Transcript Change of Condition - Southern Illinois University Carbondale

Change of Condition
Chapter 12
IT 208
Chapter 12
1
Competencies
 Describe the different methods of softening steel
 Describe the different methods of hardening steel
 Describe the difference between Martensite and
Austinsite
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Phase Diagram
 Is a graphical means of representing the phases of a
metal allow system as a function of composition and
temperature
 Discuss the Water phase system (O/H)
 Discuss the Copper-nickel Alloy system (O/H)
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Phase Diagram
 Discuss the Tin-Lead Allow system
• widely used in soldering for making electrical connection.
 The addition of two solid phases alpha (α) and beta (β).
• Alpha phase is a solid solution of tin in lead
• Beta phase is solid solution of lead in tin that occurs only at
elevated temperatures around 200 degrees C
 Between these solid solutions lies a mixture of the two solid
phases, (α) + (β).
 Two liquidus lines that begin at the melting points of the pure
metals and meet at a composition of 61.9% Sn.
• Point is called the eutectic composition for the tin-lead
system
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Phase Diagram
(Tin-Lead Allow system)
 A eutectic alloy is a particular composition in an
allow system for which the solidus and liquidus are
at the same temperature.
 The corresponding eutectic temperature, the melting
point of the eutectic composition is 183 deg C
 Eutectic temperature is always the lowest melting
point for an alloy system.
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Discuss Iron-Carbon Phase Diagram
 Steels with less than 0.3 % carbon cannot be
hardened effectively, while the maximum effect is
obtained at about 0.7 % due to an increased
tendency to retain austenite in high carbon steels
 The ferrous metals of engineering importance are
alloys of iron and carbon.
 These alloys divide into two major groups; steel and
cast iron.
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Iron-Carbon Phase
1. Pure iron melts at 1539 degrees C (2827 deg F)
during the rise in temperature from ambient, it
undergoes several solid phase transformations
a. Starting at room temperature the phase is alpha iron
or ferrite. With less than 0.025% carbon at
temperatures below 894 deg C
b. At 912 degrees C, ferrite transforms to gamma iron,
called austenite. With less than 2% carbon
c. At 1394 degrees C, austenite transforms to delta
iron, which remains until melting occurs at 1539
degrees C
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Iron-Carbon Phase
2. Solubility limits of carbon in iron are low in the ferrite phase –
only about 0.022% at 723 deg C. Austenite can dissolve up to
about 2.1 % carbon at 1130 deg C. The difference in solubility
between alpha and gamma leads to opportunities for
strengthening by heat treatment
3. The eutectoid point is the lowest temperature at which
austenite can exist (722 deg C).
a. Eutectoid – the temperature and composition (0.77 - 0.81%
Carbon) at which a single-phase solid goes directly, on
cooling, to a two-phase solid. Steels below 0.77% Carbon
are considered hypoeutectoid steels those above up to
2.1% are considered hypereutectoid steels.
b. The eutectoid composition of the Iron-Carbon system is
called pearlite.
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Iron-Carbon Phase
4. Even without head treatment, the strength of iron
increases dramatically as carbon content increases,
and we enter the region in which the metal is called
steel.
 More precisely, steel is defined as an iron-carbon
alloy containing from 0.02% to 2.1 % carbon.
5. Another prominent phase in the iron-alloy system. Is
Fe3C also known as cementite. Which is a metallic
compound of iron and carbon that is hard and brittle
Carbon content of about 6.7%.
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Iron-Carbon Phase
6. Above a carbon content of 2.1% up to about 4% or
5% is defined as cast iron
7. If sufficient time is allowed for cooling of the
austenite
 it will revert completely to pearlite
 however, if the steel is cooled quickly from the
austenite, martensite is formed
 Martensitic steel has Rockwell C hardness of about
66 and pearlite is very soft in comparison.
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 Discuss the TTT curve (12-11)
 Three major categories of heat treatments
• Methods of softening steels
• Methods of hardening steels
• Methods of modifying the properties of steels
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Methods of Softening Steels
 Annealing is the softening of a metal to its softest
possible condition. For steels, the metal must be
heated into the austenitic range and cooled very
slowly.
 Normalizing is a heat treatment used to give steel
an even GRAIN size. It is used prior to machining or
other heat treatments.
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Methods of Hardening Steels
Can be done by flame, induction, electron beams, and
laser beam
 Quenching is the rapid cooling of a metal to harden
it.
 Cryogenics, or deep freezing
• done to make sure there is no retained Austenite
during quenching.
• When steel is at the hardening temperature, there is a
solid solution of Carbon and Iron, known as Austenite.
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Methods of Hardening Steels
• The amount of Martensite formed at quenching is a
function of the lowest temperature encountered.
• At any given temperature of quenching there is a
certain amount of Martensite and the balance is
untransformed Austenite. This untransformed
austenite is very brittle and can cause loss of strength
or hardness, dimensional instability, or cracking.
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Methods of Hardening Steels
• Quenches are usually done to room temperature.
Most medium carbon steels and low alloy steels
undergo transformation to 100 % Martensite at room
temperature.
• High carbon and high alloy steels have retained
Austenite at room temperature. To eliminate retained
Austenite, the temperature has to be lowered.
• In Cryogenic treatment the material is subject to deep
freeze temperatures of as low as -185°C (-301°F), but
usually -75°C (-103°F) is sufficient.
• The Austenite is unstable at this temperature, and the
whole structure becomes Martensite.
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Methods of Hardening Steels
Surface Hardening
 If steel is hardened all the way through the part, it will be brittle.
In parts that have wearing surfaces such as gear teeth, shafts,
lathe beds, and cams, only the surface of the part should be
hardened so as to leave the inside soft and ductile.
• Flame hardening is widely used in deep hardening for
large substrates.
• Induction hardening is suitable for small parts in
production lines.
These processes are applicable only to steels that have sufficient
carbon and alloy content to allow quench hardening.
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Methods of Hardening Steels
Case Hardening
 If low-carbon steel is used and toughness is need in
the workpiece, its surface cannot be significantly
hardened. Therefore a process to add carbon or
nitrogen to the surface is done.
• Done by carburizing, nitriding, carbonitriding or
cyaniding
• These elements diffuses into the outer layers of the
steel to increase hardness.
• The steel surface can then be hardened by
QUENCHING.
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Hardness
 Is a function of the Carbon content of the steel.
 Requires a change in structure from the bodycentered cubic structure found at room temperature
to the face-centered cubic structure found in the
Austenitic region.
 Steel is heated to Autenitic region. When suddenly
quenched, the Martensite is formed. This is a very
strong and brittle structure.
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Hardenability
 The ease with which full hardness can be achieved
throughout the material.
 A measure of the depth of full hardness achieved
• Is related to the type and amount of alloying elements.
• Different alloys, which have the same amount of
Carbon content, will achieve the same amount of
maximum hardness; however, the depth of full
hardness will vary with the different alloys.
 The reason to alloy steels is not to increase their
strength, but increase their hardenability
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Methods Of Modifying The
Properties Of Steels
 Tempering – is the removal of internal stresses in a
metal by heating the part back to a temperature
between 200 – 1200 deg F, for an appropriate time
based on part size and desired tempering.
 Spheroidizing – done by heating the steel to a
temperature just under 1300 deg F and held for a
period based on size. Grains will be changed into
small spheres
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Methods Of Modifying The
Properties Of Steels
 Martempering
• steel is quenched from the austenitic temperature to
just above the MARTENSITE start temperature
• held there for a few seconds to a few minutes
• and then quenched.
• It is used to provide an even-sized martensite
throughout the part.
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 Austempering –
• steel is quenched to just above the MARTENSITE
start temperature
• held there for several hours before lowering the
temperature to room conditions.
• The grain structure of the steel will be entirely bainitic
• Bainite has some of the hardness properties of
martensite and some of the toughness properties of
pearlite
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