Page 202 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 187

Figure 3.5 Heat-affected zone cracking. (Courtesy of The Lincoln
Electric Company.)

is related to the welding process, the crack occurs in the base material,
not in the weld material. This type of cracking is also known as
underbead cracking, toe cracking, or delayed cracking. Because this
cracking occurs after the steel has cooled below approximately 200°C,
it can be called cold cracking, and because this cracking is associated
with hydrogen, it is also called hydrogen-assisted cracking.
In order for heat-affected zone cracking to occur, three conditions
must be present simultaneously: (1) there must be a sufficient level of
hydrogen, (2) there must be a sufficiently sensitive material involved,
and (3) there must be a sufficiently high level of residual or applied
stress. Sufficient reduction or elimination of one of the three variables
will eliminate heat-affected zone cracking. In welding applications,
the typical approach is to limit two of the three variables, namely, the
level of hydrogen and the sensitivity of the material. Hydrogen can
enter into a weld pool through a variety of sources. Moisture and
organic compounds are the primary sources of hydrogen. It may be
present on the steel, electrode, in the shielding materials, and in
atmospheric humidity. Flux ingredients, whether on the outside of
electrodes, inside the core of electrodes, or in the form of submerged
arc or electroslag fluxes, can adsorb or absorb moisture, depending on
storage conditions and handling practices. To limit hydrogen content
in deposited welds, welding consumables must be properly main-
tained, and welding must be performed on surfaces that are clean and
dry. The second necessary condition for heat-affected zone cracking is
a sensitive microstructure. In the case of heat-affected zone cracking,
the area of interest is the heat-affected zone that results from the
thermal cycle experienced by the region immediately surrounding the
weld nugget. As this area is heated by the welding arc during the cre-
ation of the weld pool, it transforms from its room temperature struc-
ture of ferrite to the elevated temperature structure of austenite. The
subsequent cooling rate will determine the resultant HAZ properties.
Conditions that encourage the development of crack-sensitive
microstructures include high cooling rates and higher hardenability
levels in the steel. High cooling rates are encouraged by lower heat-
input welding procedures, greater base material thicknesses, and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.accessengineeringlibrary.com)
Copyright © 2009 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.