If the force applied is within the “Yield” of the metal, it will return to its original shape.

• When steel is cooled slowly, it has longer time for the grain to grow, hence larger grain size is produced.
• When the grain size is large, the steel becomes softer and more ductile.
• When a steel is cooled rapidly, the grain size become small, and the steel become hard and brittle.
• Ship building industry, we generally use ship steel with fine grain for balance between:
  Hardness.
  Ductility
  Strength

Steel grade : Grade of steel

.steel grade

.grade of steel

.grades of steel

Based on carbon content there are various types of steel.

1. Low Carbon Steel is steel with carbon content 0.05 – 0.3%
2. Medium Carbon Steel is steel with carbon content 0.3 – 0.8%
3. High Carbon Steel is steel with carbon content 0.8-2.0%
4. Ultra-High Carbon Steel is steel with carbon content 2.0-4.0%
5. Cast Iron is steel with carbon content >4.0%

.ships material   .ship material

.Mild Steel

Mild steel or low carbon steel (0.05 to 0.3% carbon) is used as a ship structural material. It has the advantage of having a relatively good strength weight ratio. whilst the cost is low.

There are four grades of steel in common use. They are specified by the Classification Societies as Grades A. B, D E. They arebringgraded largely upon their degree of notch toughness.  

Grade A has the least resistance to brittle fracture whilst Grade E is termed •extra notch tough•.

Grade D has sufficient resistance to cracks that’s why it is used extensively for main structural material.

Which grade of steel will be used in which part of the ship depends upon the thickness of the material and which part of the ship the material going to be used and the stress of that of the ship.

For example. the bottom Shell plating of a Ship within the midship portion of the ship will have the following grade requirements.

Plate thickness              Grade

up to 20.5 mm               A

20.5 to 25.5mm            B

25.5 mm to 40 mm       D

Above 40 mm                E

The tensile strength of the different grades remains constant at between 400 MN/m2 and 490 MN/m2.

The main difference of the grades are in the chemical composition of the steel. Chemical composition of grade D and E are such that they improve the impact strength of D and E Steels. Impact resistance is measured by means of a Charpy test in which specimens may be tested at a variety of temperatures. The minimum values required by Lloyds Register are

Type Of Steel   Temperature                 Impact Resistance

B                                        0° C                     27 joules

D                                       0°                        47 joules

E                                        -40°C                  27 joules

Higher tensile steels

As oil tankers and bulk carriers increased in size the thickness of Steel required for the main longitudinal Strength  members also increased. To reduce the thickness of material higher tensile strength is used.

These steels are designated AH. BH. DH and EH and may be used to replace the normal grades for any given Structural member. Thus, a bottom Shell Plate amidships may be 30 mm in thickness of grade DH steel.

The tensile strength is increased to between 490 MN/m2 and 620 MN /m2

High Tensile Steel used where it is most effective. For example, in upper deck plating and longitudinais, and bottom shell plating and longitudinals.

To weld HTS, low hydrogen electrodes are used, with some preheating.

Arctic D steel

If any part of the ship is going to be exposed at a particularly low temperature. then the normal grades Of steel are not suitable. A special type of steel. known as Arctic D. has been developed for this purpose. It has a higher tensile strength than normal mild steel. Its most important quality it can maintain a minimum of 40J at —55° C in a Charpy impact test.

Aluminum

• Pure aluminum is soft so it is alloyed with ni,cr,mo,zn,cu etc
• Tensile strength 260 MN/m2
• Reduced weight thus reduce fuel consumption
• Increase the deadweight
• Used in accommodation and small passenger ship and boat

Tensile Strength	capacity of object to resist deformation under tension, as a result of tensile force.
Ductility	capacity of object to undergo permanent changes in shape, without loss of strength or rupture.
Toughness	capacity of object to absorb energy then deformed, but without fracturing.
Hardness	capacity of object to resist surface deformation, penetration & indentation as a result of abrasion, drilling, impact, scratching or wear.