PHASE TRANSFORMATION

The solidification of metals and their alloys is an important industrial process.

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

represent first order phase transformations in general.

HOMOGENEOUS NUCLEATION

Homogeneous nucleation occurs when there are no special objects inside a

phase which can cause nucleation.

For instance when a pure liquid metal is slowly cooled below its equilibrium

freezing temperature to a sufficient degree numerous homogeneous nuclei are

created by slow-moving atoms bonding together in a crystalline form.

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SOLIDIFICATION OF PURE METALS

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3
SOLIDIFICATION

Why do we need to know about solidification? Many metal components are

formed as castings. The primary initial form for wrought alloys is the cast ingot.

Welding processes involve solidification phenomena. Alloy powders are often

atomized, and rapidly solidified. Metastable microstructures, phases and glasses

are becoming more widely used.

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

significant super-cooling of pure metal liquids.

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Undercooling or super-cooling is achieved by suppressing heterogeneous

nucleation. “Prenucleants” are assumed to exist in the liquid metal. In many

processes, homogeneous nucleation is assumed to occur, but experimental

evidence suggests otherwise.

CRYSTAL GROWTH FROM THE LIQUID PHASE

The movement of a boundary separating liquid from solid, under the influence

of a temperature gradient normal to the boundary, is the result of two different

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

Solidification does not happen on cooling instantaneously on cooling below and

takes place via nucleation and growth.

NUCLEATION

 homogeneous – rare and only if very large undercooling ΔT;

 heterogeneous – on mould walls and/or “impurities”

(Including deliberate alloy additions to grain refine microstructure)

Growth may be affected by temperature gradients and preferred crystal growth

directions.

HOMOGENEOUS NUCLEATION

When a solid forms within its own liquid without aid of foreign materials-

nucleate homogeneously.

Homogeneous nucleation requires a large driving force (undercooling) because

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

an impurity particles, i.e. nucleation agent or mold walls - heterogeneous

nucleation. Many liquid metals start solidification in a few degrees of super-

cooling.

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 HEAT TREATMENT OF STEEL

Steels can be heat treated to produce a great variety of microstructures and

properties. Generally, heat treatment uses phase transformation during heating

and cooling to change a microstructure in a solid state.

In heat treatment, the processing is most often entirely thermal and modifies

only structure. Thermo mechanical treatments, which modify component shape

and structure, and thermochemical treatments which modify surface chemistry

and structure, are also important processing approaches which fall into the

domain of heat treatment. The iron-carbon diagram is the base of heat

treatment.

According to cooling rate we can distinguish two main heat treatment

operations:

• annealing – upon slow cooling rate (in air or with a furnace)

• quenching – upon fast cooling (in oil or in water)

Annealing - produces equilibrium structures according to the Fe-Fe3C diagram

Quenching - gives non-equilibrium structures

Among annealing there are some important heat treatment processes like:

• Normalising

• Spheroidising

• Stress Relieving

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NORMALISING

The soaking temperature is 30-50°C above A3 or Acm in austenite field range.

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).

During normalising we use grain refinement which is associated with allotropic

transformation upon heating γ→α (Fig. 2).

Important: austenite does not change grain size during cooling!!

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

hardness are reduced.

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

austenite cannot transform to ferrite and pearlite by atomic diffusion.

Martensite is supersaturated solid solution of carbon in α-iron (greatly

supersaturated ferrite) with tetragonal body centered structure. Martensite is

very hard and brittle. Martensite has a “needle-like” structure.

Kinetics of martensite transformation is presented by TTT diagrams (Time-

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

component size and quenching medium type (water, oil).

The critical cooling rate is the slowest speed of quenching that will ensure

maximum hardness (full martensitic structure).

TEMPERING

This process is carried out on hardened steels to remove the internal stresses and

brittleness created by the severe rate of cooling.

The treatment requires heating the steel to a temperature range of between 200

and 600°C depending upon the final properties desired.

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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

negligible before tempering treatment) is increased slightly. The combined

effect is to “toughen” the material which is now capable of resisting certain

degree of shock loading. The higher the tempering temperature the greater the

capacity for absorbing shock.

FURNACE TYPES

Furnaces may be grouped into two general types:

1. As a source of energy to be used elsewhere, as in firing steam boilers to

supply process steam, or steam for electric power generation, or for space

heating of buildings or open 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

industrial furnaces, which may be classified in several ways:

By function:

 Heating for forming in solid state (rolling, forging)

 Melting metals or glass

 Heat treatment to improve physical properties

 Preheating for high-temperature coating processes, galvanizing, vitreous

enamelling, other coatings

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  Smelting for reduction of metallic ores

 Firing of ceramic materials

 Incineration

By method of load handling:

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-

hearth-type car bottom designs

Continuous furnaces with loads pushed through or carried by a conveyor

Tilting-type furnace

To avoid the problem of door war page or leakage in large batch-type furnaces,

the furnace can be a refractory-lined box with an associated firing system,

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

overhead crane manipulators.

For handling heavy loads by overhead crane, without door problems, the

furnace can be a portable cover unit with integral firing and temperature control.

Consider a cover-type furnace for annealing steel strip coils in a controlled

atmosphere. The load is a stack of coils with a common vertical axis,

surrounded by a protective inner cover and an external heating cover. To

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

problems. The furnace hearth is a movable car, to allow load handling by an

overhead traveling crane. In one type of furnace, the door is suspended from a

lifting mechanism. To avoid interference with an overhead crane, and to achieve

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

handicaps in access for loading and unloading.

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

bottom. Such a furnace would have a cover displaced by a mechanical carriage

and would have a compound metal and refractory recuperator arrangement.

Loads are handled by overhead crane equipped with suitable gripping tongs.

Continuous-Type Furnaces

The simplest type of continuous furnace is the hearth-type pusher furnace.

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

through a side door by an auxiliary pusher lined up by the operator.

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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.

Long loads can be conveyed in the direction of their length in a roller-hearth-

type furnace. Loads can be bars, tubes, or plates of limited width, heated by

direct firing, by radiant tubes, or by electric-resistor-controlled atmosphere, and

conveyed at uniform speed or at alternating high and low speeds for quenching

in line.

Sequential heat treatment can be accomplished with a series of chain or belt

conveyors.

Small parts can be loaded through an atmosphere seal, heated in a controlled

atmosphere on a chain belt conveyor, discharged into an oil quench, and

conveyed through a washer and tempering furnace by a series of mesh belts

without intermediate handling.

Except for pusher-type furnaces, continuous furnaces can be self-emptying. To

secure the same advantage in heating slabs or billets for rolling and to avoid

scale loss during interrupted operation, loads can be conveyed by a walking-

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

horizontally or the strands can be conveyed in a series of vertical passes by

driven support rolls. Furnaces of this type can be incorporated in continuous

galvanizing lines.

Unit loads can be individually suspended from an overhead conveyor, through a

slot in the furnace roof, and can be quenched in line by lowering a section of the

conveyor.

Small parts or bulk materials can be conveyed by a moving hearth, as in the

rotary hearth-type or tunnel kiln furnace. For roasting or incineration of bulk

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

entering the combustion zone, where fuel is introduced through sidewalls.

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

general use for sintering ceramic materials.

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Classification by Source of Heat

The classification of furnaces by source of heat is as follows:

Direct-firing with gas or oil fuels

Combustion of material in process, as by incineration with or without

supplemental fuel

Internal heating by electrical resistance or induction in conductors, or dielectric

heating of non-conductors. Radiation from electric resistors or radiant tubes, in

controlled atmospheres or under vacuum.

THERMAL EQUILIBRIUM DIAGRAMS FOR ALLOYS

Thermal equilibrium diagrams are special diagrams which contain information

about changes that take place in alloys. The temperature at which a particular

alloy changes from liquid to solid is an example of the type of thermal

information contained in a thermal equilibrium diagram.

An alloy is a mixture or combination of two or more elements that produce a

new element with improved properties.

During the solidification which takes place on cooling, the elements of an alloy

combine in a particular way. This depends on the elements contained in the

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

cooling takes place until the metal reaches room temperature.

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.

SOLID SOLUTION ALLOYS

When metals combine they sometimes become completely soluble in each

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

greater strength. They also have poorer electrical/thermal conductivity, greater

hardness but not as elastic as pure metals.

The usual forms of solid solution are: substitutional solid solution and

interstitial solid solution.

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

(interstices) between the larger atoms.

UNDERSTANDING EQUILIBRIUM DIAGRAMS

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

graphs. There are three zones in this diagram:

 Liquid phase

 Solid phase

 Liquid + solid phase (pasty)

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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

an example of a eutectic mixture. A thermal equilibrium diagram for

cadmium/bismuth is built up in the same way as the solid solution diagram

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

called a partial solubility thermal equilibrium diagram. It is a combination of the

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:

 Body centred cubic structure (BCC) – copper, gold

 Face centred cubic structure (FCC) – zinc, cadmium

 Close packed hexagonal structure (CPH) – iron, tungsten

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

a crystalline structure or pattern.

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

different physical properties.

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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

metals with a BCC structure are brittle.

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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

crystalline structure. These faults or defects are called crystal defects.

LINE DEFECTS IN CRYSTALS

If atoms are out of line in the grain body, or lattice, this is known as a line

imperfection, or line defect. Line defects are called dislocations. Dislocations

allow the grains to distort or slip under shear stress. Slip in metals is largely due

to the presence of dislocations. Ductility in metals is a result of the distortion

allowed by slip in metals.

A method of restricting the movement of a dislocation is to alloy it with another

element OR to cold work the metal.

Point defects in metals

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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

known as a vacant site defect.

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

the defect caused is called an interstitial defect.

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

certain temperature, depending on its carbon content.

CRYSTALLINE STRUCTURES

These have regular, repeating, geometrical molecular patterns.

AMORPHOUS STRUCTURES

These have disorganised, irregular molecular patterns

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 CARBURISING

Carburising is a “thermochemical” treatment, usually conducted at temperatures

in the range 800-940°C, in the first stage of “case-hardening”. This process

changes the chemical composition of the surface of a low-carbon steel

component so that subsequent fast cooling, by “quenching”, produces a hard

“case” combined with a softer/tougher “core”. Quenching is normally followed

by a low-temperature tempering / stress relieving treatment. In carburising,

controlled levels of carbon are introduced at the surface and allowed to diffuse

to a controlled depth. The heat treater employs a variety of processing media to

achieve these objectives, including controlled gaseous atmospheres and molten

salt (“cyaniding”).

WHAT ARE THE BENEFITS?

Carburising case-hardening treatment offers a means of enhancing the strength

and wear proper-ties of parts made from relatively-inexpensive easily-worked

materials. Generally applied to near-finished components, the process impart a

high-hardness wear-resistant surface which, with sufficient depth, can also

improve fatigue strength. Applications range from simple mild steel pressings to

heavy-duty alloy-steel transmission components.

SALT BATH CARBURISING

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

quenched in water to harden the surface.

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

and greater control over the process is possible.

WHAT SORT OF STEELS CAN BE TREATED?

Steels that can be treated by these processes fall into two types:

1. Low-carbon / non-alloy (mild) steels can be case-hardened by carburising or

carbonitriding, but do not develop significant core strength. Thus they are

normally treated for increased wear resistance only.

2. Low-carbon alloy case-hardening steels, intrinsically higher-strength

materials, can be carburised to yield a high surface hardness whilst developing

significant strength and toughness in the core. They are not normally

carbonitrided. BS970 lists some case-hardening steels and their typical

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

it can exist in two different forms depending on temperature.

EUTECTIC POINT

• At this special change point, the liquid steel changes to the solid austenite +

cementite phase without going through the pasty stage.

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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

• This is almost pure iron but contains about 0.02% carbon.

• It has a BCC structure.

CEMENTITE

• This is a compound of iron and carbon.

• It is called Iron Carbide (Fe3C). It is a hard, brittle material. This is what gives

the hardness to high carbon steel.

• It has a higher melting point than either of its elements

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.

• Pearlite is a layered structure of ferrite and cementite.

AUSTENITE

• This is an FCC solid solution structure which can contain up to 2% carbon.

• It is a hard non-magnetic substance.

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