Page 53 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 42 Wednesday, October 18, 2006 8:02 AM

5
Steels

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The production of all steels used for shipbuilding purposes starts with the
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
smelting of iron ore and the making of pig-iron. Normally the iron ore is
smelted in a blast furnace, which is a large, slightly conical structure lined
with a refractory material. To provide the heat for smelting, coke is used
and limestone is also added. This makes the slag formed by the incombusti-
ble impurities in the iron ore fluid, so that it can be drawn off. Air necessary
for combustion is blown in through a ring of holes near the bottom, and the
coke, ore, and limestone are charged into the top of the furnace in rotation.
Molten metal may be drawn off at intervals from a hole or spout at the bot-
tom of the furnace and run into moulds formed in a bed of sand or into
metal moulds.
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The resultant pig-iron is from 92 to 97 per cent iron, the remainder being
carbon, silicon, manganese, sulphur, and phosphorus. In the subsequent
manufacture of steels the pig iron is refined, in other words the impurities
are reduced.

Manufacture of Steels

Steels may be broadly considered as alloys of iron and carbon, the carbon
percentage varying from about 0.1 per cent in mild steels to about 1.8 per
cent in some hardened steels. These may be produced by one of four differ-
ent processes, the open hearth process, the Bessemer converter process, the
electric furnace process, or an oxygen process. Processes may be either an
acid or basic process according to the chemical nature of the slag produced.
Acid processes are used to refine pig-iron low in phosphorus and sulphur
which are rich in silicon and therefore produce an acid slag. The furnace
lining is constructed of an acid material so that it will prevent a reaction
with the slag. A basic process is used to refine pig-iron that is rich in phos-
phorus and low in silicon. Phosphorus can be removed only by introducing
a large amount of lime, which produces a basic slag. The furnace lining
must then be of a basic refractory to prevent a reaction with the slag. About
85 per cent of all steel produced in Britain is of the basic type, and with
modern techniques is almost as good as the acid steels produced with supe-
rior ores.