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Milling Process For Food Engineering Essay

ABSTRACT

Grain milling for food use and other purposes requires a suitable machine with appropriate components; hammers being one of them. The hammer mill is designed and constructed from locally available materials for milling grain particles of sorghum which are passed through a cylindrical sieve positioned beneath the hammer assembly.

Milling process is achieved by the use of hammers in beating the grains fed into the mill through the hopper to produce fine particles; the fineness aimed depends on the detachable screen with aperture sizes ranging from 870μm to 2 mm.

Sorghum milling hammers installed at Dar Brew are faced with rapid wearing of hammers which increases the frequency of replacement and thus this project aims at introducing a new heat treatment process to increase the life of the grain mills hammers.

Contents

LIST OF FIGURES

CHAPTER 1

1.0 INTRODUCTION

The hammer mill which can otherwise be referred to as Cereal Miller was designed for grinding and sieving all kinds of cereal grains, such as maize, wheat, millet, corn, sorghum, wheat, etc. It can also process non-cereal materials such as dry cassava tuber and yam tuber.

In ancient time, the cereal grains were crushed between two stones and made into crude cake but the advent of modern automated systems employing steel material such as hammer mill have revolutionized the processing of cereals and their availability as human foods and other purposes (Donnel 1983).

According to Taylor and Dewar 2001, the lack of an efficient Cereal milling technology has been identified as a major constraint to the establishment of a vibrant Cereals food industry. Early attempts to process sorghum by roller milling, the industrial milling technology for Sorghum, wheat and maize resulted in poor separation of grain components giving poor refined products that lacked consumer appeal. One important aspect of sorghum grain structure is that the pericarp (outer bran layer) appears to be more friable than that of most other cereals. This is disadvantageous in dry milling as the flour can be become contaminated with bran. The friable nature of the sorghum pericarp is probably related to the fact that it, almost uniquely among cereals, contains starch granules. Grain shape, the large germ, pericarp friability and the very variable proportion of horny to floury endosperm texture in grain are issues that need to be addressed in the selection of suitable sorghum types for dry milling. These factors also impact on sorghum dry milling technology. The structure of a cereal grain is presented in Figure 1 below;

Figure : Structure of cereal grain

(Source: FAO, (2003), Market research for agroprocessors, Marketing Extension Guide No. 3. Rome)

In recent years, mechanized milling commonly using hammer mills has become popular for commercial small and medium-scale cereals flour production. However, this milling system is characterized by high milling losses, inconsistent quality and low production rates. The defects and shortcoming of currently used hammer mills have meant that most hammer mill operators and owners are running their business at marginal profit levels. (Taylor and Dewar 2003)

The cereal grain is fed into a grinding chamber in which a number of hammers rotate at high speed. The collision of the hammers with the grain causes a breakdown to flour. The mill outlet contains a retention screen that holds back larger particles until they are broken down further so that there will be a known maximum particle size. A size distribution test (or Sieve analysis) of the flour should be conducted regularly. Sieve analysis shows whether the hammer mill screens are in good order and whether the mill is correctly adjusted. In the same way, the hammers are to be checked regularly if they are in a good working order.

The use of hammer mill technology in Dar Brew Ltd industry has brought about a recent problem of rapid wearing of hammers which are supplied by Technology Development and Transfer Centre (TDTC). Some of the grain mills manufactured at TDTC are Small size mills with 24 hammers, Medium size mills with 32 hammers, King size mills with 32 hammers and Animal feed mills with 12 hammers.

Production of flour depends on the speed of the mill, hammers strength and rigidity and size of sieves. However, shearing and abrasion phenomenon results wearing out processes of the hammers. Wear is a significant cost factor not only due to repurchasing and replacement of parts but also due to shutdown times. Normally the hammers supplied by TDTC are turned after two days depending on usage and decision should be made as to whether or not to replace the hammers and screens.

1.1 PROBLEM STATEMENT

Hammer mills installed at Dar Brew are faced with rapid wear of hammers which results in frequent need for replacement. Dar Brew are using the mill to process red Sorghum which normally contains small stones and sand which accelerates the rate of wearing.

1.2 PROJECT OBJECTIVES

1.2.1 Main objectives

The main objective of this project is to improve the wear properties of hammers for grain mills.

1.2.2 Specific objectives

To study the wear characteristics of hammers for TDTC grain mills.

To identify an appropriate heat treatment process to increase the hardness of the hammers

To conduct tests on the hammers using the improved TDTC grain mill installed at Dar Brew.

1.3 SIGNIFICANCE OF THE PROJECT

The output of this project completion will be hardened hammers with increased lifetime and thus reduce operation costs (lower replacement frequency)

1.4 SCOPE OF THE STUDY

This project will focus on Dar Brew grain mill hammers where by sources of wearing-out will be collected. Lifetime of the hammers and costs involved in replacements of the hammers will also be determined. A heat treatment process to improve the lifetime of the hammers will be developed.

CHAPTER 2

LITERATURE REVIEW

2.0 INTRODUCTION

Milling is the process of grinding cereal grains using a special machine. The main objective of milling is to improve the digestibility of the grains for human or animal consumption. Milling for human consumption is aimed to produce a palatable meal or flour and to expose the starch in the endosperm to the digestion juices of the stomach. The objective is not to produce a very fine flour or paste but rather to mill the grain to a point of coarseness that is acceptable to the consumer.

The principal mechanisms of fracture in milling are compression and shear. Other effects occur, such as cutting, sawing, tearing and abrasion, but they are only a combination of shear and compression. The stages of compression and shear occur in all milling machines between two flat plates, or between a flat plate and a bed of grain. In some mills, the grain is suspended in the air where it is struck by a high-speed plate or hammer. A larger grain has more inertia and, therefore, fractures more easily. As the grain becomes finer, it has less inertia and fractures less.

2.1 DESCRIPTION OF HAMMER MILLS

2.1.1 Operating principles of hammer mill;

As the name implies, hammers in the mill grind grains through impact and pulverization. The grains are placed into a holding hopper on top of the hammer mill, and a small control gate allows the grains to trickle into the grinding chamber. The feed hopper is chamfered to facilitate unidirectional flow of the raw material by gravity to the milling chamber. The hammers strike the grains and shatter them before they can pass through the screen (sieves) surrounding the hammers in which the fineness of the particles is regulated by the use of sieves of different mesh sizes. The flour produced either falls by gravity into a chamber or sack below, or is propelled by air flow up through a cyclone into a holding container. The airflow is provided by either the fan effect of the hammers or by extra fan blades mounted on the hammer shaft. A hammer mill consists of a large cylinder with a horizontal shaft that drives a rotor with several rows of free-swinging hammers. The hammers rotate inside a perforated metal screen through which the flour is drawn. The hammers are driven by two or four sets of V-section belts between the motor and the mill. The hammers spin at high speed in a strong housing (usually made of thick steel sheets).

The speed of the mill has to be matched to the size of the mill as a small mill needs to run at higher revolutions than does a larger mill. Control of the feed to prevent overloading of the crushing chamber is achieved by using a cut-out mechanism to control the periodic opening and closing of the feed hopper gates in the chamber. The hammers are allowed to swing freely instead of being rigidly attached so as to absorb shock loads encountered when they come into contact with very hard substances or material.

Some hammer mills have screens that cover the mill around 360 degrees. More popular designs have screens around 180 degrees of the lower periphery as this allows easily made replacement screens to be used. Beater bars are often incorporated into the upper semi-circle against which the grain impacts. Screens are made by perforating blank sheets of steel. Figure 2 below presents the major parts of a hammer mill.

Figure : Major parts of the grain mill

(Source: http://www.journal.au.edu/au_techno/2005/jan05/vol8no3_aujtechno03.pd)

2.1.2 Modified TDTC grain mill;

The major components of the machine include the hammers, shaft, bearing, centrifugal fan, mechanical separator, cyclone casing and electric motor. Overall dimension of the mill: L x W x H = 1800 x 1100 x 1000(mm), 12 Hammers with dimension: L x W x t=110x50x6(in mm), Power rating of the motor: 30 kW and Mill Capacity: 1000Kg/hr

Hammers are made from a rectangular-shaped piece of steel with a hole at one end. As one corner of the hammer wears, it can be turned over to present the other corner. The hammers can be turned over until all four corners have worn away and then replacement of worn hammers is can be done. This has to be done after two days of running. When undertaking replacements, it is necessary to ensure that the tip clearance between hammers and screen is about 5 mm.

C:\Users\Lameck\Desktop\New Bitmap Image.jpg

Figure : Grain mill hammer

Assuming the sets on each side of the mill are referred to as the north, south, east and west, then the east ones should balance with those on the west, and the north ones with those on the south (See Figure 4). Otherwise, the mill will vibrate, which places extra load, and increases wear, on the bearings. The repair or replacement of hammers is a very time-consuming task.

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Figure Hammers balanced

2.1.3 Monitoring and maintenance of hammer mills

Before operating the mill, a checkup of the hammers should be done. In order to ensure efficient working of the machine the following should be done; preventive maintenance by adopting a periodic maintenance schedule for all the parts of mills, periodic checking of hammers and hammer pockets for proper functioning, carrying out regular maintenance, checking alignment of belts, avoiding idle operation of machines and Machines should be placed in well supported flat surface to avoid vibration as well as material wastage.

2.2 HEAT TREATMENT OF CARBON STEEL

2.2.1 Introduction:

Heat treatment is an operation involving the heating of the solid metal to definite temperatures, followed by cooling at suitable rates in order to obtain certain physical properties, which are associated with changes in the nature, form, size and distribution of the micro-constituents.

Plain carbon steels and alloy steels are among the relatively few engineering materials which can be usefully heat treated in order to vary their mechanical properties. The other main group is the heat-treatable aluminium alloys. Steels can be heat treated because of the structural changes that can take place within solid iron-carbon alloys. The various heat-treatment processes appropriate to plain carbon steels are:

Annealing

Normalizing

Hardening

Tempering

In all the above processes the steel is heated slowly to the appropriate temperature for its carbon content and then cooled. It is the rate of cooling which determines the ultimate structure and properties that the steel will have, providing that the initial heating has been slow enough for the steel to have reached phase equilibrium at its process temperature. Refer Figure 5 below.

Figure : Heat-treatment temperature Ranges of Classes of Carbon Steels in Relation to the Equilibrium Diagram

(Source: http://www.cseiq.org/pdf_files/1266913387_7.pdf)

2.2.1.1 Annealing

All annealing processes are concerned with rendering steel soft and ductile so that it can be cold worked and/or machined. There are three basic annealing processes as in figure 2 below and these are:

1) Stress-relief annealing at subcritical temperatures

It is also called 'process annealing’, 'interstage annealing' and subcritical annealing, it is often used for softening cold worked low carbon (0.3 % carbon content) steel or mild steel. To fully anneal such steel would involve heating to a temperature of more than 900˚C, with consequent high cost

2) Spheroidised annealing at subcritical temperatures

The Spheroidised condition is produced by annealing the steel at a temperature between 650 and 700˚C, just below the lower critical temperature. During this treatment cementite forms as spheroidal particles in a ferrite matrix, putting the steel into a soft, but very tough, condition.

3) Full annealing for forgings and castings.

It is the treatment given to produce the softest possible condition in hypoeutectoid steel. It involves heating the steel to a temperature within the range 30-50˚C above the upper critical temperatures and then allowing the steel to cool slowly within the furnace. This produces a structure containing coarse pearlite. Full annealing (Figure 6) is an expensive treatment and when it is not absolutely essential for the steel to be in a very soft condition, but a reasonably soft and ductile material is required, the process known as normalizing is used instead.

C:\Users\Lameck\Desktop\New Bitmap Image.jpg

Figure : Annealing temperature for plain carbon steel

(Source: http://www.cseiq.org/pdf_files/1266913387_7.pdf)

The structural effects of heating a steel casting to a temperature just above its upper critical followed by cooling to room temperature can be analyzed as follows;

Over heating

Overheating during annealing or heating for too long a period in the austenitic range will obviously cause grain growth of the newly formed austenite crystals, leading to a structure almost as bad as the original Widmanstatten structure. For this reason the requisite annealing temperature should not be exceeded, and the casting should remain in the austenitic range only for as long as is necessary to make it completely austenitic.

Burning (Excessive over heating)

Excessive overheating will probably cause oxidation, or "burning", of the surface, and the penetration by oxide films of the crystal boundaries following carburization of the surface. Such damage cannot be repaired by heat-treatment, and the castings can only be scrapped. To prevent "burning", castings are often annealed in cast-iron boxes into which they are packed with lime, sand, cast-iron turnings or carbonaceous material, according to the carbon content of the castings.

As in figure 7 the lower critical temperature (723˚C) is reached on heating, the patches of pearlite change to austenite, but these crystals of austenite are very small, since each grain of pearlite gives rise to a number of new austenite crystals. As the temperature rises, the Widmanstatten-type plates of ferrite are dissolved by the austenite until, when the upper critical temperature is reached, the structure consists entirely of fine-grained austenite. Cooling causes re-precipitation of the ferrite but since the new austenite crystals are small, the precipitated ferrite will also be distributed as small particles. Finally, as the lower critical temperature is reached, the remaining small patches of austenite will transform to pearlite. Therefore the operation needs a very specific controlling on the heat temperature of annealing to avoid undesired phases in the steel.

Figure : The structural effects of heating a steel casting to a temperature just above its upper critical followed by cooling to room temperature

(Source: http://www.cseiq.org/pdf_files/1266913387_7.pdf)

2.2.1.2 Normalizing

This process resembles full annealing except that, whilst in annealing the cooling rate is deliberately retarded, in normalizing the cooling rate is accelerated by taking the work from the furnace and allowing it to cool in free air. Provision must be made for the free circulation of cool air but draughts must be avoided.

In the normalizing process, as applied to hyper-eutectoid steels, it can be seen that the steel is heated to approximately 50˚C above the upper critical temperature line. This ensures that the transformation to fine grain austenite corrects any grain growth or grain distortion that may have occurred previously. Again, the steel is cooled in free air and the austenite transforms into fine grain pearlite and cementite. The fine grain structure resulting from the more rapid cooling associated with normalizing gives improved strength and toughness to the steel but reduces its ductility and malleability. The increased hardness and reduced ductility allows a better surface finish to be achieved when machining.The excessive softness and ductility of full annealing leads to local tearing of the machined surface.

The type of structure obtained by normalizing will depend largely upon the thickness of cross-section, as this will affect the rate of cooling. Thin sections will give a much finer grain than thick sections, the latter often differing little in structure from an annealed section. Although it is highly successful but this procedure is tied up with an excessive amount of working capital and space and nowadays heat treatment is preferred as the work in progress is turned round more quickly.

2.2.1.3 Hardening

When a piece of steel containing sufficient carbon is cooled rapidly from above its upper critical temperature it becomes considerably harder than it would be if allowed to cool slowly. This involves rapidly quenching the steel, from a high temperature into oil or water. Hypereutectoid steels are heated to 30-50˚C above the upper critical temperature prior to quenching. Consequently, hypereutectoid steels are hardened by quenching from 30-50˚C above the lower critical temperature. The degree of hardness produced can vary, and is dependent upon such factors as the initial quenching temperature; the size of the work; the constitution, properties and temperature of the quenching medium; and the degree of agitation and final temperature of the quenching medium.

To harden a piece of steel, it must be heated then quenched in some media which will produce in it the desired rate of cooling. The medium used will depend upon the composition of the steel and the ultimate properties required. The quenching medium is chosen according to the rate at which it is desired to cool the steel. The following list of media is arranged in order of quenching speeds: 5 % Caustic soda, 5 - 20 % Brine, Cold water, Warm water and Mineral oil.

2.2.1.4 Tempering

A quench-hardened plain carbon steel is hard, brittle and hardening stresses are present. In such a condition it is of little practical use and it has to be reheated or tempered to relieve the stresses and reduce the brittleness. This temperature will remove internal stress setup during quenching, remove some or all of the hardness and increase the toughness of the material. Tempering causes the transformation of martensite into less brittle structures. Unfortunately, any increase in toughness is accompanied by some decrease in hardness. Tempering always tends to transform the unstable martensite back into the stable pearlite of the equilibrium transformations.

Tempering temperatures below 200°C only relieve the hardening stresses but above 220°C the hard brittle martensite starts to transform into a fine pearlitic structure called secondary troostite (or just 'troostite'). Troostite is much tougher although somewhat less hard than Martensite and is the structure to be found in most carbon-steel cutting tools. Tempering above 400°C causes any cementite particles present to "ball-up" giving a structure called sorbite. This is tougher and more ductile than troostite and is the structure used in components subjected to shock loads and where a lower order of hardness can be tolerated, for example springs.

Iron-carbon phase diagram

Iron-carbon phase diagram describes the iron-carbon system of alloy containing up to 6.67% of carbon, discloses the phase compositions and their transformations occurring with the alloys during their cooling or heating. Carbon content 6.67% corresponds to the fixed composition of the iron carbide Fe3C.

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Figure : Iron-Carbon phase diagram

(Source: www.subtech.com

The following phases are involved in the transformation occurring with iron-carbon alloys as indicated in Figure 8 above;

L - Liquid solution of carbon in iron. δ-ferrite – Solid solution of carbon in iron. Maximum concentration of carbon in δ-ferrite is 0.09% at 1493˚C (temperature of the peritectic transformation).

The crystal structure of δ-ferrite is BCC (cubic body centered).

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

α-ferrite – solid solution of carbon in α-iron. Ferrite has BCC crystal structure and low solubility of carbon up to 0.25% at 723˚C. Ferrite exists at room temperature. Stable form of iron at room temperature transforms to FCC austenite at 912˚C.

Cementite – iron carbide, intermetallic compound, having fixed composition Fe3C. Cementite is a hard and brittle substance influencing on the properties of steels and cast irons.

The following phase transformations occur with iron-carbon alloys:

Alloys containing up to 0.51% of carbon start solidification with formation of crystals of δ-ferrite. Carbon content in δ-ferrite increases up to 0.09% in solidification and at 1493˚C remaining liquid phase and δ-ferrite perform peritectic transformation resulting information of austenite.

Alloys containing carbon more than 0.51% but less than 2.06% form primary austenite crystals in the beginning of solidification. Iron-carbon alloys, containing up to 2.06% of carbon, are called steels. Alloys, containing from 2.06 to 6.67% of carbon, experience eutectic transformation at 1147˚C. The eutectic concentration of carbon is 4.3%. All iron-carbon alloys (steels and cast irons) experience eutectoid transformation at 723˚C. The eutectoid concentration of carbon is 0.83%. When the temperature of an alloy reaches 723˚C, austenite transforms to pearlite (fine ferrite-cementite structure, forming as a result of decomposition of austenite at slow cooling conditions).

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.

Phase compositions of the iron-carbon alloys at room temperature

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

2.3 DEVELOPMENT OF MICROSTRUCTURE IN IRON - CARBON ALLOYS

Microstructure depends on composition (carbon content) and heat treatment. In the discussion below we consider slow cooling in which equilibrium is maintained.

2.3.1 Microstructure of eutectoid steel

When alloy of eutectoid composition (0.76 % C) is cooled slowly it forms pearlite, lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C) The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures: redistribution of Carbon atoms between ferrite (0.022 %C) and cementite (6.7 %C) by atomic diffusion. Mechanically, pearlite has properties intermediate to soft, ductile ferrite and hard brittle cementite. Microstructure of eutectoid steel is presented in Figure 9 below;

Figure : Microstructure of eutectoid steel

(Source: http://web.utk.edu/~prack/MSE%20300/FeC.pdf

2.3.2 Microstructure of hypoeutectoid steel

Hypoeutectoid (less than eutectoid) alloys contain proeutectoid ferrite (formed above the eutectoid temperature) plus the eutectoid pearlite that contain eutectoid ferrite

and cementite. See Figure 10 below;

Figure : Microstructure of hypoeutectoid steel

(Source: http://web.utk.edu/~prack/MSE%20300/FeC.pdf)

2.4 PHASE TRANSFORMATIONS OF FE-C KINETICS

2.4.1 Phase transformations (change of the microstructure)

Phase transformations do not occur instantaneously. Diffusion-dependent phase transformations can be rather slow and the final structure often depend on the rate of cooling/heating. We need to consider the time dependence or kinetics of the phase transformations. Phase transformations can be divided into three categories;

a) Diffusion-dependent with no change in phase composition or number of phases present. (e.g. melting, solidification of pure metal, allotropic transformations, recrystallization, etc.)

b) Diffusion-dependent with changes in phase compositions and/or number of phases (e.g. eutectoid transformations)

c) Diffusionless phase transformation - produces a metastable phase by cooperative small displacements of all atoms in structure (e.g. martensitic transformation Phase transformations do not occur instantaneously).

2.4.2 TTT Diagrams;

The TTT diagrams (as in fig.11) are for the isothermal (constant T) transformations (material is cooled quickly to a given temperature before the transformation occurs, and then keep it at that temperature).

i. At low temperatures, the transformation occurs sooner (it is controlled by the rate of nucleation) and grain growth (that is controlled by diffusion) is reduced.

ii. Slow diffusion at low temperatures leads to fine-grained microstructure with thin-layered structure of pearlite (fine pearlite).

iii. At higher temperatures, high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite (coarse pearlite).

iv. At compositions other than eutectoid, a proeutectoid phase (ferrite or cementite) coexist with pearlite. Additional curves for proeutectoid transformation must be included on TTT diagrams.

2.4.3 Martensite

Martensite forms when austenite is rapidly cooled (quenched) to room T. It forms nearly instantaneously when the required low temperature is reached. The austenite-martensite does not involve diffusion i.e. no thermal activation is needed, this is called an athermal transformation. Each atom displaces a small (sub-atomic) distance to transform FCC γ-Fe (austenite) to martensite which has a Body Centered Tetragonal (BCT) unit cell (like BCC, but one unit cell axis is longer than the other two).

Martensite is metastable; can persist indefinitely at room temperature, but will transform to equilibrium phases on annealing at an elevated temperature and can coexist with other phases and/or microstructures in Fe-C system. Since martensite is metastable non-equilibrium phase, it does not appear in phase Fe-C phase diagram. The martensitic transformation involves the sudden reorientation of C and Fe atoms from the FCC solid solution of γ-Fe (austenite) to a body-centered tetragonal (BCT) solid solution (martensite).

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Figure : TTT Diagram including Martensite

A: Austenite P: Pearlite B: Bainite M: Martensite

(Source: http://web.utk.edu/~prack/MSE%20300/FeC.pdf)

2.4.3.1 Tempered Martensite

Martensite is so brittle that it needs to be modified for practical applications. This is done by heating it to 250-650˚C for some time (tempering) which produces tempered martensite, an extremely fine-grained and well dispersed cementite grains in a ferrite matrix.

Tempered martensite is less hard/strong as compared to regular martensite but has enhanced ductility (ferrite phase is ductile).

Mechanical properties depend upon cementite particle size: fewer and larger particles means less boundary area and softer.

Particle size increases with higher tempering temperature and/or longer time (more Carbon diffusion) therefore softer and more ductile material. Tempered martensite is less hard/strong as compared to regular martensite but has enhanced ductility (ferrite phase is ductile).

2.4.3.2 Mechanical properties of martensite

Of the various microstructures in steel alloys Martensite is the hardest, strongest and the most brittle. The strength of martensite is not related to microstructure; rather, it is related to the interstitial C atoms hindering dislocation motion. Tempered martensite is less hard/strong as compared to regular martensite but has enhanced ductility (ferrite phase is ductile).

2.4.3.3 Duplex ferrite-martensite steels

The development of strong and tough structural steels with Duplex Microstructures of steel with Duplex Ferrite-Martensite is prepared from plain carbon steels. The harder martensite provides better mechanical properties. However, according to conventional understanding, hard martensite cracking can easily trigger cleavage unstable fractures. Thus it is important in designing Duplex Ferrite-Martensite steels that we find metallurgical controls to impede the extension of cleavage cracks induced by cracking of the hard martensite without decreasing the hardness of the martensite. However, the role of the hard phase on arresting cleavage cracks is difficult to assess although its effect on initiation has been clarified by many researchers (Koo and Thomas 1977).

To get the best microstructure and tool performance, the quenching rate should be rapid but in order to minimize distortion, a slow quenching rate is recommended. Slow quenching results in less temperature difference between the surface and core of a part, and sections of different thickness will have a more uniform cooling rate. Martensite formation leads to an increase in volume and stresses in the material. This is also the reason why quenching should be interrupted before room temperature has been reached, normally at 50-70°C.However, if the quenching rate is too slow, especially with heavier cross-sections, undesirable transformations in the microstructure can take place, risking a poor tool performance.

Water is used as a quenching medium for unalloyed steels. Sodium chloride (salt) (8–10%) or soda should be added to the water in order to achieve optimum cooling efficiency. Water hardening can often cause problems in the form of distortion and quench cracks. Oil hardening is safer, but hardening in air is best of all. Oil should be used for low alloyed steels. The oil should be of good quality, and preferably of the rapid quenching type. It should be kept clean and must be changed after a certain period of use.

CHAPTER 3

METHODOLOGY

The methodology towards accomplishing the main objective of this project is as described below to achieve each specific objective;

3.1. Literature review

Several literatures concerning how to improve properties of milling hammers will be done to generate ideas on how to accomplish the task. The study on reports, journals, books, websites and papers containing relevant materials concerning the project will be reviewed. The review will enable to obtain information considered necessary to provide sufficient evidence of relevance to this study. The literature review is intended to understand how other writers have dealt with a similar problem in the past including their methods used and results realized. Also it helps to pull out more knowledge and come out with some findings that help to solve the problem.

3.2 Interviewing

It is a technique which will be used to initiate and control the information exchange to obtain quantifiable and comparable information relevant to an emerging or previously stated hypothesis by asking customers of the TDTC milling hammers. This will increase the validity of the data generated concerning source of the problem.

3.3 Physical observation

Through visual observation of the mill mechanism and thus identify sources of the problem resulted by operation.

3.4 Data collection

This is for obtaining a particular targeted output and this involves number of techniques to be applied such as;

Working with Dar Brew staff to collect data on wear characteristics of hammers.

carbon test of the bars to be improved and Lab heat treatment data collection

3.5 Establishing the best material to use for hammers

The choice of a material for a particular article depends on its overall cost and on one or more outstanding properties combined with others at a lower level, e.g. a high tensile structural steel should preferably be as weldable as mild steel with increased corrosion resistance to allow a thinner section to be used. The main properties to be considered are strength at room and elevated temperature, embrittlement in service size and mass effect, surface hardness melting point and density. For this project Low carbon steel (Mild steel) bar will be used. Mild steel have a tensile strength of 380N/mm2, density of 7.8g/cm2 and specific strength of 48.

3.6 Establishing the appropriate heat treatment process to use

Mild steel will be treated in duplex microstructure martensite phase (Ferrite-Martensite).

3.7. Conducting field tests at Dar Brew to establish the preferred heat regime

Testing is a vital step in the process in order to determine the performance of the hammers. Milling hammers will be installed to determine their lifetime.

CHAPTER 4

EXPECTED OUTPUT OF THE PROJECT

At the end of this project, a heat treatment process that extend the lifetime of the grain mill hammers will be established. Development of this process will improve the wear properties of the hammers and thus replacement frequency becomes low.

CHAPTER 5

DURATION IN WEEKS

Semester 1

Semester 2

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Activities

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14

15

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2

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14

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A

2

B

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C

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D

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F

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G

8

H

9

I

10

J

ACTIVITIES SCHEDULE

KEY A. Proposal of project title B. Project title refine and confirmation C. Literature review D. Progressive report writing and Powerpoint preparation E. Oral presentation F. compilation and n of final report G. Development of Practical (carbon content test) H. Heat treatment process of the hammers I. Testing of the hammers J. Report submission

CHAPTER 6

BUDGET ALLOCATION

S/N

ITEM

UNIT

QUANTITY

UNIT PRICE

(Tshs)

TOTAL PRICE

(Tshs)

1

Stationary

Plain papers

Pens

Stapler

Ruler

Ream

Piece

1

4

1

1

7000

500

2500

300

10,300

2

Printing

Pieces

200

100

20,000

3

Transportation

Trip

6

300

1,800

4

Meals and drinks

2000

50,000

Total

Tshs 82,100

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