168 Steels:MetallurgyandApplications
9 Lloyd's Register of Shipping
9 Nippon Kaiji Kyokai
9 Registro Italiano NavaleWhereas each of these societies publishes its own design rules and steel
specifications, they collaborate closely through the International Association of
Classification Societies (IACS). Therefore there is a high degree of uniformity
in steel specifications in terms of composition, tensile properties and impact
resistance. This collaboration stems from the mid-1950s when there was an urgent
need to harmonize the approaches that had been taken individually in formulating
steel specifications with improved resistance to brittle fracture.
Until the early 1940s, shipbuilding had been based on riveted construction and
only one grade of steel was in common use, namely 'shipbuilding quality'. The
steel was specified very simply in terms of tensile and bend tests and no limi-
tations were placed on chemical composition. However, during World War II,
the emergency shipbuilding programme in the United States demanded higher
production rates for the so-called Liberty Ships and brought about the change
from riveted to welded construction. This necessitated consideration of steel
composition in relation to weldability but of far greater significance was the
emergence of the problem of brittle fracture. In some instances, brittle fracture
led to the catastrophic break-up of cargo vessels at sea and also produced the
spectacular failure of the SS Schenectady in January 1943, whilst lying alongside
the outfitting berth of a shipyard. Whereas it was shown that structural perfor-
mance could be enhanced by means of improved design in critical elements, it
was also evident that there was an urgent need to improve the toughness char-
acteristics of ship plate steel.
In the years following World War II, each of the classification societies took
independent action in formulating steel specifications, but by 1952 pressure from
shipbuilders, owners and steelmakers brought about the initial discussions for
the harmonization of specifications. A detailed account of these discussions was
published by Boyd and Bushell is and at the outset, the seven classification soci-
eties collectively had a total of 22 grades of steel. However, these steels could
be classified into three main types:
- Ordinary ship steel, which was used in modest thicknesses and in lightly
stressed areas. - An intermediate grade for areas where there was a need for some control over
notch toughness and for intermediate thicknesses. - A high-grade steel with good notch ductility and for heavy plate thicknesses.
Although the seven societies recognized these three broad categories of steel,
they were not able to rationalize their individual grades into three commonly
acceptable specifications. On the one hand, the American Bureau of Shipping
(ABS) favoured specifications based on deoxidation practice, composition and
heat treatment whereas the European societies preferred specifications based
primarily on mechanical properties. It was agreed finally to adopt unified grades
based on both approaches and this resulted in five specifications for the three