Manish Project Repoart (2).docx

PROJECT REPORT

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1. PROJECT INTRODUCTION As the title “Analysis of Heat Treatment and effect of Carbon Percentage in Metal Structure” suggests this project is to understand the effect of heat treatment technique and carbon content on the metal properties and structure. Here we shall also analyze the process of heat treatment techniques and their types.

2. HEAT TREATMENT INTRODUCTION The heat treatment includes heating and cooling operations or the sequence of two or more such operations applied to any material in order to modify its metallurgical structure and alter its physical, mechanical and chemical properties. A.Heat Treatment Definition Heat Treatment is a technique of controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product shape. B.Heat Treatments Objectives

Heat Treatment is often associated with increasing the strength of material, but it can also be used to alter certain manufacturability objectives such as improve machining, improve formability, restore ductility after a cold working operation. Steels are heat treated for one of the following reasons.

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Softening: Softening is done jto reduce strength of hardness, remove residual stresses, improve toughness, restore dutility, refine grain size or change the electromagnetic properties of the steel.

Hardening: Hardening of steels is done to increase the strength and wear properties.

Material Modification: To modify properties of materials in addition to hardening and softening, to maximize service life.

3. HEAT TREATMENT PROCESSES

A Annealing 3

Annealing is performed primarily for homogenization, recrystallization or relief of residual stress in typical cold worked or welded components.Few important variants of annealing are followings.

a. Full Annealing (conventional annealing) Full annealing process consists of three steps. First step is heating the steel component to above A3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and 0

above A1 (lower critical temperature) temperature for hypereutectoid steels by 30-50 C. The second step is holding the steel component at this temperature for a definite holding (soaking) period of at least 20 minutes per cm to assure equalization of temperatureand complete austenization. Final step is to cool the hot steel component to room temperature slowly in the furnace. The full annealing is used to relieve the internal stresses, to reduce hardness, to refine the grain structure, to make the material homogenous.

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Schematic representation of annealing operation

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Iron-carbon phase equilibrium diagram

b.Spheroidise annealing Spheroidise annealing produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite.Spheroidise annealing is achieved by holding the steel component at just below the lower critical temperature (A1) transforms the pearlite to globular cementite particles.

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A typical heat treatment cycle to produce spheroidised structure. d.Recrystallization annealing Recrystallization annealing process consists of heating a steel component below A1 0

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temperature i.e. at temperature between 625 C and 675 C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. e.Stress relief annealing Stress relief annealing process consists of three steps. The first step is heating the cold 0

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worked steel to a temperature between 500 C and 550 C i.e. below its recrystallization temperature. The second step involves holding the steel component at this temperature for 12 hours. The final step is to cool the steel component to room temperature in air. B.Normalizing Normalizing process consists of three steps. The first step involves heating the steel component above the A3 cm temperature for hypoeutectoid steels and above A(upper critical 0

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temperature for cementite) temperature for hypereutectoid steels by 30 C to 50 C. The second step involves holding the steel component long enough at this temperature for homogeneous austenization. The final step involves cooling the hot steel component to

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room temperature in still air. Due to air cooling, normalized components show slightly different structure and properties than annealed components.

The variation in the properties of the annealed and normalized components.

Normalizing

C. Hardening Different techniques to improve the hardness of the steels are conventional hardening, martempering and austempering.

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a. Conventional hardening Conventional hardening process consists of four steps. The first step involves heating the steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for 0

hypereutectoid steels by 50 C. The second step involves holding the steel components for sufficient socking time for homogeneous austenization. The third step involves cooling of hot steel components at a rate just exceeding the critical cooling rate of the steel to room temperature or below room temperature. The final step involves the tempering of the martensiteto achieve the desired hardness. In this conventional hardening process, the austenite transforms to martensite. This martensite structure improves the hardness.

Heat treatment cycle for conventional hardening process

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b. Martempering (marquenching) Martempering process overcomes the limitation of the conventional hardening process. This process follows interrupted quenching operation. In other words, the cooling is stopped at a point above the martensite transformation region to allow sufficient time for the center to cool to the temperature as the surface. Further cooling is continued through the martensite region, followed by the usual tempering. In this process, the transformation of austenite to martensite takes place at the same time throughout the structure of the metal part.

Heat treatment cycle for martempering.

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b. Austempering This process is also used to overcome the limitation of the conventional hardening process. Here the quench is interrupted at a higher temperature than for martempering to allow the metal at the center of the part to reach the same temperature as the surface. By maintaining that temperature, both the center and surface are allowed to transform to bainite and are then cooled to room temperature. Austempering causes less distortion and cracking than that in the case of martempering and avoids the tempering operation.

Heat treatment cycle for austempering. D.Tempering Tempering is achieved by heating hardened steel to a temperature below A1, which is in the 0

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range of 100 C to 680 C, hold the component at this temperature for a soaking period of 1 to 2 hours (can be increases up to 4 hours for large sections and alloy steels), and subsequently cooling back to room temperature.

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Following Figuredepicts the influence of tempering temperature on the properties of steel. It is observed that the increase in the tempering temperature decreases the hardness and internal stresses while increases the toughness.

Variation in properties with tempering temperature

E. Flame Hardening: Flame hardening is used to harden only a portion of a metal. Unlike differential hardening, where the entire piece is heated and then cooled at different rates, in flame hardening, only a portion of the metal is heated before quenching. This is usually easier than differential hardening, but often produces an extremely brittle zone between the heated metal and the unheated metal, as cooling at the edge of this heat affected zone is extremely rapid.

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F.Induction hardening Induction hardening is a surface hardening technique in which the surface of the metal is heated very quickly, using a no-contact method of induction heating. The alloy is then quenched, producing a martensite transformation at the surface while leaving the underlying metal unchanged. This creates a very hard, wear resistant surface while maintaining the proper toughness in the majority of the object. Crankshaft journals are a good example of an induction hardened surface. G.Case hardening Case hardening is a thermochemical diffusion process in which an alloying element, most commonly carbon or nitrogen, diffuses into the surface of a monolithic metal. The resulting interstitial solid solution is harder than the base material, which improves wear resistance without sacrificing toughness.

4. DEFECTS IN HEAT TREATMENT Heat treatment of steels or aluminum can lead to several defects as followings.

A.Crack: When the internal tensile stresses exceed the resistance of the steel to separation the crack occurs.

B.Distortion:Distortion occurs due to uneven heating, too fast cooling, part incorrectly supported in furnace, incorrect dipping in quenching and stresses present before preheating.

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C.Warping: Asymmetrical distortion of the work is often called warping in heat-treating practice.

5 Analyzing The Effect Of Heat Treatment Processes On TheMechanical Properties Of Medium Carbon Steel

A.METHODS OF ANALYSES a Preparation of the Tensile Specimens The material used for this study is a medium carbon steel with carbon content of 0.30% carbon.

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b Heat treating the Medium Carbon Steel Standard heat treatment procedures were adoptedto heat treat the medium carbon steel. Five different samples were prepared for each of the operation and the average values were calculated upon which the analyses were based. c Tensile Test of the Medium Carbon Steel After the specimens had been heat treated as appropriate, the tensile test were carried out on them to determine the mechanical properties of the steel and compare it with the non heat treated specimen which was also subjected to the same tensile test.

B. Heat Treatment Process Done on Specimen

a. Hardening process b. Tempering process c. Annealing process d. Normalizing process

C.Material Testing After the successful heat treatment test was performed on Standard Universal Testing chine.

Figure (b) 15

The stress/strain values obtained from the tensile test gave the engineering stress/strain values. These values were later converted to true stress/strain values using the relationship given below:

D.Results And Discussion

The heat treated specimens were now subjected to tensile test. The resulting engineering stress - strain curves obtained from the test are shown in Figures 2 to 5 for annealed, normalized, tempered and hardened specimens respectively. Table 1: The materials property for different heat treated specimens based on true-stress strain data

The value of yield strength (σy) was observed to be higher for the tempered steel specimen, possibly as a result of the grain re-arrangement due to the subsequent tempering process. The yield strength value for the hardened specimen was also observed to be greater than that of normalized and annealed specimens, while the normalized specimen also has a greater value than that of annealed specimen, which has the least value. The hardness of the steel increases 16

with cooling rate and also with increasing pearlite percentage which increased as the percentage mertensiteincreases.The increase in the hardness was due to the delay in the formation of peatrlite and martensite at a higher cooling rate. The value of ultimate tensile strength (σu) were observed to be in the order; hardened > tempered > normalized > annealed, possibly as a result of the refinement of the primary phase after the subsequent cooling processes. Beyond the yield point, the stress continuously increases with further plastic strain, while the slope of the stress-strain curves, representing the strain hardening steadily decreases with increasing stress. It was also observed from the graphs that for all the heat treated specimens, except for the hardened specimen, there were tremendous increase in the toughness of the material which indicates that hardened material, though have a very high ultimate tensile stress (σu), but at the expense of its toughness, hence where toughness is a major concern, the material should be oil tempered for a satisfactory results.

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The strain produced for each of the specimen was in the order of annealed > normalized > tempered > hardened.

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Empirical relationships were also developed to determined various value of stresses at any given strain and strain rate for each of the specimen. The empirical relationships were given in equation 3 to 7 for normalized, tempered, annealed, hardened and as received specimen respectively.

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E.CONCLUSION From the results obtained, it can be inferred that mechanical properties depends largely upon the various form of heat treatment operations and cooling rate. Hence depending upon the properties and the applications that may be required for any design purpose, a suitable form of heattreatment should be adopted.For high ductile and minimum toughness, annealing the medium carbon steel will give satisfactory results.

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6 GENERAL HEAT TREATMENT DATA

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7 EFFECT OF CARBON CONTENT IN METAL STRUCTURE A Introduction As an elemental metal, pure iron has only limited engineering usefulnessdespite its allotropy. Carbon is the main alloying addition that capitalizeson the allotropic phenomenon and lifts iron from mediocrity intothe position of a unique structural material, broadly known as steel. To analize the effect of carbon content in iron we would go through the Iron carbon phase diagram and study the effect of carbon content on microstructure of alloy.

B.Allotropy of Iron

The Iron Carbon Phase Diagram 22

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This diagrame graphically represents the effects of temperature and composition on all the phases present in iron carbon alloys. 1) All the alloys in the temperature range above the curve ABCD are in liquid state. 2) Point A on the curve represent the melting point(1539˚C) of pure iron. Point D represent melting point(1550˚C) of cementite. With the fall in temperature of the lquid along the curve ABC Austenite crystals separate from the liquid metal. Similarly, the crystal of cementite will separate from the liquid with fall in temperature of the latter along line CD. The horizontal line HJB represent a pearlite reaction in which austenite is formed.Crystal of delta (δ) iron separate from the liquid along line AB 3) The curve HJECF represents the temperature line along which all carbon always solidify completely.It is known as solidline.all the alloys containing 0.18 to 2.0% carbon will solidify at temperatures represented by the solidus line HJE and all

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those containing 2.0 to 6.7% carbon at 1130 ˚C, represented by the solidus line ECF.Point C corresponds to 4.3% carbon.At this point austenite and cementite are precipitated from the liquid alloy and form an eutectic alloy called Ledeburite.point J,C and F are respectively known as Peritectic,Eutectic and Eutectoid. 4) Two types of transformation are represented by this diagram, called the Primary Transformation and Secondary Transformations.The Primary Transformation include primary solidification i.e. , the change of alloy phase from liquid to solid and the secondary transformation include the phase change in solid state. 5) The area of ferrite formation is represented by the region GPQ. Solubility of carbon in α iron at 723 ˚C is indicated by point p.At this point it is 0.025% carbon and goes on reducing as cooling proceeds. This aspect is represented by point PQ. 6) Steels having less than 0.8% carbon are called Hypoeutectoide steels and those having more than 0.8% carbon Hypereutectoid steels.Those steels which contain exactly 0.8% carbon are known as Eutectoid steels.Thehypoeutectoid steels below the line ES will consist of austenite and cementite. Similarly, cast iron with carbon below 4.3% will have pearlite and ledeburite, those with exactly 4.3% carbon only ledeburite having above 4.3% carbon cementite and ledeburite as their phase structures below the line EF.

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8 FINAL OUTCOME After analyzing the process of heat treatment it is observed as final outcome that it develops following general effects in mechanical properties. 

Increase the hardness of metal

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Relieves the internal stresses set up in the material after hot or cold working

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Improves machinability

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Softens the metal

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Modifies theintesrnal structure of material

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Improves the electrical and magnetic properties

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Increases quality of metal to increase better resistance to heat, corrosion and wear

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Improves mechanical properties like tensile strength, ductility and shock resistance etc.

After analyzing the effects of carbon content it is observed as final outcome that with increase in carbon in steel 

Decreases the ductility of steel.

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Increases the tensile strength of steel

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Increases the hardness of steel.

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Decreases the ease with which steel can be machined.

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Lowers the melting point of steel.

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Makes steel easier to harden with heat treatments.

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Lowers the temperature required to heat treat steel.

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Increases the difficulty of welding steel. 26

9 SCOPE OF FUTURE STUDY: Scope for future study regarding heat treatment and carbon percentage in metals is more. I have learnt a lot from this project by analyzing heat treatment by various method and carbon percentage. 

By changing the percentage of carbon in steel we can control the hardness of steel.

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Better optimization of temp. & process can reduce the defects of heat treatment.

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Better control of temperature in heat treatment can increase the hardness as well as toughness.

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10 REFERENCES: 

Workshop technology by R S Khurmi.

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Material Science by W D Callister.

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Material Science by R K Rajput.

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Material Science by O P Khanna.

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Visit to SMMS (Specialist Materials & Maintenance Services) department of EIL (Engineers India Limited). Internet.

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11 FINAL REMARKS OF PROJECT GUIDE

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