Page 359 - Introduction to Marine Engineering

Engineering materials 331

burning process. The iron ore is reduced to iron and falls to the base of
the furnace, becoming molten as it falls. Various impurities, such as
carbon, silicon, manganese and sulphur, are absorbed by the iron as it
descends. A slag of various materials, combined with the limestone,
forms on top of the iron. The slag is tapped or drawn off from the
furnace as it collects. The molten iron may be tapped and run into
moulds to make bars of pig iron. Alternatively it may be transferred
while molten to a steel manufacturing process.
Various processes are used in the manufacture of steel, such as the
open hearth process, the oxygen or basic oxygen process and the electric
furnace process. The terms 'acid' or 'bask' are often used with
reference to steels. These terms refer to the production process and the
type of furnace lining, e.g. an alkaline or basic lining is tised to make
basic steel. The choice of furnace lining is decided by the raw materials
used in the manufacture of the steel. In all the steel producing processes
the hot molten steel is exposed to air or oxygen which oxidises the
impurities to refine the pig iron into high-quality steel.
Steels produced in the above processes will all contain an excess of
oxygen which will affect the material quality. Several finishing
treatments are used in the final steel casting. Rimmed steel has little or
no oxygen removing treatment, and the central core of the solidified
ingot is therefore a mass of blow holes. Hot rolling of the ingot usually
welds up most of these holes. Killed steel is produced by adding
aluminium or silicon before the molten steel is poured. The oxygen
forms oxides with this material and a superior quality steel compared
with rimmed steel is produced. Vacuum degassed steels result from
reducing the atmospheric pressure while the steel is molten. This
reduces the oxygen content and a final deoxidation can be achieved with
small additions of silicon or aluminium.
Cast iron is produced by rernelting pig iron under controlled
conditions in a miniature type of blast furnace known as a 'cupola'.
Variations of alloying additions may also be made. Two main types of
cast iron occur—'white' and 'grey'. The colour relates to the appearance
of the fractured surface. White cast iron is hard and brittle; grey is
softer, readily machinable and less brittle.

Heat treatment consists of heating a metal alloy to a temperature below
its melting point and then cooling it in a particular manner. The result is
some desired change in the material properties. Since most heat
treatment is applied to steel, the various terms and types of treatment
will be described with reference to steel.