HEAT TREATMENT

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Mohammud Hanif Dewan, Maritime Lecturer and Trainer, Bangladesh

What is Heat Treatment?• Heat treatment is a general term referring to a cycle of heating and cooling which alters the internal structure of a metal and thereby changes its properties• Metal and alloys are heat treated for a number of purposes however the primarily to:-

1. Increase their hardness and strength

2. To improved ductility

3. To soften them for subsequent operations (cutting etc)

4. Stress relieving

5. Eliminate the effects of cold work

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HEAT TREATMENT OF STEEL The mechanical properties of materials can be changed by heat treatment. Let’s first examine how this applies to carbon steels. CARBON STEELS In order to understand how carbon steels are heat treated we need to re-examine the structure. Steels with carbon fall between the extremes of pure iron and cast iron and are classified as follows.

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All metals form crystals when they cool down and change from liquid into a solid. In carbon steels, the material that forms the crystals is complex. Iron will chemically combine with carbon to form IRON CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very hard and brittle. The more cementite the steel contains, the harder and more brittle it becomes.

When it forms in steel, it forms a structure of 13% cementite and 87% iron (ferrite) as shown. This structure is called PEARLITE. Mild steel contains crystals of iron (ferrite) and pearlite as shown. As the % carbon is increased, more pearlite is formed and at 0.9% carbon, the entire structure is pearlite. 4

NAME Dead mild

CARBON % 0.1 – 0.15

TYPICAL APPLICATION

pressed steel body panels Mild steel Medium carbon steel High carbon steels Cast iron

0.15 – 0.3 0.5 – 0.7 0.7 – 1.4 2.3 – 2.4

steel rods and bars forgings springs, drills, chisels engine blocks

1538

1130

2.0

oC

695

910

0.4 0.8 1.2

AUSTENITE

AUSTENITE + FERRITE

FERRITE + PEARLITE

HYPO-EUTECTOID STEELS

PEARLITE

Mixture of Ferrite & Cementite

EUTECTOID STEELS

AUSTENITE

AUSTENITE + CEMENTITE

AUSTENITE + CEMENTITE

HYPER-EUTECTOID STEELS

IRON – CARBON EQUILIBRIUM DIAGRAM

5

1538

1130

2.0

oC

695

910

0.4 0.8 1.2

AUSTENITE

FERRITE + CEMENTITE

AUSTENITE + CEMENTITEAUSTENITE + FERRITE

FERRITE

+

PEARLITE

CEMENTITE

+

PEARLITE

AUSTENITE + LIQUID

IRON – CARBON EQUILIBRIUM DIAGRAM

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AUSTENITE

• A solid solution of Carbon in face-centred cubic iron (Allotropic), containing a maximum 0f 1.7 % carbon at 1130oC

• It is soft, ductile and non-magnetic and also exist in the plain carbon steel above the upper critical range.

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FERRITE

• Ferrite is nearly pure iron. A solid solution of Carbon in body-centred cubic iron, containing a maximum of 0.04 % Carbon at 695oC.

• At room temperature, small amounts of manganese, silicon and other elements may be dissolved in iron as well as up to 0.007 % Carbon.

• Found only in Hypoeutectoid steel

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CEMENTITE

• A hard brittle compound of iron and Carbon with the formula Fe3C

• The hardest constituent of steel• This may exist in the free state usually as a

grain boundary film, or as a constituent of the eutectoid pearlite

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PEARLITE

• This is the eutectoid structure consisting of alternate lamination of ferrite and cementite.

• It contains 0.83% Carbon and is formed by the breakdown of the austenite solid solution at 695oC

• The properties of pearlite are harder and stronger than ferrite, but softer and more ductile than cementite

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If the carbon is increased further, more cementite is formed and the structure becomes pearlite and cementite as shown.

 

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HEAT TREATMENT of CARBON STEELS Steels containing carbon can have their properties (hardness, strength, toughness etc) changed by heat treatment. Basically if it is heated up to red hot and then cooled very rapidly the steel becomes harder. Dead mild steel is not much affected by this but a medium or high carbon steel is.

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Principle of heat treatment of steel

• Metals are never heated to the melting point in heat treatment.

• Therefore, all the reactions within the metal during the heating and cooling cycle, take place while the metal is in the solid state

• During ordinary heat treating operations, steel is seldom heated above 983oC.

• In using the iron-iron carbide diagram, we need only to concern ourselves with that part which is always solid steel.

• The area where the Carbon content is 2% or less and the temperature is below 1130oC

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COOLING RATE• Cooling rate is the most important part of heat treatment.

• Different cooling rates are now considered as they have a

significant effect on the properties of the metal.

SLOW COOLING

• Austenite is transformed to course pearlite.

• Slightly more rapid cooling may produce fine pearlite in which

the layers of ferrite and cementite are thinner.

INTERMEDIATE COOLING

• Austenite transforms to a material called Bainite instead of the

usual pearlite.

• When etched, Bainite gives a dark appearance and shows a

circular or needle like form. 14

FAST COOLING• By quenching in water, the transformation of

austenite is suppressed until about 318oC at which point a new constituent called Martensite(quite brittle) begins to form instead of the Bainite or pearlite of slower cooling rate.

• As the temperature drops lower, the transformation become complete.

• This temperature vary with the alloy content of the steel

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TIME TEMPERATURE TRANSFORMATION

• In order to obtain steels with the desired properties, we must have some control over the transformation process, and this is indicated in the TTT diagram

• TTT diagram are used to predict the metallurgical structure of a steel sample which is quenched in the austenite region and held to constant elevated temperature below 729oC.

• This is known as Isothermal transformation

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Time (sec)

oC

0

760

725

650

590

540

430

316

260

190

90

1 2 5 10 20 50

100

200

500

100

0 200

0 500

0 1000

0

2000

0

5000

0

1000

00

1 m

in

15 m

in

30 m

in

1 hr 10

hrs

1 da

y

TIME TEMPERATURE TRANSFORMATION DIAGRAM

Ferrite form

Pearlite starts

Pearlite forms

Pearlite is complete Coarse

Pearlite Fine Pearlite

Bainite forming

Upper Bainite

Lower Bainite

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HOT ROLLING This is used to produce sheets, bars and sections. If the rollers are cylindrical, sheet metal is produced. The hot slab is forced between rollers and gradually reduced in thickness until a sheet of metal is obtained. The rollers may be made to produce rectangular bars, and various shaped beams such as I sections, U sections, angle sections and T sections. Steel wire is also produced this way. The steel starts as a round billet and passes along a line of rollers. At each stage the reduction speeds up the wire into the next roller. The wire comes of the last roller at very high speeds and is deflected into a circular drum so that it coils up. This product is then used for further drawing into rods or thin wire to be used for things like springs, screws, fencing and so on.

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COLD ROLLING The process is similar to hot rolling but the metal is cold. The result is that the crystals are elongated in the direction of rolling and the surface is clean and smooth. The surface is harder and the product is stronger but less ductile. Cold working is more difficult that hot working.

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FORGING In this process the metal is forced into shape by squeezing it between two halves of a die. The dies may be shaped so that the metal is simply stamped into the shape required (for example producing coins). The dies may be a hammer and anvil and the operator must manipulate the position of the billet to produce the rough shape for finishing (for example large gun barrels).

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COLD WORKING Cold working a metal by rolling, coining, cold forging or drawing leaves the surface clean and bright and accurate dimensions can be produced. If the metal is cold worked, the material within the crystal becomes stressed (internal stresses) and the crystals are deformed. For example cold drawing produces long crystals. In order to get rid of these stresses and produce “normal” size crystals, the metal can be heated up to a temperature where it will re-crystallise. That is, new crystals will form and large ones will reduce in size. If the metal is maintained at a substantially higher temperature for a long period of time, the crystals will consume each other and fewer but larger crystals are obtained. This is called “grain growth”. Cold working of metals change the properties quite dramatically. For example, cold rolling or drawing of carbon steels makes the stronger and harder. This is a process called “work hardening”.

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HOT WORKING Most metals (but not all) can be shaped more easily when hot. Hot rolling, forging, extrusion and drawing is easier when done hot than doing it cold. The process produces oxide skin and scale on the material and producing an accurate dimension is not possible. Hot working, especially rolling, allows the metal to re-crystallise as it is it is produced. This means that expensive heat treatment after may not be needed. The material produced is tougher and more ductile.

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LIQUID CASTING AND MOULDING When the metal cools it contracts and the final product is smaller than the mould. This must be taken into account in the design. The mould produces rapid cooling at the surface and slower cooling in the core. This produces different grain structure and the casting may be very hard on the outside. Rapid cooling produces fine crystal grains. There are many different ways of casting.

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SAND CASTING A heavy component such as an engine block would be cast in a split mould with sand in it. The shape of the component is made in the sand with a wooden blank. Risers allow the gasses produced to escape and provide a head of metal to take up the shrinkage. Without this, the casting would contain holes and defects. Sand casting is an expensive method and not ideally suited for large quantity production. Typical metalsused are cast iron. Cast steel and aluminium alloy.

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DIE CASTING Die castings uses a metal mould. The molten metal may be fed in by gravity as with sand casting or forced in under pressure. If the shape is complex, the pressure injection is the best to ensure all the cavities are filled. Often several moulds are connected to one feed point. The moulds are expensive to produce but this is offset by the higher rate of production achieved. The rapid cooling produces a good surface finish with a pleasing appearance. Good size tolerance is obtained. The best metals are ones with a high degree of fluidity such as zinc. Copper, aluminium and magnesium with their alloys are also common.

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CENTRIFUGAL CASTING This is similar to die casting. Several moulds are connected to one feed point and the whole assembly is rotated so that the liquid metal is forced into the moulds. This method is especially useful for shapes such as rims or tubes. Gear blanks are often produced this way.

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MACHINING Machining processes involve the removal of material from a bar, casting, plate or billet to form the finished shape. This involves turning, milling, drilling, grinding and so on. Machining processes are not covered in depth here. The advantage of machining is that is produces high dimensional tolerance and surface finish which cannot be obtained by other methods. It involves material wastage and high cost of tooling and setting.

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Heat treatment Methods

• Annealing

• Normalizing

• Hardening

• Tempering

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Annealing

• heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility

• It the process of heating solid metal to high temperatures and cooling it slowly so that its particles arrange into a defined lattice

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Stages in annealing

Heating to the desired temperature ,Holding or soaking at that temperature,Cooling or quenching ,usually to room

temperature .

• In practice annealing concept is most widely used in heat treatment of iron and steals

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Purpose of annealing

• It is used to achieve one or more of the following purpose .

1. To relive or remove stresses

2. To include softness

3. To alter ductility , toughness, electrical, magnetic.

4. To Refine grain size

5. To remove gases

6. To produce a definite microstructure . 31

Application

Annealing process is employed in following application • Casting • Forging • Rolled stock • Press work ….

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3 Types of Annealing:

I. Process Annealing

II. Full annealing

III. Spheroidising

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i. Process Annealing Carried out on cold-worked low carbon steel

sheet or wire in order to relieve internal stress and to soften the metals.

• The steel is heated to 550 to 650oC below the critical point.

34Increase in ductility reduce in TS & hardness

ii. Full Annealing

It carried out on hot-worked and cast steels in order to obtain grain refinement with high ductility.

It also produces a softer steel with better machinability

• For steels

– heating above critical point (30 - 50oC) then

- holding at this temperature for a time (thickness) - followed by slow cooling usually in furnace.

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iii. Spheroidizing Annealing

To remove coarse pearlite and making machining process easy .

It forms spherodite structure of maximum soft and ductility easy to machining and deforming.

• The process is limited to steels in excess of 0.5% carbon. This steel can be softened by annealing at 650 – 750oC just below the lower critical point, when the cementite of the pearlite balls up or spheroidizes.

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Defects uncontrolled temperature

i. Overheating Heated above the actual temperature or to long

maintained at annealing temperature: austenite grain growth will occur and make the metal weak and brittle

ii. Burning If heated above the upper critical point to temperature,

Brittles films of oxide are formed which make the steel unsuitable. For further use and must be remelted.

iii. Under annealing The original pearlite will have change to several small

austenite grains.

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NORMALIZING

For hypoeutectoid steels heating above critical point (30 - 50oC) holding at this temperature for a time (thickness) & followed by cooling in still air.

• Produces maximum grain refinement and consequently the steel slightly harder and stronger than a fully annealed steel.

• However the properties will vary with section thickness

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HARDENING

Hardening is process in which Medium and High carbon steels (0.4 – 1.2%) is heated to a temperature above the critical point (until red hot), held at this temperature and quenched (rapidly cooled) in water, oil or molten salt baths.

• Hardening producing a very hard and brittle metal. At 723 Deg C, the ferrite changes into Austenite structure.

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TEMPERING

Tempering is a process of heat treating, which is used to

increase the toughness of iron-based alloys.

To remove some of the brittleness from hardened steels,

tempering is used. The metal is heated to the range of 220-

300 deg C and cool in the air.

• Tempering is usually performed after hardening, to reduce

some of the excess hardness, and is done by heating the

metal to some temperature below the critical temperature

for a certain period of time, then allowed to cool in still air.

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QUENCHING

• To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled.

• Quenching Media: Brine (water and salt solution) Water Oil Air Turn off furnace

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CASE HARDENING

• Low carbon steels cannot be hardened by heating due to the small amounts of carbon present. So, Case hardening seeks to give a hard outer skin over a softer core on the metal.

• The addition of carbon to the outer skin is known as carburising.

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• When temperature region 200 – 450oC the martensite decomposes into ferrite and the precipitation of the fine particles of carbide occurs known. as troostite

• At higher temperatures 450 – 650oC the carbide particles coalesce thus producing fewer and larges particles which provide fewer obstacles to dislocations resulting further increasing toughness while decrease in strength and hardness and known as sorbite.

• Sorbite is ideal for components subject to dynamic stresses such as crankshaft and connecting rod

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SORBITEMARTENSITE TROOSTITE

200 400 600

oC

Hardness

200

800

600

400

1000

EFFECT OF TEMPERING

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Any Question?

Thank you!