Phase Transformation
Phase Transformation
Phase Transformation
Not only do structural alloys start with the casting of ingots for processing into
reinforcing bars or structural shapes, but when a metal is welded a small portion
of metal near the weld melts and resolidifies. It also serves as a model to
HOMOGENEOUS NUCLEATION
For instance when a pure liquid metal is slowly cooled below its equilibrium
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SOLIDIFICATION OF PURE METALS
2
3
SOLIDIFICATION
formed as castings. The primary initial form for wrought alloys is the cast ingot.
NUCLEATION
The solidification of metals occurs by nucleation and growth. The same is true
of melting (maybe) but the barriers are much less. Thus, it is possible to achieve
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Undercooling or super-cooling is achieved by suppressing heterogeneous
The movement of a boundary separating liquid from solid, under the influence
atomic movements.
Atoms leave the liquid and join the solid = rate of attachment
Atoms leave the solid and join the liquid = rate of detachment
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PURE METALS: NUCLEATION & GROWTH
NUCLEATION
directions.
HOMOGENEOUS NUCLEATION
When a solid forms within its own liquid without aid of foreign materials-
nucleate homogeneously.
of the relatively large contribution of surface energy to the total free energy of
small particles.
HETEROGENEOUS NUCLEATION
Nucleation occurs on preferential sites, such that a solid forms in contact with
cooling.
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HEAT TREATMENT OF STEEL
In heat treatment, the processing is most often entirely thermal and modifies
and structure, are also important processing approaches which fall into the
treatment.
operations:
Among annealing there are some important heat treatment processes like:
• Normalising
• Spheroidising
• Stress Relieving
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NORMALISING
The temperature depends on carbon content. After soaking the alloy is cooled in
still air. This cooling rate and applied temperature produces small grain size.
The small grain structure improve both toughness and strength (especially yield
strength).
SPHEROIDISING
The process is limited to steels in excess of 0.5% carbon and consists of heating
the steel to temperature about A1 (727°C). At this temperature any cold worked
ferrite will recrystallize and the iron carbide present in pearlite will form as
spheroids or “ball up”. As a result of change of carbides shape the strength and
QUENCHING
Soaking temperature 30-50°C above A3 or A1, then fast cooling (in water or oil)
with cooling rate exceeding a critical value. The critical cooling rate is required
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to obtain non-equilibrium structure called martensite. During fast cooling
Temperature-Transformation).
With the quenching-hardening process the speed of quenching can affect the
amount of marteniste formed. This severe cooling rate will be affected by the
The critical cooling rate is the slowest speed of quenching that will ensure
TEMPERING
This process is carried out on hardened steels to remove the internal stresses and
The treatment requires heating the steel to a temperature range of between 200
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This heat energy allows carbon atoms to diffuse out of the distorted lattice
structure associated with martensite, and thus relieve some of the internal
stresses. As a result the hardness is reduced and the ductility (which was
degree of shock loading. The higher the tempering temperature the greater the
FURNACE TYPES
supply process steam, or steam for electric power generation, or for space
2. As a source of energy for industrial processes, other than for electric power
The primary concern of this chapter is the design, operation, and economics of
By function:
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Smelting for reduction of metallic ores
Incineration
Batch furnaces for cyclic heating, including forge furnaces arranged to heat one
end of a bar or billet inserted through a wall opening, side door, stationary-
Tilting-type furnace
To avoid the problem of door war page or leakage in large batch-type furnaces,
mounted above a stationary hearth, and arranged to be tilted around one edge of
the hearth for loading and unloading by manual handling, forklift trucks, or
For handling heavy loads by overhead crane, without door problems, the
furnace can be a portable cover unit with integral firing and temperature control.
improve heat transfer parallel to coil laminations, they are loaded with open coil
separators between them, with heat transferred from the inner cover to coil ends
by a recirculating fan. To start the cooling cycle, the heating cover is removed
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by an overhead crane, while atmosphere circulation by the base fan continues.
Cooling may be enhanced by air blast cooling of the inner cover surface.
For heating heavy loads of other types, such as weldments, castings, or forgings,
car bottom furnaces may be used with some associated door maintenance
overhead traveling crane. In one type of furnace, the door is suspended from a
some economy in construction, the door may be mounted on one end of the car
and opened as the car is withdrawn. This arrangement may impose some
Loads such as steel ingots can be heated in pit-type furnaces, preferably with
units of load separated to allow radiating heating from all sides except the
Loads are handled by overhead crane equipped with suitable gripping tongs.
Continuous-Type Furnaces
Pieces of rectangular cross section are loaded side by side on a charge table and
pushed through the furnace by an external mechanism. In the design shown, the
furnace is fired from one end, counter flow to load travel, and is discharged
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Furnace length is limited by thickness of the load and alignment of abutting
edges, to avoid buckling up from the hearth. A more complex design would
provide multiple zone firing above and below the hearth, with recuperative air
preheating.
type furnace. Loads can be bars, tubes, or plates of limited width, heated by
conveyed at uniform speed or at alternating high and low speeds for quenching
in line.
conveyors.
secure the same advantage in heating slabs or billets for rolling and to avoid
beam mechanism. Such a walking beam- type slab heating furnace would have
loads supported on water-cooled rails for over and under firing, and would have
an overhead recuperator.
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Thin strip materials, joined in continuous strand form, can be conveyed
galvanizing lines.
slot in the furnace roof, and can be quenched in line by lowering a section of the
conveyor.
materials, the shaft type furnace provides a simple and efficient system. Loads
are charged through the open top of the shaft and descend by gravity to a
discharge feeder at the bottom. Combustion air can be introduced at the bottom
of the furnace and preheated by contact with the descending load before
Combustion gases are then cooled by contact with the descending load, above
the combustion zone, to preheat the charge and reduce flue gas temperature.
With loads that tend to agglomerate under heat and pressure, as in some ore-
roasting operations, the rotary kiln may be preferable to the shaft-type furnace.
The load is advanced by rolling inside an inclined cylinder. Rotary kilns are in
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Classification by Source of Heat
supplemental fuel
about changes that take place in alloys. The temperature at which a particular
During the solidification which takes place on cooling, the elements of an alloy
alloy in question. To understand this we will look at the cooling of a pure metal.
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At 1500oC the pure metal is fully liquid. As time passes the temperature of the
metal falls. At 1083oC for copper the liquid metal begins to change into solid.
This change does not happen instantly but takes a little time. When this time has
passed, the solidification ends and all of the metal has changed to solid. More
If a metal is 100% pure and contains no traces of other elements then some
under cooling may occur before solidification begins. Under cooling is when the
temperature drops below the liquid to solid temperature for a short period.
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CONSTANT TEMPERATURE CHANGE
From the graph we can see that the temperature does not change while the metal
is changing from liquid to solid. This is similar to when water changes into ice,
which we call freezing. In metallurgy the term freeze point is used. Another
example of this is the boiling of water. Water boils at 100oC. After this the
water turns into steam but the temperature does not continue to rise. This extra
heat that changes the water into steam is called latent heat.
other. Metals which combine in this way are said to form solid solutions. When
this type of alloy solidifies, only one type of crystal is formed. Under a
microscope the crystalline structure of a solid solution alloy looks very like a
pure metal. Solid solution alloys have similar properties to pure metals but have
The usual forms of solid solution are: substitutional solid solution and
Substitutional solid solution – this is when atoms of the parent metal are
replaced or substituted by atoms of the second metal. In this case the atoms of
the two metals are of similar size and direct substitution takes place.
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Interstitial solid solution – this is when the atoms of the parent metal are bigger
than those of the alloying metal. The smaller atoms fit into the spaces
There are a few different types of thermal equilibrium diagram. We have seen
how a thermal equilibrium diagram for copper/nickel can be prepared from six
Liquid phase
Solid phase
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These are divided on the graph by the solidus and liquidus lines.
EUTECTIC ALLOY
Another type of thermal equilibrium diagram is one that can be prepared from a
eutectic alloy. In a eutectic alloy the two metals are completely soluble in the
liquid phase but are insoluble in the solid phase. The cadmium/bismuth alloy is
although they do differ. There is one point on the diagram where the liquid alloy
changes to solid without going through a liquid/solid state is called the eutectic
point. This is lowest melting point of any composition of the alloy. The
temperature at which this occurs is very important as all alloys become solid at
this temperature.
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PARTIAL SOLUBILITY ALLOY
Some metals in an alloy only partially dissolve in each other. Solder (lead/tin
alloy) is an example of this. The equilibrium diagram for this type of alloy is
solid solution and eutectic diagrams and is a little more complex. The solvus
line in the diagram plots the amount that metal ‘A’ that dissolves in metal ‘B’
up to a certain temperature. A solid solution exists between the solidus line and
the solvus line. These are present both on the left and right of the diagram.
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CRYSTAL PATTERNS AND UNIT CELLS
In metals, atoms bond together in a pattern which is repeated over and over,
giving a crystalline structure. Most metals crystallise with one of the following
crystalline structures:
The BCC structure has atoms arranged so that their centres are positioned on the
corners of a cube, with one atom in the centre. This unit cell is repeated to form
Atoms in the FCC and CPH unit cell are more tightly packed together than the
atoms in a BCC structure. This helps to explain why different metals have
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SLIP IN BCC AND FCC STRUCTURES
Slip means that part of a metal can slip over itself. Slip can take place in metals
when they are subjected to certain shear type forces. If you take a look at the
BCC and FCC structures you will see that slip is more likely in an FCC
structure. This would explain why metals with an FCC structure are ductile and
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ATOMIC IMPERFECTIONS IN METALS
Crystalline structures in metals have many imperfections. Atoms are not always
where they should be if the crystal structure was adhered to. In reality there are
often atoms missing, too many atoms, impure atoms, or distortions in the
If atoms are out of line in the grain body, or lattice, this is known as a line
allow the grains to distort or slip under shear stress. Slip in metals is largely due
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As already mentioned, it is rare that an ideal crystalline structure exists in a
metal. Sometimes an atom may be missing from a line or a row and the lattice is
placed under strain. When this occurs, a vacancy exists. Type of defect is
When an atom from another element, which is not the same size as the other
atoms, is present it also causes distortion in the lattice. This atom can be larger
or smaller than the other atoms. This type of defect is called a substitute defect.
If an atom from an impurity finds its way into a space or interstice in the lattice
ALLOTROPY OF METALS
This is the ability of some elements to exist in more than one crystalline
structure. Steel can exist as BCC when cold and as FCC when heated above a
CRYSTALLINE STRUCTURES
AMORPHOUS STRUCTURES
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CARBURISING
controlled levels of carbon are introduced at the surface and allowed to diffuse
salt (“cyaniding”).
improve fatigue strength. Applications range from simple mild steel pressings to
Case hardening without causing course structures is possible using salt bath
carburising. The component is placed in the salt bath at 9000C for one hour.
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This gives a thin carbon case and not too much grain growth. It is then
GAS CARBURISING
This is carried out in a special sealed furnace. The carburising agent is a carbon
rich gas circulating in the furnace chamber. This is a fast method of carburising
Steels that can be treated by these processes fall into two types:
carbonitriding, but do not develop significant core strength. Thus they are
significant strength and toughness in the core. They are not normally
mechanical properties. Consult your heat treater when selecting steels for case-
hardening.
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IRON CARBON EQUILIBRIUM DIAGRAMS
ALLOTROPIC
Iron, when cooling from a high temperature, displays two special points known
o
as arrest points or critical points. These change points occur at 1390 C and
o o o
910 C. Above 1390 C Iron exists with a BCC lattice but between 1390 C and
o
910 C it exists with a FCC lattice. Iron is said to be allotropic, which means that
EUTECTIC POINT
• At this special change point, the liquid steel changes to the solid austenite +
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EUTECTOID POINT
• At this special change point the solid austenite changes into solid pearlite.
o
• This occurs at 723 C for steel when 0.83 % carbon is contained in the alloy.
• Eutectoid– Solid
FERRITE
CEMENTITE
• It is called Iron Carbide (Fe3C). It is a hard, brittle material. This is what gives
PEARLITE
• At the eutectoid point (0.83% carbon) solid austenite changes into two solid
phases - ferrite and cementite. These two solids combine to form pearlite.
AUSTENITE
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