©
COPYRIGHT 2000 THE ESAB GROUP, INC. LESSON
IV COVERED
ELECTRODES FOR
WELDING LOW
ALLOY STEELS
4.1 LOW
ALLOY STEELS Low
alloy steels, as discussed in Lesson I, are those steels to that have small amounts
of alloying elements
added for specific purposes; i.e., to increase strength, toughness, corrosion
and rust resistance, or to alter the
response to heat treatment. Nearly every steel manufacturer
makes a family of low alloy steels
that are usually sold under trade names such as Maynari R,
Cor-ten, Man-ten, and many others.
Many of the steels are designed to develop their specific
properties such as high strength or
toughness in the hot rolled and controlled cooling condition,
rather than by subsequent heat treatment.
Other compositions of low alloy steels are designed
to develop specific properties following
heat treatments. Examples of these types are U.S. Steel
T-1, Armco Steel SS-100, Great Lakes Steel NA XTRA 100, all of which are quenched
and tempered to reach high strength
with good toughness. Covered low alloy welding elec-
trodes are designed, in most cases,
to match the properties of the low alloy steels rather than
to match the exact chemical composition
of the steel. Exceptions to this are the chromium molybdenum
electrodes that need to contain about the same amounts of the alloy ingredients
as the steel in order to match the
properties of the steel. 4.2 CONSEQUENCE
OF HYDROGEN IN LOW ALLOY STEEL One
of the reasons that low alloy steels are becoming more popular is because of the
exten- sive research
that was conducted in the development of electrodes for welding them. Although
special precautions and care are required
in welding the low alloy steels, they can now be joined
with a high degree of reliability. But that was not always so. During
World War II when there
was a dramatic increase in the use of high strength low alloy steel, there was
also a corresponding
increase in weld defects. It was quickly realized that hardenable steels
could not be welded
in the same manner and with the same electrodes as were then commonly used
for welding the lower strength mild
steels. Through extensive research, it was found that en-
trapped hydrogen was the culprit in
causing weld defects, and the term "hydrogen embrittlement"
became synonymous with red flags warning of impending disaster.
4.2.0.1 When
hydrogen bearing compounds such as water, minerals, or chemicals are
present in the electrode coating, as
is common with mild steel electrodes, the chemically combined
hydrogen is dissociated into atomic hydrogen by the heat of the welding arc. The
molten weld metal has the capacity
to dissolve the atomic hydrogen. However, as soon as the
