ENGINEERING MATERIALS

• Heat Treatment • Critical Temperatures• Transformation on Heating / Cooling

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Heat Treatment (HT) • INTRODUCTION:- There are interrelationship among the

structure, properties, processing and performance of engineering materials.

• In processing structure and companion properties are manipulated and controlled.

• HT is the term used to describe the controlled heating & cooling of materials for the primary purpose of altering their structures and properties.

• Both physical and mechanical properties can be altered by HT (e.g. strength, ductility, hardness, toughness, machine ability, wear & corrosion resistance, etc)

• Can be done for strengthening purpose (Converting structure to martensite)

• Can be done for softening & conditioning purposes Annealing, tempering, etc.)

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Heat Treatment (HT) • These changes can be induced with no concurrent1

change in product shape.• HT is one of the most important and widely used

manufacturing processes.• Technically the term heat treatment applies only to

processes where heating & cooling are performed for the specific purpose of altering properties; but heating & cooling often occur as incidental phases of other manufacturing processes such as hot forming or welding.

• The material properties will be altered, however, just as though an intentional HT had been performed, & the result can be either beneficial or harmful

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Heat Treatment (HT)

• For this reason both the individual who selects material & the person who specifies its processing must be fully aware of the possible changes that can occur during heating or cooling activities.

• HT should be fully integrated with other manufacturing processes if effective results are to be obtained.

• More than 90 % of all HT is performed on steel & other ferrous metals.

• Steel=0.06% to 1.0 % C, 0.6% ideal content for HT.• The FCC can hold more carbon in solution and on

rapid cooling the crystal structure wants to return to its BCC structure. It cannot due to trapped carbon atoms. The net result is a distorted crystal structure called body centered tetragonal (BCT) called martensite.

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Heat Treatment (HT)• 5.2 Processing heat treatments(PHT) The term

HT is often associated with those thermal processes that increase the strength of a material, but the broader definition permits inclusion of an other set of processes that we will call processing heat treatments.

• These are often performed as a means of preparing the material for fabrication.

• Specific objectives may be the improvement of machining characteristics, the reduction of forming forces or the restoration of ductility to enable further processing.

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Heat Treatment (HT)• Equilibrium Diagrams as Aids:-Most of the PHTs

involves rather slow cooling or extended times at elevated temperatures.

• These conditions tend to approximate equilibrium, and the resulting structures, therefore, can be reasonably predicted through the use of an equilibrium phase diagram.

• These diagrams can be used to determine the temp that must be attained to produce a desired starting structure, & to describe the changes that will then occur upon subsequent cooling.

• It should be noted, however, that these diagrams are for true equilibrium conditions, and any departure from equilibrium may lead to substantially different results

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Solid Phases in the Iron Carbon Phase Diagram• α-ferrite, Austenite (γ), Cementite(Fe3C), and , δ-

ferrite• α-ferrite – solid solution of carbon in α-iron. α-

ferrite has BCC crystal structure and low solubility of carbon – up to 0.025% at (723ºC). α-ferrite exists at room temperature. The solubility of carbon in α-ferrite decreases to 0.005 % at 0ºC

• Austenite (γ)– interstitial solid solution of carbon in γ-iron. Austenite has FCC crystal structure, permitting high solubility of carbon – up to 2.06% at 1147 ºC. Austenite does not exist below 723ºC and maximum carbon concentration at this temperature is 0.83%.

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Solid Phases in the Iron Carbon Phase Diagram

• Cementite(Fe3C)Unlike ferrite and austenite, cementite is a very hard inter metallic compound consisting of 6.67% carbon and the remainder (93.3%) iron, but when mixed with soft ferrite layers its average hardness is reduced considerably. Slow cooling gives course perlite; soft easy to machine but poor toughness. Faster cooling gives very fine layers of ferrite and cementite; harder and tougher.

• δ-ferrite – The interstitial Solid solution of carbon in iron. Maximum concentration of carbon in δ-ferrite is 0.09% at 1493ºC– temperature of the peritectic

transformation. The crystal structure of δ-ferrite is like α-ferrite (BCC) but with a greater lattice constant.

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Critical temperatures• Upper critical temperature (point) A3 is the

temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.

• Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys

• Lower critical temperature (point) A1 is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.

• Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic.

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Transformation on heating cooling• Hypoeutectoid steels (carbon content from 0 to 0.83%) consist

of primary (proeutectoid) ferrite (according to the curve A3) and pearlite.

• Eutectoid steel (carbon content 0.83%) entirely consists of pearlite.

• Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid)cementite (according to the curve ACM) and pearlite.

• Cast irons (carbon content from 2.06% to 4.3%) consist of proeutectoid cementite C2 ejected from austenite according to the curve ACM , pearlite and transformed ledeburite (ledeburite in which austenite transformed to pearlite).

• Carbon A very small interstitial atom that tends to fit into clusters of iron atoms. It strengthens steel and gives it the ability to harden by heat treatment. 11/10/2011 12

Heat Treatment (HT)• It also causes major problems for welding , particularly

if it exceeds 0.25% as it creates a hard microstructure that is susceptible1 to hydrogen cracking.

• Carbon forms compounds with other elements called carbides. Iron Carbide, Chrome Carbide etc.

• The diagram shown below is based on the transformation that occurs as a result of slow heating. Slow cooling will reduce the transformation temperatures; for example: the A1 point would be reduced from 723°C to 690 °C.

• However the fast heating and cooling rates encountered in welding will have a significant influence on these temperatures, making the accurate prediction of weld metallurgy using this diagram difficult.

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Heat Treatment (HT)• Pearlite  A mixture of alternate strips of ferrite and

cementite in a single grain. • The distance between the plates and their thickness

is dependant on the cooling rate of the material; fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness.

• The name for this structure is derived from its mother of pearl appearance under a microscope.

• A fully pearlitic structure occurs at 0.8% Carbon. • Further increases in carbon will create cementite at

the grain boundaries, which will start to weaken the steel.

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Heat Treatment (HT)

• Cooling of a steel below 0.8% carbon   When a steel solidifies it forms austenite.

• When the temperature falls below the A3 point, grains of ferrite start to form.

• As more grains of ferrite start to form the remaining austenite becomes richer in carbon.

• At about 723°C the remaining austenite, which now contains 0.8% carbon, changes to pearlite.

• The resulting structure is a mixture consisting of white grains of ferrite mixed with darker grains of pearlite.

• Heating is basically the same thing in reverse.

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Heat Treatment (HT)• Martensite  If steel is cooled rapidly from austenite, the F.C.C

structure rapidly changes to B.C.C leaving insufficient time for the carbon to form pearlite.

• This results in a distorted structure that has the appearance of fine needles.

• There is no partial transformation associated with martensite, it either forms or it doesn’t.

• However, only the parts of a section that cool fast enough will form martensite; in a thick section it will only form to a certain depth, and if the shape is complex it may only form in small pockets.

• The hardness of martensite is solely dependant on carbon content, it is normally very high, unless the carbon content is exceptionally low.

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• There are five basic heat treating processes: hardening, case hardening, annealing, normalizing, and tempering.• Although each of these processes bring about different results in metal, all of them involve three basic steps: heating, soaking, and cooling. Heating is the first step in a heat-treating process. Many alloys change structure when they are heated to specific temperatures.

Heat Treatment (HT)

• The structure of an alloy at room temperature can be either a mechanical mixture, a solid solution, or a combination solid solution and mechanical mixture.

• A mechanical mixture can be compared to concrete. Just as the. sand and gravel are visible and held in place by the cement.

• The elements and compounds in a mechanical mixture are clearly visible and are held together by a matrix of base metal.

• A solid solution is when two or more metals are absorbed, one into the other, and form a solution.

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Heat Treatment (HT)• When an alloy is in the form of a solid solution,

the elements and compounds forming the metal are absorbed into each other in much the same way that salt is dissolved in a glass of water.

• The separate elements forming the metal cannot be identified even under a microscope.

• A metal in the form of a mechanical mixture at room temperature often goes into a solid solution or a partial solution when it is heated.

• Changing the chemical composition in this way brings about certain predictable changes in grain size and structure. This leads to the second step in the heat treating process: soaking.

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Heat Treatment (HT)• SOAKING:- Once a metal part has been

heated to the temperature at which desired changes in its structure will take place, it must remain at that temperature until the entire part has been evenly heated throughout. This is known as soaking.

• The more mass the part has, the longer it must be soaked.

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Heat Treatment (HT)• Cooling:- After the part has been properly

soaked, the third step is to cool it.• Here again, the structure may change from one

chemical composition to another, it may stay the same, or it may revert to its original form. For example, a metal that is a solid solution after heating may stay the same during cooling, change to a mechanical mixture, or change to a combination of the two, depending on the type of metal and the rate of cooling.

• All of these changes are predictable. For that reason, many metals can be made to conform to specific structures in order to increase their hardness, toughness, ductility, tensile strength, and so forth.

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Transformation on heating cooling• Martensite (FeC):- If a sample of a plain carbon steel

in the austenitic condition is rapidly cooled to room temp by quenching it in water, its structure will be changed from austenite to (FeC).

• (FeC). in plain carbon steels is a metastable phase consisting of a supersaturated interstitial solid solution of carbon in BCC iron or BCT iron (The tetragonality is caused by a slight distortion of the BCC iron unit cell.

• The Ms temp for FeC alloys decreases as the weight % C increases in these alloys.

• Ms is martensite start temp and Mf is martensite finish temp

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Transformation on heating cooling

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Fig 9.17