Page 55 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 44 Wednesday, October 18, 2006 8:02 AM
44 Ship Construction
The amount of deoxidizing elements added determines whether the
steels are ‘rimmed steels’ or ‘killed steels’. Rimmed steels are produced
when only small additions of deoxidizing material are added to the molten
metal. Only those steels having less than 0.2 per cent carbon and less than
0.6 per cent manganese can be rimmed. Owing to the absence of deoxidiz-
ing material, the oxygen in the steel combines with the carbon and other
gases present and a large volume of gas is liberated. So long as the metal is
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molten the gas passes upwards through the molten metal. When solidifica-
tion takes place in ingot form, initially from the sides and bottom and then
across the top, the gasses can no longer leave the metal. In the central
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portion of the ingot a large quantity of gas is trapped with the result that the
core of the rimmed ingot is a mass of blow holes. Normally the hot rolling
of the ingot into thin sheet is sufficient to weld the surfaces of the blow
holes together, but this material is unsuitable for thicker plate.
The term ‘killed’ steel indicates that the metal has solidified in the ingot
mould with little or no evolution of gas. This has been prevented by the
addition of sufficient quantities of deoxidizing material, normally silicon or
aluminium. Steel of this type has a high degree of chemical homogeneity,
and killed steels are superior to rimmed steels. Where the process of deoxi-
dation is only partially carried out by restricting the amount of deoxidizing
material a ‘semi-killed’ steel is produced. ึ
In the ingot mould the steel gradually solidifies from the sides and base
as mentioned previously. The melting points of impurities like sulphides
and phosphides in the steel are lower than that of the pure metal and these
will tend to separate out and collect towards the centre and top of the ingot
which is the last to solidify. This forms what is known as the ‘segregate’ in
way of the noticeable contraction at the top of the ingot. Owing to the high
concentration of impurities at this point this portion of the ingot is often
discarded prior to rolling plate and sections.

Heat Treatment of Steels
The properties of steels may be altered greatly by the heat treatment to
which the steel is subsequently subjected. These heat treatments bring about
a change in the mechanical properties principally by modifying the steel’s
structure. Those heat treatments which concern shipbuilding materials are
described.

ANNEALING This consists of heating the steel at a slow rate to a tempera-
ture of say 850°C to 950 °C, and then cooling it in the furnace at a very slow
rate. The objects of annealing are to relieve any internal stresses, to soften
the steel, or to bring the steel to a condition suitable for a subsequent heat
treatment.