Subject: Manufacturing Processes Class: 3rd Mechanical Engineering Department Tikrit University Prepared by: Assistant Prof.Dr.Farouk Mansour Mahdi

Lecture No.1 Week No.1 No. of hours: 2 theoretical and 1 tutorial Metal Casting Metal Casting 1- Casting Furnaces ( Melting Furnaces )

Melting furnaces used in the foundry industry are of many diverse configurations. The selection of the melting unit is one of the most important decisions foundries must make. Several important factors must be considered for proper selection, these includes:

1. The temperature required to melt the metal or alloy. 2. The melting rate and quantity of molten metal required. 3. The required quality of the melt and subsequent final product. 4. The economy of installation, operation and maintenance. 5. Environmental and waste disposal requirements.

Furnaces can be classified according to the type of lining:

1- Acidic lined furnaces ( e.g. SiO2 )

2- Basic lined furnaces ( e.g. MgO, MgCO3 , CaO)

Lining materials are characterized by:

1-Refractoriness.

2-High wear resistance.

3-Low coefficient of thermal expansion.

4-High resistance to thermal shock.

5-Heat insulation. 6-passive towards molten metal, furnace gases and ( chemically inert).

1-1- Cupola Furnaces

coke type

Description: A cupola or cupola furnace is a melting device used in foundries to melt cast , some bronzes and even aluminum when attention is paid to keep the temperature low. The construction of a conventional cupola consists of a vertical shell which is lined with a refractory brick. The size of a cupola is expressed in diameters and can range from 0.5 to 4.0 m while the stack height is between 6 to 11 m. The bottom of the cylinder is fitted with doors which swing down and out. The top, where gases escape can be opened or fitted with a cap to prevent rain from entering the cupola.

Operation: To begin a production run, called a 'cupola campaign', the furnace is charged with layers of coke and ignited with torches. When the coke is ignited, air is introduced to the coke bed through ports in the sides called tuyeres. When the coke is very hot, solid pieces of metal are charged into the furnace through the charging door. The metal is alternated with additional layers of fresh coke. Limestone ( CaCO3 ) is added to act as a flux. As the heat rises within the stack the metal is melted. It drips down through the coke bed to collect in a pool at the bottom, just above the bottom doors. Some of the is picked up by the falling droplets of molten metal which raises the carbon content of the iron. Additions to the molten iron such as ferro-manganese, ferro-silicon, silicon carbide and other alloying agents are used to alter the molten iron to conform the required composition. When the metal level is sufficiently high in the well, the cupola operator opens the "tap hole" to let the metal flow into a ladle or other container to hold the molten metal. When slag will rise to the top of the formed iron pool. The slag hole is opened to permit the slag flow out.

Advantages:

1. Lower initial cost on a small floor area comparing with those furnaces with the same capacity. 2. The cupolas is the only continuous melting method. 3. High melt rates. 4. Relatively low operating costs. 5. Ease of operation. 6. From a life-cycle perspective, cupolas are more efficient and less harmful to the environment than electric furnaces. This is because they derive energy directly from coke rather than from electricity that first has to be generated. 7. Adequate temperature control. 8. Adequate Chemical composition control.

9. Efficiency of cupola varies from 30 to 50%.

Disadvantages:

1- Since molten iron, coke and oxygen are in contact with each other, certain elements like Si and Mn are lost and others like S and C are picked up. This changes the final analysis of molten metal. 2- Close temperature control is difficult to maintain. 3- Accurate control of chemical composition is not possible.

1-2 Reverberatory Furnaces

( Pb remelting)

Reverberatory furnaces which are also called "air furnaces" are used for (refining) or melting processes, in which the fuel is not in direct contact with the contents but heats it by a flame and hot gases blown over it from another chamber. Such furnaces are used in copper, tin, nickel production, and in aluminum recycling. In , this process is called the open-hearth process ( which will be explained later ). The basic idea of a reverberatory furnace is to use the heat reflecting off a surface, usually brick, to heat the metal.

Operation: The material to be heated is placed on the hearth and is heated by the hot gases or flame produced by the burning of fuel. The waste gases escape out of the chimney. This way, the metal does not come into direct contact with the fuel or the flame. By placing the metal in a shallow depression and then directing an intense flame over that depression and to the wall, the heat rebounds (radiates back or reverberates) to melt the metal. Many casters will adjust the length of the flame since a longer path will mean that the heat will be more intense. Reverberatory furnaces are available with capacities of up to 150 tons of molten aluminum.

Advantages:

1-Low operating and maintenance costs.

2-High volume processing rate.

3-Adequate temperature control.

4-Adequate Chemical composition control.

Disadvantages:

1-High initial cost.

2-The reverberatory process is a batch type.

3-Large floor space requirements.

4-Precious control of melt temperature cannot be made (Wide Metal Temperature Variations +/- 50º F). 5-Accurate control of chemical composition cannot be satisfied.

6-Limited control of furnace atmosphere.

7- Typical aluminum reverberatory furnaces have melting efficiencies of 15 - 39%.

8- Greater hydrogen gas pick up in aluminum melting.

1-3 Open hearth furnace

Description: It is an alternative steelmaking process in which natural gas, oil, atomized heavy oils, tar, or pulverized coal are used as fuel. Both air and fuel are preheated to about 800o C before combustion. A flame temperature of about 2,000° C could be obtained, and this is sufficient to melt the charge. Initially, charges of 10 tons were made, but furnace capacity gradually increased to 100, 300 and eventually to 600 tons. Operation: In case of re-melting of steel scrap, the furnace is charged with light scrap, such as sheet metal, shredded vehicles or waste metal. When light scrap has melted, heavy scrap, such as building, construction or steel milling scrap is added, together with from blast furnaces. After all steel has been melted, slag forming agents, such as limestone, are added. The oxygen in iron oxide and other impurities decarburize the pig iron by burning the carbon away, forming steel. To increase the oxygen content of the hearth, iron ore can be added. Preparing a hearth usually takes 8 h to 8 h and 30 minutes. Additions can be made to the steel to produce the desired composition. After a period of time, the direction of air and fuel flow is reversed. The chambers heated from the previous cycle, in turn, heat the incoming fuel and air. Most open hearth furnaces are chemically basic. The basic furnaces can remove phosphorous, sulfur, silicon, carbon, and manganese from the charge metal.

The furnace is tapped through a tap hole located at the side of the hearth and liquid steel is let to flow out. Once all the steel has been tapped, the slag is skimmed away. The tapped steel may be cast into ingots or it may be used in continuous casting for the rolling mill.

Advantages:

1-The great advantage of the open hearth was its flexibility: the charge could be all molten pig iron, all cold scrap, or any combination of pig iron and scrap. 2-Basic open hearth furnaces are capable of processing iron of almost any chemical composition.

3-The process is suited to handle any amount of low cost steel scrap.

4-Open hearth furnaces can operate on any kind of fuel.

5-The quality of open hearth furnace is the highest among commercial steel making processes ( Bessemer, Thomas and oxygen converter techniques ).

6-Adequate temperature control.

7-Adequate Chemical composition control.

Disadvantages: 1-High initial cost.

2-Large floor space requirements.

3- The open hearth process is a batch type.

4-Low productivity as compared with oxygen converter process.

5-The necessity of providing fluxes and regenerators ( such as alloying elements) raises the costs of construction and running of open hearth furnaces.

6-Precious control of melt temperature cannot be made.

7-Accurate control of chemical composition cannot be satisfied.

8-Limited control of furnace atmosphere.

1-4 Electric Arc Furnaces

Electric arc furnaces may be categorized as direct arc and indirect arc. Both types of units are suited for the melting of high melting point alloys such as . They may be lined with acid or basic refractories. The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces (BOF) is their capability to treat charges containing up to 100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces

Direct arc furnaces are very popular for the melting of alloy steels and range in size from a few kilograms, for laboratory units, to about 400 tons per batch. Typical units found in foundries are in the range of 1 to 10 tons. The furnace generally consists of steel shell lined with acid or basic refractories. The roof which can normally swing away to facilitate charging, generally contains three carbon electrodes operate on a high tension three-phase power supply. These electrodes protrude vertically through the roof and an electric current passes directly through them into the metal bath. The entire unit is capable of being tilted for discharge of the melt through the pouring spout. Indirect arc furnaces: generally consist of a horizontal barrel shape steel shell lined with refractories. Melting is effected by the arcing between two horizontally opposed carbon electrodes. Heating is via radiation from the arc to the charge. The barrel shaped shell is designed to rotate and reverse through approximately 180° in order to avoid excessive heating of the refractories above the melt level and to increase the melting efficiency of the unit.

Indirect arc furnaces are suitable for melting a wide range of alloys but are particularly popular for the production of copper base alloys. The units operate on a single-phase power supply and hence the size is usually limited to relatively small units.

Direct Arc Furnace

indirect arc furnace Advantages

1- High melt rates. 2- High pouring temperatures. 3- Excellent control of melt chemistry. 4- Accurate control of melt temperature. 5- Furnace atmosphere can be controlled. 6- Flexibility i.e. can be rapidly started and stopped, allowing the to vary production according to demand.

Disadvantages

1- High initial cost. 2- High operation and maintenance costs. 3- The process is a batch type.

1-5 Electric Induction Furnaces The principle of induction melting is that a high voltage electrical source from a primary coil induces a low voltage, high current in the metal, or secondary coil.

Coreless induction furnaces

The heart of the coreless is the coil, which consists of a hollow section of heavy duty, high conductivity copper tubing which is wound into a helical coil. Coil shape is contained within a steel shell and magnetic shielding is used to prevent heating of the supporting shell. To protect it from overheating, the coil is water-cooled, the water being re-circulated and cooled in a cooling tower. The shell is supported on trunnions on which the furnace tilts to facilitate pouring.

The crucible is formed by ramming a granular refractory between the coil and a hollow internal former which is melted away with the first heat leaving a sintered lining.

When the charge material is molten, the interaction of the magnetic field and the electrical currents flowing in the induction coil produce a stirring action within the molten metal. This stirring action forces the molten metal to rise upwards in the center causing the characteristic meniscus on the surface of the metal. The stirring action within the bath is important as it helps in thorough mixing of alloying elements and homogenizing of temperature throughout the melt. Excessive stirring can increase gas pick up, lining wear and oxidation of alloys.

The heating system in an induction furnace includes:

1- Induction heating power supply. 2- Induction heating coil. 3- Water-cooling source, which cools the coil and several internal components inside the power supply.

schematic representation of coreless induction furnace

Advantages

1- Induction heating is a clean form of heating. No by-products of combustion means a cleaner melting environment and no associated products of combustion pollution control systems. 2- High melt rate or high melting efficiency. Cold charge-to-tap time of one to two hours are common. 3- Alloyed steels can be melted without any loss of alloying elements. 4- Controllable and localized heating. 5- ThoroughInduction mixing unit of melt constituents by electromagnetic stirring action. 6- Compact Installation. High melting rates can be obtained from small furnaces. 7- Better Working Environment. Induction furnaces are much quieter than gas furnaces, arc furnaces, or cupolas. 8- Energy Conservation. Overall energy efficiency in induction melting ranges from 55 to 75%, and is significantly better than combustion processes. 9- Reduced Refractory. The compact size in relation to melting rate means induction furnaces require much less refractory than fuel-fired units. 10-Furnace atmosphere can be controlled.

Disadvantages

1- High capital cost of the equipment. 2- High operating cost. 3- Refining in induction furnace is not as effective as in . 4- Life of Refractory lining is low as compared to Electric Arc Furnace. 5- Removal of S & P is limited, so selection of charges with less impurity is required.

1-6 Crucible Furnaces

Crucible furnaces are one of the oldest and simplest types of melting units used in the foundry. The furnaces uses a refractory crucible which contains the metal charge. The charge is heated via conduction of heat through the walls of the crucible. The heating fuel is typically coke, oil, gas or electricity. Crucible melting is commonly used where small batches of low melting point alloy are required. The capital outlay of these furnaces makes them attractive to small non-ferrous foundries. Its capacity may range from 30 to 150 kg.

Tilting crucible furnaces

Advantages of crucible furnaces

1- Low investment (initial) and maintenance costs. 2- Crucibles have the unique ability to melt, hold and transfer metal using a single vessel. 3- The melt can be treated directly in the crucible. 4- Allowing incompatible alloy changes to be made simply by switching vessels. 5- Even when fixed within the furnace structure, crucibles offer significant advantages when compared to directly-heated fuel–fired furnaces. These important benefits include: i- Lower Metal Loss. ii- Cleaner Metal.

iii- Alloy Flexibility. iv- Quick Replacement.

Disadvantages of crucible furnaces

1- Low efficiency (7 to 19%). 2- Low melt rate. 3- High emissions. 4- Size limitations. 5- Manual charging causes very high operational costs.

Only completely dry metal must be used for subsequent charging, since wet charge material causes ejection of metal resulting in great risks for personnel

Lecture No.2 Week No.2 No. of hours: 2 theoretical and 1 tutorial 1-2 CASTING PROCESSES

1-2-1 Expandable Mold Casting

In expendable mold casting, the mold is destroyed to remove the casting and a new mold is required for each new casting. Expendable mold casting include the following processes:

1-Sand Casting

Sand casting, also known as sand molded casting, is a metal casting process characterized by using sand as the mold material. The term "sand casting" can also refer to an object produced via the sand casting process. Sand castings are produced in specialized factories called foundries. Over 70% of all metal castings are produced via a sand casting process. Sand casting is relatively cheap and sufficiently refractory even for steel foundry use. The next figure illustrates the basic production steps in sand casting:

Patterns

Patterns in sand casting are used to form the mold cavity. One major requirement is that patterns (and therefore the mold cavity) must be oversized (i) to account for shrinkage in cooling and solidification, and (ii) to provide enough metal for the subsequence machining operation(s).

Fig.(1-12): Split pattern showing the two sections together and separated.

Fig.(1-13): Solid pattern for a pinion gear

Cores

Cores serve to produce internal surfaces in castings In some cases, they have to be supported by chaplets for more stable positioning:

(a) (b) (c) Fig.(1-14): (a) Core held in place in the mold cavity by chaplets, (b) chaplet design, (c) casting with internal cavity.

Cores are made of foundry sand with addition of some resin for strength by means of core boxes:

Fig.(1-15): Core box, two core halves ready for baking, and the complete core made by gluing the two halves together

Foundry sands

In addition to the sand, a suitable bonding agent (usually clay) is mixed or occurs with the sand. The mixture is moistened, typically with water, but sometimes with other substances, to develop strength and plasticity of the clay and to make the aggregate suitable for molding. The sand is typically contained in a system of frames or mold boxes known as a flask. The mold cavities and gate system are created by compacting the sand around models, or patterns, or carved directly into the sand.The typical foundry sand is a mixture of fresh and recycled sand, which contains 90% silica (SiO2 ), 23% water, and 7% clay. The grain size and grain shape are very important as they define the surface quality of casting and the major mold parameters such as strength and permeability:

results Bigger grain size in a worse surface finish Irregular grain shapes produce stronger mold

Larger grain size ensures better permeability

Fig.(1-16): effect of grain size and shape on mold properties

Casting Sand Properties

Refractoriness — This refers to the sand's ability to withstand the temperature of the liquid metal being cast without breaking down. For example some sands only need to withstand 650 °C(1,202 °F) if casting aluminum alloys, whereas steel needs a sand that will withstand 1,500 °C (2,730 °F). Sand with too low a refractoriness will melt and fuse to the casting. Chemical inertness — The sand must not react with the metal being cast. This is especially important with highly reactive metals, such as magnesium and titanium. Permeability — This refers to the sand's ability to exhaust gases. This is important because during the pouring process many gases are produced, such as hydrogen, nitrogen, carbon dioxide, and steam, which must leave the mold otherwise casting defects, such as blow holes and gas holes, occur in the casting. Note that for each cubic centimeter (cc) of water added to the mold 16,000 cc of steam is produced. Surface finish — The size and shape of the sand particles defines the best surface finish achievable, with finer particles producing a better finish. However, as the particles become finer (and surface finish improves) the permeability becomes worse. Cohesiveness (or bond) — This is the ability of the sand to retain a given shape after the pattern is removed. Flowability – The ability for the sand to flow into intricate details and tight corners without special processes or equipment. Collapsibility — This is the ability of the sand to be easily stripped off the casting after it has solidified. Sands with poor collapsibility will adhere strongly to the casting. When casting metals that contract a lot during cooling or with long freezing temperature ranges a sand with poor collapsibility will cause cracking and hot tears in the casting. Special additives can be used to improve collapsibility. Availability/cost — The availability and cost of the sand is very important because for every ton of metal poured, three to six tons of sand is required. Although sand can be screened and reused, the particles eventually become too fine and require periodic replacement with fresh sand. In large castings it is economical to use two different sands, because the majority of the sand will not be in contact with the casting, so it does not need any special properties. The sand that is in contact with the casting is called facing sand, and is designed for the casting on hand. This sand will be built up around the pattern to a thickness of 30 to 100 mm (1.2 to 3.9 in). The sand that fills in around the facing sand is called backing sand. This sand is simply silica sand with only a small amount of binder and no special additives.

Types of Base Sand

Silica sand

Silica (SiO2) sand is the sand found on a beach and is also the most commonly used sand. It is made by either crushing sandstone or taken from natural occurring locations, such as beaches and river beds. The fusion point of pure silica is 1,760 °C (3,200 °F), however the sands used have a lower melting point due to impurities. For high melting point casting, such as steels, a minimum of 98% pure silica sand must be used; however for lower melting point metals, such as and non-ferrous metals, a lower purity sand can be used (between 94 and 98% pure). Silica sand is the most commonly used sand because of its great abundance, and, thus, low cost (therein being its greatest advantage). Its disadvantages are high thermal expansion, which can cause casting defects with high melting point metals, and low thermal conductivity, which can lead to unsound casting. It also cannot be used with certain basic metal because it will chemically interact with the metal forming surface defect. Finally, it causes silicosis in foundry workers.

Olivine sand Olivine is a mixture of orthosilicates of iron and magnesium from the mineral dunite. Its main advantage is that it is free from silica, therefore it can be used with basic metals, such as manganese steels. Other advantages include a low thermal expansion, high thermal conductivity, and high fusion point. Finally, it is safer to use than silica, therefore it is popular in Europe.

Chromite sand Chromite sand is a solid solution of spinels. Its advantages are a low percentage of silica, a very high fusion point (1,850 °C (3,360 °F)), and a very high thermal conductivity. Its disadvantage is its costliness, therefore its only used with expensive alloy steel casting and to make cores.

Zircon

Zircon sand is a compound of approximately two-thirds zircon oxide (Zr2O) and one-third silica. It has the highest fusion point of all the base sands at 2,600 °C (4,710 °F), a very low thermal expansion, and a high thermal conductivity. Because of these good properties it is commonly used when casting alloy steels and other expensive alloys. It is also used as a mold wash (a coating applied to the molding cavity) to improve surface finish. However, it is expensive and not readily available. Chamotte sand

Chamotte is made by calcining fire clay (Al2O3-SiO2) above 1,100 °C (2,010 °F). Its fusion point is 1,750 °C (3,180 °F) and has low thermal expansion. It is the second cheapest sand, however it is still twice as expensive as silica. Its disadvantages are very coarse grains, which result in a poor surface finish, and it is limited to dry sand molding. Mold washes are used to overcome the surface finish problem. This sand is usually used when casting large steel workpieces.

Binders

Binders are added to a base sand to bond the sand particles together (i.e. it is the glue that holds the mold together). Clay and water A mixture of clay and water is the most commonly used binder. There are two types of clay commonly used: bentonite and kaolinite, with the former being the most common.

Oil Oils, such as linseed oil, other vegetable oils and marine oils, used to be used as a binder, however due to their increasing cost, they have been mostly phased out. The oil also required careful baking at 100 to 200 °C (212 to 392 °F) to cure (if overheated the oil becomes brittle, wasting the mold). Resin Resin binders are natural or synthetic high melting point gums. The two common types used are urea formaldehyde (UF) and phenol formaldehyde (PF) resins. PF resins have a higher heat resistance than UF resins and cost less. There are also cold-set resins, which use a catalyst instead of a heat to cure the binder. Resin binders are quite popular because different properties can be achieved by mixing with various additives. Other advantages include good collapsibility, low gassing, and they leave a good surface finish on the casting.

MDI (methylene diphenyl diisocyanate) is also a commonly used binder resin in the foundry core process.

Sodium silicate

Sodium silicate [Na2SiO3 or (Na2O)(SiO2)] is a high strength binder used with silica molding sand. To cure the binder carbon dioxide gas is used, which creates the following reaction:

The advantage to this binder is that it can be used at room temperature and it's fast. The disadvantage is that its high strength leads to shakeout difficulties and possibly hot tears in the casting.

Additives Additives are added to the molding components to improve: surface finish, dry strength, refractoriness, and "cushioning properties". Up to 5% of reducing agents, such as coal powder, pitch, creosote, and fuel oil, may be added to the molding material to prevent wetting (prevention of liquid metal sticking to sand particles, thus leaving them on the casting surface), improve surface finish, decrease metal penetration, and burn-on defects. These additives achieve this by creating gases at the surface of the mold cavity, which prevent the liquid metal from adhering to the sand. Reducing agents are not used with steel casting, because they can carburize the metal during casting. Up to 3% of "cushioning material", such as wood flour, saw dust, powdered husks, peat, and straw, can be added to reduce scabbing, hot tear, and hot crack casting defects when casting high temperature metals. These materials are beneficial because burn-off when the metal is poured creating voids in the mold, which allow it to expand. They also increase collapsibility and reduce shakeout time. Up to 2% of cereal binders, such as dextrin, starch, sulphite lye, and molasses, can be used to increase dry strength (the strength of the mold after curing) and improve surface finish. Cereal binders also improve collapsibility and reduce shakeout time because they burn-off when the metal is poured. The disadvantage to cereal binders is that they are expensive. Up to 2% of iron oxide powder can be used to prevent mold cracking and metal penetration, essentially improving refractoriness. Silica flour (fine silica) and zircon flour also improve refractoriness, especially in ferrous castings. The disadvantages to these additives is that they greatly reduce permeability.

Parting compounds To get the pattern out of the mold, prior to casting, a parting compound is applied to the pattern to ease removal. They can be a liquid or a fine powder (particle diameters between 75 and 150 micrometres (0.0030 and 0.0059 in)). Common powders include talc, graphite, and dry silica; common liquids include mineral oil and water- based silicon solutions. The latter are more commonly used with metal and large wooden patterns.

Mixing of foundry sands

Mixing of foundry sand is an essential step in sand preparation to satisfy homogeneous distribution of its constituents.