©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
VIII e.
Preheat
and Interpass
Temperature -
The importance of observing preheat and interpass
temperatures cannot be overstressed. Problems, such as spalling,
cracking, and distortion can be minimized
by proper preheating, interpass tem- perature,
and slow or retarded cooling. f.
Dilution
- Dilution of the hardfacing deposits is expected in
all cases where the hardfacing
alloy is fused to the base metal and should be kept to a minimum.
Excessive dilution with the base metal
will alter the hardness of the deposit and in
part, is a result of the heat input. Heat input is a function of the heat
(amper- age and
voltage) and deposition rate (travel speed). Note: As
an example, a coated electrode, which operates at 225 amps and has a low deposition
rate, may put
more heat into the workpiece than an open arc continuous electrode, which operates
at 400 amps but
has a deposition rate three times higher than the coated electrode. The
electrode manufacturer’s
recommended welding current should be used. Dilution
will be greater in stringer beads (straight) than in a weaving bead. A weaving
bead is recommended
wherever possible. Electrical
stickout (the amount of wire between the contact tip and the arc) must be kept
relatively constant
to control penetration in open arc welding. Long stickout decreases penetration
and thereby, the
amount of dilution. Short stickout can drastically increase penetration
and dilution. g.
Hardfacing
Thickness -
Too much hardfacing can cause more problems than too
little. The hardfacing deposit should consist of no more than two layers
and the total
thickness should not exceed ¼” in most cases. 8.6 ECONOMICS
OF HARDFACING Hardfacing
filler metals are quite costly as noted earlier. The iron based alloys are
the lowest in
cost with the cobalt based alloys being the highest. 8.6.0.1
Consider a steel mill application requiring
hardfacing on the guide blocks which will
be subjected to abrasion and intermittent contact with hot billets at temperatures
of approximately
1800-2000°F. Logically, one might choose a cobalt base surfacing alloy,
which will withstand continuously applied
higher temperatures than the iron base types for this
application. However, since the guide blocks are in contact with the billets
intermit- tently
for short periods of time, the constant operating temperature is well below 800°F.
Iron base hardfacing alloys, which
retain hardness at a constant 1000°F, are used in this
application quite successfully at a
considerable savings over the cobalt base types.
