Page 58 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 47 Wednesday, October 18, 2006 8:02 AM
Steels 47
been suggested. Also, the effects of corrosion with lesser thicknesses of
plate and section may require more vigilant inspection.
Higher tensile steels used for hull construction purposes are manu-
factured and tested in accordance with Lloyds Register requirements. Full
specifications of the methods of manufacture, chemical composition, heat
treatment, and mechanical properties required for the higher tensile steels
are given in Chapter 3 of Lloyds Rules for the Manufacture, Testing and
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Certification of Materials. The higher strength steels are available in three
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strength levels, 32, 36, and 40 (kg/mm ) when supplied in the as rolled or
normalized condition. Provision is also made for material with six higher
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strength levels, 42, 46, 50, 55, 62 and 69 (kg/mm ) when supplied in the
quenched and tempered condition. Each strength level is subdivided into
four grades, AH, DH, EH and FH depending on the required level of
notch-toughness.

Corrosion Resistant Steels

Steels with alloying elements, that give them good corrosion resistance and
colloquially referred to as stainless steels are not commonly used in ship
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structures, primarily because of their higher initial and fabrication costs.
Only in the fabrication of cargo tanks containing highly corrosive cargoes
might such steels be found.
For oil tankers the inner surfaces, particularly the deckhead and bot-
tom, are generally protected by high cost corrosion resistant coatings that
require vigilant inspection and maintenance (see Chapter 27). A recent
development in the manufacture of an alloyed shipbuilding steel with
claimed improved corrosion resistance properties and its approval by
Lloyds Register for use in certain cargo tanks of a 105,000 dwt tanker
indicate that in the future the need to coat oil cargo tanks might be dis-
pensed with.

Steel Sandwich Panels

As an alternative to conventional shipyard fabricated stiffened steel plate
structures proprietary manufactured steel sandwich panels have become
available and used on ships where their lighter weight was important. Such
panels consist of a steel core in the form of a honeycomb with flanges to
which the external steel sheets are resistance (spot) or laser (stake) welded.
Early use of these bought in steel sandwich panels was primarily for non-
hull structures in naval construction where their light weight was important.
Also when fabricated using stainless steel their corrosion-resistance and
low maintenance properties have been utilised.