Page 100 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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CONNECTIONS

3.32 CHAPTER THREE

In shop fabrication of cover-plated beams and built-up members, each component requiring
splices should be spliced before it is welded to other parts of the member. Up to three subsections
may be spliced to form a long girder or girder section.
With too rapid cooling, cracks may form in a weld. Possible causes are shrinkage of weld and
heat-affected zone, austenite–martensite transformation, and entrapped hydrogen. Preheating the
base metal can eliminate the first two causes. Preheating reduces the temperature gradient between
the weld and the adjacent base metal, thus decreasing the cooling rate and resulting stresses. Also, if
hydrogen is present, preheating allows more time for this gas to escape. Use of low-hydrogen elec-
trodes, with suitable moisture control, is also advantageous in controlling hydrogen content.
High cooling rates occur at arc strikes that do not deposit weld metal. Hence, strikes outside the
area of permanent welds should be avoided. Cracks or blemishes resulting from arc strikes should
be ground to a smooth contour and checked for soundness.
To avoid cracks and for other reasons, AWS specifications require that under certain conditions, before
a weld is made, the base metal must be preheated. Table 3.9 lists typical preheat and interpass tempera-
tures. The table recognizes that as plate thickness, carbon content, or alloy content increases, higher pre-
heats are necessary to lower cooling rates and to avoid microcracks or brittle heat-affected zones.
Preheating should bring the surface of the base metal to the specified preheat temperature within a
distance equal to the thickness of the part being welded, but to not less than 3 in of the point of welding.
This temperature should be maintained as a minimum interpass temperature while welding progresses.
Preheat and interpass temperatures should be sufficient to prevent crack formation. Temperatures
above the minimums in Table 3.9 may be required for highly restrained welds.
To prevent cracking, peening sometimes is used on intermediate weld layers for control of shrink-
age stresses in thick welds. Peening should be done with a round-nose tool and light blows from a
power hammer after the weld has cooled to a temperature that feels 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.

TABLE 3.9 Requirements of AWS D1.1 for Minimum Preheat and Interpass Temperatures, °F, for Welds in Buildings for Some
Commonly Used Structural Steels a
Shielded metal-arc with
low-hydrogen electrodes;
submerged-arc, with carbon
or alloy steel wire neutral
flux, gas-metal arc, or flux-
cored arc with electrodes or
electrode-flux combination
Shielded metal-arc Shielded metal-arc capable of depositing weld
with low-hydrogen with low-hydrogen metal with a maximum dif-
Thickness at thickest Shielded metal-arc electrodes; submerged- electrodes; submerged- fusible hydrogen content of
part at point of with other than low- arc, gas-metal arc, or arc, gas-metal arc, or 8 Ml/100 g when tested in
welding, in hydrogen electrodes flux-cored arc flux-cored arc accordance with AWS A4.3
ASTM A36; A53
Grade B; A242; A441;
ASTM A36, A53 A501; A529; A572
c
Grade B, A501, Grades 42, 50, and 55; ASTM A572 Grades ASTM A913 Grades 50,
A529 A588; A992 60 and 65 60, and 65
3 32 b 32 b 50 32 b
To / 4
3 1 150 50 150 32 b
Over / 4 to 1 / 2
1 1 225 150 225 32 b
1 / 2 to 2 / 2
1 300 225 300 32 b
Over 2 / 2
a
In joints involving different base metals, preheat as specified for higher-strength base metal.
b
When the base-metal temperature is below 32°F, the base metal shall be preheated to at least 70°F and the minimum interpass temperature shall
be maintained during welding.
c
The heat input limitations of AWS D1.1, paragraph 5-7, shall not apply to A913.
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