China precision machining of heat
treatment is embraced by the broader study of metallurgy. Metallurgy is the
physics, chemistry, and engineering related to metals from ore extraction to
the final product.
Heat treatment is the operation of heating
and cooling a metal in its solid state to change its physical properties.
According to the procedure used, steel can be hardened to resist cutting action
and abrasion, or it can be softened to permit machining.
China precision machining With the proper heat treatment internal
stresses may be removed, grain size reduced, toughness increased, or a
hard surface produced on a ductile
interior. The analysis of the steel must be known because small percentages of certain elements, notably carbon, greatly affect the
physical properties.
Alloy steel owe their properties to the presence of one or more elements
other than carbon, namely nickel, chromium, manganese, molybdenum, tungsten,
silicon, vanadium, and copper. Because of their improved physical properties
they are used commercially in many ways not possible with carbon steels.
China precision machining The following discussion applies principally
to the heat treatment of ordinary commercial steels known as plain carbon
steels. With this process the rate of
cooling is the controlling factor, rapid cooling from above the critical
range results in hard structure, whereas
very slow cooling produces the opposite
effect.
A Simplified Iron-carbon Diagram
If
we focus only on the materials normally
known as steels, a simplified diagram is
often used.
Those portions of the iron-carbon diagram
near the delta region and those above 2% carbon content are of little
importance to the engineer and are deleted. A simplified diagram, focuses on
the eutectoid region and is quite useful in understanding the properties and
processing of steel.
The key transition described in this diagram is the decomposition of
single-phase austenite(γ) to the two-phase
ferrite plus carbide structure as
temperature drops.
Control of this reaction, which arises due
to the drastically different carbon solubility of austenite and ferrite,
enables a wide range of properties to be
achieved through heat treatment.
To
begin to understand these processes,
consider a steel of the eutectoid
composition, 0.77% carbon, being slow cooled along line x-x’ At the upper temperatures, only austenite is
present, the 0.77% carbon being dissolved in solid solution with the iron. When
the steel cools to 727℃(1341℉), several changes occur simultaneously.
China precision machining The iron wants to change from the
FCC austenite structure to the BCC ferrite structure, but the ferrite can only
contain 0.02% carbon in solid solution.
The rejected carbon forms the carbon-rich
cementite intermetallic with composition
Fe3C. In essence, the net reaction at the eutectoid is austenite 0.77%C→ferrite
0.02%C+cementite 6.67%C.
Since this chemical separation of the
carbon component occurs entirely in the
solid state, the resulting structure is a fine mechanical mixture of
ferrite and cementite. Specimens prepared by polishing and etching in a weak
solution of nitric acid and alcohol reveal the lamellar structure of
alternating plates that forms on slow cooling.
This structure is composed of two distinct
phases, but has its own set of characteristic properties and goes by the name
pearlite, because of its resemblance to mother- of- pearl at low magnification.
Steels having less than the eutectoid
amount of carbon (less than 0.77%) are
known as hypo-eutectoid steels. Consider now the transformation of such
a material represented by cooling along line y-y’.
At high temperatures, the material is
entirely austenite, but upon cooling enters a region where the stable phases
are ferrite and austenite. Tie-line and level-law calculations show that
low-carbon ferrite nucleates and grows, leaving the remaining austenite richer
in carbon.
At 727℃(1341℉), the austenite is of eutectoid
composition (0.77% carbon) and further
cooling transforms the remaining austenite to pearlite. The resulting structure
is a mixture of primary or pro-eutectoid
ferrite (ferrite that formed
above the eutectoid reaction) and regions of pearlite.
Hypereutectoid steels are steels that contain greater than the eutectoid
amount of carbon. When such steel cools,
the process is similar to the hypo-eutectoid case, except that the
primary or pro-eutectoid phase is now cementite instead of ferrite.
As the carbon-rich phase forms, the
remaining austenite decreases in carbon content, reaching the eutectoid
composition at 727℃(1341℉). As before, any remaining austenite
transforms to pearlite upon slow cooling through this temperature.
It
should be remembered that the transitions that have been described by the phase
diagrams are for equilibrium conditions, which
can be approximated by slow cooling. With slow heating, these transitions occur in the
reverse manner.
China precision machining However, when alloys are cooled rapidly, entirely different results may be
obtained, because sufficient time is not provided for the normal phase reactions
to occur, in such cases, the phase
diagram is no longer a useful tool for
engineering analysis.
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