Page 54 - Ship Construction.DJ Eyres 6Ed

Ch05-H8070.fm Page 43 Wednesday, October 18, 2006 8:02 AM
Steels 43
Only the open hearth, electric furnace, and oxygen processes are described
here as the Bessemer converter process is not used for shipbuilding steels.

OPEN HEARTH PROCESS The open hearth furnace is capable of
producing large quantities of steel, handling 150 to 300 tonnes in a single
melt. It consists of a shallow bath, roofed in, and set above two brick-lined
heating chambers. At the ends are openings for heated air and fuel (gas or
--- ใช้เพื่อการศึกษาเท่านั้น---
oil) to be introduced into the furnace. Also these permit the escape of the
burned gas which is used for heating the air and fuel. Every twenty minutes
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
or so the flow of air and fuel is reversed.
In this process a mixture of pig-iron and steel scrap is melted in the
furnace, carbon and the impurities being oxidised. Oxidization is produced
by the oxygen present in the iron oxide of the pig iron. Subsequently carbon,
manganese, and other elements are added to eliminate iron oxides and give
the required chemical composition.

ELECTRIC FURNACES Electric furnaces are generally of two types, the
arc furnace and the high-frequency induction furnace. The former is used for
refining a charge to give the required composition, whereas the latter may
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only be used for melting down a charge whose composition is similar to that
finally required. For this reason only the arc furnace is considered in any
detail. In an arc furnace melting is produced by striking an arc between elec-
trodes suspended from the roof of the furnace and the charge itself in the
hearth of the furnace. A charge consists of pig-iron and steel scrap and the
process enables consistent results to be obtained and the final composition
of the steel can be accurately controlled.
Electric furnace processes are often used for the production of high-grade
alloy steels.

OXYGEN PROCESS This is a modern steelmaking process by which a
molten charge of pig-iron and steel scrap with alloying elements is con-
tained in a basic lined converter. A jet of high purity gaseous oxygen is then
directed onto the surface of the liquid metal in order to refine it.
Steel from the open hearth or electric furnace is tapped into large ladles
and poured into ingot moulds. It is allowed to cool in these moulds, until it
becomes reasonably solidified permitting it to be transferred to ‘soaking
pits’ where the ingot is reheated to the required temperature for rolling.

CHEMICAL ADDITIONS TO STEELS Additions of chemical elements
to steels during the above processes serve several purposes. They may be
used to deoxidize the metal, to remove impurities and bring them out into
the slag, and finally to bring about the desired composition.