SHIPBUILDING MATERIALS Steel is the most widely used material in shipbuilding.  It is a mixture of various elements.It is an alloy.  Main component is iron with content around 98%.  Other Elements are Carbon,Chromium, Copper, Magnesium, Manganese, Nickel, Phosphorus, Silicon & Sulphur.  The properties of Steel are dependant upon type & amount of alloying materials used. Steel Making : It involves melting of iron concentrate at a high temperature(17000C) with coke. Two basic methods used are: - Blast Furnace Method - The Electric Arc Furnace or Induction Furnace method Sponge iron is produced from iron ore by reducing it with coal or natural gas. It is so named because it has a great affinity for water. When wetted, it reacts violently giving out great heat. This is known Direct Reduced Iron (DRI) or Hot Briquetted Iron(HBI). Expensive Ferro Alloys are now added to produce a clean steel satisfying stringent classification & quality requirements. The steel so obtained is normally Mild Steel. Carbon content is between 0.15 & 0.33%. Grades of Steel  All of IACS (The International Association of classification Societies) have agreed to standardize the regulations pertaining to the manufacture of steel for shipbuilding use.

 Finished item carries the society’s brand clearly marked on it.  The ring around the mark is painted in different colours to indicate the grade of steel:  Grade A ---- White  Grade B ---- Green  Grade C ---- Blue  Grade D ---- Red  Grade E ---- Yellow  Grades A & B are ordinary mild steel  Grades C & D are notch Tough  Grade E is extra notch tough. It is used in areas of maximum stress concentrations and bending moments such as the bilge keel, sheer strake and in midships area of the hull of vessels greater than 200 metres. High Tensile Steel : It has greater tensile strength for equivalent thickness. This strength is retained even at low temperatures. High tensile steels are used in large tankers & ore carriers. Permission has been granted by classification societies for a reduction in scantlings where high tensile steels are used. High tensile steels are denoted by AH, BH, CH…….. for the various grades. A vessel should have a plan indicating the location of High Tensile Steel plates used, so that repairs & renewals can be correctly carried out. The reduction in scantlings granted, has come under scrutiny in the wake of several losses of bulk carriers. Corrosion affects both ordinary steels as well as high tensile steels equally. After a period of time, the corrosion being equal, the reduction in thickness for a high tensile

steel plate as a percentage of original thickness is greater, leaving the HTS plate weaker than an ordinary plate. Definitions:  Stress : It is the force exerted on a body  Strain : It is the effect of stress on a body. It results in deformation of the body.It may also be defined as a ratio of extension (elongation) to the Original length of a material that is subjected to a tensile test.  Deformation: Change of shape or dimensions of a body without alteration of its mass.  Elasticity : The ability of a metal to recover its original configuration i.e. shape & dimensions, when the stress is removed.  Elastic Deformation : It is deformation of a body which occurs when a stress is applied and which disappears on removal of that stress.  Plasticity : is the ease with which a metal may be bent or moulded into a given shape. Plasticity usually increases with rise in temperature.  Plastic Deformation: It is that part of the deformation which remains after the stress has been removed.  Malleability : is the property of a metal of becoming permanently flattened or stretched by hammering or rolling without cracking or breaking.  Ductility : It is the property of a metal which permits it to be drawn into wires. If a bar or wire is drawn out lengthwise the amount of extension or reduction in cross section area measures its ductility.  Strength : It is the ability of a metal to bear loads without strain or deformation. The higher the load bearing capacity, the stronger is the metal.

 Hardness : is the property of a material to resist indentation, abrasion or wear by some other body. Hardness generally decreases with rise in temperature. The degree of hardness is measured by certain destructive & non destructive tests. Properties like tensile strength, ductility and elasticity may in general, also be predicted from hardness tests.  Toughness : is the ability of a material to bear variable load conditions without failure. A material could be strong but brittle while a material which is tough has strength & resilience.  Creep : is the property of a metal to permit small amounts of plastic deformation to occur over long periods of time at elevated temperatures. It occurs as a result of metal being stressed within the elastic limit at a particular temperature.  Fatigue : It is the development and propagation of cracks in a metal as a result of great number of repeated alternating stresses.  Brittle Fracture: It is the fracture of a metal caused by lack of ductility in the crystal structure of the metal due to low temperatures.  Hooke’s Law : For an elastic body, strain is proportional to stress. When a body changes its shape within its elastic limits, the ratio stress/strain is a constant = Young’s modulus of Elasticity. The Stress-strain curve if plotted would be a straight line curve.  Limit of Proportionality : It is the highest stress prior to which deformation increases proportionally to the load applied.  Yield Point : The point at which a body ceases to be elastic and becomes permanently distorted or set is termed the yield point. The load which is applied to cause this is called a

“Yield Point Load”. The body is then said to have undergone plastic deformation or flow.  Tensile strength is indicated by the maxima of a stressstrain curve and, in general, indicates when necking will occur.Its value does not depend on the size of the test specimen. It is, however, dependent on the preparation of the specimen and the temperature of the test environment and material. Tensile strength, along with elastic modulus and corrosion resistance, is an important parameter of engineering materials used in structures and mechanical devices.  Ultimate Tensile strength : It is the highest stress preceeding fracture of a sample.  Elongation : The ratio of increase in length of a sample, produced by tensile stresses, to its original length is termed as elongation. Stress-Strain Diagram Graph of stress as a function of strain. It can be constructed from data obtained in any mechanical test where load is applied to a material, and continuous measurements of stress and strain are made simultaneously. It is constructed for

compression, tension and torsion tests.

Yield Point

Stress-Strain curve for a material such as mild steel.

The total area under the curve indicates how tough the material is - how much energy it can absorb while deforming plastically and not breaking. The stress-strain curve for each material is different and unique. From these curves it is possible to extract a number of the materials properties.

Stress-Strain curves for different materials. The stress-strain curve for concrete is nearly straight and then stops. This shows a brittle material. Cast iron is also a brittle material. The mild steel curve extends further and the material continues to strain (stretch if under tension) with the stress remaining relatively constant. This shows a high ductility. Properties of a good Ship-building Steel : a) The carbon content should be between 0.15 & 0.33%. b)It should be easily welded & chemical composition should be suitable for flame cutting. c) It should be ductile. d)The composition should be homogenous to prevent any flaws or weakness in the metal. e) It should be resistant to corrosion. f) It should be available at a reasonable cost. g)The yield point should be high. Fractures : Stress fractures may be initiated by a small crack or notch in the plate.

Cold conditions increase chances of a brittle fracture. Mild Steel plates used in construction of Titanic became brittle at 00C(because of high sulphide content), Whereas the Mild steel plates used in construction of ships today will achieve same brittleness at -600C to -700C. Mild Steel Unsuitable for Liquefied Gas Carriers: Steels of grade D & E are acceptable for temperatures upto 00C. Where low temperatures are encountered, carbon-manganese steels having manganese content 0.7-1.6% and small percentages of Nickel,Chromium, Molybdenum, Copper & Vanadium are used upto temperatures of -550C. For lower temperatures, nickel steel (nickel content 9%) is used for temperatures upto -1650C. Pig Iron : It is raw material for cast & Wrought iron. It is obtained from sponge iron after same has been treated to loose it’s water affinity & has become more stable.

Cast Iron / Steel : It is produced by melting pig iron with layers of coke mixed with a little limestone & then putting it in a mould. Carbon content is around 2-4%. It has high strength and can withstand severe wear & tear but is brittle & liable to fracture under shock. To improve its shock resilience it is heat treated. It is used in Stern Frames / Pump casings / Cylinder Heads, Windlass Drums/gypsy, Valve bodies etc. where intricate shapes are involved. Wrought Iron / Steel Forgings :

It is manufactured by melting pig iron with silica in a coal fired furnace. It is then drawn and beaten into shape while still hot. A heat treatment is given afterwards. The final product is extremely ductile & free from brittleness. It is used for Anchors, Anchor cables etc. Use of Aluminium in shipbuilding: Aluminium alloys are tested & graded by classification society surveyors in the same manner as mild steel. Advantages: 1) Light weight. Its density is 2.72 t/cum as compared to 7.84 t/cum for Steel. Saving in deadweight, leading to more cargo carrying capability. A lot of vessels now have their superstructures made of aluminium. This has lead to lowering of centre of gravity. Therefore improved stability. 2)Lower hull weight requires less power for propulsion or more speed for the same power. Many fast ferries, hovercrafts & catamarans have their entire hull built of aluminium alloy. 3)Stregthwise comparable to steel. Also it is comparatively more durable at lower temperatures than steel. Steel becomes brittle at low temperatures, forming cracks which rapidly propagate. Aluminium does not have this disadvantage, which makes it useful as a tank material on LPG and LNG carriers. 4)Aluminium has high corrosion resistant properties. 5)Aluminium is non magnetic. Non existence of induced magnetism benefits the ship’s com & other such equipment.This quality is particularly useful in warships making them immune to magnetic mines.

Disadvantages: 1) Aluminium has low melting point (6590C) as compared to steel(15000C). In normal conditions this temperature is sufficiently high. However in case of fire the aluminium structure could melt & collapse.Therefore fire protection regulations are more stringent in these vessels. Aluminium bulkheads on enger vessels are to be insulated to a fire resistant standard equivalent to steel bulkheads. Machinery casings must be made of steel & lifeboat davits should not be made of aluminium alloy components. 2)Difficulty in welding aluminium is another factor against its use in shipbuilding. The metals affinity for oxygen causes aluminium alloys in the molten state to readily absorb oxygen causing excessive corrosion, thereby weakening the metal. Welding of aluminium has to be done in a gas shield and only Metal Inert Gas(MIG) and Tungsten Inert Gas(TIG) welding processes are suitable. On board welding thus becomes a costly affair. 3)It is very expensive. Aluminium at $1250 per tonne , is almost ten times as expensive as steel. 4)Aluminium has to be insulated from steel to prevent a galvanic cell being set up and bimetallic corrosion taking place. If not, it will act as an anode and get wasted away in the galvanic reaction, protecting the steel. It may be concluded that while use of aluminium is feasible for hatches, superstructures, gas tanks & even for the entire hulls of hovercrafts & ferries, until the cost is brought down, steel will continue to dominate the shipbuilding industry. How strength is preserved in Aluminium superstructures in case of fire : Steel or Other Equivalent Material. Where the words "steel

or other equivalent material" occur, "equivalent material" means any material which, by itself or due to insulation provided, has structural and integrity properties equivalent to steel at the end of the applicable fire exposure to the standard fire test (e.g. aluminium alloy with appropriate insulation).

standard fire test” means a test in which a specimen of the relevant “A” Class or “B” Class Division, having an exposed surface area of not less than 4.65 square metres and a bulkhead height or deck length of 2.44 metres, resembling as closely as possible the intended construction and including where appropriate at least one t, is exposed in a test furnace to a series of time temperature relationships defined by a smooth curve drawn through the following temperature points measured above the initial furnace temperature At the end of the first 5 minutes 556°C “ “ “ “ “ 10 minutes 659°C “ “ “ “ “ 15 minutes 718°C “ “ “ “ “ 30 minutes 821°C “ “ “ “ “ 60 minutes 925°C • Class A • Are divisions forming bulkheads and decks that are; – Constructed of steel or equivalent – suitably stiffened – Prevent age of smoke and flame to the end of one hour standard fire test

– Insulated using non-combustible material so that average temperature on un-exposed side does not rise above 140oC and point temperature above 180oC. The time duration for which the bulkhead complies with this , governs its class A-60--- 60 Min A-30--- 30 Min A-15 ---15 Min A-0 ---0 Min

THE FIRE TEST FOR A & B CLASS DIVISIONS

• Class B • These are divisions formed by bulkheads, decks, ceilings and lining – Prevent age of flame for first half hour of standard fire test – Insulated so average un-exposed side temperature does not rise more than 140oC above original and

no single point rises more than 225oC above original . – B-15 -15 Min B-0 -0 Min – Constructed of non-combustible material and all materials entering the construction are similarly non-combustible except where permitted.

• Class C • These are divisions constructed of approved noncombustible materials. Combustible veneers are allowed where they meet other criteria. (Veneer refers to Wooden laminates covering the divisions for good finish)

Special precautions against corrosion when aluminium alloy is connected to the steelwork: 

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One way is to electrically insulate the two metals from each other. Unless they are in electrical , there can be no galvanic couple set up. This can be done using plastic or another insulator to separate aluminium and steel parts. Another way is to keep the metals dry and/or shielded from ionic compounds (salts, acids, bases), for example by painting or encasing the protected metal in plastic or epoxy, and allowing them to dry. Coating the two materials or if it is not possible to coat both, the coating shall be applied to the more noble, the material with higher potential(Steel). This is necessary because if the coating is applied

only on the more active material, in case of damage of the coating there will be a large cathode area and a very small anode area, and for the area effect the corrosion rate will be very high.