©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A AN
INTRODUCTION TO METALS 1.1
SOURCE AND MANUFACTURING
Metals come from natural deposits of
ore in the earth’s crust. Most ores are contaminated
with impurities that must be removed
by mechanical and chemical means. Metal extracted from
the purified ore is known as primary or virgin metal, and metal that comes
from scrap is
called secondary metal. Most mining of metal bearing ores is done by either
open pit or underground
methods. The two methods of mining employed are known as “selective”
in which small
veins or beds of high grade ore are worked, and “bulk”
in which large quantities of
low grade ore are mined to extract a high grade portion.
1.1.0.1 There
are two types of ores, ferrous and nonferrous. The term ferrous comes
from the Latin word “ferrum”
meaning iron, and a ferrous metal is one that has a high iron
content. Nonferrous metals, such
as copper and aluminum, are those that contain little or
no iron. There is approximately
20 times the tonnage of iron in the earth’s crust compared
to all other nonferrous products combined;
therefore, it is the most important and widely used
metal. 1.1.0.2
Aluminum, because of its attractive
appearance, light weight and strength, is the next
most widely used metal. Commercial aluminum ore, known as bauxite, is a
residual deposit
formed at or near the earth’s surface. 1.1.0.3
Some of the chemical processes that
occur during steel making are repeated during
the welding operation and an understanding of welding metallurgy can be gained
by imagining the
welding arc as a miniature steel mill. 1.1.0.4
The largest percentage of commercially
produced iron comes from the blast furnace
process. A typical blast furnace is a circular shaft approximately 90 to
100 feet in height
with an internal diameter of approximately 28 feet. The steel shell of the
furnace is lined
with a refractory material, usually a hard, dense clay firebrick.
1.1.0.5 The
iron blast furnace utilizes the chemical reaction between a solid fuel charge
and the resulting rising column of
gas in the furnace. The three different materials used for
the charge are ore, flux and coke.
The ore consists of iron oxide about four inches in diameter.
The flux is limestone that decomposes into calcium oxide and carbon dioxide.
The lime reacts with impurities in
the ore and floats them to the surface in the form of a slag.
Coke, which is primarily carbon, is the ideal fuel for blast furnaces because
it produces carbon
monoxide gas, the main agent for reducing iron ore into iron metal.
