Page 57 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 46 Wednesday, October 18, 2006 8:02 AM
46 Ship Construction

FLAT OFFSET ANGLE
BAR BULB BAR
PLATE

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งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี

TEE CHANNEL TEE
BULB BAR BAR

FIGURE 5.1 Steel sections of shipbuilding
ึ

Ship classification societies originally had varying specifications for steel:
but 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 Lloyds 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. Lloyds Register
requirements for Grades A, B, D and E steels may be found in Chapter 3 of
Lloyds Rules for the Manufacture, Testing and Certification of Materials.

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; it does, however, lead to larger deflections. The weldability
of higher tensile steels is an important consideration in their application in
ship structures and the question of reduced fatigue life with these steels has