Page 44 - Handbook of Structural Steel Connection Design and Details

Fasteners and Welds for Structural Connections

Fasteners and Welds for Structural Connections 29

temperatures. The table recognizes that as plate thickness, carbon
content, or alloy content increases, higher preheats are necessary to
lower cooling rates and to avoid microcracks or brittle heat-affected
zones.
Preheating should bring to the specified preheat temperature the
surface of the base metal within a distance equal to the thickness of the
part being welded, but not less than 3 in of the point of welding. This
temperature should be maintained as a minimum interpass tempera-
ture while welding progresses.
Preheat and interpass temperatures should be sufficient to prevent
crack formation. Temperatures above the minimums in Table 1.10 may
be required for highly restrained welds.
Peening sometimes is used on intermediate weld layers for control of
shrinkage stresses in thick welds to prevent cracking. It should be done
with a round-nose tool and light blows from a power hammer after the
weld has cooled to a temperature warm to the hand. The root or surface
layer of the weld or the base metal at the edges of the weld should not
be peened. Care should be taken to prevent scaling or flaking of weld
and base metal from overpeening.
When required by plans and specifications, welded assemblies should
be stress-relieved by heat treating. (See AWS D1.1 for temperatures
and holding times required.) Finish machining should be done after
stress relieving.
Tack and other temporary welds are subject to the same quality
requirements as final welds. For tack welds, however, preheat is not
mandatory for single-pass welds that are remelted and incorporated
into continuous submerged-arc welds. Also, defects such as undercut,
unfilled craters, and porosity need not be removed before final
submerged-arc welding. Welds not incorporated into final welds should
be removed after they have served their purpose, and the surface should
be made flush with the original surface.
Before a weld is made over previously deposited weld metal, all slag
should be removed, and the weld and adjacent material should be
brushed clean.
Groove welds should be terminated at the ends of a joint in a manner
that will ensure sound welds. Where possible, this should be done with
the aid of weld tabs or runoff plates. AWS D1.1 does not require removal
of weld tabs for statically loaded structures but does require it for
dynamically loaded structures. The AISC Seismic Provisions (2005) also
require their removal in zones of high seismicity. The ends of the welds
then should be made smooth and flush with the edges of the abutting
parts.
After welds have been completed, slag should be removed from
them. The metal should not be painted until all welded joints have

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