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Steel used in Ship Building

Transition from Wood to Steel

18th century- Wooden ships were used. No material was easier to be worked with available tools. Low strength, small ships(<60 m)

1807 - Steam propulsion introduced

1830- Iron ships were build. Riveting was joining technology. Bigger ships possible.

Contd.

1870- Steel ships introduced. riveting contd. Steel made of Bessemer process. (costly and brittle)

1890- Welding introduced in small scale.

1920- Welding introduced for repairs.

1930- All welded tugs. Steel made of open hearth process. Improves cost and quality.

2nd world- All welded steel ships built in large numbers. war-

Steel

Steel may be broadly considered as alloy of iron and carbon, the carbon percentage varying from 0.1 per cent in mild steels to about 1.8 per cent in some Hardened steels

TYPES OF STEEL

MILD STEEL & LOW CARBON STEEL Mild steel is the most common form of steel because

its price is relatively low while it provides material

properties that are acceptable for many applications. Low carbon steel contains approximately 0.05–0.15%

carbon and mild steel contains 0.16–0.29% carbon, therefore it is neither brittle nor ductile

  Mild steel has a relatively low tensile strength, but it

is cheap and malleable

Medium carbon steel Approximately 0.30–0.59% carbon

content. Balances ductility and strength and has good wear resistance; used for large parts, forging and automotive components.

High carbon steel Approximately 0.6–0.99% carbon content.Very

strong, used for springs and high-strength wire

Ultra-high carbon steel Approximately 1.0–2.0% carbon content and

can be tempered to great hardness

High Tensile Steel 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.

The weldability of higher tensile steels and reduced

fatigue life with these steels is an important consideration in their application in ship structures .

The effects of corrosion with lesser thicknesses of plate and section may require more vigilant

inspection

GRADES OF STEEL

STRENGTH CATEGORY GRADE

NORMAL MS STRENGTH A,B,D,E

HT STEEL STRENTH LEVEL AH,DH,EH 32 Kg/mm2

HT STEEL STRENTH LEVEL AH,DH,EH 34 Kg/mm2

HT STEEL STRENTH LEVEL AH,DH,EH 36 Kg/mm2

CHEMICAL COMPOSITION

A B D E

C % 0.23 max 0.21 max 0.21 max 0.18 max

Mn % 0.58 min 0.80 min 0.6 min 0.7 min

Si % 0.5 max 0.5 max 0.1-0.5 0.1-0.5

S % 0.04 max 0.04 max 0.04 max 0.04max

P % 0.04 max 0.04 max 0.04 max 0.04 max

PROCESS

OPEN HEARTH PROCESS

ELECTRIC FURNACE PROCESS

OXYGEN PROCESS

BESSEMER CONVERTOR PROCESS

FLOW DIAGRAM

HEAT TREATMENT OF STEEL

The properties of steels may be altered greatly by the heat treatment to which the steel is subsequently subjected. These heat treatments bring about a change in the mechanical properties principally by modifying the steel’s

structure. Those heat treatments which concern shipbuilding materials are described.

ANNEALING

This consists of heating the steel at a slow rate

to at temperature of say 850 °C to 950 °C, and then cooling it in the furnace at a very slow rate.

The objects of annealing are to relieve any internal stresses, to soften the steel, or to bring the steel to a condition suitable for a subsequent heat treatment.

NORMALIZING

This is carried out by heating the steel slowly to a temperature similar to that for annealing and allowing it to cool in air.

The resulting faster cooling rate

produces a harder stronger steel than annealing, and also refines the grain size.

QUENCHING (HARDENING)

Steel is heated to temperatures similar to that for annealing and normalizing,

and then quenched in water or oil. The fast cooling rate produces a very

hard structure with a higher tensile strength.

TEMPERING

Quenched steels may be further heated to a temperature somewhat between atmospheric and 680 °C, and some alloy steels are then cooled fairly rapidly by quenching in oil or water.

The object of this treatment is to relieve the severe internal stresses produced by the original hardening process and to make the material less brittle but retain the higher

tensile stress.

PROPERTIES

COMPARISON

Strengths Yield stress (N/mm2) UTS(N/mm2)

Mild Steel 250 400

High Tensile Steel 1000 1500

Stainless Steel 316 325 575

MATERIAL CLASS

SELECTION OF GRADE

ADVANTAGES OF STEEL

Structural steel’s low cost, strength, durability, design flexibility, adaptability and recyclability make it a good material of choice for Ships

Steel has the highest strength to weight ratio

of any building material.

Provides consistent material quality; because it is produced in strict accordance with national standards, there is no regional variance in quality.

Fire resistant, does not burn and will not contribute

fuel to the spread of fire.

Inorganic; it does not rot, split, crack.

Produces less scrap and waste (2% for steel vs. 15-20% for wood).

Scrap is 100% recyclable can be recycled indefinitely without losing any of its qualities.

Slower aging process with less maintenance.

Enhanced resale value.

Environment friendly advantages

Almost half the world’s steel production now takes place in electric plants that operate exclusively with recycled scrap and generate no CO2 emissions.

The by-products arising from steel production are all re-used. For example, slag is employed as a high-value mineral material for highway construction, as ballast, and for the manufacture of cement.

 

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