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Metallurgy encompasses both the science and the technology of metals; that is, the way in which science is applied to the production of metals, and the engineering of metal components used in products for both consumers and manufacturers.

1) Distinguish between low carbon steel and mild steel based on carbon content.
Low carbon steel (Dead mild steel)
– Steels containing carbon contents of 0.05 % to 0.15 %.
– High ductility
– Used for press work and other applications.
– Eg. Motor car bodies, tin cans, nails and wires.
Mild steel
– steels containing carbon contents of 0.15% to 0.3 %
– higher tensile strength and hardness
– used for structural members, shafting. Levers and various forging.
– steels used for casting usually contained carbon of 0.3 % to 0.35 %
Mild steel is used in shipbuilding

2) Distinguish between different grades of mild steel plates as per Lloyd’s Register of Shipping grades and indicate some of the areas of usage.
OR
What are the different grades of mild steels used in ship construction under the classification societies requirements.
– There are five different qualities of steel used in shipbuilding.
– They are graded as A, B, C, D & E.
– But Lloyd’s Register of Shipping grades are only A, B, D & E.
– these grades are distinguished by a property known as ” Notch-Toughness.”

Grade A
– ordinary mild steel with carbon content of 0.23 %
– subjected to tensile test.
– contains manganese 2.5 of the carbon content
– other of silicon, sulfur and phosphorous.
– used for structural members, shafting. Levers and various forging.
Grade B
– ordinary mild steel,
– better quality than grade A.
– subjected to tensile test and impact test.
– used in places where thicker quality plates are required in the more critical regions. – used in welded structures which are highly stress or thick or subject to low temperatures. – used in areas, sheerstrake, the deck stringer strake, around corner of hatchway, sheerstrake at ends of the superstructure, Bilge strake, keel and bottom plating, if the thickness of those areas are more than 20.5 mm
Grade D
– ordinary mild steel,
– better quality than grade A, B
– subjected to tensile test and impact test.
– used in welded structures which are highly stress or thick or subject to low temperatures. – used in areas, especially at sheerstrake where it is more than 25.5 mm thickness,
In a ship longer than 200 m, at hatch way corner where it is more than 35.5 mm thickness. Bilge strake, keel and bottom plating, if the thickness is more than 20.5 mm.

Grade E
– ordinary mild steel,
– better quality than grade A, B & D.
– subjected to tensile test and impact test.
– used in welded structures which are highly stressed or thick or subject to low temperatures.
– used in areas, especially in refrigerated spaces when required to carry cargo down to -10’C.

3) Some of the alloying elements have beneficial effects, whilst others are undesirable in the manufacture of steel. Highlight the effects of Carbon, Manganese, Sulphur, Phosphorous, Nitrogen and Oxygen, in general terms.
Hardness, Tensile strength, ductility, toughness
Carbon
– Increases hardness and tensile strength but reduces ductility.
Manganese
– increases tensile strength, ductility and notch-toughness.
Silicon
– in small quantity increases hardness and tensile strength without making welding difficult.
Sulphur
– an impurity which tends to produce hot shortness when steel is stressed during welding.
Phosphorus
– an impurity which reduces ductility and toughness and may create faults when welding.
Nitrogen
– if absorbed by the weld metal will cause it to become brittle, hard and liable to cracking.
Oxygen
– if present, combines with the hot steel to form iron oxides which may remain in and weaken the joint.
– when combine with carbon in the steel to form carbon monoxide gas.
– if gas bubbles are trapped in the steel when it cools it will cause the cavities called “blow holes”

4) What are high tensile steels and how are they named differently from normal grade steel?

Special stress-strain curve for a ductile metal

High tensile steel
– Greater UTS (ultimate tensile strength) and yield strength than mild steel.
– strength of the steel can be increased in several different ways,
For that amount of carbon and manganese can be increased or various alloying may be introduced. Heat treatment can also be used.
– Lloyd Rules specify the chemical composition, method of manufacture, heat-treatment and tests required for various grades. They are suffix with ‘H’ as AH, DH, EH which are commonly used. Advantage of high tensile steel
1) saving structural weight
2) saving of weld metal
3) ease of handling
4) possibility of building bigger fabricated units.
Disadvantages of high tensile steels
1) likelihood to damage to the bottom , due to reduced thickness in the metal.
2) difficulty in effecting repairs where no HT steel is available.
3) smaller allowable amount of wastage by corrosion. Maintenance and protection on HT steel must be improved for that reason.
4) possibility of increased vibration, because of reducing of mass.
5) greater care required when bending and welding
6) increased bending or deflection (hogging or sagging).

5) What does the term ‘Notch-tough’ imply and how is notch toughness of steel determined?
Notch-Toughness steel
– Mild steels of grade A, B, D, E are subject to tensile test to ensure their value of yield strength, tensile strength and elongations are met with the requirements.
– Additionally, grade B, D, and E are subjected to another test called “impact test” to ensure that they They have a minimum standard of ‘notch toughness’.
– if they passed the test they can be named as “notch tough steels.
Notch tough steels are those which have a greater ability to resist the spreading of a crack or which reduce the possibility of a fracture occurring.

– the impact test for notch-toughness can be done by the following.
1) use a small specimen of metal which has a vee-shaped notch cut in one face.
2) clamp it in a test machine as shown in figure.
3) the pivoted pendulum is weighted and raised to an angle to the opposite side of the test piece’s notch. Take note that angle on the scale (‘A’).
Then release the pendulum to strike the test piece and brake it.
4) take note the angle on the scale (‘B’) after the pendulum has broken the test piece and reached to the farthest point.
5) compare the angle before and after braking the test piece.
By that mean the amount of energy used to fracture the test piece can be obtained. Three test pieces are normally used and the results are averaged.

6) Outline the advantages and disadvantages of incorporating aluminum alloy in ship building. List some areas of usage.
Advantages
1) Weight saving
2) Corrosion resistance
3) Non magnetic
4) High thermal conductivity
5) Notch tough at low temperature
Disadvantages
1) Costly
2) Lower melting point
3) Galvanic corrosion
4) Vibration
5) Static electricity (rubbing aluminum against with steel structure)
Usage of aluminum
1) Superstructures, Deck houses, funnels, masts, guard rails, vent trunks etc.
Connection aluminum to steel
– aluminum should never be connected to the steel structure due to increase wear and tear of aluminum in contact with steel and risk of static electricity.
– they should be connected by using neoprene (synthetic rubber) sheet, Teflon or other similar materials as an intervening medium

7) Discuss the advantages and disadvantages of incorporating aluminum into structures of vessels. In the construction of many ships, aluminum and steel are closely associated. How is the maintenance of these parts affected.

Advantages
1) Weight saving
2) Corrosion resistance
3) Non magnetic
4) High thermal conductivity
5) Notch tough at low temperature
Disadvantages
1) Costly
2) Lower melting point
3) Galvanic corrosion
4) Vibration
5) Static electricity (rubbing aluminum against with steel structure)
Connection aluminum to steel
– aluminum should never be connected to the steel structure due to increase wear and tear of aluminum in contact with steel and risk of static electricity.
– they should be connected by using neoprene (synthetic rubber) sheet, Teflon or other similar materials as an intervening medium
Maintenance
– the aluminum resists to corrosion by the formation of inert gas of oxide which forms naturally on the surface. So that very little maintenance is required.
– Painting is only required to improve the dull unattractive appearance of the film and is required at long interval.
– Paint containing lead, mercury and copper cannot be used because of the reactions.
– usually zinc chromates or zinc oxide based paints are used especially as primers.

8) Explain with the aid of a diagram
Yield Point
Ultimate Tensile Stress
Modulus of Elasticity

Special stress -strain curve for a ductile metal

Yield Point (elastic limit)

– a point on a stress-strain diagram (Point ‘B’ in the diagram) from which a material will completely recover to its original shape without becoming Permanently deformed.
– till at that point a material can be subjected to maximum stress with no permanent deform.

Modulus of Elasticity
– An modulus of elasticity (elastic modulus) , is the mathematical description of an object or substance’s tendency to be deformed elastically (i.e., non-permanently) when a stress is applied to it.
– The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region. (the curve from point ‘O’ to point ‘B’ in the diagram)

Ultimate Tensile Strength
– if the stress has been continually increased beyond the elastic limit, eventually a point will be reached Where the strain will increase with no increase in stress.
– indeed beyond this point loads may even be lightened without halting continuing deformation. – the greatest stress that can be applied to an object before experiencing necking is called the ultimate tensile strength

9) With the aid of the iron carbide diagram, explain any two heat treatment processes.

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A, B, C – Upper critical
D, E – Lower critical

– iron carbide diagram are drawn from the result of heat treatment of the steel.
– it illustrates the upper critical and lower critical temperatures of steels based on carbon content.
– it also indicates that if you heat steels above these two temperatures the steel undergoes a change in its properties.
– these two temperatures vary for steels based on carbon content,
– however for steels whose carbon content is 0.85 %, the upper and lower critical temperatures are the same.

Quench hardening
– it is carried out by heating the material to about 30’C above the upper critical temperature and holding it at this temperature until the structure is completely austenite. (a solid solution of carbon in iron).
– material is then quench rapidly in water.
– the resulting structure is very hard and brittle and is known as ‘martensite’. (iron-carbon material)
– if the material is cooled at a slower rate by quenching in oil, a softer structure results which is known as ‘bainite’
– in both cases (rapid and slow), the resulting material will be hard, but too brittle for general service.
Tempering
– the resulting material from the quench hardening is too brittle for normal use.
– thus that brittleness to be removed by using the method called tempering.
– tempering reduces internal stresses and also soften the material to a small extent.
– tempering is basically heating the steel after hardening to a given temperature and then cooling.
– temperature is always below the lower critical temperature.
– at different temperature, the surface oxides form different colors, known as temper colors.
– the temper color may be used to determine the relative software of the material.

• This topic was modified 2 years, 3 months ago by Admin.
• This topic was modified 2 years, 2 months ago by icedcappucino.

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