©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A
1.1.0.6 The
basic operation of the blast furnace is to reduce iron oxide to iron metal and
to remove impurities from the metal.
Reduced elements pass into the iron and oxidized elements
dissolve into the slag. The metal that comes from the blast furnace is called
pig iron and is
used as a starting material for further purification processes.
1.1.0.7 Pig
iron contains excessive amounts of elements that must be reduced before
steel can be produced. Different
types of furnaces, most notably the open hearth, electric
and basic oxygen, are used to continue
this refining process. Each furnace performs the task
of removing or reducing elements such as carbon, silicon, phosphorus, sulfur and
nitrogen by saturating the molten metal
with oxygen and slag forming ingredients. The oxygen
reduces elements by forming gases that are blown away and the slag attracts
impurities as it separates from the
molten metal. 1.1.0.8
Depending upon the type of slag that
is used, refining furnaces are classed as either
acid or basic. Large amounts of lime are contained in basic slags and high
quantities of
silica are present in acid slags. This differential between acid and basic
slags is also present
in welding electrodes for much of the same refining process occurs in the welding
operation. 1.1.0.9
After passing through the refining
furnace, the metal is poured into cast iron ingot molds.
The ingot produced is a rather large square column of steel. At this
point, the metal is
saturated with oxygen. To avoid the formation of large gas pockets in the
cast metal, a substantial
portion of the oxygen must be removed. This process is known as deoxidation,
and it is accomplished through additives
that tie up the oxygen either through gases or in slag.
There are various degrees of oxidation, and the common ingots resulting
from each are
as follows: 1.1.1
Rimmed
Steel
- The making of rimmed steels involves the least deoxidation.
As the
ingots solidify, a layer of nearly pure iron is formed on the walls and bottom
of the mold, and
practically all the carbon, phosphorus, and sulfur segregate to the central core.
The oxygen
forms carbon monoxide gas and it is trapped in the solidifying metal as blow holes
that disappear in the hot rolling process.
The chief advantage of rimmed steel is the excel- lent
defect-free surface that can be produced with the aide of the pure iron skin.
Most rimmed
steels are low carbon steels containing less than .1% carbon.
1.1.2
Capped
Steel - Capped steel regulates
the amount of oxygen in the molten metal
through the use of a heavy cap that is locked on top of the mold after the metal
is allowed to
reach a slight level of rimming. Capped steels contain a more uniform core
composition than the rimmed steels.
Capped steels are, therefore, used in applications
