©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
IV 4.5.1.1
Some of the low alloy high strength
steels are intended for use at subzero tempera- tures.
Nickel bearing low hydrogen electrodes (C1, C2, C3 types) are available
for such low temperature
applications. 4.5.1.2
Chromium molybdenum low alloy steels
are used for moderately high temperature service.
Piping, tubing, boilers, etc., that are used extensively in power generating
plants, are fabricated
from these steels. Chrome-moly low hydrogen electrodes (B1, B2, B3, etc.)
are produced to
weld these steels. 4.5.1.3
Many bridges and outdoor structures
are constructed from "weathering" grade steels.
These are low alloy steels that, on exposure to the atmosphere, develop
a thin, tightly adhering
layer of rust that prevents further rusting and eliminates the need for painting.
Low alloy
electrodes with additions of chromium and copper are available for welding these
steels. 4.5.1.4
Quenched and tempered low alloy steels
usually develop high strength with good toughness.
These types are used where substantial savings in the weight of the structure
is important.
Quite often, but not exclusively, these steels are used by the military.
One of the more
exotic applications for quenched and tempered low alloy steels is in the fabrication
of the pressure
hulls for nuclear submarines. The "M" series of high tensile low hydrogen
electrodes is
intended to weld these steels. 4.5.1.5
High tensile line pipe for the transmission
of oil and gas is being used with greater frequency
today. Low alloy cellulosic electrodes of the 7010 and 8010 variety are
used for field
welding. 4.5.2
Joint
Design -
In fillet welding of high strength quenched and tempered steels, toe
cracking alongside the welds (see Figure
4) is frequently a problem. The toe cracking is caused
by the high strength weld metal having a higher yield point and tensile strength
than the steel.
4.5.2.1 When
the weld area shrinks on cooling
from the welding temperature, something must
give, and because the yield and strength levels
of the steel are lower than those of the weld
metal, cracking occurs in the heat affected zone
of the steel. The solution to this problem is to
use a lower strength weld metal and increase the
fillet size to meet the weld joint strength requirements.
HEAT AFFECTED
ZONE CRACK
AT TOE OF WELD
WELD METAL BASE
METAL TOE
CRACKING FIGURE
4
