrosion and heat resistance. Typical applications
include deep-drawn parts.

Type 442
Has increased chromium to improve scaling resistance. Typical applications include
furnace and heater parts.

Type 446
Contains even more chromium added to further improve corrosion and scaling resistance
at high temperatures. Especially good for oxidation resistance in sulfuric atmospheres.

Duplex Grades
Duplex grades are the newest of the stainless steels. This material is a combination of
austenitic and ferritic material. This material has higher strength and superior resistance
to stress corrosion cracking. An example of this material is type 2205. It is available on
order from the mills.

Precipitation Hardening Grades
Precipitation hardening grades, as a class, offer the designer a unique combination of
fabricability, strength, ease of heat treatment, and corrosion resistance not found in any
other class of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-5Ni (15-5PH).

The austenitic precipitation-hardenable alloys have, to a large extent, been replaced by
the more sophisticated and higher strength superalloys. The martensitic precipitationhardenable
stainless steels are really the work horse of the family. While designed primarily
as a material to be used for bar, rods, wire, forgings, etc., martensitic precipitationhardenable
alloys are beginning to find more use in the flat rolled form.

While the semiaustenitic precipitation-hardenable stainless steels were primarily designed
as a sheet and strip product, they have found many applications in other product forms.


Developed primarily as aerospace materials, many of these steels are gaining commercial
acceptance as truly cost-effective materials in many applications.
Superalloy Grades

Superalloys are used when 316 or 317 are inadequate to withstand attack.They contain
very large amounts of nickel and/or chrome and molybdenum. They are usually much
more expensive than the usual 300 series alloys and can be more difficult to find. These
alloys include Alloy 20 and Hastelloy.
 
INFO FROM : 800/523-7532, 610/384-3910 fax, http://www.sppusa.com





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