©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART A
depending on the material being tested,
the shape of the indenter and the load applied, the conversion
tables may differ. For example, a material listed as having a hardness of
Rb or Rc means
its hardness has been determined from the Rockwell "B" scale or the Rockwell
"C" scale. 1.6.11
Coefficient
of Expansion
- All metals expand when heated and contract when
cooled. This dimensional change
is related to the crystalline structure and will vary with
different materials. The different
expansion and contraction rates are expressed numeri- cally
by a coefficient of thermal expansion. When two different metals are heated
to the same temperature
and cooled at the same rate, the one with the higher numerical coeffi-
cient will expand and contract more
than the one with the lesser coefficient. 1.6.12
Thermal
Conductivity -
Some metals will absorb and transmit heat more readily
than others. They are categorized
as having high thermal conductivity. This characteristic
contributes to the fact that some metals
will melt or undergo transformations at much lower temperatures
than others. 1.7
EFFECTS OF THE ALLOYING ELEMENTS
Alloying is the process of adding a
metal or a nonmetal to pure metals such as copper, aluminum
or iron. From the time it was discovered that the properties of pure metals
could be improved
by adding other elements, alloy steel has increased by popularity. In fact,
metals that are welded are rarely in
their pure state. The major properties that can be
improved by adding small amounts of
alloying elements are hardness, tensile strength, ductility
and corrosion resistance. Common alloying elements and their effect on the
properties of metals are as follows:
1.7.1
Carbon
- Carbon is the most effective, most widely used and
lowest in cost alloying
element available for increasing the hardness and strength of metal. An
alloy containing
up to 1.7% carbon in combination with iron is known as steel, whereas the
combination above 1.7% carbon is known
as cast iron. Although carbon is a desirable alloying
element, high levels of it can cause problems; therefore, special care is required
when welding high carbon steels and
cast iron. 1.7.2
Sulphur
- Sulphur is normally an undesirable element in steel
because it causes brittleness.
It may be deliberately added to improve the machinability of the steel.
The sulphur
causes the machine
chips to break rather than form long curls and clog the machine. Normally,
every effort is
made to reduce the sulphur content to the lowest possible level because it can
