Thermal Processing/Heat Treatment of
Steel
Heat Treatment Fundamentals
It is an operation or combination of operations involving
heating a metal or alloy in its solid state to a certain temperature
holding it there for some times, and
cooling it to the room temperature at a predetermined rate to obtain desired
properties.
All basic heat-treating processes for steels involve the transformation of
austenite.
the nature and appearance of these transformation products determine
the physical and mechanical properties of heat-treated steels.
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Heating Period
Holding
Heating steel to above critical temperature range
Temperature
(A3 or Acm) in order to form single-phase austenite.
Rate of heating is usually less important than other
factors, except for Time
[1] highly stressed materials, or
[2] thick-sectioned materials.
Usually, slow heating rate is preferable.
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Holding / Soaking Period
Holding
Holding at the austenitizing temperature for
Temperature
complete homogenization of structure.
Usually, 1 hour per inch section is enough for holding.
Time
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Cooling Period
Cooling rate determines the nature of transformation products of austenite.
Depending on cooling rate,
common heat treatment of steels are classified as:
[1] annealing
[2] normalizing
[3] hardening
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Annealing of Steels
Annealing is a heat treatment process that consists of heating to and holding at a suitable
temperature followed by cooling slowly through the transformation range preferably in the
furnace, primarily for the softening of metallic materials.
Generally, in plain carbon steels, full annealing (commonly known as annealing) produces
ferrite-pearlite structures.
Purposes of annealing:
May be to refine grain
Inducing ductility, toughness, softness
Relieve residual stresses
Improving electrical and magnetic properties
In some cases, to improving machinability
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1.Full Annealing
Heating and holding steels to austenitizing temperature and then cooling very
slowly through the transformation range preferably in the furnace.
Improves ductility
utilized in low- and medium-carbon steels that will be machined or will
experience extensive plastic deformation during a forming operation.
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2.Stress Relief Annealing
Heating and holding steels to below lower critical temperature and then
cooling to room temperature (sub-critical annealing).
Relieve residual stresses due to heavy machining/grinding or other cold-
working processes, non-uniform cooling (during weld/casting), and phase
transformations.
Distortion may result if these residual stresses are not removed.
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3.Process Annealing
Similar to stress relief annealing.
Process annealing is a heat treatment that is used to negate the effects of cold
work.
i.e; to soften and increase the ductility of a previously strain-hardened metal.
It is commonly utilized during fabrication procedures that require extensive
plastic deformation, to allow a continuation of deformation without fracture
or excessive energy consumption.
Structure refined by a process of recovery and recrystallization.
Ordinarily a fine-grained microstructure is desired
Designed to restore ductility of steels between processing steps and facilitate
further cold working.
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4.Spheroidizing Annealing
Make very soft steels for good machining (for hypereutectoid steels).
Prolonged heating breaks pearlite and cementite network and spheroids
of cementite in ferrite matrix forms.
Both sub-critical and inter-critical annealing practices are used.
Prolonged heating at a temp. just below the lower critical temp.
Heating and cooling alternately between temperatures that are just above and just
below the lower critical line.
Heating to a temperature above the lower critical line and then either cooling very
slowly in the furnace or holding at a temperature just below the lower critical line.
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4.Spheroidizing Annealing
The spheroidized structure is desirable when minimum hardness, maximum
ductility or maximum machinability is important.
Low carbon steels are seldom spheroidized for machining because in the
spheroidized condition they are excessively soft and gummy. The cutting tool
will tend to push the material rather than cut it.
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Temperature Range
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Annealing of Hypoeutectoid Steels
AUSTENITE Since cooling is very slow,
annealing comes closest to
austenite
follow the iron – iron carbide
912 C equilibrium diagram
A3 (c)
FERRITE +
(b) AUSTENITE
727 C
Temperature
A1 0.76
ferrite
Pearlite Austenite
pearlite FERRITE + PEARLITE (d)
(a)
0.2
wt.% C
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Annealing of Hyper-eutectoid Steels
Refinement of the grain size of hypereutectoid steel will occur about 30°C (50°F)
above the lower-critical-temperature (A1) line.
Heating above this temperature will coarsen the austenitic grains, which, on cooling,
will transform to large pearlitic areas.
The microstructure of annealed hypereutectoid steel will consist of coarse lamellar
pearlite areas surrounded by a network of proeutectoid cementite.
Because this excess cementite network is brittle and tends to be a plane of
weakness, annealing should never be a final heat treatment for hypereutectoid steels.
The presence of a thick, hard, grain boundary will also result in poor machinability.
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A careful estimation of the proportions of pearlite and/or ferrite present in an
annealed steel can be used to determine the approximate carbon content of the
steel:
Wt.% C = (0.76) x (%Pearlite area) + (0.008) x (%Ferrite area)
Wt.% C = (0.76) x (%Pearlite area) + (6.7) x (%Cementite area)
An approximate tensile strength of a hypoeutectoid steel can also be determined
in a similar manner:
Approx. Tensile Strength, psi = (120,000) x (%Pearlite area)
+ (40,000) x (%Ferrite area)
Tensile strength of hypereutectoid steels can not be estimated similarly, since their strengths
are determined by the cementite network only.
Problem
Microstructure of an annealed steel sample is found to contain 25% ferrite area and 75%
pearlite area. Identify the steel and determine its approximate tensile strength.
Wt.% C in steel = (0.76) 0.75 + (0.008) 0.25 = 0.602 %
Since the carbon content is less than 0.76, the eutectoid composition, the sample is a
hypoeutectoid steel.
Approx. Tensile Strength = (120,000) 0.75 + (40,000) 0.25 psi
= 100,000 psi
