carbon steels and carbon alloy steels
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Heat Treatment
It is a process utilized to change certain characteristics of metals and alloys in order to make them more suitable
for a particular kind application Heat Treatment can greatly influence mechanical properties such as strength,
hardness, ductility, toughness, and wear resitence of the alloys.
Heat Treatment of Carbon Steels and Carbon Alloy Steels:
Heat Treatment on both type of the steel is done for improving mechanical properties such as tensile and yieldstrength. This is accomplished by altering the molecular structure of steel in order to produce more durable
microstructure. The structure of steel is composed of two variables:
Grain Structure: The arrangement of atoms in a metal.
Grain Size: The size of the individual crystals of metal. Large grain size is generally associated with low
strength, hardness, and ductility.
During the alloy process elements such as carbon are introduced to the metal. These added elements interrupt theflow of the individual grains, increasing strength. Thus, control of the metal crystal structure is a key element in
successful heat treating.
A Metal can also exist in various phases: Ferrite, austenite and cementite. To better understand these phases, look
at the Iron-Carbon Phase Diagram. The Y-axis (vertical) is a measurement of temperature while the X-Axis
(Horizontal) is a measurement of the carbon content of the steel. The far left hand side of the X-axis represents
the Ferrite phase of steel (low carbon content) while the far right hand. Side represents the cementite phase ofsteel (high carbon content), which is also known as iron carbide. The curved horizontal line that occurs just above
1333 F represents the austenite phase of steel.
The following phases are involved in the transformation, occurring with iron-carbon alloys:
L L represents a liquid solution of carbon in iron.1.
-ferrite -ferrite is a solid solution of carbon in iron. Maximum concentration of carbon in -ferrite is
0.09% at 2719 F (1493C) temperature of the peritectic transformation. The crystal structure of -ferrite
is BCC (cubic body centered).
2.
Austenite Austenite is an interstitial solid solution of carbon in -iron. Austenite has FCC (cubic face
centered) crystal structure, permitting high solubility of carbon up to 2.06% at 2097 F (1147 C).
Austenite does not exist below 1333 F (733C) and maximum carbon concentration at this temperature is0.83%.
3.
-ferrite -ferrite is a solid solution of carbon in -iron. -ferrite has BCC crystal structure and low
solubility of carbon up to 0.25% at 1333 F (733C). It exists at room temperature.
4.
Cementite Cementite is an iron carbide, intermetallic compound, having fixed composition Fe3C.
Cementite is a hard and brittle substance, influencing on the properties of steels and cast irons.
5.
When ferrite (low carbon steel) is at room temperature, it has a body-centered-cubic structure, which can onlyabsorb a low amount of carbon. Because Ferrite can only absorb a very low amount of carbon at room
temperature, the un-absorbed carbon separates out of the body-centered-cubic structure to form carbides which
join together to create small packets of an extremely hard crystal structure within the ferrite called cementic.
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However, when ferrite is heated to a temperature above the transformation line( austenite line) the
body-centered-cubic structure changes to a face-centered-cubic structure, thus allowing for absorption of the
carbon into the crystal structure.
Once the steel enters the austenitic phase all of the cementite dissolves into austenite. If the steel is allowed to
cool slowly, the carbon will separate out of the ferrite as a cubic-structure reverts from face-centered back to
body-centered. The islands of cementite will reform within the ferrite, and the steel will have the same propertiesthat it did before it was heated. However, when the steel is rapidly cooles, or quenched, in a quenching medium
(such as oil, water or cold air) the carbon does not have time to exit the cubic structure of the ferrite and it
becomes trapped within it. This leads to the information of martensitic; microstructure that produces the mostsought after mechanical properties in steel fasteners.
During quenching, it is impossible to cool the specimen at a uniform rate throughout. The surface will always coolmore rapidly than the interior of the specimen. Therefore, the austenite will transform over a range oftemperatures, yielding a possible variation of microstructure and properties depending on the position within the
material.
The successful heat treatment of steels to produce a predominantly martensitic micro structure throughout the
cross section depends mainly on three factors:
The composition of the alloy.1.
The type and character of the quenching medium.2.
The size and shape of the specimen.3.
Hardenability : It is the ability of steel to transform into martensite with a particular quenching treatment. This is
directly affected by the alloy composition of the steel. For every different steel alloy there is a specific relationship
between its mectanical properties and its cooling rate. It is not hardness which is a resistance to indentation;
rather, hardness measurements are utilized to determine the extent of a martensitic transformation in the interior
of the material.
Tempering : It involves heating the steel to a specific temperature below that of ausenite and allowing it to cool
slowly. This cause the crystal structure to relax, thereby increasing the ductility and decreasing the hardness tospecified levels. The specific tempering temperature will vary based on the desired result for the steel.
The following example will demonstrate the effectiveness of the tempering:
ASTM A193 Grade B7 , SAE J429 Grade and ASTM A574 Socket Head cap Screws are all made from alloy steels. Infact some alloy steel grades can be used to manufacture any of the three final products. Such as 4140 and 4142
alloy steel. The final mechanical properties apper in the table.
Fasteners Produced From AISI 4140& 4142 Steel
Fastener ASTM A193 B7 SAE J429 Gr. 8 ASTM A574 SHCS
Tempering
Temp.
1150F 800F 650F
Tensile strength 125,000 PSI min (2 1\2in
and under)
150,000 PSI min 180,000 PSI min
( through in ) 170,000 PSI
min( above in)
Yield
Strength
105,000 PSI min
(2 1/2in and under)
130,000 PSI min 153,000 PSI min
Proof
Strength
N/A 120,000 PSI 140,000 PSI min
( through in ) 135,000 PSI
min( above in )
Hardness HRC 35 max. HRC 33-39 HRC 39-45( through in ) HRC 37-45
( above in )
The initial heat treating process is relatively the same for the entire three products. The parts are heat treated
until fully austenitized and then are quenched and tempered in the oil. This tempering temperature dictates the
final product. A lower tempering temperature will produce a harder and higher tensile strength part for these
alloys steels. However, the lower tempering temperatures will also mean lower ductility, impact strength, andpossibly lower fatigue life.
Annealing
It is the heat treating process used to soften previously cold-worked metal by allowing it to re-crystallize.The term
annealing refers to a heat treatment in which a material is exposed to an elevated tempertature for an extended
period and then slowly cooled. Ordinarily, annealing is carried out to, (1) Relieve stress (often introduced whencold-working the part.) ;(2) Increase softness, ductility and toughness; and (3) Produce a desired microstructure.
Any annealing process consists of the three stages:
Heating to the desired temperature1.
Holding or soaking at that temperature2.Slowly cooling, usually to room temperature3.
Time is the important parameters in these procedures. It is used to negate the effects of cold work that is to be
soften and increase the ductility of a previous strain-hardened metal.
Stress relieving
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It is a process that is utilized when internal residual stress develop in metal pieces in response to such thing as cold
working.Failure to remove these internal stress may result in distottion and warping. A stress relief anneaing heat
treatment removes these stress heating the piece to a recommended temperature, held there long enough to
attain a uniform temperature, and finally cooled to a room temperature in air.
Normalizing
It is an annealing heat treatment used to refine the grains and produce a more uniform and desirable sizedistribution. Medium and high carbon steels having microstructure containing pearlite may still be too hard to
conveniently machine or plastically deform. These steels (and in fact,any steel) may be annealted to develop the
spheroidite structure . Spheroidized steels have a maximum softness and ducility and are easlly machined ordeformed.
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