Basics of Materials Science and Engineering Short Notes Part-5

1. Ferrous & Non-Ferrous Materials

1.1 Ferrous Materials

Ferrous Metals mostly contain iron. They have small amounts of other metals or elements added, to give the required properties.  Ferrous Metals are magnetic and give little resistance to corrosion.

In ferrous materials the main alloying element is carbon (C).

Depending on the amount of carbon present, these alloys will have different properties, especially when the carbon content is either less/higher than 2.14%.

Thus the ferrous alloys with less than 2.14% C are termed as steels, and the ferrous alloys with higher than 2.14% C are termed as cast irons.

1.2 Classification of Metals

Metals are classified into two types: Ferrous & Non-Ferrous Materials

Pig Iron: The raw materials used to produce pig iron in a blast furnace are

Iron Ore: The iron content of these ores ranges from 70% down to 20% or less.

Coke is a substance made by heating coal until it becomes almost pure carbon. 

Sinter is made of lesser grade, finely divided iron ore which, is roasted with coke and lime to remove a large amount of the impurities in the ore.

Limestone: Iron ore often has silicon dioxide (sand) impurity which will be removed by adding limestone.

1.3 Classification of Ferrous Materials

1.4 Classification of Steels

Mechanical properties of steels are very sensitive to carbon content.

Thus steels are basically three kinds based on carbon content

Low- carbon steels (% wt of C < 0.3)

• Their microstructure consists of ferrite and pearlite
• Because of above atomic structure, these alloys are thus relatively soft, ductile combined with high toughness.
• Hence these materials are easily machinable and weldable.
• They are used in different applications like structural shapes, tin cans, automobile body components, buildings, etc.

Medium carbon steels (0.3 <% wt of C < 0.6)

• These are stronger than low carbon steels. However these are of less ductile than low carbon steels.
• They are used in different applications like railway tracks & wheels, gears, other machine parts which may require good combination of strength and toughness. 

High- carbon steels (% wt of C > 0.6).

• These are strongest and hardest of carbon steels, and of course their ductility is very limited.
• They possess very high wear resistance, and capable of holding sharp edges.
• They are Used as tool and die steels owing to the high hardness and wear resistance property,
• They are used in tool application such as knives, razors, hacksaw blades, etc

HSLA (high-strength low-alloy) steels

A special group of ferrous alloys with noticeable amount of alloying additions are known as HSLA (high-strength low-alloy) steels.

• Common alloying elements are: Cu, V, Ni, W, Cr, Mo, etc.
• These alloys can be strengthened by heat treatment, and yet the same time they are ductile, formable.
• HSLA steels are more resistant to corrosion than the plain carbon steels, which they have replaced in many applications where structural strength is critical.

Applications:

• Support columns, bridges, pressure vessels.
• Effects of Alloying Elements on Steel

1.5 Special Alloy Steels

By adding alloys, specials alloy steels can be produced to suit specific requirements by modifying the properties

These are

• Stainless steels

• The name comes from their high resistance to corrosion i.e. they are rustless (stain-less).
• Steels are made highly corrosion resistant by addition of special alloying elements,
• especially a minimum of 12% Cr along with Ni and Mo.

• Magnetic steels

• Alloys like carbon.chromium, tungsten and cobalt are used to produce permanent magnetic steels having high magnetic retentive power
• Typical alloy has 60% iron, 20% nickel, 8% cobalt and 20% aluminium

• High resistance steels

• These are high nickel based steels which have high red hot hardness.
• These are also called as Online Classroom Program alloys

• High speed steels

High Speed Steels are high-performance special steels offering high hardness at temperatures up to 500°C and high wear resistance, thanks to alloying elements like tungsten, molybdenum, vanadium and chromium which are able to form carbides. To improve hot hardness, cobalt may also be added.

     The characteristic properties of all high speed steels grades include.       

• High working hardness   
• High wear hardness   
• Excellent toughness   
• High retention of hardness and red hardness

2. Cast Iron

Ferrous alloys with more than 2.14 wt.% C are designated as cast irons

Commercially cast irons contain about 3.0-4.5% C along with some alloying additions.

Alloys with this carbon content melt at lower temperatures than steels i.e. they are responsive to casting. Hence casting is the most used fabrication technique for these alloys.

Based on the form of carbon present, cast irons are categorized as gray, white, nodular and malleable cast irons.

2.1 Gray cast iron

• These alloys consists carbon in form graphite flakes, which are surrounded by either ferrite or pearlite.
• Because of presence of graphite, fractured surface of these alloys look grayish, and so is the name for them.

Properties:

• Due to graphite flakes, gray cast irons are weak and brittle in tension.
• Stronger in compression
• However they possess good damping properties
• They also show high resistance to wear.

Applications:

• Because of its damping property, these are used as base structures, bed for heavy machines, etc.

2.2 White cast iron

How name came:

• When Si content is low (< 1%) in combination with faster cooling rates, there will be no time left for cementite to get decomposed and retains the brittle cementite.
• Because of presence of this cementite, fractured surface appear white, hence the name.

Properties:

• They are very hard and brittle because of this, extremely difficult to machine.
• Hence their use is limited to wear resistant applications.

Applications:

• These are used as rollers in rolling mills.
• Usually white cast iron is heat treated to produce malleable iron. 

2.3 Nodular (or ductile) cast iron

Alloying additions are of prime importance in producing these materials.

Constituents:

• Small additions of magnesium or cerium (Mg / Ce) to the gray cast iron melt before casting produces distinctly different microstructure.
• This result in graphite to form nodules or sphere-like particles.
• Matrix surrounding these particles can be either ferrite or pearlite depending on the heat treatment.

Properties:

• These are stronger and ductile than gray cast irons.

Applications:

• Pump bodies, crank shafts, automotive components, etc. 

2.4  Malleable cast iron

• These formed after heat treating white cast iron.
• Heat treatment involves heating the material up to 800-900 ْC, and keep it for long hours, before cooling it to room temperature.
• High temperature incubation causes cementite to decompose and form ferrite and graphite.

Properties:

• These materials are stronger with appreciable amount of ductility.

Applications:

• Railroad, connecting rods, marine and other heavy- duty services.

3. Non Ferrous Metals

Non-ferrous metals are those which do not contain significant quantity of iron or iron as base metal. These metals possess low strength at high temperatures, generally suffer from hot shortness and have more shrinkage than ferrous metals.

They are utilized in industry due to following advantages:

• High corrosion resistance
• Easy to fabricate, i.e., machining, casting, welding, forging and rolling
• Possess very good thermal and electrical conductivity
• Attractive colour and low density

   The various non-metals used in industry are: copper, aluminium, tin, lead, zinc, and nickel, etc., and their alloys.

3.1 Aluminium

Aluminium is white metal which is produced by electrical processes from clayey mineral known as bauxite. However, this aluminium ore bauxite is available in India in plenty and we have a thriving aluminium industry.

Properties:

• These are characterized by low density, high thermal & electrical conductivities,
• Good corrosion resistant characteristics because of formation of Al₂O₃ protective layer.
• As Al has FCC crystal structure, these alloys are ductile even at low temperatures and can be formed easily.
• However, the great limitation of these alloys is their low melting point (660 ْC), which restricts their use at elevated temperatures.

Applications:

Aerospace, Automotive, Marine, Rail, Packaging etc

Alloys of Aluminium

3.2 Copper

The crude form of copper extracted from its ores through series of processes contains 68% purity known as Blister copper. By electrolytic refining process, highly pure (99.9%) copper which is remelted and casted into suitable shapes.

Copper is a corrosion resistant metal of an attractive reddish brown colour.

Properties and Uses:

• High Thermal Conductivity: Used in heat exchangers, heating vessels and appliances, etc.
• High Electrical Conductivity: Used as electrical conductor in various shapes and forms for various applications.
• Good Corrosion Resistance:Used for providing coating on steel prior to nickel and chromium plating
• High Ductility:Can be easily cold worked, folded and spun. Requires annealing after cold working as it loses its ductility.

Alloys of Copper

4. Other metals

4.1 Lead

Lead is the heaviest of the common metal. Lead is extracted from its ore known as galena. It is bluish grey in colour and dull lusture which goes very dull on exposure to air.

Properties and Uses:

• Its specific gravity is 7.1(w.r.to water) and melting point is 360°C.
• It is resistant to corrosion and many chemicals do not react with it (even acids).
• It is soft, heavy and malleable, can be easily worked and shaped.
• Lead is utilized as alloying element in producing solders and plumber’s solders.
• It is alloyed with brass as well as steel to improve their machinability.
• It is utilized in manufacturing of water pipes, coating for electrical cables, acid tanks and roof covering etc.

4.2 Tin

It is a brilliant white metal with yellowish tinge. Melting point of tin is 240°C

Properties and Uses:

(1) Tin is malleable and ductile, it can be rolled into very thin sheets.

(2) It is used for tinning of copper and brass utensils and copper wire before its conversion into cables.

(3) It is useful as a protective coating for iron and steel since it does corrode in dry or wet atmosphere.

(4) It is utilized for making important alloys such as fine solder and moisture proof packing with thin tin sheets.

4.3 Zinc

The chief ores of zinc are blende (ZnS) and calamine (ZnCO₃).

Zinc is a fairly heavy, bluish-white metal principally utilized in view of its low cost, corrosion resistance and alloying characteristics. Melting point of zinc is 420°C and it boils at 940°C.

Properties and Uses:

• High corrosion resistance: Widely used as protective coating on iron and steel. Coating may be provided by dip galvanizing or electroplating.
• High fluidity and low melting point: Most suitable metal for pressure die casting generally in the form of alloy.
• When rolled into sheets, zinc is utilized for roof covering and for providing a damp proof non-corrosive lining to containers.
• The galvanized wires, nails, etc. are produced by galvanizing technique and zinc is also used in manufacture of brasses.

4.4 Nickel

About 85% of all nickel production is obtained from sulphide ores.

Properties and Uses:

• Pure nickel is tough, silver coloured metal, harder than copper having some but less ductility but of about same strength.
• It is plated on steel to provide a corrosion resistance surface or layer.
• Widely used as an alloying element with steel. Higher proportions are advantageously added in the production of steel such as monel or inconel.
• It possesses good resistance to both acids and alkalis regarding corrosion so widely utilized in food processing equipment.

4.4.1 Alloys of Nickel

a) German silver

Composition:

• The composition of this alloy is 60% Cu, 30% Ni and 10% zinc.

Properties:

• It displays silvery appearance and is very ductile and malleable.

Applications:

• It is utilized for electrical contacts, casting of high quality valves, taps and costume jewellery. It is also used in producing electrical wires,

b) Monel metal

Composition:

• It contains 68% Ni, 30% Cu, 1% iron and remainder small additions of Mn and other elements.

Properties:

• It is corrosion resistant and possesses good mechanical properties and maintains them at elevated temperatures.

c) Nichrome

It is an alloy of nickel and chromium which is utilized as heat resistant electrical wire in electrical appliances such as furnaces, geysers and electric iron, etc.

d) Inconel and incoloy

• These alloys principally contain, Ni, Cr, Fe, Mo, Ti and very small proportions of carbon.
• These are used as high temperature alloys. Inconel does not respond to heat treatment.

4.5 Magnesium

Principal ores of magnesium are magnesite, carnallite and dolomite. Magnesium is extracted by electrolytic process.

Properties and Uses:

• It is the lightest of all metals weighing around two-thirds of aluminium.
• The tensile strength of cast metal is the same as that of ordinary cast aluminium, i.e., 90 MPa.
• The tensile strength of rolled annealed magnesium is same as that of good quality cast iron.
• Magnesium can be easily formed, drawn forged and machined with high accuracy. (5) In powdered form it is likely to burn, in that situation adequate fire protection measures should be strictly observed.
• It’s castings are pressure tight and achieve good surface finish. Magnesium castings include motor car gearbox, differential housing and portable tools.

4.6 Vanadium

It occurs in conjunction with iron pyrite, free sulphur and carbonaceous matter.

Properties and Uses:

(1) It is silvery white in colour.

(2) Its specific gravity is 5.67.

(3) Its melting point is 1710°C.

(4) When heated to a suitable temperature it can be hammered into any shape or drawn into wires.

(5) It is used in manufacture of alloy steels.

(6) Vanadium forms non-ferrous alloys of copper and aluminium from which excellent castings can be produced.

4.7 Antimony

Chief ore of antimony is stibnite. To a small extent, antimony is obtained as a by-product in refining of other metals such as lead, copper silver and zinc.

Properties and Uses:

• It is silvery white, hard, highly crystalline and so brittle that it may be readily powdered.
• Its specific gravity is 6.63 and melting point is 630°C.
• It is generally used as an alloying element with most of heavy metals.
• Lead, tin and copper are the metals which are most commonly alloyed with antimony.

4.8 Cadmium

It is obtained commercially as a by-product in the metallurgy of zinc and to some extent of lead.

Properties and Uses:

• White metal with bluish tinge, capable of taking a high polish.
• Its specific gravity is 8.67 and melts at 321°C.
• It is slightly harder than tin but softer than zinc.
• It is malleable and ductile and can be readily rolled and drawn into wires.

It is chiefly utilized in antifriction alloys for bearings. It is also used as rust proof coating for iron and steel. Components of automobiles and refrigerator such as nuts, bolts and trimmings, locks and wire products are plated with it.

4.9 Bearing Materials

A bearing material should possess the following characteristics:

• It should possess enough compressive strength to provide adequate load carrying capacity.
• It should possess good plasticity to negate small variations in alignment and fitting.
• Its wear resistance should be adequate to maintain a specified fit.
• The coefficient of friction of the bearing material should be low to avoid excessive heating.

Some significant bearing metals are as follows:

Babbitt’s metal: It is utilized for production of heavy duty bearings. It is white in colour containing 88% tin, 8% antimony and 4% copper. It is a soft material with a low coefficient of thermal expansion.

4.10 Titanium alloys

• Ti and its alloys are of relatively low density, high strength and have very high melting point.
• At the same time they are easy to machine and forge.
• However the major limitation is Ti’s chemical reactivity at high temperatures, which necessitated special techniques to extract. Thus these alloys are expensive.
• They also possess excellent corrosion resistance in diverse atmospheres, and wear properties.

Applications:

• Common applications include: space vehicles, airplane structures, surgical implants, and petroleum & chemical industries.

4.11 Refractory metals

• These are metals of very high melting points. For example: Nb, Mo, W and Ta.
• They also possess high strength and high elastic modulus.

Applications:

• Space vehicles, x-ray tubes, welding electrodes, and where there is a need for corrosion resistance.

4.12 Noble metals

• These are eight altogether: Ag, Au, Pt, Pa, Rh, Ru, Ir and Os.
• All these possess some common properties such as: expensive, soft and ductile, oxidation resistant.

Ag, Au and Pt are used extensively in jewelry, alloys are Ag and Au are employed as dental restoration materials; Pt is used in chemical reactions as a catalyst and in thermocouples.

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