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, 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 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 manufactured and tested in accordance with Lloyd’s 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 Lloyd’s Rules for the Manufacture, Testing and Certification of Materials. The higher strength steels are available in three strength levels, 32,36, and 40 (kg/mm2) when supplied in the as rolled or normalized condition.Provision is also made for material with six higher strength levels, 42, 46, 50,55, 62 and 69 (kg/mm2) 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.
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