ENGINEERING MATERIALS

Engineering Materials

IINTRODUCTION:

Choice of materials for a machine element depends very much on its properties, cost, availability and such other factors. It is therefore important to have some idea of the common engineering materials and their properties before learning the details of design procedure. This topic is in the domain of material science or metallurgy but some relevant discussions are necessary at this stage.

SELECTION OF METERIALS:

Selection of proper materials for the machine components is one of the most important steps in the process of machine design. The best material is one which will serve the desired objective at minimum cost. The following factors should be considered while selecting the materials:

1.      Availability

2.      Cost

3.      Mechanical properties

4.      Manufacturing considerations

1.      AVAILABILITY: The material should be readily available in the market, in large enough quantities to meet the requirement.

2.      COST: For every application, there is a limiting cost beyond which the designer cannot go. When this limit exceeded, the designer has to consider other alternative materials. In cost analysis, there are two factors, namely cost of materials and the cost of processing the material into finished goods. It is likely that the cost of material might be low, but the processing may involve costly machining operations.

3.      MECHANICAL PROPERTIES: The important mechanical properties of materials from the consideration of design are strength, rigidity, toughness, resilience, shock resistance, wear resistance, creep characteristics and hardness. These properties govern the selection of materials. Depending upon the service conditions and the functional requirement, different mechanical properties are considered and a suitable material is selected.

4.      MANUFACTURING CONSIDERATIONS: Machinability of material is an important consideration in selection. Sometimes, an expensive material is more economical than a low priced one, which is difficult to machine. Where the product is of a complex shape, casting properties are important. The manufacturing processes, such as casting, rolling, forging welding and machining govern the selection of the material

Past experience is a good guide for the selection of material.

CLASSIFICATION OF MATERIALS:

FERROUS METALS:

Those metals which have the iron as their main constituents. The principle raw material for all ferrous metals is pig iron which is obtained by smelting iron ore with coke and limestone, in the blast furnace.

1.      Cast Iron

2.      Wrought Iron

3.      Steel

CAST IRON:

ü  It is an alloy of iron, carbon and silicon.

ü  It is hard and brittle, so it cannot be used for machine which is subjected to shocks.

ü  Carbon content may be within 1.7% to 3% .

ü  Carbon may be present as free carbon or iron carbide Fe₃C.

ü  Low cost

ü  Good casting characteristics

ü  Excellent machinability

In general the types of cast iron are

1.      Grey cast iron

2.      White cast iron

3.      Malleable cast iron

4.      Spheroidal or nodular cast iron

5.      Austenitic cast iron

6.      Abrasion resistant cast iron.

GREY CAST IRON: Carbon here is mainly in the form of graphite.

Carbon – 3 to 3.5%

Silicon – 1 to 2.75%

Manganese – 0.40 to 1.0%

Phosphorous – 0.15 to 1%

Sulphur – 0.02 to 0.15%

And the remaining is iron

ü  This type of cast iron is inexpensive

ü  Has high compressive strength.

ü  Graphite is an excellent solid lubricant and this makes it easily machinable but brittle.

Some examples of this type of cast iron are

FG20, FG35 or FG35Si15.

The numbers indicate ultimate tensile strength in MPa and 15 indicates 0.15% silicon.

So, FG20 means grey cast iron with 20 MPa tensile strength.

USES: Machine tools bodies, automobile cylinder blocks, heads housings, flywheels, pipes and pies fitting and agricultural implements.

WHITE CAST IRON: The white cast iron shows a white fracture and has the following compositions

Carbon – 1.75 to 2.3%

Silicon – 0.85 to 1.2%

Manganese – less than 0.40%

Phosphorous – less than 0.2%

Sulphur – less than 0.12%

And the remaining is iron

ü  In these cast iron carbons is present in the form of iron carbide (Fe₃C) which is hard and brittle.

ü  The presence of iron increases hardness and makes it difficult to machine.

ü  These cast iron are abrasion resistant

ü  High tensile and low compressive strength

USES: car wheels, rolls for crushing grains and jaws crusher plates.

MALLEABLE CAST IRON: These are the white cast irons rendered malleable by annealing.

ü  These are tougher than grey cast iron

ü  They can be twisted or bent without fracture

ü  Excellent machining properties

ü  Inexpensive

USES for making parts where forging is expensive, like hubs of wagon wheels, small fittings for railway rolling stock, brake supports, parts of agricultural machinery, pipe fittings, door hinges,

locks etc.

The white heart malleable cast iron obtained after annealing in a decarburizing atmosphere have a silvery-grey fracture with a heart dark grey to black. The microstructure developed in a section depends upon the size of the section. In castings of small sections, it is mainly ferrite with certain amount of pearlite.

White heart malleable cast iron — WM 350 and WM 400

The blackheart malleable cast iron obtained after annealing in an inert atmosphere have a black fracture. The microstructure developed in the castings has a matrix essentially of ferrite with temper carbon and shall not contain flake graphite.

Blackheart malleable cast iron — BM 300; BM 320 and BM 350

The pearlitic malleable cast iron obtained after heat-treatment have a homogeneous matrix essentially of pearlite or other transformation products of austenite. The graphite is present in the form of temper carbon nodules. The microstructure shall not contain flake graphite

Pearlitic malleable cast iron — PM 450 ; PM 500 ; PM 550 ; PM 600 and PM 700

Spheroidal or nodular graphite cast iron:  In these cast irons graphite is present in the form of spheres or nodules. The nodular or spheroidal graphite cast iron is also called ductile cast iron or high strength cast iron. This type of cast iron is obtained by adding small amounts of magnesium (0.1 to 0.8%) to the molten grey iron.

ü  They have high tensile strength and good elongation properties.

ü  It has high fluidity, castability, tensile strength, toughness, wear resistance, pressure tightness weldability and machinability.

USES: It is generally used for castings requiring shock and impact resistance along with good machinability, such as hydraulic cylinders, cylinder heads, rolls for rolling mill and centrifugally cast products.

They are designated as, for example,

SG50/7, SG80/2

Where the first number gives the tensile strength in MPa and the second number indicates percentage elongation.

Austenitic cast iron:

ü  These are alloy cast irons and they contain small percentages of silicon, manganese, sulphur, phosphorus etc.

ü  They may be produced by adding alloying elements viz. nickel, chromium, molybdenum, copper and manganese in sufficient quantities.

ü  These elements give more strength and improved properties

Depending on the form of graphite present these cast iron can be classified broadly under two headings:

Austenitic flake graphite iron designated, for example, AFGNi16Cu7Cr2

Austenitic spheroidal or nodular graphite iron designated, for example,

ASGNi20Cr2.

USES: They are used for making automobile parts such as cylinders, pistons, piston rings, brake drums etc.

Abrasion resistant cast iron: These are alloy cast iron and the alloying elements render abrasion resistance.

A typical designation is

ABR33 Ni4 Cr2

Which indicates a tensile strength in 33kg/mm² with 4% nickel and 2% chromium.

WROUGHT IRON:

This is a very pure iron where the iron content is of the order of 99.5%. It is produced by re-melting pig iron and some small amount of silicon, sulphur, or phosphorus may be present.

ü  It is tough, malleable

ü  Ductile

ü  Can easily be forged or welded.

ü  It cannot however take sudden shock.

USES: Chains, crane hooks, railway couplings and such other components may be made of this iron.

Steel: This is by far the most important engineering material and there is an enormous variety of steel to meet the wide variety of engineering requirements. Steel is basically an alloy of iron and carbon in which the carbon content can be less than 1.7% and carbon is present in the form of iron carbide to impart hardness and strength.

Two main categories of steel are

ü  Plain carbon steel

ü  Alloy steel.

PLAIN CARBON STEEL: Most of the steel produced now-a-days is plain carbon steel or simply carbon steel. Carbon steel is defined as a steel which has its properties mainly due to its carbon content and does not contain more than 0.5% of silicon and 1.5% of manganese.

The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content.

1.      Dead mild steel — up to 0.15% carbon

2.      Low carbon or mild steel — 0.15% to 0.45% carbon

3.      Medium carbon steel — 0.45% to 0.8% carbon

4.      High carbon steel — 0.8% to 1.5% carbon

Steels Designated on the Basis of Mechanical Properties

These steels are carbon and low alloy steels where the main criterion in the selection and inspection of steel is the tensile strength or yield stress.

According to Indian standard, these steels are designated by a symbol ‘Fe’ or ‘Fe E’ depending on whether the steel has been specified on the basis of minimum tensile strength or yield strength, followed by the figure indicating the minimum tensile strength or yield stress in N/mm2.

For example

‘Fe 290’

Means a steel having minimum tensile strength of 290 N/mm² and

‘Fe E 220’

Means a steel having yield strength of 220 N/mm².

Steels Designated on the Basis of Chemical Composition:

According to Indian standard, the carbon steels are designated in the following order:

1.      Figure indicating 100 times the average percentage of carbon content,

2.      Letter ‘C’, and

3.      Figure indicating 10 times the average percentage of manganese content. The figure after multiplying shall be rounded off to the nearest integer.

For example

20C8

Means a carbon steel containing 0.15 to 0.25 per cent (0.2 per cent on an average) carbon and 0.60 to 0.90 per cent (0.75 per cent rounded off to 0.8 per cent on an average) manganese.

Free Cutting Steels

The free cutting steels contain sulphur and phosphorus. These steels have higher sulphur content than other carbon steels. In general, the carbon content of such steels varies from 0.1 to 0.45 per cent and sulphur from 0.08 to 0.3 per cent. These steels are used where rapid machining is the prime requirement.

According to Indian standard, carbon and carbon manganese free cutting steels are designated in the following order:

1.      Figure indicating 100 times the average percentage of carbon,

2.      Letter ‘C’,

3.      Figure indicating 10 times the average percentage of manganese, and

4.      Symbol ‘S’ followed by the figure indicating the 100 times the average content of sulphur. If instead of sulphur, lead (Pb) is added to make the steel free cutting, then symbol ‘Pb’ may be used.

For example

10C8S10

Means 0.10 percent carbon, 0.8% manganese, 0.10% sulphur.

Alloy steel:  These are steels in which elements other than carbon are added in sufficient quantities to impart desired properties, such as wear resistance, corrosion resistance, electric or magnetic properties.

Chief alloying elements added are usually

ü  Nickel for strength and toughness

ü  Chromium for hardness and strength

ü  Tungsten for hardness at elevated temperature

ü  Vanadium for tensile strength

ü  Manganese for high strength in hot rolled and heat treated condition

ü  Silicon for high elastic limit

ü  Cobalt for hardness

ü  Molybdenum for extra tensile strength.

Indian Standard Designation of Low and Medium Alloy Steels

According to Indian standard, low and medium alloy steels shall be designated in the following order:

1.      Figure indicating 100 times the average percentage carbon.

2.      Chemical symbol for alloying elements each followed by the figure for its average percentage content multiplied by a factor as given below :

Element Multiplying factor	Multiplying factor
Cr, Co, Ni, Mn, Si and W	4
Al, Be, V, Pb, Cu, Nb, Ti, Ta, Zr and Mo	10
P, S and N	100

For example

40 Cr 4 Mo 2

Means alloy steel having average 0.4% carbon, 1% chromium and 0.25% molybdenum.

STAINLESS STEEL:

Stainless steel is one such alloy steel that gives good corrosion resistance. One important type of stainless steel is often described as 18/8 steel where chromium and nickel percentages are 18 and 8 respectively.

USES: These steels are used in the manufacture of pump shafts, rail road car frames and sheathing, screws, nuts and bolts and small springs

Indian Standard Designation of High Alloy Steels (Stainless Steel and Heat Resisting Steel)

According to Indian standard, the high alloy steels (i.e. stainless steel and heat resisting steel) are designated in the following order:

1.      Letter ‘X’.

2.      Figure indicating 100 times the percentage of carbon content.

3.      Chemical symbol for alloying elements each followed by a figure for its average percentage content rounded off to the nearest integer.

4.      Chemical symbol to indicate specially added element to allow the desired properties.

For example,

X 10 Cr 18 Ni 9

Means alloy steel with average carbon 0.10 per cent, chromium 18 per cent and nickel 9 per cent.

High Speed Tool Steels These steels are used for cutting metals at a much higher cutting speed than ordinary carbon tool steels. Most of the high speed steels contain tungsten as the chief alloying element, but other elements like cobalt, chromium, vanadium, etc. may be present in some proportion.

Indian Standard Designation of High Speed Tool Steel

According to Indian standard, the high speed tool steels are designated in the following order :

1.      Letter ‘XT’.

2.      Figure indicating 100 times the percentage of carbon content.

3.      Chemical symbol for alloying elements each followed by the figure for its average percentage content rounded off to the nearest integer, and

4.      Chemical symbol to indicate specially added element to attain the desired properties.

For example,

XT 75 W 18 Cr 4 V 1

Means a tool steel with average carbon content 0.75 per cent, tungsten 18 per cent, chromium 4 per cent and vanadium 1 per cent.

Non-ferrous Metals:

Metals containing elements other than iron as their chief constituents are usually referred to as non-ferrous metals. The non-ferrous metals are usually employed in industry due to the following characteristics:

1.      Ease of fabrication (casting, rolling, forging, welding and machining),

2.      Resistance to corrosion,

3.      Electrical and thermal conductivity, and

4.      Weight.

There is a wide variety of non-metals in practice. However, only a few exemplary ones are discussed below:

Aluminium: This is the white metal produced from Alumina. In its pure state it is weak and soft but addition of small amounts of Cu, Mn, Si and Magnesium makes it hard and strong. It is also corrosion resistant, low weight and non-toxic.

1.      Duralumin: This is an alloy of 4% Cu, 0.5% Mn, 0.5% Mg and aluminium. It is widely used in automobile and aircraft components.

2.      Y-alloy: This is an alloy of 4% Cu, 1.5% Mn, 2% Ni, 6% Si, Mg, Fe and the rest is Al. It gives large strength at high temperature. It is used for aircraft engine parts such as cylinder heads, piston etc.

3.      Magnalium: This is an aluminium alloy with 2 to 10 % magnesium. It also contains 1.75% Cu. Due to its light weight and good strength it is used for aircraft and automobile components.

Copper alloys: Copper is one of the most widely used non-ferrous metals in industry. It is soft, malleable and ductile and is a good conductor of heat and electricity. The following two important copper alloys are widely used in practice

1.      Brass (Cu-Zn alloy) It is fundamentally a binary alloy with Zn upto 50%. As Zn percentage increases, ductility increases upto ~37% of Zn beyond which the ductility falls. Small amount of other elements viz. lead or tin imparts other properties to brass. Lead gives good machining quality and tin imparts strength. Brass is highly corrosion resistant, easily machinable and therefore a good bearing material.

2.      Bronze (Cu-Sn alloy): This is mainly a copper-tin alloy where tin percentage may vary between 5 to 25. It provides hardness but tin content also oxidizes resulting in brittleness. Deoxidizers such as Zn may be added. Gun metal is one such alloy where 2% Zn is added as deoxidizing agent and typical compositions are 88% Cu, 10% Sn, 2% Zn. This is suitable for working in cold state. It was originally made for casting guns but used now for boiler fittings, bushes, glands and other such uses.

Gun Metal: It is an alloy of copper, tin and zinc. It usually contains 88% copper, 10% tin and 2% zinc. This metal is also known as Admiralty gun metal. It was made for casting guns. It is extensively used for casting boiler fittings, bushes, bearings, glands, etc.

Lead: It is a bluish grey metal having specific gravity 11.36 and melting point 326°C. It is so soft that it can be cut with a knife. It is extensively used for making solders, as a lining for acid tanks, cisterns, water pipes, and as coating for electrical cables. The lead base alloys are employed where a cheap and corrosion resistant material is required. Alloy containing 83% lead, 15% antimony, 1.5% tin and 0.5% copper is used for large bearings subjected to light service.

Tin: It is brightly shining white metal. It is soft, malleable and ductile. It can be rolled into very thin sheets. It is used for making important alloys, fine solder, as a protective coating for iron and steel sheets and for making tin foil used as moisture proof packing. A tin base alloy containing 88% tin, 8% antimony and 4% copper is called babbit metal. It is a soft material with a low coefficient of friction and has little strength. It is the most common bearing metal used with cast iron boxes where the bearings are subjected to high pressure and load.

Non-metals:

Non-metallic materials are also used in engineering practice due to principally their low cost, flexibility and resistance to heat and electricity. Though there are many suitable non-metals, the following are important few from design point of view:

Timber: This is a relatively low cost material and a bad conductor of heat and electricity. It has also good elastic and frictional properties and is widely used in foundry patterns and as water lubricated bearings.

Leather: This is widely used in engineering for its flexibility and wear resistance. It is widely used for belt drives, washers and such other applications.

Rubber: It has high bulk modulus and is used for drive elements, sealing, vibration isolation and similar applications.

Plastics: These are synthetic materials which can be moulded into desired shapes under pressure with or without application of heat. These are now extensively used in various industrial applications for their corrosion resistance, dimensional stability and relatively low cost.

1.      Thermosetting plastics: Thermosetting plastics are formed under heat and pressure. It initially softens and with increasing heat and pressure, polymerisation takes place. This results in hardening of the material. These plastics cannot be deformed or remoulded again under heat and pressure. Some examples of thermosetting plastics are phenol formaldehyde (Bakelite), phenol-furfural (Durite), epoxy resins, phenolic resins etc.

2.      Thermoplastics: Thermoplastics do not become hard with the application of heat and pressure and no chemical change takes place. They remain soft at elevated temperatures until they are hardened by cooling. These can be re-melted and remoulded by application of heat and pressure. Some examples of thermoplastics are cellulose nitrate (celluloid), polythene, polyvinyl acetate, polyvinyl chloride ( PVC) etc.

Heat Treatment of Steels

The term heat treatment may be defined as an operation or a combination of operations, involving the heating and cooling of a metal or an alloy in the solid state for the purpose of obtaining certain desirable conditions or properties without change in chemical composition.

The aim of heat treatment is to achieve one or more of the following objects:

ü  To increase the hardness of metals.

ü  To relieve the stresses set up in the material after hot or cold working.

ü  To improve machinability.

ü  To soften the metal.

ü  To modify the structure of the material to improve its electrical and magnetic properties.

ü  To change the grain size.

ü  To increase the qualities of a metal to provide better resistance to heat, corrosion and wear.

Normalising: The main objects of normalizing are:

ü  To refine the grain structure of the steel to improve machinability, tensile strength and structure of weld.

ü  To remove strains caused by cold working processes like hammering, rolling, bending,etc., which makes the metal brittle and unreliable.

ü  To remove dislocations caused in the internal structure of the steel due to hot working.

ü  To improve certain mechanical and electrical properties.

The process of normalising consists of heating the steel from 30 to 50°C above its upper critical temperature (for hypoeutectoid steels) or Acm line (for hypereutectoid steels). It is held at this temperature for about fifteen minutes and then allowed to cool down in still air.

AnnealinG: The main objects of annealing are:

ü  To soften the steel so that it may be easily machined or cold worked.

ü  To refine the grain size and structure to improve mechanical properties like strength and ductility.

ü  To relieve internal stresses which may have been caused by hot or cold working or by unequal contraction in casting.

ü  To alter electrical, magnetic or other physical properties.

ü  To remove gases trapped in the metal during initial casting.

The annealing process is of the following two types:

Full annealing: The purpose of full annealing is to soften the metal to refine the grain structure, to relieve the stresses and to remove trapped gases in the metal. The process consists of

ü  Heating the steel from 30 to 50°C above the upper critical temperature for hypoeutectoid steel and by the same temperature above the lower critical temperature i.e. 723°C for hypereutectoid steels.

ü  Holding it at this temperature for some time to enable the internal changes to take place. The time allowed is approximately 3 to 4 minutes for each millimetre of thickness of the largest section, and

ü  Cooling slowly in the furnace. The rate of cooling varies from 30 to 200°C per hour depending upon the composition of steel.

Process annealing: The process annealing is used for relieving the internal stresses previously set up in the metal and for increasing the machinability of the steel. In this process, steel is heated to a temperature below or close to the lower critical temperature, held at this temperature for some time and then cooled slowly. This causes complete recrystallisation in steels which have been severely cold worked and a new grain structure is formed. The process annealing is commonly used in the sheet and wire industries.

Spheroidising:  It is another form of annealing in which cementite in the granular form is produced in the structure of steel. This is usually applied to high carbon tool steels which are difficult to machine. The operation consists of heating the steel to a temperature slightly above the lower critical temperature (730 to 770°C). It is held at this temperature for some time and then cooled slowly to a temperature of 600°C. The rate of cooling is from 25 to 30°C per hour.

The spheroidising improves the machinability of steels, but lowers the hardness and tensile strength. These steels have better elongation properties than the normally annealed steel.

Hardening: The main objects of hardening are:

ü  To increase the hardness of the metal so that it can resist wear.

ü  To enable it to cut other metals i.e. to make it suitable for cutting tools.

The process of hardening consists of

ü  Heating the metal to a temperature from 30 to 50°C above the upper critical point for hypoeutectoid steels and by the same temperature above the lower critical point for hypereutectoid steels.

ü  Keeping the metal at this temperature for a considerable time, depending upon its thickness.

ü  Quenching (cooling suddenly) in a suitable cooling medium like water, oil or brine.

It may be noted that the low carbon steels cannot be hardened appreciably, because of the presence of ferrite which is soft and is not changed by the treatment. As the carbon content goes on increasing, the possible obtainable hardness also increases.

Tempering: The steel hardened by rapid quenching is very hard and brittle. It also contains internal stresses which are severe and unequally distributed to cause cracks or even rupture of hardened steel. The tempering (also known as drawing) is, therefore, done for the following reasons:

ü  To reduce brittleness of the hardened steel and thus to increase ductility.

ü  To remove the internal stresses caused by rapid cooling of steel.

ü  To make steel tough to resist shock and fatigue.

The tempering process consists of reheating the hardened steel to some temperature below the lower critical temperature, followed by any desired rate of cooling. The exact tempering temperature depends upon the purpose for which the article or tool is to be used.

Surface hardening or case hardening: In many engineering applications, it is desirable that steel being used should have a hardened surface to resist wear and tear. At the same time, it should have soft and tough interior or core so that it is able to absorb any shocks, etc. This is achieved by hardening the surface layers of the article while the rest of it is left as such. This type of treatment is applied to gears, ball bearings, railway wheels, etc.

Following are the various surfaces or case hardening processes by means of which the surface layer is hardened:

1.      Carburising,

2.      Cyaniding,

3.      Nitriding

4.      Induction hardening, and

5.      Flame hardening.

PROPERTIES OF ENGINERING MATERIALS:

Materials are characterized by their properties. They may hard ductile or heavy, conversely they may be soft, brittle or light. The mechanical properties of materials are the properties that describe the behavior of materials under the action of external forces. These mechanical properties of the metal include strength, stiffness, elasticity, plasticity, ductility, brittleness, malleability, toughness, resilience, creep and hardness.

Strength: It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress. Depending upon the types of stresses induced by extermal loads, strength is expressed as tensile , compressive or shear strength.

ELASTICITY: It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

Plasticity: It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

DIFFERENCE BETWEEN ELASTICITY AND PLASTICITY:

1.      Elasticity is the ability of material to regain its original shape after temporary deformation under external force. Plasticity is the ability to retain the deformation permanently even after the load is removed.

2.      The amount of elastic deformation is very small while plastic deformation is relatively more.

3.      During elastic deformation, atoms of metal are temporarily displaced from their original positions but return back when load is removed. During plastic deformation, atoms of metal are permanently displaced from their original positions and take up new positions.

4.      For majority of materials, the stress-strain relationship is linear in elastic range and non-linear in plastic range.

5.      Elasticity is an important consideration in machine tools components while plasticity is desirable for components made by press working operations.

STIFFNESS OR RIGIDITY: It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness

RESILIENCE: It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials.

TOUGHNESS: It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed upto the point of fracture. This property is desirable in parts subjected to shock and impact loads.

DIFFERNCE BETWEEN RESILIENCE AND TOUGHNESS:

1.      Resilience is the ability of material to absorb energy with in elastic range. Toughness is the ability to absorb energy within elastic limit.

2.      Modulus of resilience is the area below stress-strain curve in tension test upto yield point. Modulus of toughness is the total area below the stress-strain curve.

3.      Resilience is essential is in spring applications while toughness id required for components subjected to bending, twisting, stretching or to impact loads, spring steels are resilient while structural steels are tough.

MALLEABILITY: It is defined as the ability of the material to deform to a greater extent before the sign of crack, when it is subjected to compressive force. It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron,

DUCTILITY:  It is defined as the ability of the material to deform to a greater extent before the sign of crack, when it is subjected to tensile force. It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead.

DIFFERNCE BETWEEN MALLEABILITY AND DUCTILITY:

1.      Malleability is the ability of the material to deform under compressive force. Ductility is the ability of material to deform under tensile force.

2.      Malleability increases with temperature while ductility deceases with increasing temperature.

3.      All ductile materials are also malleable, but the converse is not true.

4.      Malleability is important property when component is forged, rolled or extruded. Ductility is desirable when the component is formed or drawn. It is also desirable when the machine component is subjected to shock loads.

BRITTLENESS:  It is that property of the material which shows negligible plastic deformation before fracture takes place. It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads, snap off without giving any sensible elongation. Cast iron is a brittle material.

DIFFERENCE BETWEEN DUCTILITY AND BRITTLENESS:

1.      Ductile material deform to a greater extent before fracture in tension test. Brittle materials show negligible plastic deformation prior to fracture.

2.      Steels, copper and aluminium are ductile materials while cast iron is brittle.

3.      The energy absorbed by ductile specimen before fracture in tension testy is more, while brittle fracture is accompanied by negligible energy absorption.

4.      In ductile materials, failure takes place by yielding which gradual. Brittle materials fail by sudden fracture.

Machinability: It is the property of a material which refers to a relative ease with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel.

Creep: When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.

 Fatigue: When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc.

Hardness: It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test. The hardness of a metal may be determined by the following tests:

1.      Brinell hardness test,

2.      Rockwell hardness test,

3.      Vickers hardness (also called Diamond Pyramid) test

Questions with Answers

Q: Classify common engineering materials.

A: Common engineering materials can be broadly classified into metals and non-metals. Metals include ferrous and non-ferrous metal and the nonmetals include timber, leather, rubber and a large variety of polymers. Among the ferrous metals different varieties of cast iron, wrought iron and alloy steels are extensively used in industry. There are also a large variety of timber, leather and polymers that are used in industry.

Q: What are the advantages of malleable cast iron over white or grey cast iron?

A: Malleable cast iron are tougher than grey or white cast iron and can be twisted or bent without fracture. They also have excellent machining properties and are relatively inexpensive.

Q: A standard alloy steel used for making engineering components is 20Cr18 Ni2. State the composition of the steel.

A: The composition of the steel is 0.2% carbon, 18% chromium and 2% nickel.

Q: Name two important copper alloys and give their typical compositions.

A: Two most important copper alloys are bronze and brass. Bronze is a Cu-Sn alloy with the typical composition of 88% Cu, 10% Sn and 2% Zn. Brass is a Cu-Zn alloy with the typical composition of red brass of 85% Cu , 15% Zn.

Q: List at least five important non-metals commonly used in machine design.

A: Some important non-metals for industrial uses are: Timber, leather, rubber, bakelite, nylon, polythene, polytetraflutoethylene (PTFE).

Q: State at least 5 important mechanical properties of materials to be considered in machine design.

A.7: Some important properties of materials to be considered in design are: Elastic limit, yield and ultimate strength, hardness and toughness.

Q: Define resilience and discuss its implication in the choice of materials in machine design.

A : Resilience is defined as the property of a material that enables it to resist shock and impact. The property is important in choosing materials for machine parts subjected to shock loading, such as, fasteners, springs etc.

Q:  What is grey cast iron?
A: It is ordinary commercial iron having following composition

1. Carbon 3-3.5% in the form of free graphite
2. Silicon 1-7.5%
3. Mn 0.40-1.0%
4. Phos0.15-1%
5. Sulphur 0.02-0.15%
6. Remaining iron

Q:  Name the important properties of grey cast iron?
A: The important properties of grey cast iron are:-                                                                  

ü  High compressive strength

ü  Low tensile strength

ü  No ductility

ü  Easy machiniability

ü  Free graphite in the structure acts of a lubricant

ü  Damping of vibrations

Q:  How is grey cast iron design at end?
A: As per Indian standard code 210-1993 the grey cast iron is designated by the alphabet FG followed by a figure indicating the minimum tensile strength

Q: What is Mild steel?
A: It is an alloy of iron and carbon with carbon content upto a maximum of 1.5-4.5%

Q: What is steel?
A: Steel is an alloy of iron and carbon with carbon content upto a maximum of 1.5%.The carbon is in the combined form as ferric carbide

Q:  How are steels designated based on their composition?
A: As per Indian standard code IS-1570-1979 steels are designated e.g: 30C8 or 35C4 where

1. 30 or 35 indicates 100 times the average carbon content
2. Letter ‘C’ means Plain carbon steel
3. 8 or 4 indicate 10 times the average Manganese content.

Q: State the important mechanical properties of materials?
A:

ü  Strength: It is the ability of a material to resist external applied load.

ü  Stiffness: It is the ability of a material to resist deformation.

ü  Elasticity: It is the ability to regain original shape when loads are removed.

ü  Ductility: It is the property of a material to be drawn into wire.

ü  Malleability: It is the property of a material to be hammered into thin sheets.

ü  Toughness: It is the property of a material to resist fracture.

ü  Hardness: It is the property to resist wear, searching, indentation, machining etc.

Q: What is static loading?
A: It is defined as a loading which is applied to a part very gradually and after the load reaches its final value, it does not change in magnitude, direction or point of application with time.

Q:  Name some machine parts which are subjected to variable or fatigue loads.
A: Axels, shafts, crank shafts, connecting rod, springs, gear teeth etc.

Q: Whatis“fatigue”?
A: When a material is subjected to Repeated loading, it fails at stresses below the yield point stresses. Such a failure is called “Fatigue”.