Welding Slag has to be removed in order to be able to view the welded area for inspection as well for aesthetics and visual appearance. Welding slag can be chipped away with a pointed hammer. Sometimes, it is also removed by grinding.
Welding Slag
Welding Slag has to be removed in order to be able to view the welded area for inspection as well for aesthetics and visual appearance. Welding slag can be chipped away with a pointed hammer. Sometimes, it is also removed by grinding.
Welding Flux
When the arc is initiated, the high temperature melts the work piece and the welding rod. The flux coating on the rod also melts and forms a protective shield over the weld poor, protecting it from the atmospheric gases.
Shielded metal arc welding
The welding transformer can produce high currents. Current from the welding transformer passes through a cable and then through the electrode into the work piece which is earthed or connected directly to the transformer.
An arc is generated by touching the work piece with the electrode and then breaking the contact. The high current which is interrupted now forms an arc as it tries to jump across the gap between the electrode and the work piece.
Shielded metal arc welding is widely used in almost all industries. It is also used in construction. Shielded metal arc welding can be used to weld both ferrous and non-ferrous metals.
Porosity in Welding
Some of the causes of porosity are
- Improper shielding
- Presence of water or moisture on the weld surface
- Presence of paint, which can get vaporised during the weld process.
- Improper gas flow setting in gas welding
Shielding Gas
Inert and semi-inert gases are used for shielding. Examples are Helium, Argon, Carbondioxide, etc. Shielding gases are generally denser than air (an exception is helium). They also have good thermal conductivity.
Rockwell Scale
The Rockwell test is a non-destructive testing. Its setup is easy to install.
The indentation hardness is linearly related to tensile strength. Thiss permits the quick and reliable testing of bulk materials.
The test
A minor load is placed on the specimen. The depth of the indentation formed is noted. This is the zero point. A bigger load, called the major load, is now placed upon the minor load. The depth of the indentation with reference to the zeropoint is noted.
The depth of indentation and the hardness are inversely related. A hard material will have lesser depth of indentation while a softer material will have a greater depth of indentation.
The hardness of a material can be directly calculated from the formula
HR= N- d/s
where N is the Rockwell scale used and s is the scale factor
High speed steels
The name High Speed Steels was used as they cut faster than other types of steels.
High speed steels are categorized after the name of their constituents, such as Molybdenum high speed steels, Vanadium high speed steels, Chromium high speed steels and so on.
High Speed steels are used in cutting equipments, such as blades and saws. They are also used in tool bits and in dies.
Quenching
Quenching is done in oil or in water. The medium is chosen depending on the level of quenching desired. Quenching in water can sometimes cause too much hardness that the material may crack. In such situations, oil may be chosen. Salt baths and special polymers are also used in special applications.
Quenching Rate
Quenching rate is the rate at which the drop in temperature has to occur. Oil has a quenching rate lesser than water. It means that the temperature will fall less faster in oil than in water.
Tempering
It is used particularly in iron alloys, such as steel. It is generally used after quenching. Quenching causes the material to become hard and brittle. The metal, in that state, may not be suitable for many applications.
Tempering is done by heating the material and then gradually cooling it. The heating should be very gradual to prevent the formation of cracks. The temperature to which the metal must be heated is based on the desired toughness and hardness.
Galvanizing
The layer of zinc protects the steel from the environment. In addition, galvanizing also provides galvanic protection.
Galvanic Corrosion
Galvanic corrosion when two different metals are joined together. When placed in a conductive environment, one of the metals becomes the anode while the other metal becomes the cathode. The anode corrodes and is deposited in the cathode. The anode thus wears off.
In galvanized steel, the zinc becomes the anode and wears off. The steel is thus protected.
The coating can be of different thickness depending on the application. Different surface finishes can also be made.
There are different methods of galvanizing
Hot Dip Galvanizing
Hot dip Galavanizing involves immersing a steel component in a molten bath of zinc. A coat of zinc is applied to the component.
Electrogalvanizing
Electrogalvanizing involves electroplating a layer of zinc on a steel component. Zinc is connected to a positive terminal of a DC power source while Steel is connected to the negative terminal of the source. Zinc gets deposited on the cathode.
Tool Steels
They are used to make tools, dies, molds and hammers using in metalworking. Knives are also made from tool steels.
Colour coated steels
Paints, laminates and vinyl dispersions can be used for the coatings.
Color coated steels can be directly used in the applications. This saves time during the manufacturing or construction process. These steels are used in the construction sector, the automotive industry and in making home furniture.
TMT bars
TMT bars are made by rolling steel wires and passing them through water whose pressure at an optimal pressure. The cooling creates a hard outer surface while the inner core is soft.
They are used in the constructions of houses, bridges, dams, etc. They have high thermal resistance and can withstand high temperatures.
Galvalume
Galvalume has high thermal resistance. It is also resistant to corrosion. Galvalume can also be easily formed. It has smooth appearance and is preferred for roofing applications in the construction industry.
Galvalume is a good reflector and can reflect thermal radiation. This reduces the cooling costs and increases comfort.
Drill bits
Drill bits come in various shapes and sizes. There are standardized size charts based on which the bits can be chosen.
Drill bits are made using different types of steel. Drill bits made with low carbon steels are softer and may require frequent sharpening or replacement. Drill bits can be made with high carbon steel, which is harder. High speed steels are also used to make drill bits.
Diamond coated drill bits can be used to drill holes in glass and other hard materials.
Hole Saws
Holes saws are more efficient than drill bits as all the material need not be removed by drilling. Once the outer periphery of the material is cut, the material to be removed in pieces. Less power is therefore required.
The downside is that more torque is required to use the hole saw. This requires more powerful drills as compared to the drill bit. Holesaws need to be guided carefully by the operator in the desired direction. They can easily drift away from the intended direction.
Industrial Diamonds
Natural diamonds as well as synthetic diamonds can be used as industrial diamonds. High grade diamonds are used for jewellery while low grade diamonds can be used for industrial applications.
Aerosols
Other examples of aerosols, we come across in daily life, spray painting, perfume sprays, pesticide spray guns, etc.
Aerosols find wide applications in the field of medicine. Many medicines, such as those for asthma, are delivered as aerosols. Aerosols are effective in delivered precise amounts of medications to the correct location using metered dose inhalers.
Compressed air is used as the propellant in many applications. In medicine, aersols are delivered using hydrofluoroalkanes.
Colloids
A colloidal solution has two main components. They are the colloidal particles and the dispersing medium. A key feature, which distinguishes colloids from suspensions is that a colloidal solution never settles. The particles will always remain suspended.
Colloidal solution exhibits a property called the Brownian movement. The particles have a random, zig zag motion, when observed under the microscope. This is due to collisions between the particles in the dispersing medium.
When a beam of light is shone on to a colloidal solution, the particles on the path of the light will be illuminated. For examples, on a foggy night, the path of light from a car head light can be seen.
Foam
Foams can be open celled, in which the gas can enter or leave the foam or close celled, such as bubble wrap, where the gas is trapped and cannot move.
Foams find application in fire extinction. Many fire extinction methods use foams, particularly for fires caused by oils and other flammable materials. Mattresses and cushions are also examples of foams.
Foams are used for thermal insulation and for shock protection.
Emulsion
If there is more water than oil in the solution, the solution is called water in oil solution. If there is more oil than water, the solution is called oil in water solution.
The yolk of an egg is also an emulsion.
Stabilization of an emulsion
An Emulsion is an unstable solution. In a solution of oil in water, over time, the oil and water will start to separate. This will eventually result in two separate constituents. Emulsifiers are substances which will stabilize an emulsion. Emulsifiers work by making the droplets in an emulsion remain separate. This ensures that one constituent remains suspended in the other constituents.
Emulsions paints are used to paint homes. They have a rich matte finish. The emulsions consist of small droplets, such as vinyl, which are suspended in water. They can be applied on walls and ceilings.
Boronizing
Boron carbide is commonly used in the boriding mixture. The atoms of boron diffuse into the surface of the steel material, hardening it. The boron reacts with the constituents elements in the steel such as iron, nickel and cobalt, forming Iron borides, nickel borides and cobalt borides.
Boronized metals have high wear resistances and can be used in applications such as drilling and cutting. Boronizing also makes metals resistant to corrosion.
Knoop Hardness Test
The hardness test is given by
HK = Load/ (Correction Factor x Area of the impression in sq.mm)
The correction factor is unique to the shape of the indentor and is provided by the manufacturer of the test apparatus.
The advantages of this test are that only a small specimen is required. A microscope is required to measure the dimensions of the indentation.
Silicon Carbide
Silicon Carbide is also used in hard ceramics. These ceramics are formed by sintering grains of Silicon Carbide. Such ceramics are used as in car brakes and in clutches.
In Electronics, Silicon Carbide is used as semiconductors and in LEDs (Light Emitting Diodes). Silicon Carbide is used in the making of bullet proof vests. It is also used as a refractory material in furnaces as it can with stand high temperatures.
Boron Carbide
Boron Carbide is made by heating carbon with boron oxide in an electric furnace. The boron oxide is reduced, forming boron carbide. The resulting powder is pressed at high temperature to form the material.
Boron Carbide is has a tendency to absorb neutrons. Hence, it is also used as control rods in nuclear reactors to control the chain reaction.
Boron Nitride
Boron Nitride is used in electric insulators and in cutting tools. It also has good chemical resistance. Its resistance to abrasion makes it suitable for use in nozzles which handle slurry and in high pressure water jet cutters.
Mohs' Scale of Hardness
The scale is from 1 to 10, 1 being the hardest and 10, the softest. Talc, a soft mineral used for making powders, is at scale 10. All minerals will be able to scratch talc. Talc cannot scratch any other mineral. Diamond, being the hardest mineral, is at position 1. Diamond can scratch all materials. No mineral will be able to scratch diamond.
To find the Mohs' value for an unknown mineral, the mineral is used to scratch a mineral of known value, say glass. It is able to scratch glasss, its value is more than that of the glass, which is 5. The mineral can be made to scratch minerals upper in the ranking. The Mohs' value can thus be found.
Heat Deflection Temperature
Some polymers can withstand light loads at high temperatures while other will soften and lose rigidity at those temperatures.
Heat Aging
The samples of the polymers are placed in an oven and exposed to high temperatures for a specified period of time. The specimens are then taken out and their mechanical properties are tested. Heat aging tests are sometimes combined with UV exposure tests as well.
Softening in Plastics
The temperature at which a certain extent of softening occurs is called the softening point for that polymer.
The softening temperature for a particular material is determined by the Vicat test.
In this test, a specimen of the polymer to be tested is placed in a Vicat apparatus. A needle with a cross section area of 1 sq.mm is placed on the specimen with a load of 10 N for the Vicat A test and 50 N for the Vicat B test.
The temperature at which the needle makes an impression 1mm deep is noted as the softening temperature.
Brittleness Temperature
Brittleness Test
The brittleness test is done by placing a number of specimens of a polymer in a bath. The temperature is gradually lowered. The specimens are continually impacted with a hammer at a speed of 2000 mm/s.
At a specific temperature, the specimens will start cracking. The temperature at which half of the specimens crack is called the Brittleness Temperature of the polymer.
Liquid Nitrogen
In industries, it is used for shrink fitting, in superconducting systems, and in machining. It also finds application in medicine, where it is used in removing warts and in cryogenic surgery.
Liquid Nitrogen is commercially produced by the cryogenic distillation of liquid air. Pure Nitrogen can also be liquified to produce liquid nitrogen.
Liquid Nitrogen should be handled with care as it may cause cold burns if it comes in contact with skin.
Throttling
Throttling is used to limit the flow in industrial systems. For example, the flow of air or fuel to an engine can be throttled. This would vary the power output of the engine. The speed control in a vehicle is called the throttling system.
The throttling process can result in a drop in temperature due to the Joule Thomson effect.
Triple Point
Triple points are useful in calibrating thermometers. The temperature of a pure substance is adjusted such that all three phases are in equilibrium. This temperature, the triple point, is then used to calibrate the thermometers.
Liquid Air
Other gases such as liquid nitrogen and oxygen can be distilled from liquid air at specific temperatures. For example, liquid nitrogen will separate from the mixture and evaporate at -195.79 deg.C. The vaporized gas can be distilled separately to produce liquid nitrogen.
Liquid air can also be used for cooling in many applications.
The Joule Thomson Effect
Joule Thomson Coefficient
The ratio of the rate of change of temperature to the rate of change of pressure is known as the Joule Thomson Coefficient of the gas at a given pressure.
The Joule Thomson effect also occurs in piping when the pipe diameter changes suddenly. This has to be factored during the design.
Transient Stability and Steady State Stability
Transient Stability is the ability of a power system to return to its normal state after a major disturbance, such as a fault or a disconnection or connection of a large load.
When there is a disturbance in the system, there are oscillations. These oscillations are called swings. Transient stability analysis is concerned with the response of the power system to such oscillations. A power system with proper response will bring the system back to steady state operations within a short period of time.
Steady State Stability
Steady State Stability is the ability of a power system to respond to slow or gradual changes in its operating parameters. When a number of power sources and loads are connected to a system, there will be gradual shifting of loads from one generator to another. These oscillations, if not properly controlled, can develop into large oscillations which can cause bigger disturbances.
Diamond based Thermal Compounds for IC Heat Sinks
The heat generated should be conducted and safely dissipated to the outside environment. This is done using heat sinks. The heat sink is mounted on the IC. The heat conducting compound is applied between the heat sink and the IC.
Integrated Circuits developed large amounts of heat during operation. This is particularly high in large scale integrated circuits, where thousands of components are crowded within a small area.
BH Analyzer
BH analyzers are used to analyze the behavior of circuit components at different frequencies. The B-H curves are plotted across a wide frequency spectrum.
BH Analyzers are also used to determine the permeability of materials used in the construction of electric machines.
Coercivity
Materials with high coercivity are made into permanent magnets, such as Alnico. The unit of coercivity is ampere/meter.
Materials with low coercivity are made into electromagnets, such as soft iron.
The coercivity can be calculated from the B-H curve of a material. The horizontal distance between points b and a in the BH curve in the right is the coercivity
The unit of coercivity is Ampere/metre
Sandpaper
Originally, Sand, glass or seashells were used as abrasives. Today, however, the abrasives used are aluminium oxide and silicon carbide.
Sand papers are also known as emery papers. The sandpapers are standardized according the size of particles they contain. The sand paper number specifies the finenes of the abrasive used. A higher number indicates greater fineness. A number 6 will have coarser particle than a number 20 sand paper.
Backing refers to the paper or fabric to which the abrasive crystals are bonded. The backing can be cloth, polyester, rayons.
The abrasive is bound to the backing by means of a adhesive substance called the bonding. The bonding should be strong enough to hold the abrasive while the rubbing action takes place.
Clogging is the accumulation of dirt and removed material between the abrasive crystals. This can affect the effectiveness of sandpaper action. Wet Sandpapers can be used with a liquid, such as water to remove the clogged impurities.
Shock Absorber
In automobiles, shock absorbers serve to absorbs the bumps and irregularities on the road surface. They also make sure that the tyres are in contact with the roads at all times. This is essential for reliable braking and steering action.
Shock absorbers are designed in many ways. The simplest shock absorber is a spring based one. The load of the chassis is transmitted through a spring to the axle. Both coiled springs and leaf springs are used in shock absorbers.
Leaf springs consist of strips of metals placed on top of one another and rivetted together. The energy of the shock is dissipated as heat as the strips rub against each other.
Hydraulic shock absorbers consist of a piston and a cylinder containing hydraulic oil. The piston pushes against the oil which is forced trough a small opening in the piston. The oil takes time to enter the opening due to its viscosity. The resistance offered by the oil serves to retard the movement of the piston and absorbs the vibration.
Bumpers in Automobiles
The bumper serves to absorbs the energy of low velocity collisions. The bumper deforms and absorbs the energy of a collision. Bumpers are designed to regain their original dimension after the energy of the collision has been dissipated.
Besides absorbing energy of collisions, they also serve to protect pedestrians by minimizing the impact. Some bumpers are designed to guide the people to topple over the vehicle rather than under it.
Air Conditioning in Vehicles
The Air conditioning cycle or the refrigeration cycle works by compressing a gas, cooling it, allowing it to expand and absorb heat from the air to be cooled and the compressing it to a liquid again. The refrigerant is the gas which is used as the working fluid. In earlier times, ammonia was used as the refrigera
The air conditioner consists of the following components.
Compressor: The compressor is the central part of the air conditioning system. The compressor compresses the refrigerant (the gas used for refrigeration). It is driven by the engine of the automobile through a belt.
Condenser: The compressed air from the condenser is cooled by the condenser by means of a cooling fan.
Evaporator: The Evaporator is place where the liquid refrigerant transforms into a gas. As this happens, heat is absorbed from the surrounding regions. There is a expansion valve which regulates the evaporation of the liquid and consequently the temperature. Outside, air is passed through the tubes. The air loses heat and becomes cool and is blown into the car as cool air.
Accumulator. The accumulator is used as reservoir for the liquid has not still evaporated. This can cause damage to the compressor. The Accumulator serves to hold the liquid refrigerant.
Aerodynamic Drag in Automobiles
The power developed by the engine of the automobile is used to overcome the drag. This lost power affects the mileage. Aerodynamic drag also creates noise. Therefore, aerodynamic drag is not desirable.
The aerodynamic drag increases with the speed of the vehicle. Automobiles are designed to overcome drag and air resistance. This is done by making the vehicle streamlined with a smooth surface, avoiding sharp edges. In many vehicles, the lights and the wheel arcs are designed as part of the vehicle.
Air fuel ratio
If the air is more relative to the fuel, the mixture is called a lean mixture. This results in lesser power generation. The required torque will not be developed.
If the air is less compared to the fuel, the mixture is called a rich mixture. This results in incomplete combustion. The smoke emitted is dark in color due to the presence of carbon particles which have not been oxidised. The engine will not develop the required power.
Stoichiometric combustion is the ideal combustion in which all the fuel is combusted with no excess air. It is not possible to mix the ideal amount of fuel with the ideal amount of gas in practice.
The ratio of the mass of air to the mass of fuel is called the Air-Fuel ratio or AFR.
λ the air-fuel equivalence ratio
Using the air equivalence ratio, λ is convenient in situation where engines use different fuels.
When the engine is running in rich air-fuel mixture i.e when there is excess fuel, λ is less than one. When the engine is running with lean air fuel mixture, i.e. when air is more than required, λ is greater than one.
Afterburners for Pollution Control
In an afterburner, the harmful gases of an industrial process are driven into a burner by means of a powerful blower. The high temperature of the burner causes the molecules of the pollutants to breakdown into simpler compounds like carbondioxide and water.
The gases are then led to the chimney where they are released into the atmosphere.
Regenerative Thermal Oxidizer
The Regenerative Thermal Oxidizer is another variant of the afterburner in which the heat used to decompose the molecules of the Volatile Organic Compounds is retrieved from the exhaust gas itself.
Cupola Furnaces
The molten metal is extracted through taps which are, usually, in the bottom of the cupola. Cupolas have provisions called tuyeres in the side to feed fresh air to the coke to obtain higher temperatures.
The cupola furnace is filled with coke. At the bottom, a layer of wood is placed and ignited. Air is blown through the side channels. The metal to be melted is placed on top of the cupola. The coke burns in the air to form carbon monoxide, which on further oxidation becomes carbon dioxide.
As the molten metal comes down the cupola, it reacts with the oxides of carbon. This produces metal with a high carbon content.
Crucible Furnace
The crucible is a container made of graphite and clay. It can withstand very high temperatures. Crucible furnaces are used to melt non-ferrous metals which have relatively lower melting points. These furnaces are powered by gas, electricity or coke.
Crucible furnaces are classified into
Tilting furnaces where the crucible is tilted and the molten metal spills out.
Ladle furnaces where the molten metal is removed by means of a ladle.
Bale out furnacez where the crucible is lifted and the molten metal is poured into the mould.
Brazing
Due to capillary action, the molten metal is able to flow into even minute gaps in the joint. Brazing can result in joints, which have good strength.
To prevent the formation of oxides when the material is heated, a layer of flux, such as borax is applied over the metal which is heated. The type of flux depends on the base metals and the filler metals used for the joint.
Kiln
In some kilns such as lime kilns and brick kilns, the material to be processed are exposed to the flames while in others the material is kept in a separate chamber which is heated.
The kiln is usually lined with refractory bricks which can withstand high temperatures.
Curing
Curing is usually done by heating. It can also be done by exposing the material to be cured to ultraviolet radiation. The curing process in rubber is called vulcanization.
Refractory bricks
These bricks are fired at high temperatures. The bricks vitrify and are then glazed. Refractory bricks are made of the oxides of aluminium, magnesium and silicon.
The type of refractory brick depends on the material the furnace will handle and the temperatures it will operate in.
The refractory bricks are attacked to the furnace walls by means of a special metallic support called an anchorage. The anchorage is a support structure made of metals which can withstand the high temperatures.
Anchorage in Furnaces
Thermal Shock
The stress caused by such sudden change in temperature is called Thermal Shock.
The intensity of the thermal shock is dependent on the temperature coefficient of expansion of the material. Materials with lower temperature coefficient of expansion can withstand temperature differences better than those with higher temperature coefficients.
Borosilicate glass is an example of a material which can withstand sudden temperature differences.
Thermal shock resistance is the property of a material to resist thermal shock. Thermal Shock Testing involves subjecting the test material alternatively to high and low temperatures.
Stress
For example, consider, a loaded truck standing on a bridge. The weight of the truck exerts a force on the material in the bridge. The bridge is thus stressed by the weight of the truck.
Stress can be due to an external force or due to internal flaws and defects in the material. Examples of materials having internal stresses are prestressed concrete, tempered glass, etc.
Stress is defined as the force per unit cross sectional area.
Stress = Force/ Area
Stress can be expressed in many units such as kg/sq.m, pascals, pounds/sq. inch, etc.
Normal Stress and Shear Stress
The stress on an object can be resolved into two types stresses They are normal stress and shear stress.
Normal Stress is the stress that is normal to the plane of the material. Shear Stress is the stress that is parallel to the plane of the material.
The deformation suffered by the body as a result of stress is called strain.
Strain
True Strain
True Strain is the natural logarithm of the ratio of the final length to the original length.
True Strain = Ln (Final Length/Original Length)
Engineering Strain
Engineering Strain expresses strain as the ratio of the change in length to the original length.
Engineering Strain = Change in Length/ Original Length
Strain Hardening
For instance, low carbon steel is stretched beyond the yield point and aged (left for a few days). The material will have a higher yield stress. Steel can be hardened by rolling it between a pair of rollers. The dimension of the steel sheet is reduced and the metal is hardened.
Strain hardening occurs as the dislocations cause the atoms to move to other locations and are anchored there. Strain hardening is a cold working process. It is done below the recrystallization temperature of the material.
A simple example would be to bend a piece of wire or a paper clip in opposite directions for a few times. Fatigue sets in the material. The point of the bending becomes hardened. Further bending will occur at a point above or below the earlier point.
Composite materials
Composite materials are generally lighter, stronger and less expensive than ordinary materials. They can be easily moulded in a the desired shapes. Examples of composite materials are concrete. plywood and fibre-reinforced plastics.
Each of the constitutent materials will give a specific property to the composite. One material may give strength while another will give rigidity or resistance to corrosion. In Fibre reinforced plastic, the fibre gives strength to the material while the plastic resin holds the fibre together and gives shape to the material.
Composite materials find wide applications in automobiles, buildings, medicine and in space technology.
SuperAlloys - An Overiew
The basemetals used in superalloys are usually iron, nickel or cobalt. Aluminium or Titanium are added to the alloys. Metals lose their strength as the temperature increases. However, superalloys retain their mechanical properties even at temperatures up 70 percent of their melting points. Superalloys are used in spacecrafts and in aircraft jet engines.
Superalloys can also be developed as single crystals.
Oxidation of the superalloysalloys is a concern at high temperatures. To prevent this, a layer of oxide is allowed to form at the surface. This layer of oxide prevents further oxidation.
Superalloys can be processed in a number of methods such as investment casting, sintering, directional solidification, single crystal growth, etc.
Creep deformation
Creep increases as the temperature increases. Hence, the study of creep is important for components which will operate at high temperatures.
Soft metals such as lead, aluminium or solder can creep even at room temperatures. Material such as tungsten can resist creep deformation even at high temperatures. As a rule of thumb, creep can occur at around 35 percent of the melting point for metals.
Creep deformation can be classified into three stages
Stage 1
This is the primary stage. At this stage, the strain rate or the deformation rate is very high. The strain rate slows gradually and stabilizes.
Stage 2
At this stage, the strain rate stabilizes and is almost constant.
Stage 3
In this stage, the strain rate again increases. This is due to failure processes such as necking or the formation of cracks. The material is permanently deformed at this stage.
Creep can be prevented by choosing materials which have higher melting point. Special alloys which resist creep can be chosen for critical applications. Materials with bigger grain size will also have lesser creep.
Viscous and Elastic Deformation
Viscous deformation is the deformation in which the deformation varies linearly with time. The material does not come back to its original position. Examples of viscous deformation are honey, wax, etc.
Elastic Deformation is the deformation in which the material returns to its original shape after the stress is removed. In elastic deformation, the deformation is proportional to the stress. Eg. Metals.
Viscoelastic deformation.
Certain materials exhibit both viscous and elastic deformation. In these materials, some deformation is elastic while some deformation is viscous. Examples are rubber, plastics, etc. Wood is also a viscoelastic material.
Applications
Viscoelastic materials are used in applications where vibration damping is required. Viscoelastic materials are also used where impact and shock absorption are required, such as the bumper of a car.
Bingham Plastic
The mathematical model for a Bingham Plastic is
Shear Stress = minimum yield stress + plastic viscosity * shear rate
In a Bingham plastic, flow does not occur until the stress reaches a specific value called the minimum yield stress.
Viscoplasticity
Creep deformation is an example of viscoplasticity.
Viscoplasticity is studied in the analysis of crash tests in automobiles. It is an important characteristic when choosing materials, which can withstand high loads.
Hooke's law
Mathematically, it can be expressed as
F = kX
where
F is the force in newtons
k is the spring constant
X is the distance travelled in metres
Hooke's law for stress and strain
Hooke's law can be generalized elasticity to say that "the strain of an elastic object is proportional to the stress". Once the elastic limit is exceeded, Hooke's law will not hold true.
For plastic materials
Strain = Proportionality constant x Stress
The proportionality constant will be unique for different materials.
Inconel
Inconel is a difficult alloy to shape. It gets hardened during the working process itself (work hardening). Special manufacturing techniques are thus required.
Inconel is a trademark of Special Metals Corporation.
Inconel finds application in cryogenic tanks, jet engines, gas turbines, etc.
Nimonic
It is an alloy of Nickel and Chromium. It also has cobalt, titanium and aluminium. It is available in different grades, such as nimonic 75, nimonic 80A and nimonic 90.
Passivation
Passivation involves the creation of a protective layer on the surface of the material. This is usually an oxide. Iron can be passivated by heating it in an atmosphere of oxygen. A layer of oxide is formed. This layer prevents further attack by oxygen.
Nickel pipes can be used to handle fluorine by passivation. A layer of fluoride is allowed to form on the surface. This prevents further reaction.
Rolling
Rolling results in a reduction of the thickness and a hardening of the material. Cold rolling is done below the recrystallization temperature while hot rolling is done above the recrystallization temperature.
Hot Rolling improves the grain structure and makes it more uniform. It also removes porosity and other casting defects from the metal.
Rolling is used to produce metal sheets. Metal can be passed through consecutive sets of rollers to progressively reduce the thickness. Metal foils used for wrappings are also produced by rolling.
Rolling can also produce shapes such as T and L shaped profiles
Sheet metal
Sheet metals are used in a variety of applications like creating ducts and pipes for an HVAC system, in the building of the automobile body, to create an enclosure for panels, etc.
Sheet metal is usually obtained by rolling. Sheet metal is cut by special tools and bent to form the required shapes.
Malleability is the property of a material to be made into sheets.
Cryogenic Machining
Conventional machining causes high temperatures. These high temperatures create thermal stress on the tool and can decrease its life. Cryogenic machining extends the life of the tool as high temperatures are avoided.
Cryogenic machining is cleaner than conventional machining. Burr formation is greatly reduced. Residual stress is also lessened.
Cryogenic machining can be retrofitted in normal machining equipment.
Cryogenic Treatment - An Overview
Cryogenic treatment is used to relieve residual stresses and to increase wear resistance. The component to be treated is immersed in a bath of liquid nitrogen or any other refrigerant and slowly cooled till the target temperature. It is then gradually brought back to room temperature.
Corrosion resistance is also improved as a result of cryogenic treatment. Electrical and mechanical properties are also improved.
Induction Hardening
Due to the alternating electromagnetic field, eddy currents are induced in the surface of the material. The depth of the hardening can be controlled by controlling the current through the induction coil.
After the heating, the material is quenched in a cooling medium, such as oil. Carbon alloy steels are generally hardened in this manner.
Gears, pinion shafts, bearing races are some of the components which can be hardened by induction hardening.
Flame hardening
It is difficult to control depth of hardening in flame hardening. Along with the surface getting hardened, other properties such as bending and torsional strength and fatigue resistance are also improved.
Flame hardening can be used to harden the surface of any metal.
Case hardening
In case hardening, a "case" of hard material is created around the relatively soft body.
The case hardening process involves heating the component to a high temperature . The hot steel component is immersed in a case hardening compound. The case hardening compound is high in carbon.
The component is again heated and then quenched in water. The process can be repeated till the desired depth of hardness is obtained. In case hardening, the outer surface of the metal is hardened while the inner surface is kept soft.
Nitriding
The formation of nitrides hardens the surface. Nitriding can be done only in steels that contain elements, which can form nitrides, such as molybdenum, aluminium and chromium.
Unlike other methods of hardening, nitriding does not require quenching. After nitriding, the components are subject to stress relieving processes. Sometimes, the surface formed is ground to remove the outermost layer which may be brittle.
Abrasive blasting
The abrasives which impinge on the surface at high velocity carry away the undesirable materials. Different textures, from matte to shiny, can be obtained depending on the abrasive used.
Workers should were proper protective devices such as masks and suits during the procedure.
In addition to cleaning, abrasive blasting is also used as a finishing process. It is used to give a specific desired texture to material. In certain situations, it is used to roughen the surface for processes such as thermal spraying.
Annealing
Annealing increases the ductility and increases its workability of steel. The hardness of the steel is reduced by annealing. Metals like copper, brass and gold can also be annealed.
At high temperatures, the metal is susceptible to oxidation. This can be preventing by conducting the heating in an inert atmosphere of special gases such as forming gas ( a mixture of hydrogen and nitrogen) or endothermic gas ( a mixture of carbonmonoxide, nitrogen and hydrogen).
Glass is also annealed to remove internal stresses and increase the strength.
Machining Coolants
Coolant liquids are broadly classified into
Oil based coolants (straight and soluble oils) and
Chemical machine coolants (synthetic and semisynthetic)
The functions of the coolant are
To maintain a low temperature.
The tolerances specified during the machining process may become distorted due to expansion at high temperature. Hence, the temperature should be maintained at specified range to ensure correct measurement.
To lubricate the cutting surface
Excessive friction at the cutting surface can cause hardening of the material, making cutting difficult.
Clean the surface
The coolant keeps the surface by removing material like burrs, dust, etc
Prevent rust
oxidation can occur at the high temperature caused by cutting. Coolants prevent rust formation by forming a protective surface.
The coolant is delivered to the desired location by different methods. Some machines use spraying or flooding. In some machines, a jet of coolant liquid is focussed on the work area.
The coolant liquid is also a fertile medium for bacterial growth. This can cause infections over a period of time. Sometimes, antiseptics are added to prevent bacterial growth.
Crankcase relief valves
Under normal conditions, they are mounted on a seat and are in the closed conditions. When they are pushed open from the inside, the release the high pressure gases in the crankcase.
They are usually provided with a flame trap which prevents flames, if any, from escaping outside. Once the pressure has been relieved, the valves will return to their closed position.
Flame Traps
The trap works by absorbing the heat from the fire and preventing it from reaching the other side. A flame trap can limit a fire accident by limiting the flame from spreading.
Flame traps are located at specific intervals in the pipeline. Flame industries are used in many industries such as petrochemical, refining, paper manufacturing, etc. Flame traps should only be used for applications they have been designed for. They also need to be periodically checked for corrosion and insect infestation.
Crankcase Ventilation
Crankcase ventilation refers to the system involved in evacuating gases from the crankcase. In small engines, the gas is allowed to leave the crankcase by means of a ventilation pipe by natural draft. In bigger engines, a pump is used to aid the removal of gases.
Positive Crankcase Ventilation
Positive crankcase ventilation is a system used in automobiles to improve emission control. In this sytem, the blowby which leaks into the crankcase is drawn and released into the intake manifold. The gases are thus sent again into the combustion chamber. This is done only when the vehicle is moving at low speeds. It is not done during higher speeds as the air/fuel mixture may become lean.
Monoblock Engines
Besides, thermal stress from unequal temperatures are minimized. There is also no need for cylinderhead bolts, gaskets, which reduces maintenance costs. The design has higher manufacturing cast. It is also difficult to service the valve seats.
Since the cylinder head cannot be removed, the pistons, the connecting rods and the crankshaft may have to be removed to gain access to the valve seats.
Cylinder Head
Aluminium cylinders are also used where better heat dissipation is required. Common defects in cylinder heads are cracks which can cause water leakage. Any drop in water level or pressure should be investigated. A gasket is placed between the cylinder head and the liner and the engine block, which needs to be periodically renewed.
Profilometers
Contact Profilometers have a stylus which is run across the surface. The profilometers can measure variations in height from a few nanometers to 1 millimeter. The stylus generates an analog signal, which is subsequently converted to a digital signal and saved. Contact Profilometers are considered to be more accurate as they physically touch the surface they are measuring.
Non-contact profilometers These profilometers work without physically coming in contact with the surface. They use laser based techniques.
Lapping - An Overview
Common abrasives used are Aluminium Oxide, Silicon Carbide, Diamond, emery, etc. Lapping is used in wide variety of industrial applications and also in making lens and mirrors, where it is used to obtain finishes with fine dimensions. Lapping is also widely used in the semiconductor industry.
Cast Aluminium
Aluminium Castings are used extensively in cooking. Aluminium can be made into plates and utensils. Aluminium castings are also used to create outdoor furniture, such as benches, chairs, and tables.
Planing
Foundry - An Overview
Electric arc furnaces, cupola furnaces and reverbatory furnaces are some of the furnace types. The choice of furnace type depends on the alloy used for the castings.
A Captive Foundry is a foundry which is attached to an industry and produces castings only for the industry. The Foundry process consists of a series of steps.
They are
- Melting the metal
- Mould making
- Pouring the metal into the mould
- Shaking out the cast from the mould
- Heat Treatment Surface
- Cleaning and Finishing
Hot Dip Galvanization
The inner layers of the protective coat are made of an alloy of the zinc and steel ( as a result of the metallurgical reaction) while the outer layers are of pure zinc.
Since the object is completely immersed in the molten zinc in the zinc kettle. All parts of the objects get a uniform coat of protective zinc.
Pyrometallurgy
Roasting
Here, the ore is converted into an oxide of the metal.
Reduction
The oxygen in the ore is removed by heating with coke or charcoal. The oxygen escapes as carbondioxide or carbonmonoxide.
Refining
The metal thus obtained is further purified by electrolysis.
Porosity in metals
Porosity is usually discovered when a casting is being machined. As layers of metal are removed, voids and channels will become obvious. Porosity can be classified into three main categories Gas Porosity caused by gas formation and trapped air during the casting process.
Flow Porosity caused by unequal pressure on the molten metal during the casting process which causes flow porosity Shrink Porosity caused by changes in volume as the metal solidifies. One technique of addressing porosity is by impregnating the pores with an inert material which can withstand high temperature and vibration.
Sintered bearings
Powder Metallurgy
Plane Bearings
Plane bearings can be integral, that is, the shaft sits directly on the groove. or as a form of bushing in which a semicylindrical sleeve is placed below the shaft. This has the advantage of being replaced when it worn. The sleeve is made of soft material as compared to the shaft.
Some plane bearings have two shells or sleeves. One above the shaft and one below. These bearings are used in the crankshaft of the engine.
Nylon bearings
They can be used in environments which are corrosive for normal bearings, such as in saltwater. They are relatively inexpensive and can withstand high temperature ranges.
Cast Iron bearings
The coefficient of friction between steel and cast iron is very less. The bearings will have a grease reservoir which can be used to store grease.
Preventing Crankcase explosions
Crankcase explosions can be prevented by
Ensuring proper lubrication to all parts.
Operating at the correct load.
Using bearings which prevent overheating such as white metal bearings.
The crankcase ventilation should be functional.
Having a reliable Oil Mist Detecting system
Crankcase explosions
Crankcase explosion occurs due to the ignition of oil mist. The temperature inside the crankcase is quite high. Certain parts of the crankcase will be higher still. These are called hotspots. For example, certain bearings may have higher temperature. Certain regions of the piston can be hotter due to some abnormality. Leaks can occur in the combustion chamber (crankcase blowby). This can contain sparks which can ignite the oil mist.
Oil mist is formed when oil reaches the hot areas of the crankcase and gets vapourised. As the vapour drifts in the crankcase, it can get condensed into a white mist once it reaches the relatively cooler areas. This white mist is highly atomised. When this mist comes in contact with a hot surface, it can ignite causing the explosion.
Preventing Crankcase explosions
- Monitor the oil mist levels in large engines continually using oil mist detectors.
- Monitor the temperature of the engine and connecting rod bearings.
- Do not operate the engine above its rated load.
- Perform specified maintenance regularly
Some of the indications of an explosion are
- Change in the engine speed.
- Unusual Noise.
- Sudden increase in the temperature of the exhaust.
- Increase in the load.
- Smell of the oil mist in the engine room.
When any of the indications are noticed,
- the fuel and air supply should be cut off.
- the engine should be stopped and allowed to cool.
- Do not open the crankcase doors until the engine has cooled.
Blowby in Crankcases
The leakage of the exhaust gases can also occur at the turbocharger. Crankcase pressure should be continually monitored. Increase of crankcase pressure can lead to safety issues such as crankcase explosions
Crankcase in Engines
It is the largest enclosure in the engine. In wet sump engines, the oil sump is located at the bottom of the crankcase.
Balancing Shafts in Engines
In engines with certain cylinder configurations, there is an inherent balance due to the firing of the cylinders. For instance, V engines with six cylinders are inherently unbalanced as they have an odd number of cylinders on each bank.
Four cylinder engines are another configuration where inertial forces cause vibrations. These vibrations are offsets by two balancing shafts which rotate at twice the speed of the engine in the opposite direction. The balancing shaft contains counterweights. They are located in the crankcase of the engine
Prelubrication in Engines
This can result in wear of the engine components during the starting process. To prevent this, a special pump is used to pump the lubricating oil throughout the engine at the time of starting. This pump is driven by a motor. The engine will be permitted to started only after the lubricating oil has reached a certain value.
Drystarting an engine
Oils for automobile engines, today, have special properties such that the oil clings to the surface of the automotive parts.
Interference fits - An Overview
Examples of interference fits are the fitment of a wheel on an axle, the fitment of a vibration damper on the crankshaft, etc
Interference fits can be done using methods such as by the application of huge amounts of force by hydraulic means, by heating the components such that it expands and allows the smaller part to enter.
Cooling the smaller part with liquid nitrogen or solid carbondioxide is also one method. The part contract which makes it to fit the other component.
Inteference fits provide excellent grip and are very reliable.
Vibration Damper in Crankshaft
The Vibration Damper contains an inertia. rings which rotates in a layer of silicone fluid. The vibrations are dissipated in the fluid. Overtime, the fluid can become thick and hard. This reduces the damping property
This torque needs to be smoothened. This is done by the flywheel which stores the power and delivers it uniformly. However, this can produce oscillations. These oscillations can damage the crankshaft if they reach resonance or if they become large.
The vibration damper prevents this by absorbing the vibrations. The vibrations are not transmitted to other components such as the couplings or the drive train or any other load.
Vibrations dampers can deteriorate over age. As this happens, increased vibrations and noise can be observed. Hence, the should be examined periodically.
The Vibration Damper contains an inertia. rings which rotates in a layer of silicone fluid. The vibrations are dissipated in the fluid. Overtime, the fluid can become thick and hard. This reduces the damping property.
Counterweights in the Crankshaft
The crankshaft is said to be "internally balanced" if the counterweights alone can balance the crankshaft.
If the counterweights are not properly chosen, the crankshaft can be seriously damaged during rotation.
Crankshaft - An Overview
The Crankshaft rests on the engine block. The crankshaft is supported by bearing journals. The crankshaft is made of ductile iron. It is designed to withstand heavy impact loads and high centrifugal forces. The Crankshaft also has high fatigue strength to withstand cyclic loading. Crankshafts can be made by either forging or casting. Forged castings are stronger. They are used for heavy duty applications. For small and moderate sized engines, cast crankshafts are used.
Crankshafts are made of steels containing manganese, chromium, molybdenum, etc. They are hardened using techniques such as surface hardening and nitriding.
Poppet Valve
The valve is operated by the pushrods which are driven by the camshaft. When the valve is seated in the seat, the opening is closed and the valve is considered closed. When the valve is pushed out of the seat by the pushrod, it is open.
Thermo Plastics and Thermosetting Plastics
Thermoplastics are plastics which can be moulded again and again. These plastics do not undergo any chemical change during the heating and moulding process. Examples include polyethylene, polystyrene, polyvinylchloride.
Thermosetting Plastics
In contrast, thermosetting plastics can be used only once. These plastics are chemically changed during the moulding process. They are hard and brittle. Examples of thermosetting plastics are bakelite, vulcanised rubber. Thermosetting plastics form cross links when heated. This gives them the hardness and ability to withstand high temperature. A property which makes them ideal for high temperature applications.
Hydrometallurgy - An Overview
There are three steps to this process. They are
Leaching
The ore is immersed in the solution or in certain cases, the aqueous solution is pumped into the deposits. After the reaction with the ore is complete. The solution leaches from the deposits where it is collected or pumped out.
Concentration of the Solution and purification
The leached solution contains the required metal. This has to be removed. This can by done using chemical process such as precipitation, solvent extraction, ion exchange, etc. The desired metal is now obtained.
Metal Recovery
The metal recovered may not be in the desired purity. To increase the purity, the metal is subject to electrolysis with the impure metal as the anode. The pure metal is obtained at the cathode
Ingots
Ingots of precious metals, such as gold are also used as currency and media of exchange.
Billet in an Extrusion Process
Smelting - An Overview
Iron oxide is an ore of iron. It contains oxygen in addition to iron. The oxygen is removed by heating the ore with charcoal or coke. At high temperatures, the oxygen combines with carbon to form carbon monoxide and carbondioxide.
Other impurities are removed as slag and iron is obtained.
Smelting consists of two main processes.
Roasting
Roasting is the process in which the ore is thermally decomposed into simpler compounds, usually oxides of the metal.
Reduction
This is the process in which oxygen is removed from the oxide are removed to obtain the elemental metal.
Wrought Iron and Cast Iron
Cast Iron is iron which has been melted and poured into a mould. Cast Iron has high quantity of carbon, as a result of which, it is highly brittle and hard. It has, however, high compressive strength which is desirable. It has poor malleability and ductility.
Both Wrought iron and Cast Iron are vulnerable to corrosion, particularly, when exposed outside. Protective coatings can be applied to prevent this.
Plastic - Plasticizers
Plastic
Plasticizers are additives which are added to a plastic material to increase its viscosity or flow properties. This is necessary to mould plastics into complex shapes which require lower viscosity at a specific temperature. Plasticizers can be in the liquid or solid form.
Plasticizers work by decreasing the length of the polymer chains. This makes the polymers more flexible. For instance, plasticizers are added to PVC to make it workable. The PVC is heated such that the distance between the molecules becomes greater due to thermal agitation. The plasticizer is then added. The molecules of the plasticizers get into the gaps between the PVC molecules. The long chains of the PVC are thus interrupted.
Plasticizers are usually esters, which are organic compounds made by the reaction of an alcohols and an acid.
Plasticizers are also added to concrete to make it more workable.
Ferrous Metals and Non Ferrous Metals
Non Ferrous metals are metals or alloys which do not contain iron. Examples are zinc, aluminium, brass and bronze. They are used for specific qualities such as electric conductivity, corrosion resistance, etc.
Heat Deflection Temperature
The Heat Distortion Temperature for a material can be determined experimentally.
ASTM D8 defines the Heat Distortion Temperature as the temperature at which a bar of standard dimension experiences a deflection of .25mm when loaded with a flexural load of 455 kPa. The heating is done at the rate of 2K per minute.
A material cannot be used for structural applications at a temperature above its Heat Distortion Temperature.
In Injection moulding, the part which is moulded can be removed from the mould once it is near or below the Heat Distortion Temperature.
Squeeze Casting - An Overview
It is possible to create parts with great complexity and detail using this process. Automotive components are generally made using this process. The process is cheap and simple.
A comparison of Casting and Forging
| Casting | Forging |
There is no size limit to casing. | Very large components are difficult to forge |
Since the working is done in the molten stage, metals can be added to form custom alloys during the process itself. | Adding metals to the material is not possible |
Very complex parts can be designed. | Difficult to make complex parts |
Castings are hard and brittle. | Have excellent properties such as strength, toughness and ductility. |
Risk of formation of cavities, porosity, etc. | No Risk of cavities, porosity, etc. |
Lesser Wear Resistance | Greater Wear Resistance |
Requires close supervision of the process by trained personnel. | The process is predictable and easy to monitor. |
Grain direction cannot be controlled. | Grain direction can be controlled and oriented by design in order to achieve desirable qualities. |
Longer lead times | Shorter lead times |
Types of Extrusion
Direct Extrusion
In Direct Extrusion, the die is stationary. The billet is put into the die and the force is applied by a ram. A dummy block is added between the molten metal and the die. As force is applied, the molten metal is forced through the shape of the die. In direct extrusion, friction develops between the billet and the die. This requires the greater force to move the billet forward.
Indirect Extrusion
In Indirect Extrusion, the billet is stationary. The die is pressed against the billet by means of rams. The force required is lesser as the friction is minimized. However, long components cannot be made using the indirect extrusion as the die needs to be supported
The risk of cracking is minimized as lesser temperature is produced.
Hydrostatic extrusion
In hydrostatic extrusion, the extrusion container is filled with a hydraulic liquid. Unlike other forms of extrusions, where the force is transmitted by means of a ram, in hydrostatic extrusion, the force is transmitted hydraulically.
The high pressure required can be generated by a pump or by a press.
Hydraulic extrusion does not generate friction as there no contact between the billet and the walls of the container. It is used in the extrusion of brittle materials, such as ceramics.
Impact Extrusion
This form of extrusion is used to create objects such as small cups and tubes. Here, the billet is placed above the die. A high impact force is applied using a piston or a hammer. The billet is extruded through the die in the form of the desired shape.
Defects in Extrusion
Some of the common defects of the Extrusion process are
Surface Cracks
These are formed when excessive tensile stress is applied. The cracks, usually, occur at the ends. In many cases, they can be found along the length of the product. Surface cracks can occur if the temperature of the billet is very high. They can also occur if the billet is pushed through the die too fast.
Surface cracks can be prevented by extruding the material at the optimum temperature and speed.
Surface Lines
These are formed when foreign and unwanted material is present in the surface of the die. These lines occur in dies which are not maintained properly. Dies have to be properly maintained and cleaned at regular intervals.
Internal Cracks
Internal cracks occur when the extrusion rate is very low or if the die has a very high angle. The cracks are caused by excessive tensile stress in the centre of the material. Cracks can also occcur if the material used is not of good quality.
Operating centrifugal pumps in series or in parallel
There are certain points to be kept in mind and certain precautions to be taken when operating pumps in this manner.
For Series operation
Both the pumps should have the same capacity.
The pumps should run at the same speed.
The first pump should be started first and then the second pump should be started.
The casing of the second pump will have a higher pressure. The casing should be designed to withstand it.
The mechanical seal of the second pump should be able to withstand the high pressure from the outlet of the first pump.
For Parallel operation
When pumps are operated in parallel, their output increases to close to double the output of an individual pump.
The pumps must have the same flow rate
Drop Forging
Drop Forging can be done above the recrystallization temperature when it is called hot forging and below the recrystalization temperature, when it is called cold forging.
Drop Forging can be done using an open die where the flow of the material has to be guided by the operator.
In closed die forging, the ram which hits the billet will have the shape of a die. The material is enclosed between the fixed die and the die on the ram. The material flows to take the shape of the die.
Subscribe to:
Posts (Atom)