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 lements are !arbon"!hromium" !opper" Magnesium" Manganese" #ic$el" hosphorus" Silicon & Sulphur. 'he properties of Steel are dependant upon type & amount of alloying materials used. Steel Making : It involves melting of iron concentrate at a high temperature()*+++!, with co$e. 'wo basic methods used are /last 0urnace Method 'hee le 'h lect ctrric 1rc 0urn urnace ace or or In Indu duct ctio ion n 0ur 0urn nace ace met meth hod
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. 2hen wetted" it reacts violently giving out great heat. 'his is $nown 3irect 4educed Iron (34I, or 5ot /ri6uetted Iron(5/I,. xpensive 0erro 1lloys 1lloys are now added to produce a clean steel satisfying stringent classification & 6uality re6uirements. 'he steel so obtained is normally Mild Steel. !arbon content is between +.)7 & +.%. Grades of Steel 1ll members of I1!S ('he International 1ssociation 1ssociation of classification Societies, have agreed to standardie the regulations pertaining to the manufacture of steel for shipbuilding use.
0inished item carries the society:s brand clearly mar$ed on it. 'he ring around the mar$ is painted in different colours to indicate the grade of steel;rade 1 2hite ;rade / ;reen ;rade ! /lue ;rade 3 4ed ;rade
High Tensile Steel : It has greater tensile strength for e6uivalent thic$ness. 'his strength is retained even at low temperatures. 5igh tensile steels are used in large tan$ers & ore carriers. ermission has been granted by classification societies for a reduction in scantlings where high tensile steels are used. 5igh tensile steels are denoted by 15" /5" !5>>.. for the various grades. 1 vessel should have a plan indicating the location of 5igh 'ensile Steel plates used" so that repairs & renewals can be correctly carried out. The red!tion in s!antlings granted" has !o#e nder s!rtin$ in the %ake of se&eral losses of 'lk !arriers( )orrosion affe!ts 'oth ordinar$ steels as %ell as high tensile steels e*all$( After a +eriod of ti#e" the !orrosion 'eing e*al" the red!tion in thi!kness for a high tensile
steel +late as a +er!entage of original thi!kness is greater" lea&ing the HTS +late %eaker than an ordinar$ +late( 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 sub?ected to a tensile test. 3eformation- !hange of shape or dimensions of a body without alteration of its mass. lasticity - 'he ability of a metal to recover its original configuration i.e. shape & dimensions" when the stress is removed. lastic 3eformation - It is deformation of a body which occurs when a stress is applied and which disappears on removal of that stress. lasticity - is the ease with which a metal may be bent or moulded into a given shape. lasticity usually increases with rise in temperature. lastic 3eformation- 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 crac$ing or brea$ing. D!tilit$ - 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. 'he 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. 5ardness generally decreases with rise in temperature. 'he degree of hardness is measured by certain destructive & non destructive tests. roperties li$e tensile strength" ductility and elasticity may in general" also be predicted from hardness tests. Toghness - is the ability of a material to bear variable load conditions without failure. 1 material could be strong but brittle while a material which is tough has strength & resilience. )ree+ - is the property of a metal to permit small amounts of plastic deformation to occur over long periods of time at elevated temperatures. It occurs as a result of metal being stressed within the elastic limit at a particular temperature. ,atige - It is the development and propagation of crac$s in a metal as a result of great number of repeated alternating stresses. Brittle ,ra!tre- It is the fracture of a metal caused by lac$ of ductility in the crystal structure of the metal due to low temperatures. Hooke-s La% - 0or an elastic body" strain is proportional to stress. 2hen a body changes its shape within its elastic limits" the ratio stress@strain is a constant A
B
Stress-Strain Diagram ;raph 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. ast 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. Pro+erties of a good Shi+/'ilding Steel : a, 'he carbon content should be between +.)7 & +.%. b,It should be easily welded & chemical composition should be suitable for flame cutting. c, It should be ductile. d,'he composition should be homogenous to prevent any flaws or wea$ness in the metal. e, It should be resistant to corrosion. f, It should be available at a reasonable cost. g,'he yield point should be high. ,ra!tres : Stress fractures may be initiated by a small crac$ or notch in the plate.
!old conditions increase chances of a brittle fracture. Mild Steel plates used in construction of 'itanic became brittle at ++!(because of high sulphide content," 2hereas the Mild steel plates used in construction of ships today will achieve same brittleness at E++! to *++!. Mild Steel Unsita'le for Li*efied Gas )arriers: Steels of grade 3 & are acceptable for temperatures upto + +!. 2here low temperatures are encountered" carbonmanganese steels having manganese content +.*).E% and small percentages of #ic$el"!hromium" Molybdenum" !opper & Fanadium are used upto temperatures of 77+!. 0or lower temperatures" nic$el steel (nic$el content 9%, is used for temperatures upto )E7+!. Pig Iron : It is raw material for cast & 2rought iron. It is obtained from sponge iron after same has been treated to loose it:s water affinity & has become more stable.
)ast Iron 0 Steel : It is produced by melting pig iron with layers of co$e mixed with a little limestone & then putting it in a mould. !arbon content is around =G%. It has high strength and can withstand severe wear & tear but is brittle & liable to fracture under shoc$. 'o improve its shoc$ resilience it is heat treated. It is sed in Stern ,ra#es 0 P#+ !asings 0 )$linder Heads" 1indlass Dr#s0g$+s$" 2al&e 'odies et!( %here intri!ate sha+es are in&ol&ed( 1roght Iron 0 Steel ,orgings :
It is manufactured by melting pig iron with silica in a coal fired furnace. It is then drawn and beaten into shape while still hot. 1 heat treatment is given afterwards. 'he final product is extremely ductile & free from brittleness. It is sed for An!hors" An!hor !a'les et!( Use of Al#ini# in shi+'ilding:
1luminium alloys are tested & graded by classification society surveyors in the same manner as mild steel. Ad&antages: ), Cight weight. Its density is =.*= t@cum as compared to *.8G t@cum for Steel. Saving in deadweight" leading to more cargo carrying capability. 1 lot of vessels now have their superstructures made of aluminium. 'his has lead to lowering of centre of gravity. 'herefore improved stability. =,Cower hull weight re6uires less power for propulsion or more speed for the same power. Many fast ferries" hovercrafts & catamarans have their entire hull built of aluminium alloy. ,Stregthwise comparable to steel. 1lso it is comparatively more durable at lower temperatures than steel. Steel becomes brittle at low temperatures" forming crac$s which rapidly propagate. 1luminium does not have this disadvantage" which ma$es it useful as a tan$ material on C; and C#; carriers. G,1luminium has high corrosion resistant properties. 7,1luminium is non magnetic. #on existence of induced magnetism benefits the ship:s compass & other such e6uipment.'his 6uality is particularly useful in warships ma$ing them immune to magnetic mines.
Disad&antages: ), 1luminium has low melting point (E79 +!, as compared to steel()7+++!,. In normal conditions this temperature is sufficiently high. 5owever in case of fire the aluminium structure could melt & collapse.'herefore fire protection regulations are more stringent in these vessels. 1luminium bul$heads on passenger vessels are to be insulated to a fire resistant standard e6uivalent to steel bul$heads. Machinery casings must be made of steel & lifeboat davits should not be made of aluminium alloy components. =,3ifficulty in welding aluminium is another factor against its use in shipbuilding. 'he metals affinity for oxygen causes aluminium alloys in the molten state to readily absorb oxygen causing excessive corrosion" thereby wea$ening the metal. 2elding of aluminium has to be done in a gas shield and only Metal Inert ;as(MI;, and 'ungsten Inert ;as('I;, welding processes are suitable. On board welding thus becomes a costly affair. ,It is very expensive. 1luminium at H)=7+ per tonne " is almost ten times as expensive as steel. G,1luminium has to be insulated from steel to prevent a galvanic cell being set up and bimetallic corrosion ta$ing place. If not" it will act as an anode and get wasted away in the galvanic reaction" protecting the steel. It may be concluded that while use of aluminium is feasible for hatches" superstructures" gas tan$s & even for the entire hulls of hovercrafts & ferries" until the cost is brought down" steel will continue to dominate the shipbuilding industry. Ho% strength is +reser&ed in Al#ini# s+erstr!tres in !ase of fire :
Steel or Other Equivalent Material. 2here the words steel
or other e6uivalent material occur" e6uivalent material means any material which" by itself or due to insulation provided" has structural and integrity properties e6uivalent to steel at the end of the applicable fire exposure to the standard fire test (e.g. al#ini# alloy with appropriate insulation,.
standard fire testD means a test in which a specimen of the relevant B1D !lass or B/D !lass 3ivision" having an exposed surface area of not less than G.E7 s6uare metres and a bul$head height or dec$ length of =.GG metres" resembling as closely as possible the intended construction and including where appropriate at least one ?oint" 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 1t the end of the first 7 minutes 77EJ! B B B B B )+ minutes E79J! B B B B B )7 minutes *)8J! B B B B B + minutes 8=)J! B B B B B E+ minutes 9=7J! • •
)lass A 1re divisions forming bul$heads and dec$s that areK !onstructed of steel or e6uivalent suitably stiffened revent passage of smo$e and flame to the end of one hour standard fire test – – –
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Insulated using noncombustible material so that average temperature on unexposed side does not rise above )G+o! and point temperature above )8+o!. 'he time duration for which the bul$head complies with this " governs its class A/34/// 34 Min A/54/// 54 Min A/67 ///67 Min A/4 ///4 Min
THE ,IRE TEST ,8R A 9 B )LASS DI2ISI8NS • •
)lass B 'hese are divisions formed by bul$heads" dec$s" ceilings and lining revent passage of flame for first half hour of standard fire test Insulated so average unexposed side temperature does not rise more than )G+o! above original and –
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no single point rises more than ==7o! above original . B/67 /67 Min B/4 /4 Min !onstructed of noncombustible material and all materials entering the construction are similarly noncombustible except where permitted.
)lass ) 'hese are divisions constructed of approved non combustible materials. !ombustible veneers are allowed where they meet other criteria. (Feneer refers to 2ooden 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. Lnless they are in electrical contact" there can be no galvanic couple set up. 'his can be done using plastic or another insulator to separate aluminium and steel parts. 1nother way is to $eep 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. !oating 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,. 'his 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.
