Page 279 - Handbook of Structural Steel Connection Design and Details
P. 279
Welded Joint Design and Production
264 Chapter Three
require that weld bead shapes meet prescribed requirements of
maximum height and widths. This has an indirect effect of limiting
heat input.
Current density is determined by dividing the welding amperage by
the cross-sectional area of the electrode. The current density is there-
2
fore proportional to I/d . As the current density increases, there will
be an increase in deposition rates as well as penetration.
Preheat and interpass temperatures are used to control cracking
tendencies, typically in the base material. Excessively high preheat
and interpass temperatures will reduce the yield and tensile strength
of the weld metal as well as the toughness. When base metals receive
little or no preheat, the resultant rapid cooling can promote cracking
as well as excessively high yield and tensile properties in the weld
metal, and a corresponding reduction in toughness and elongation.
All of the preceding variables are defined and controlled by the
welding procedure specification. Conformance to these requirements
is particularly sensitive for critical fabrication such as seismically
loaded structures, because of the high demand placed upon welded
connections under these situations.
Fit-up. Fit-up is the term that defines the orientation of the various
pieces prior to welding. The AWS D1.1 code has specific tolerances
that are applied to the as-fit dimensions of a joint prior to welding. It
is critical that there is ample access to the root of the joint to ensure
good, uniform fusion between the members being joined. Excessively
small root openings or included angles in groove welds do not permit
uniform fusion. Excessively large root openings or included angles
result in the need for greater volumes of weld metal, with their corre-
sponding increases in shrinkage stresses. This in turn increases dis-
tortion and cracking tendencies. The D1.1 tolerances for fit-up are
1
generally measured in / -in increments. As compared to the overall
16
project, this is a very tight dimension. Nevertheless, as it affects the
root opening condition, it is critical in order to avoid lack of fusion,
slag inclusions, and other unacceptable root conditions.
Field versus shop welding. Many individuals believe that the highest-
quality welding naturally is obtained under shop welding conditions.
While some aspects of field welding are more demanding than shop
welding situations, the greatest differences are not technical but
rather are related to control. For shop fabrication, the work force is
generally more stable. Supervision practices and approaches are well
understood. Communication with the various parties involved is gen-
erally more efficient. Under field welding conditions, maintaining and
controlling a project seems to be more difficult. There are environ-
mental challenges to field conditions, including temperature, wind,
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.