Types of Welding in Shipbuilding

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Steels are alloys of iron, with properties dependent upon the

type and amounts of alloying materials used .
Steels may be broadly considered as alloys of iron and carbon,
the carbon percentage varying from about 0.1% in mild steels to
about 1.8% in some hardened
• Steel for hull construction purposes is usually mild steel
containing 0.15–0.23% carbon and a reasonably high
manganese content.
• Both sulfur and phosphorus in the mild steel are kept to a
minimum (less than 0.05%). Higher concentrations of both are
• detrimental to the welding properties of the steel, and cracks
can develop during the rolling process if the sulfur content is
high.
• Steel for a ship classed with Lloyd’s Register is produced by an
approved manufacturer, and inspection and prescribed tests
are carried out at the steel mill before dispatch.
• All certified materials are marked with the society’s brand and
other particulars as required by the rules.
• Ship classification societies originally had varying specifications
for steel.
• However, in 1959, the major societies agreed to standardize their
requirements in order to reduce the required grades of steel to a
minimum.
• There are now five different qualities of steel employed in
merchant ship construction and now often referred to as IACS
steels.
• These are graded A, B, C, D, and E.
• Grade A being an ordinary mild steel to Lloyd’s Register
requirements and generally used in shipbuilding.
• Grade B is a better quality mild steel than Grade A and specified
where thicker plates are required in the more critical regions.
Grades C, D, and E possess increasing notch-tough characteristics,
• Grade C being to American Bureau of Shipping requirements.
High tensile steels
Steels having a higher strength than that of mild steel are employed
in the more highly stressed regions of large tankers, container ships,
and bulk carriers.
Use of higher strength steels allows reductions in thickness of deck,
bottom shell, and framing where fitted in the midships portion of
larger vessels.
Benefits arising from the use of these steels in ship construction
include reduced structural weight, since smaller sections may be
used; larger unit fabrications are possible for the same weight and
less welding time.
Tensile strength: This is the main single criterion with reference to
metals. It is a measure of the material's ability to withstand the
loads upon it in service.
Terms such as stress, strain, ultimate tensile strength, yield stress
and proof stress are all different methods of quantifying the tensile
strength of the material.
The two main factors affecting tensile strength are the carbon
content of the steel and its heat treatment following manufacture.
Ductility: The ductility is a property of a material which enables it to
be drawn out into a thin wire.
Mild steel, copper, aluminium are the good examples of a ductile
material.
This is the ability of a material to undergo permanent changes in
shape without rupture or loss of strength. It is particularly
important where metals undergo forming processes during
manufacture
Hardness:
• This is a measure of the workability of the material. It is used as
an
• assessment of the machinability of the material and its resistance
to abrasion.
• The resistance of a material to force penetration or bending is
hardness.
• The hardness is the ability of a material to resist scratching,
abrasion, cutting or penetration.
• Hardness indicates the degree of hardness of a material that can
be imparted particularly steel by the process of hardening.
• It determines the depth and distribution of hardness is introduce
by the quenching process.
Hardness:
Toughness
It is the property of a material which enables it to withstand shock
or impact.
Toughness is the opposite condition of brittleness.
.Manganese steel, wrought iron, mild steel etc are examples of
toughness materials.
Brittleness
The brittleness of a property of a material which enables it to
withstand permanent deformation.
Cast iron, glass are examples of brittle materials. They will break
rather than bend under shock or impact. Generally, the brittle
metals have high compressive strength but low in tensile strength.
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Stress and strain
Stress or intensity of stress, its correct name, is the force acting on a
unit area of the material.
Strain is the deforming of a material due to stress.
Stress and strain of mild steel
Stress and strain of mild steel
Stress fracture: Stress fracture may be initiated by a small crack or
notch in a plate .
Brittle fracture:
Brittle fracture is an unstable failure process that occurs in fibre–
polymer composite materials, metals with high strength and low
ductility, and in some metal types at low temperature (i.e. below
the ductile/brittle transition temperature).
Brittle fracture:
• Brittle fracture is an unstable failure process that occurs in fibre–
polymer composite materials, metals with high strength and low
ductility, and in some metal types at low temperature (i.e. below
the ductile/brittle transition temperature).
• Because of Brittle fracture at low temperature, mild steel is
unsuitable for the very low temperatures involved in the
containment of liquefied gases.
Casting and forging
Casting and forging
Casting and forging
• The larger castings used in ship construction are usually
manufactured from carbon or carbon manganese steels.
• Examples of large castings are the sternframe, bossings, A-
brackets and parts of the rudder.
• The examples mentioned may also be manufactured as forgings.
Aluminium alloys
• The increasing in use of aluminium alloy has resulted from its
several advantages over steel.
• Aluminium is about one·third the weight of steel for equivalent
volume of material. The use or-aluminium alloys in a structure can
result in reduction of the 60% weight of an equivalent steel
structure. This reduction in weight. particularly in the upper
regions of the structure, can improve the Stability of vessel. This
follows from the lowering of the vessel's centre of gravity.
• The other two advantages of aluminum being a high resistance to
corrosion and its nonmagnetic properties. The nonmagnetic
properties can have advantages in warships and locally in the way
of the magnetic compass, but they are generally of little
importance in merchant vessels.
Aluminium alloys
Fire protection
It was considered necessary to mention when discussing aluminum
alloys that fire protection is more critical in ships in which this
material is used because of thelow melting point of aluminum
alloys.
During a fire the temperatures reached may be sufficient to cause a
collapse of the structure unless protection is provided.
The insulation on the main bulkheads in passenger ships will have
to be sufficient to make the aluminum bulkhead equivalent to a
steel bulkhead for fire purposes.
For the same reason it is general practice to fit steel machinery
casings through an aluminum superstructure on cargo ships.
Aluminium alloys
Special precautions against corrosion
• Where aluminium alloys join the steel structure, special insulating
• arrangements must be employed to avoid galvanic corrosion
where the metals meet.
• Where rivets are used, they should be manufactured from a
corrosion-resistant alloy

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Steels are alloys of iron, with properties dependent upon the

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