Basic Structures Of Ferrous Metals

Ferrous metals is mainly based on iron-carbon alloy with the combination of other alloys such as plain carbon steels, alloy, tools steels, stainless steels and cast iron. Alloys having iron with a valance of +2 are known as ferrous; those alloys which have iron with a valence of +3 are called as ferric. Ferrous metals or alloys are metals that contain the element iron in it. Depending on the end of use, metals can be simply cast into the finished part or cast into an intermediate form, such as an ingot, then worked, wrought by rolling, or processed by forging, extruding or another deformation process. All ferrous metals are magnetic. They contain a small quantity of other metals in order to give the correct properties. Manipulation of atom-to-atom relationships between iron, carbon and various alloying elements establishes the specific properties of ferrous metals. As atoms transform from one specific arrangement, or crystalline lattice, to another its gives good mechanical properties.

Pure iron: It is also called as Pure Ferrite. The carbon content is calculated. From 0 to 0.5%.It has the BCC structure when it is in room temperature. Also known as Alpha iron.

Plain Carbon Steel: Consists of iron containing small amounts of carbon. The carbon content can vary from 0.008% to approximately 2.0%.

Low- Alloy steel: Steel containing alloy additions which usually do not exceed a total about 10% are referred to allow-alloy steels

Ultra-High-Strength steel: Steel capable of developing yield strength greater than about 1104 Mpa are considered ultra-high-strength alloys.

Medium-carbon low-alloy steel: These alloys consists of grades such as 4130,4330 and 4340, which can be quenched and tempered to yield strengths on the order of 1725 Mpa

Maraging steel: This class of steel consists basically of extra-low-carbon (less than 0.3%) iron-based alloys to which a high percentage of nickel has been added.

Corrosion-Resistant (stainless) steel: Stainless steel may be divided into four categories: ferritic, martencitic, austenitic, and age-hardenable.

Ferritic stainless Steels: This group of stainless steel contains between 11.5 and 27% chromium as the only major alloying element in addition to a maximum of 0.25% carbon

Martensitic stainless steels: This type of stainless steel is also primarily chromium steel, but in contrast to the ferritic group, consists enough carbon to produce martensite by quenching 0.15 and 0.75% carbons.

Austenitic stainless steels: This Stainless Steel is alloyed to the extent that they remain austenitic at low temperatures. The principal alloying elements added to the chromium and nickel, generally totaling than 23%

Precipitation-hardening stainless steels: The last class of stainless steel we will discuss depends on precipitation hardening for the optimum development of properties. Very high strength together with corrosion resistance

Cast iron: Cast irons are iron-carbon-silicon alloys. More than 2% of carbon

Grey cast-iron: Also known as graphite cast iron. They depend on the distribution size and amount of the graphite flakes and matrix structure.

Spheroid graphite cast-iron: Also known as Ductile or nodular iron. It has high modulus of elasticity.

Austempered Ductile iron: Recent addition to cast iron family, outstanding combination of high strength, toughness, wears resistance.

Compacted cast iron: Referred as vermicular iron. Consists of 80% graphite and 20% spherodial graphite

Malleable Cast iron: Carbons present as an irregular shaped nodules of graphite. Also classified as white heart malleable cast iron. Blackheart malleable cast iron.Pearlitie malleable cast iron

Austentic carbon: They are high alloy cast iron. Mainly nickel in which carbon is present

List of advantages

These are materials with high specific strengths when compared with weight that is high strength to weight ratio.

High quality materials exist in abundant quantities within earth’s crust and are readily available worldwide in various certificate grades.

It increases the speed of construction in the field of civil engineering.

Versatility;steel suits range of construction methods & sequences.

Modification & repair can be easily done with left effort.

Recycling can be done easily.

Durability of these materials are very high

Aesthetics;steel has a broad architectural possibilities

Limitation of the material in engineering applications:

The principal limitation of many ferrous alloys is their susceptibility to corrosion

Costly waste as scrap

High cost of final finishing & polishing

Environmental issuebecause of improper disposal

Ferrous metals get rusted easily (oxidize) unless protected eg. with oil

b) Non-ferrous metal

Non-ferrous metals are metals other than iron and they do not contain an appreciable amount of iron in them. Non-ferrous metals are aluminum, magnesium, titanium alloys, copper, zinc and miscellaneous alloys like nickel, in, lead, zinc as base metals. The precious metals silver, gold and platinum are also coming under non-ferrous group. Non ferrous metals are alloys which are non magnetic.

Non ferrous metals:

Aluminum: Abundant element of 8% on earth crust and normally found in Oxide forms (Al2O3), i.e., bauxite, kaolinite, nepheline and alunite

Aluminum – base alloys: Aluminum is used in its commercially pure state as well as in its many alloy forms. The heat –treatable types have the advantage of being relatively easy to fabricate in their soft condition, after which they are heat treated to develop their higher strengths.

Copper- base alloys: Copper is seldom industrially employed in its pure state. Copper has its most value when alloyed with other elements. It dissolves with elements such as tin, zinc, and silver in rather wide proportions.

Magnesium – base alloys: Magnesium are noted for their lightness. The specific gravity of magnesium is 0.064 lb per cu.; in comparison, aluminum, steel, and titanium are 0.09, 0.28, and 0.16 lb per cu., respectively. Magnesium alloys lend themselves to welding and filler are protected by an inert gas. They are relatively easy to cast by most foundry methods, particularly die casting.

Nickel –base alloys: Nickel is one of the oldest metals known to man. Currently this metal is almost indispensable in the alloying of steels to confer toughness, uniformity of hardness, and good workability; and as a basic alloy to resist high corrosion and high temperatures

Lead-Tin alloys: The principal lead –tin alloys consist of solders and bearing materials.

The 70% tin -30% lead solder is used mainly in the joining and coating of metals. The 63% tin-37% lead is a eutectic type solder developed primarily for making electrical joints.

Zinc-base alloys: Zinc base alloys predominate as die casting materials. These alloys have high cast ability and favorable mechanical and chemical properties. Zinc base alloys can be cast in the range 750-800 º F, and, therefore, have a low –temperature advantage over other alloys

Less common metals and alloys:

Titanium and its alloys: Because of their high strength- weight ratio, titanium and its alloys have received a great amount of attention from the aircraft and missiles industries.

Molybdenum: This element has long been known for its ability to confer the property of high temperature stability to steels.

Zirconium: Zirconium metal has a density of 0.24 lb per cu in. And a melting point of 3355ºF. The metal has fair tensile strength, depending somewhat upon its method of manufacture. It fabricates similar to titanium, and it’s eminently suited to the resistance to corrosion.

List of advantages

Non ferrous metal do not corrode (aluminum for example)

High thermal conductivity

High electrical conductivity

Non ferrous metals have relatively high density

Nonmagnetic properties

Higher melting points

Resistance to chemical

They are also specified for electrical applications

They are comparatively low in electrical conductivity

Non ferrous have inherent susceptibility to corrosion in some common environment

Non ferrous metals are usually light weight but ferrous metals are heavier

Limitation of the material in engineering applications

They are not as strong as carbon steel (ferrous metal).

Non ferrous metals are typically not used in structural applications.

Non ferrous metals are usually more expensive by the pound than are ferrous metals.

Low tensile strength but excellent specific strength.

They don’t show ductile to brittle transition in low temperature.

c) Polymers:

Compounds that are formed by the joining of smaller layers, usually repeating, units linked by covalent bonds are called polymer. A polymer is a large molecule consists of repeating structural units connected by covalent bonds. Polymer in popular used as plastic; the term polymer refers to a large category of natural and synthetic materials with a wide spectrum of properties. Natural polymers are those which come from plants and animals have been used for many centuries; these materials include wood, rubber, cotton, wool, leather, and silk. Other polymers such as proteins, enzymes, starches, and cellulose are important in biological and physiological processes in plants and animals. The backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. Polymer in popular used as plastic, the term actually refers to a large class of natural and synthetic materials with a wide variety of properties

Polymers: Polymers are classified into several ways, by how the molecules are synthesized, by their molecular structure, or by their chemical family.

Linear polymer – Any polymer in which molecules are in the form of spaghetti-like chains.

Thermoplastics – Linear or branched polymers in which chains of molecules are not interconnected to one another.

Thermosetting polymers – Polymers that are heavily cross-linked to produce a strong three dimensional network structure.

Elastomers – These are polymers (thermoplastics or lightly cross-linked thermo sets) that have an elastic deformation > 200%.

Polymers are classified into three main categories;

Thermoplastics:

Branched polymer – Any polymer consisting of chains that consist of a main chain and secondary chains that branch off from the main chain. Crystalline is important in polymers since it affects mechanical and optical properties. Tacticity – Describes the location in the polymer chain of atoms or atom groups in nonsymmetrical monomers.

Liquid-crystalline polymers – Exceptionally stiff polymer chains that act as rigid rods, even above their melting point.

Elastomers (Rubbers):

Geometric isomer: A molecule that has the same composition as, but a structure different from, a second molecule.

Diene: A group of monomers that contain two double-covalent bonds. These monomers are often used in producing elastomers.

Cross-linking: Attaching chains of polymers together to produce a three-dimensional network polymer.

Vulcanization: Cross-linking elastomer chains by introducing sulfur or other chemicals.

List of advantages

Polymers are ultra durable

Flexible

doesn’t rust

slow to degrade

They can be molded into virtually any shape conceivable

can be custom colored in the production stage

Polymers are recyclable

quite a good electrical insulator and has a low dielectric constant

The biggest advantage for PP is its low cost

It also has a flexibility in cold whether with ultraviolet stability

can be easily repaired from mechanical damage with simple field tools

Limitation of the material in engineering applications

In the production stage, polymers are susceptible to contamination

The least bit of dirt or cross-contamination w/other polymers, and at best the end product is corrupt, at worst the polymers are rendered useless

Any variances in heat and timing in the molding process and, again, the final product will be corrupt or useless.

lower melting point

flammability

Elevated temperatures will make any crystalline more isotropic

non bio-degradable

easily breakable

when polymers incorporated with additives are burnt they emit a lot of poisonous gases into the atmosphere

improper disposal leads to environmental pollution

undergo oxidation and ozonation easily

d) ceramics:

These are materials that are produced when two materials are joined together to give a combination of properties that cannot be achieved in the original state. Ceramics can be divided into two classes: advanced and traditional. Advanced ceramics consist of carbides, pure oxides, nitrides, non-silicate glasses and many others; while Traditional ceramics include clay products, silicate glass and cement. A ceramic is an inorganic, non-metallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous.

Agglomerated materials:

Concrete: This is one of the oldest agglomerated composite materials to be used for engineering construction, and consists of a mixture aggregate and sand bonded together by the hydrated silicate the gel formed when the Portland cement “sets” with water.

Ratio of aggregate, sand and cement: A very common mix consists of 4parts aggregate, 2parts sand and 1 part cement powder.

The water-cement ratio: The water added to the concrete is used in the hydration of the cement itself, and any water in excess of the amount required for setting reactions has a weakening effect upon the concrete.

The nature of the aggregate and sand: The bond between the hydrated cement and the aggregate and sand is improved if the both the aggregate and sand are sharp-cornered rather than rounded. Strong fine-grained igneous rocks like basalt, dolerite, and quantize are commonly used for concrete aggregate, the size of which varies with the size of the job.

Mixing and laying: Under-or over-mixing gives a poor concrete, and the method of lying is of the utmost importance. Concrete vibrated into place is always stronger than concrete poured and hand-screwed

Curing time: The hardening of cement occurs over a considerable length of time and it is important to prevent the evaporation of moisture .during the initial stages. Concrete is often covered with wet sand or bags for seven days to prevent the evaporation of moisture, and concrete cured under water after taking its initial set achieves its maximum strength.

Asphalt paving: This is composite in which rock aggregate is bounded by viscous asphalt: it is used extensively for road surfacing. The material is not as rigid as concrete, this being an advantage for road construction.

Cermets: These are agglomerates that consist of combinations of metal and ceramics, the metal acting as the binder. Cermets are made using the techniques of powder metallurgy, the sintering temperature usually being above the melting point of the metal powder.

Laminates: Many different types of laminated materials are made of different applications, the mild-steel-stainless combination being a good example of a modern metal-to-metal laminate.

Plywood: This is made by bonding together an odd number of sheets of wood veneer so that the grain directions of alternate sheets are perpendicular to each other.

Laminated plastic sheet: This is usually made from sheet of paper or cloth and a suitable thermosetting resin. The paper or cloth passes or cloth passes through a tank containing the resin solution, between rollers that squeeze out the excess resin, and then through a drying oven in which excess solvents are removed and the resin is partially cured.

Reinforced Materials: It forms the biggest and most important group of composite materials, the purpose of reinforcement always being the improvement of strength properties. Reinforcement may involve the use of a dispersed phase, or strong fiber, thread, or rod

Reinforced concrete: This is the most widely used of all construction materials, since it is not only comparatively easy to place into position and finish, but is also maintenance free during its service life.

Glass-fiber reinforced plastics: These combine the strength of glass fiber with the shock resistance and formability of a plastic. The usual types of reinforcement are the chopped strand mat and the woven fabric, the latter giving increased strength to the composite.

List of advantages

They are harder and stiffer than steel

more heat and corrosion resistant than metals or polymers

less dense than most metals and their alloys

plentiful and inexpensive

doesn’t conduct electricity

Ceramics are used in the manufacture of knives. The blade of the ceramic knife will stay sharp for much longer than that of a steel knife, although it is more brittle and can be snapped by dropping it on a hard surface

Ceramic engines are made of lighter materials and do not require a cooling system and hence allow a major weight reduction

Ceramics are also more chemically resistant and can be used in wet environments where steel bearings would rust

High-tech ceramic is used in watch making for producing watch cases

scratch-resistance

In very high speed applications, heat from friction during rolling can cause problems for metal bearings; problems which are reduced by the use of ceramics

Durability and smooth touch.

ceramic materials may be used as bone replacements

Limitation of the material in engineering applications

The main disadvantage of medical ceramic materials is their fragility

The ceramic materials cannot deform under the stress, as can do plastics and metals

Ceramics do not perform well with tension or tensional loads.

A hard, brittle material that can withstand high temperatures and resist corrosion

Ceramics cannot be joined (and repaired) by welding.

The other disadvantage is that ceramics are strong in compression, but weak in tension

Ceramics don’t bend much, and when they break, instead of slowly pulling apart the way metals will, they generally snap

they have a tendency to shatter when something hits them hard

Q-2 An overview of the engineering properties and behavior of ferrous metals, Non-ferrous metals, polymers & composites, and ceramics

a) Ferrous metals.

Pure iron: Easily weld able, good corrosion resistance, effective electrical conductivity. Used in iron rods

Plain Carbon Steel: Expensive, soft and weak, easily weld able, good ductility, Good toughness. Used in hammers, chisels, a drill, knives, wire and dies for all purposes.

Low- Alloy steel: Machinable, ductility of more, than 25%. Used in transportation, agriculture, construction, and military applications.

Ultra-High-Strength steel: Ductile, Formable, and Machinable. Has higher strength that other steel. Mainly used in Bridges, towers, and pressure vessels.

Medium-carbon low-alloy steel: Has low Harden ability. Used in rocket motor cases, aircraft components, including bolts, pins, main landing gears, and brake housings, and a wide variety of structural and machinery parts.

Ferritic stainless Steels: Good resistant to wear and tear, highly ductile. Tensile strength – 380Mpa, Yield strength 205Mpa, Ductility 20%, High tensile strength. Good corrosion resistant. Used in furnace parts, boiler baffles, kiln linings, stack dampers, chemical processing equipment, automobile trim, catalytic converters, and decorative purposes in general.

Martensitic stainless steels: Tensile strength 485Mpa, Yield strength 275Mpa. Used in cutlery, surgical instruments, valves, turboine parts, pump parts, and oil well equipment.

Austenitic stainless steels: Outstanding resistance too many types of corrosion and erosion. Superior cast ability, Good mach inability, and Tensile strength 515Mpa, and Yield strength 170Mpa. Used in decorative purposes, interior show cases, automobile trim, aircraft is fitting, food handling.

Precipitation-hardening stainless steels: Very high strength towards corrosion and resistance. Used for aircraft parts, nuclear reactor components, landing gear parts, high-performance shafting and petrochemical applications requiring stress corrosion resistance.

Grey cast-iron: Ease of melting and casting process. Air-cooled cylinders clutch housing clutch plates.

Spheroid graphite castiron: Modulus of elasticity, Wear resistance, excellent machinability, High thermal conductivity, Outstanding cast ability.

Austempered Ductile iron: Higher tensile strength, higher ductility, Machinability and corrosion resistance are similar to g.c iron. Automotive and agricultural products like Axle housing, brake calipers, brake cylinders. Boiler segments, conveyor frames, bulldozer parts.

Compacted cast iron: Good wear resistance used in automotives and engineering applications. Used in shafts, helical gears, couplings, and conveyor rollers.

Malleable Cast iron: Higher tensile strength ductility. Fatigue life impact strength. Brake drums, discs. Cylinder heads piston rings. Used in Automotive transmission parts, clutch pedals. Steering knuckle, wheel hubs.

Austentic carbon: Good fatigue strength, good damping capacity. Used in pump components valves, compressors. Alloy steels have greater harden ability than plain carbon steels

Alloy steel have greater harden ability than plain carbon: The difference between the two is somewhat arbitrary definition. However, most agree that while the steel alloyed with more than eight percent of its weight of other elements besides iron and carbon steel is a strong ally. Low alloy steel is slightly higher. The physical properties of these steels are modified by other factors, making them more hardness, strength, corrosion resistance or hardness compared to carbon steel. For these properties, these alloys are often heat-treated.

Carbon steel is steel that does not contain significant amounts of alloying elements other than carbon. There are three major categories of carbon steel. A low-carbon steel, medium carbon and alloy.

Alloy steel is a type of steel that many advantages over steel offers. It is much harder and stronger than ordinary carbon steel by. It is used in cars, trucks, cranes, bridges and other structures can handle a large number of strains

The difference between the two is defined somewhat arbitrarily. However, most agree that while the steel is alloyed with more than eight per cent of its weight of other elements being next to iron and carbon steel is strong ally. low alloy steels are slightly more frequent. The physical properties of these steels are modified by other elements, giving them a greater hardness, strength, corrosion resistance, or hardness compared to carbon steel. To achieve these properties, these alloys often require heat treatment.

Carbon steel is a steel which does not contain significant amounts of alloying materials other than carbon. There are three major categories of carbon steel. low carbon steel, medium carbon steel and alloy.

Alloy steel is a type of steel that offers many advantages over steel. It is much harder and stronger than ordinary carbon steel by. It is used in cars, trucks, cranes, bridges and other structures to be able to handle a large number of strainsThe difference between the two is defined somewhat arbitrarily. However, most agree that while the steel is alloyed with more than eight per cent of its weight of other elements being next to iron and carbon steel is strong ally. low alloy steels are slightly more frequent. The physical properties of these steels are modified by other elements, giving them a greater hardness, strength, corrosion resistance, or hardness compared to carbon steel. To achieve these properties, these alloys often require heat treatment.

alloy steel is a type of steel that offers many advantages over steel. It is much harder and stronger than ordinary carbon steel by. It is used in cars, trucks, cranes, bridges and other structures to be able to handle a large number of strainsThe difference between the two is defined somewhat arbitrarily. However, most agree that while the steel is alloyed with more than eight per cent of its weight of other elements being next to iron and carbon steel is strong ally. low alloy steels are slightly more frequent. The physical properties of these steels are modified by other elements, giving them a greater hardness, strength, corrosion resistance, or hardness compared to carbon steel. To achieve these properties, these alloys often require heat treatment.

b) Non ferrous alloys

Aluminum: Weak and ductile, Electrical conductivity is better. High thermal conductivity, Good resistance towards corrosion. Used in Aircraft, boats, pistons and cranks.

Aluminum – base alloys: copper has high electrical and thermal conductivity. Tensile strength and hardness can be improved. Used in Power lines, controllers, signaling devices.

Miscellaneous copper base alloys: Electrical conductivity of 60%, Good corrosion resistance, has the Hcp structure. Used in applications like Aircraft and Spacecraft.

Magnesium – base alloys: Has the melting point of 1455’C. Good formability. Good Corrosion Resistance. The pure Zinc has the melting point of 419’cIt has two types of alloys; Alloy A – Good ductility Alloy B- Higher effective strength. Used in Petroleum industry, Chemical industry Food processing plants, Fuel pump, optical instruments, car doors etc.

Lead-Tin alloys: Excellent corrosion resistance, Good strength. Resistant to high temperatures. Some important types of alloys, alpha titanium alloys, near alpha titanium alloys, Alpha-beta titanium alloys, Beta titanium alloys. Used in Compressor blades, Engine forging and space craft’s.

Differences between non-ferrous alloys in the cast vs. wrought forms

Nonferrous Alloy

Specified for use in electrical and electronic applications.

Reduced weight

Higher strength

Nonmagnetic properties

Higher melting points

Resistance to chemical and atmospheric corrosion.

A type of cutting material is relatively expensive and must be directly casted into the form. Non-ferrous cast alloy tools have largely been replaced by carbide.

Wrought alloy: Solid metal that has been bent, hammered, or physically formed into a desired shape.

Wrought copper alloys can be utilized in the annealed, cold-worked, stress-relieved, or hardened-by-heat-treatment conditions, depending on composition and end use.

Bronzes comprise four main groups:

copper-tin-phosphorus alloys (phosphor bronze)

copper-tin-lead-phosphorus alloys (leaded phosphor bronze)

copper-aluminum alloys (aluminum bronzes)

copper-silicon alloys (silicon bronze)

Wrought copper-nickel alloys, like the cast alloys, have nickel as the principal alloying element. The wrought copper-nickel-zinc alloys are known as “nickel silvers” because of their color.

c) Polymers:

Polymers are classified in various ways, by the way the molecules are synthesized, their molecular structure, or chemical family. One way of classifying polymers is to indicate whether the polymer is a linear polymer or branched polymer composed of linear polymer. Spaghetti as chains. A branched polymer molecular chains of polymers composed of primary and secondary shoots small chains that results as main chain.

Thermo plastics: these are composed of long chains produced by joining together monomers.

Thermosetting polymers: these are composed of long chains of molecules that are cross linked to one another to form three-dimensional network structures.

Elastomers: these are known as rubbers. They have an elastic deformation>200%.These may be thermoplastics or lightly cross linked thermosets.the polymer chain consists of coil- like molecules that can be reversibly stretch by applying a force.

Engineering polymers include natural materials such as rubber and synthetic materials such as plastics and elastomers. Polymers are very useful materials because their structures can be altered and tailored to produce materials

With a range of mechanical properties.

In a wide spectrum of colors.

Different transparent properties.

Types of polymers: Commodity plastics, Polyethylene, Polystyrene, Polypropylene, Poly vinyl chloride, Poly ethylene terephthalate, Specialty or Engineering Plastics, Teflon, Polycarbonate, Polyesters and Polyamides.

Addition polymerization: Process by which polymer chains are built up by adding monomers together without creating a byproduct.

Unsaturated bond: The double- or even triple-covalent bond joining two atoms together in an organic molecule.

Functionality: The number of sites on a monomer at which polymerization can occur.

Degree of polymerization – The average molecular weight of the polymer divided by the molecular weight of the monomer.

Effects of temperature on thermoplastics: Properties of thermoplastics change depending upon temperature. We need to know how these changes occur because this can help us (a) better design components, and (b) guide the type of processing techniques that needed to be used.

Degradation Temperature: At very high temperatures, the covalent bonds between, the atoms in the linear chain may be destroyed, and the polymer may burn or char, in the moplastics decomposition occurs in the liquid state, in thermo sets the decomposition occurs in the solid state temperature Td is the degradation temperature.

Liquid polymers: Thermoplastics usually do not melt at a precise temperature. Instead there is usually a range of temperatures over which melting occurs. At or above the melting temperature Tm bonding between the twisted and intertwined chains is weak.

Mechanical properties of Thermoplastics: Most Thermoplastics exhibit a non-Newtonian and viscoelastic behavior, the behavior is non-Newtonian. The viscoelastic behavior means when an external force is applied to a thermoplastic polymer, both elastic and plastic deformation occurs. The mechanical behavior is closely tied to the manner in which the polymer chains move relative to one another under load. Deformation is more complicated in thermoplastics.

Plastic Behavior of Amorphous Thermoplastics: These polymers deform plastically when the stress exceeds the yield strength. Unlike deformation in the case of metals, however, plastic deformation is not a consequence of dislocation movement. Instead chains stretch, rotate, slide, and disentangle under load to cause permanent deformation. The drop in the stress beyond the yield point can be explained by this phenomenon. Initially, the chains may be highly tangled and intertwined.

Creep and stress Relaxation: Thermoplastics also exhibits creep, a time-dependent permanent deformation with constants stress or load. They also show stress relaxation .Stress relaxation, like creep, is a consequence of viscoelastic behavior of the polymer. Perhaps the most familiar example of the behavior is rubber band stretched around a pile of books.

Impact behavior: Viscoelastic behavior also helps us understand the impact properties of polymers. At the high rates of strain, as in an impact test, there is sufficient time for the chains to slide and cause plastic deformation.

Deformation of crystalline polymers: A number of polymers are used in the crystalline state. As we discussed earlier, however, the polymer are never completely crystalline. Polymer chains in the crystalline region extend into these amorphous regions as tie chains.

Crazing: Crazing occurs in thermoplastics when localized regions of plastic deformation occur in a direction perpendicular to that of the applied stress. In transparent thermoplastics such as some of the glassy polymers, the craze produces a translucent or opaque region that looks like a crack. The craze can grow until it extends across the entire cross section of the polymer part.

Blushing: Blushing or whitening to failure a plastic because of a localized crystallization that ultimately causes voids to form. A number of natural and synthetic polymers called elastomers display a large amount of elastic deformation when a force is applied.

Mechanical Properties of Thermoplastics:

Viscoelasticity: The deformation of a material by elastic deformation and viscous flow of the material when stress is applied.

Relaxation time: A property of a polymer that is related to the rate at which stress relaxation occurs.

Uses of polymers

The polymers play very important role in our daily life as

Polyethene in shopping bags.

nylon in suit cases, purses, school bags.

polyvinylchloride in waste pipes, electric wiring pipes.

polyvinylacetate in plastic bottles, in utensils etc. all the plastics are polymers. The backlight plastic is used to make the body of electronic devices as T.V the inner part of passenger compartment of aero plane is mostly composed of backlight.

Principal differences between dispersion strengthening and fiber strengthening of composite material

Dispersion strengthening

A method for producing a dispersion strengthened metal matrix composites containing a phase of agitation of manure mixed solid-liquid as a means of propagating a reduced pressure, the melting process of overheating is achieved by an increase to 150 ° C. above the liquids line for metal spreading said medium. Stirring is performed in an inert gas under a reduced pressure of 100 Torre to 1 × 10-4 Torr.reduced pressure is within a range of 1 Torre to 1 × 10 – 4 Torre using an enhanced ultra-fine dispersion material. Dispersing middle is a pure metal or an alloy of very low-

Fiber strengthening:

Improves strength, fatigue resistance, rigidity and resistance to ratification weight, stiff, brittle fibers in a softer, more ductile matrix. The matrix material provides strength to the fibers and the ductility and toughness provide, while the fibers carry most of the applied force. The fibers from the output of polyester polymer matrix for the desired application transportation aerospace. The boron fibers, graphite and polymers provide a windfall. Even small crystals single ceramic materials, are called whiskers are developed. The fibers can be woven into the fabric or produced in the form of tapes. Alternating layers of tape can be changed course.

d) Ceramics

Families of ceramics:

Silicates and glasses, oxides, ferrites and titivates, carbides, nitrides, borides, silicates, fluorides, carbons and graphite’s.

Glass: It is an organic product of the merger leading to a rigid state without crystallising.there cooled many types of glass; they are in soda-lime glass, lead glass, and soda, borosilicate, aluminosilicate glass, silica glass, fused silica, special glasses. A composite material is defined as a combination of two or more materials exposure, the distinctive characteristics of those particular materials used to make composites. With regard to a specific property such as resistance to heat resistance, or stiffness, the composite is better than each individual component materials or radically different from both. Composite materials can be classified as agglomerated materials,

laminates,
surface-coated materials, or
Reinforced materials.

Plywood: This is made by bonding together an odd number of sheets of wood veneer so that the grain directions of alternate sheets are perpendicular to each other. The wood veneer used is normally between 0.05mm and 6mm thick, and is usually produced by slicing or peeling suitable log.

Laminated plastic sheet: This is usually paper or cloth and a thermosetting resin suitable. The paper or canvas or fabric passes passes a tank with the resin solution, between the roles that squeeze the excess resin, followed by a furnace in which solvents are then removed and the resin is partially cured. The impregnated material is then cut into appropriate lengths, some of which are stacked, the stack is then pressed into a machine for hot pressing. Heat and pressure to soften the resin flow to fill the cracks in the laminated and hard sets that polymerization occurs.

Glass-fibre reinforced plastics: Combining the strength of fiberglass, with the impact resistance and formability of plastic. The usual types of reinforcement are chopped carpets and fabrics, the latest increase in the strength of the composite. Resins that can be used by setting phenol-type under considerable pressure, or without epoxy or polyester polymerization types of pressure under the influence of chemical catalyst.

Asbestors-reinforced plactics: These are used in the aircraft industry and offer the advantage of increased stiffness. Aphenolic resin is usually industry and offer the advantage of increased stiffness.

Surface coatings: The primary function of asurface coating is the protection of the material to which it is applied: however, surface coating may also perform decorative functions. It is usual to classify surface coatings as metallic coatings, inorganic chemical coatings, and organic chemical coatings.

Metallic coatings: These are usually applied by hot dipping, electroplating, cladding, or spraying techniques and server to protect the base metal from corrosion.

Inorganic chemical coatings: These are conveniently divided into vitreous coatings, oxide coatings, and phosphate coatings. Citreous coatings are commonly applied to steel in the form of a powder or frit and are then fused to steel surface by heat. Such coatings are relatively brittle, but offer absolute protection against corrosion.

Organic chemical coatings: These include paints, Varnishes and lacquers and all serve both to protect the base material and to enhance its appearance. It is doubtful whether the application of such coatings constitutes the formation of a true composite material in the modern sense of the term.

Principal differences between industrial ceramics and domestic ceramics:

Properties of industrial ceramics:

Should have a very low coefficient of thermal expansion so that they can withstand a high temperature. This low expansion also helps the ceramics to be manufactured without much error, or else there could be a dimension difference when the ceramic is cooling down after firing.

Should have high melting points so that they can sustain the high temperatures used in most industrial processes.

Good insulator of electricity.

Provide good insulation to heat.

Should have a high wear resistance

The materials used in the production of industrial ceramics are different from those used in ceramic art forms. These materials need to be strong and durable and be able to withstand very high temperatures. The common materials used are oxides, carbides and nitrides of nonmetallic inorganic minerals.

Domestic ceramics

hard,
wear-resistant,
brittle,
refractory,
thermal insulators,
electrical insulators,
nonmagnetic,
oxidation resistant,
prone to thermal shock, and
chemically stable.

Used for glassware, windows, pottery, Corning ware, magnets, dinnerware, ceramic tiles, lenses, home electronics, microwave transducers, orthopedic joint replacement, prosthesis, dental restoration, bone implants, fiber optic communications, TV, radio , microphones.

Q-3 An overview of strengthening mechanisms, corrosion mechanisms, and prevention and wear mechanisms and resistance.

a) Compare the relative merits of pack carburizing with nitriding as surface hardening treatments

Pack Carburizing the oldest method in the method of cementing components packaged in a mixture of coke and charcoal with activators, and then heated in a closed container. Although a laborious process, carbonation Pack still practiced in some rooms installation tool requirements are minimal.Parts are packed in a high carbon medium such as carbon chips or iron powder and heated in an oven for 10 to 70 hours at 910 º C. At this temperature CO gas is produced, which is a strong reducing agent. The reduction reaction occurs on the surface of carbon steel in bulk, which is then distributed in the surface due to high temperatures.

Mechanical merits such as

Increased surface hardness
Increased wear resistance
Increased fatigue/tensile strengths
wear and corrosion resistance,
hardness and load-bearing capacity
toughness and ductility

Physical merits such as

Grain Growth may occur
Change in volume may occur

Chemical merits such as

Increased surface carbon content

b) Need to carburise while other can be surface hardened by re-heating and quenching the surface layers

Carburizing is a heat treatment process in which iron or steel is heated in the presence of another material (but below the metal’s melting point) which liberates carbon as it decomposes. The outer surface or case will have higher carbon content than the original material. When the iron or steel is cooled rapidly by quenching, the higher carbon content on the outer surface becomes hard, while the core remains soft and tough.

This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25inches (6.4mm).

Low carbon steels have low strength and hardness, but a good ductility and toughness, all-steel high carbon content has the opposite behavior. We can by an appropriate heat treatment to produce a structure that hard and resistant surface so that wear and fatigue resistance are obtained, but also gives a soft, hard, hard core of resistance to gives an impact to failure.

Carburization of steel involves a heat treatment of the metallic surface using a gaseous, liquid, solid or plasma source of carbon. It also provides an even treatment of components with complex geometry making it very flexible in terms of component treatment. In general, pack carburizing equipment can accommodate larger workpieces than liquid or gas carburizing equipment, but liquid or gas carburizing methods are faster and lend themselves to mechanized material handling.

To obtain high surface hardness

To increase wear resistance

To improve fatigue life

To improve corrosion resistance (except for stainless steels)

To obtain a surface that is resistant to the softening effect of heat at temperatures up to the carburizing temperature

c) Process of erosion corrosion:

Erosion corrosion is an acceleration in the rate of corrosion attack in metal due to the relative motion of a corrosive fluid and a metal surface.

Erosion corrosion can also be aggravated by faulty workmanship.

A combination of erosion and corrosion can lead to extremely high pitting rates.

Erosion-corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys).

With the exception of cavitation, flow induced corrosion problems are generally termed erosion-corrosion, encompassing flow enhanced dissolution and impingement attack

Erosion-corrosion is associated with a flow-induced mechanical removal of the protective surface film that results in a subsequent corrosion rate increase via either electrochemical or chemical processes.

Cavitation sometimes is considered a special case of erosion-corrosion and is caused by the formation and collapse of vapor bubbles in a liquid near a metal surface.

Cavitation removes protective surface scales by the implosion of gas bubbles in a fluid.

Cavitation damage often appears as a collection of closely spaced, sharp-edged pits or craters on the surface.

In offshore well systems, the process industry in which components come into contact with sand-bearing liquids, this is an important problem.

Materials selection plays an important role in minimizing erosion corrosion damage. Caution is in order when predicting erosion corrosion behavior on the basis of hardness. High hardness in a material does not necessarily guarantee a high degree of resistance to erosion corrosion. Design features are also particularly important.

It is generally desirable to reduce the fluid velocity and promote laminar flow; increased pipe diameters are useful in this context. Rough surfaces are generally undesirable. Designs creating turbulence, flow restrictions and obstructions are undesirable. Abrupt changes in flow direction

The thickness of vulnerable areas should be increased. Replaceable ferrules, with a tapered end, can be inserted into the inlet side of heat exchanger tubes, to prevent damage to the actual tubes.

Several environmental modifications can be implemented to minimize the risk of erosion corrosion. De-aeration and corrosion inhibitors are additional measures that can be taken. Cathodic protection and the application of protective coatings may also reduce the rate of attack.

Several methods of prevention of erosion corrosion

selection of alloys with greater corrosion resistance and/or higher strength.

re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the environment.

reduction in the corrosive severity of the environment.

use of corrosion resistant and/or abrasion resistant coatings.

cathodic protection.

One of the best ways to reduce erosion corrosion is to change the design to eliminate fluid turbulence and impingement effects.

Other materials may also be utilized that inherently resist erosion. Furthermore, removal of particulates and bubbles from the solution will lessen its ability to erode.

The present invention relates to a method for preventing erosion-corrosion of inner walls of a hydraulic capsule transportation apparatus capable of transporting metallic capsules introduced into a pipe line, by means of a fluid flowing through the pipe line, and also for preventing erosion-corrosion of capsule surfaces.

To prevent corrosion of inner walls of a pipe line caused by a fluid flowing through the pipe line, for example, a method for adding a corrosion inhibitor to the fluid has been so far proposed. The corrosion inhibitor includes an inhibitor capable of oxidizing the inside wall surfaces of a pipe to form a stable oxide film,

An object of the present invention is to prevent erosion and corrosion of inside wall surfaces of pipe line in a hydraulic capsule transportation apparatus for transporting metallic capsules, introduced into the pipe line, by means of a fluid flowing through the pipe line, and also prevent erosion-corrosion of the surfaces of the capsules.

d) The principal characteristics of three main classes of stainless steel and indicate where each type is used.

The stainless steels are highly resistant to corrosion in a variety of environment, especially the ambient atmosphere. Their predominant alloying element is chromium; a concentration of at least 11 wt% Cr is required. Corrosion resistance may also be enhanced by nickel and molybdenum additions.

Stainless steels are divided into three classes on the basis of the predominant phase constituent of the microstructure;

Martensitic
Ferritic
Austenitic

A wide range of mechanical properties combined with excellent resistance to corrosion make stainless steels very versatile in their applicability.

Martensitic: Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment. It is quenched and magnetic. Martensitic stainless steels, the first stainless steels commercially developed for cutlery It has poor weldability and is magnetic. It is commonly used for knife blades, surgical instruments, shafts, spindles and pins.

Ferritic: Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, due to the lower chromium and nickel content. They are also usually less expensive.. Most compositions include molybdenum; some, aluminium or titanium. They have a moderate to good corrosion resistance, are not hardenable by heat treatment and always used in the unnealed conditions. They are magnetic. These are commonly used in computer floopy disk hubs , automotive trim , automotive exhausts , material handling equipment and in hot water tanks .

Austenitic: Austenitic, or 300 series, stainless steels make up over 70% of total stainless steel production. Superaustenitic stainless steels, exhibit great resistance to chloride pitting and crevice corrosion due to high molybdenum. The higher alloy content of superaustenitic steels makes them more expensive. Austenitic stainless steels have high ductility, low yield stress and relatively high ultimate tensile strength, when compare to a typical carbon steel.

They properties like corrosion resistance, weldability, formability fabricability, ductility, cleanability, non magnetic (if annealed) and are hardenable by cold work only.

e) Three major wear processes are termed: adhesive, abrasive and erosive wear. Explain each wear processes.

Adhesive: There are two types of adhesive friction.

Cohesive adhesive forces, holds two surfaces together even though they are separated by a distance.

Adhesive wear, material transfer from one surface to another caused by direct contact and plastic deformation.

Adhesive wear occurs when two bodies slides over each other, or are pressed into one another, which promote material transfer between the two surfaces.

Adhesive wear can be described as plastic deformation of very small fragments within the surface layer when two surfaces slides against each other.

The asperities found on the mating surfaces will penetrate the opposing surface and develop a plastic zone around the penetrating asperity.

Dependent on the surface roughness and depth of penetration will the asperity cause damage on the oxide surface layer or even the underlying bulk material. In initial asperity/asperity contact, fragments of one surface are pulled off and adhere to the other, due to the strong adhesive forces between atoms

Adhesive wear is the most common form of wear and is commonly encountered in conjunction with lubricant failures. It is commonly referred to as welding wear due to the exhibited surface characteristics.

The tendency of contacting surfaces to adhere arises from the attractive forces that exist between the surface atoms of the two materials. The type and mechanism of attraction varies between different materials.

The mechanism of adhesive wear occurs due to contact possibly producing surface plastic flow, scraping off soft surface films or breaking up and removing oxide layers.

Abrasive:

Abrasive wear occurs when a hard rough surface slides across a softer surface.defines it as the loss of material due to hard particles or hard protuberances that are forced against and move along a solid surface.

Abrasive wear is commonly classified according to the type of contact and the contact environment.The type of contact determines the mode of abrasive wear. The two modes of abrasive wear are known as two-body and three-body abrasive wear. Two-body wear occurs when the grits, or hard particles, are rigidly mounted or adhere to a surface, when they remove the material from the surface.

There are a number of factors which influence abrasive wear and hence the manner of material removal. Several different mechanisms have been proposed to describe the manner in which the material is removed. Three commonly identified mechanisms of abrasive wear are:

Plowing, Cutting, and Fragmentation

Plowing occurs when material is displaced to the side, away from the wear particles, resulting in the formation of grooves that do not involve direct material removal.

Cutting occurs when material is separated from the surface in the form of primary debris, or microchips, with little or no material displaced to the sides of the grooves.

Fragmentation occurs when material is separated from a surface by a cutting process and the indenting abrasive causes localized fracture of the wear material.

Erosive wear

Erosive wear is caused by the impact of particles of solid or liquid against the surface of an object. The impacting particles gradually remove material from the surface through repeated deformations and cutting actions.It is a widely encountered mechanism in industry.

The rate of erosive wear is dependent upon a number of factors. The material characteristics of the particles, such as their shape, hardness, impact velocity and impingement angle are primary factors along with the properties of the surface being eroded.

The abrasive wear process includes the mechanisms: micro-ploughing, micro-cutting, micro-fatigue and micro-cracking. Clearly explain these mechanisms.

Abrasive wear occurs when a hard surface slides against and cuts grooves from a softer surface. This condition is frequently referred to as two-body abrasion. Particles cut from the softer surface or dust and dirt introduced between wearing surfaces also contribute to abrasive wear. This condition is referred to as three-body abrasion..

Abrasives are extremely commonplace and are used very extensively in a wide variety of industrial, domestic, and technological applications. This gives rise to a large variation in the physical and chemical composition of abrasives as well as the shape of the abrasive.

Common uses for abrasives include grinding, polishing, buffing, honing, cutting, drilling, sharpening, lapping, and sanding. Abrasive wear takes three different modes: ploughing, microcutting, and wedge forming.

Ploughing/plowing or burnishing takes place in sliding motion, if the contact interface between two surfaces has interlocking of an inclined or curved contact. The consequence is ploughing, a certain volume of surface material is removed and an abrasive groove is formed on the weaker surface. By assuming a single contact point model where a hard, sharp abrasive is intended against a flat surface and forms a groove on it by ploughing. In ploughing mode, wear particle is not generated by a single pass of sliding and only shallow grove is formed.

In microcutting mode, long and curled ribbon-like wear particles are formed, this is called chips which are wear particles. Low friction assists in this wear mode.

In the wedge-forming mode, a wedge-like wear particle is formed at the tip of the grooving asperity as shown in the figure3 below and stays there working as a kind of built –up wedge to continue grooving. Sliding takes place at the bottom of the wedge where adhesive transfer of a thin layer from the underlying counterface continues to grow the wedge slowly. This wear mode appears as a combined effect of adhesion at an inclined or curved contact interface and shear fracture at the bottom of the wedge. High friction or strong adhesion assists in this wear mode.

In all these three abrasive wear modes, grooves are formed as the result of wear particle generation and plastic flow of material to form ridges on both sides of a groove.

Q-4 An overview of other failure such as fatigue and creep

a) Briefly explain the three stages- stage 1 , stage 2 , stage 3 – involved in a typical fatigue failure.

Fatigue: The majority of engineering failures are caused by fatigue. Fatigue failure is defined as the tendency of a material to fracture by means of progressive brittle cracking under repeated alternating or cyclic stresses of an intensity considerably below the normal strength. The number of cycles required to cause fatigue failure at a particular peak stress is generally quite large, but it decreases as the stress is increased.

A good example of fatigue failure is breaking a thin steel rod or wire with your hands after bending it back and forth several times in the same place. Another example is an unbalanced pump impeller resulting in vibrations that can cause fatigue failure.

The type of fatigue of most concern in circuit cards, gasoline, diesel, gas turbine engines and many industrial applications is thermal fatigue. Thermal fatigue can arise from thermal stresses produced by cyclic changes in temperature.

Fundamental requirements during design and manufacturing for avoiding fatigue failure are different for different cases and should be considered during the design phase.

The process of fatigue consists of three stages:

Initial crack initiation
Progressive crack growth across the part
Final sudden fracture of the remaining cross section

Crack Initiation: The initial crack occurs in this stage. The crack may be caused by surface scratches caused by handling, or tooling of the material; threads ( as in a screw or bolt); slip bands or dislocations intersecting the surface as a result of previous cyclic loading or work hardening. The most common reasons for crack initiation in a component include, Notches, corners, or other geometric inconsistencies in the component. Material inclusions, impurities, defects, or material loss due to wear or corrosion. Mechanical or thermal fatigue

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