Hard-
Surfacing,
Building
Fusion
Welding
Carbon
Welding
Non-Ferrous
Metals
Heating
& Heat
Treating
Braze
Welding
Welding
Cast Iron
Welding
Ferrous
Metals
Brazing
&
Soldering
Equipment
Set-Up
Operation
Equipment
For
OXY-Acet
Structure
of
Steel
Mechanical
Properties
of Metals
Oxygen
&
Acetylene
OXY-Acet
Flame
Physical
Properties
of Metals
How Steels
Are
Classified
Expansion
&
Contraction
Prep
For
Welding
OXY-Acet
Welding
& Cutting
Safety
Practices
Manual
Cutting
Oxygen
Cutting By
Machine
Appendices
Testing
&
Inspecting
8
However, a steel containing 0.8% carbon
is classified as a high-carbon steel, and is seldom encountered by a
welder. The metallurgist terms such
a steel eutectoid;
it can form a structure that is 100% pearlite. (Don’t get
eutectoid confused
with eutectic,
which applies to the 95.7% iron -4.3% carbon composition
which melts completely
at a fixed temperature.) A welder is almost always working on
hypoeutectoid steels, containing less –
usually much less – than 0.8%
carbon. (The steels with more than 0.8% carbon are termed hypereutectoid.)
It would be wrong
to close this chapter without looking a bit more closely at the hypoeutectoid
steels, and without discussing
the various types of heat treatment (other than simple rapid quenching) which
are used to modify the structure
and the properties of carbon steels. Most
of the steels used to form sheet (thickness less than about 0.5 cm or 1/8 in.)
contain 0.13% carbon or less; extra-low-carbon
sheet may contain as little as 0.03% carbon. Steels in the upper part of this
range (0.08 to 0.13% C),
when allowed to cool slowly, usually contain some grains of pearlite, although
the grains of pearlite are much smaller
than the grains of ferrite. Steels in the lower part of the range (below 0.08%
C) seldom contain pearlite. The
carbon usually winds up in particles of iron carbide (cementite) scattered between
grains of ferrite. The mechanical
properties of the finished steel are affected by the size of the ferrite grains,
the shape of the ferrite grains
(if the steel is cold-rolled the grains will be distorted and elongated by the
rolling process), and by the size and
shape of the iron carbide (cementite) particles, which may be large and irregular,
or small and more-or-less round
in cross-section (spheroidical). The
carbon steels used to form steel plate (thickness greater than 0.5 cm), and most
steel pipe and tubing, usually contain
more than 0.13% carbon. Unless it has been given special heat-treatment at the
steel mill after rolling, carbon
steel plate will usually show a mixture of ferrite and pearlite grains, with the
pearlite grains smaller than the ferrite
grains. However, this structure can be modified greatly by heat treatment.
Fig. 9-6. A photomicrograph of a piece
of steel containing
about 0.3% carbon will look something
like this, showing a mixture of ferrite
grains (white) and pearlite grains.
