©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
IV 4.2.0.3
This hydrogen caused defect, known
as underbead cracking (See Figure 2), begins in
the HAZ making it particularly sinister since the crack is not immediately apparent
to the eye. It
occurs after the metal has cooled from about 400°F to room temperature, and
it is sometimes
referred to as "cold
cracking". The defect may
occur immediately after cooling,
or it may take hours, days,
or even months before it
happens. 4.2.1
Preheat
- Steels that
are highly hardenable by a
rapid cooling in the heat affected
zone require pre- heat
and interpass temperature control. As
preheat is applied to the steel, the cooling
rate of the steel from higher temperatures is slowed. Maintaining a constant
tempera- ture
between each welding pass also helps to control this cooling. Slower cooling
rates pre- vent
the steel from being excessively hardened and thus, minimizes the chance of underbead
cracking. When this technique
is combined with the use of low hydrogen electrodes, a high
degree of reliability can be expected
from the welds. 4.3 MANUFACTURING
LOW HYDROGEN ELECTRODES The
discovery of hydrogen related weld defects initiated the development of low hydrogen
electrodes. The functions of
the coating with low hydrogen electrodes (i.e., shielding, arc
stabilizers, alloy additions, etc.)
are much the same as those listed in Lesson III for Mild Steel
Covered Electrodes, but the coating
is formulated with ingredients that lack hydrogen in their
chemical composition. This is
primarily accomplished by eliminating organic and chemical
compounds high in moisture content.
In fact, control of the moisture levels in the coating is
critical in the manufacture and use
of low hydrogen electrodes. 4.3.0.1
In addition to eliminating hydrogen
in the coating formula, the manufacturing process entails
a high temperature bake cycle. After the coating is extruded onto the core
in the same manner
as a mild steel coated electrode, the low hydrogen electrodes are given an initial
low temperature
bake (300-400°F), and then rebaked in a separate high temperature oven (800-
900°F) for a specified period
of time. This procedure practically eliminates all moisture, and
to guard against the reabsorbing of
moisture that is naturally present in the atmosphere, the
BASE METAL WELD
METAL HEAT AFFECTED
ZONE HYDROGEN
INDUCED CRACKS UNDERBEAD
CRACKING FIGURE
2
