Page 276 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 261

may be practically and economically unacceptable. Research will be
conducted to determine actual toughness requirements.
Heat-affected zones. As illustrated in Fig. 3.36, the base metal heat-
affected zones represent material that may affect connection perfor-
mance as well. The heat-affected zone (HAZ) is defined as that base
metal which has been thermally changed due to the energy introduced
into it by the welding process. In the small region immediately adja-
cent to the weld, the base metal has gone through a different thermal
history than the rest of the base material. For most hot-rolled steels,
the area of concern is a HAZ that is cooled too rapidly, resulting in a
hardened heat-affected zone. For quenched-and-tempered steels, the
HAZ may be cooled too slowly, resulting in a softening of the area. In
columns, the HAZ of interest is the Z direction properties immediately
adjacent to the groove weld. For the beam, these are oriented in the
X direction.
Heat-affected zone properties are a function of base metal chem-
istry and specific welding procedures. Steel makers consider HAZ
properties when developing a specific steel composition, and for
quenched-and-tempered steels, guidelines are available from the steel
producer indicating what precautions must be taken during welding
to preclude the formation of undesirable heat-affected zones. The pri-
mary welding variables that affect HAZ properties are the preheat
and interpass temperatures (both minimum and maximum), and the
heat input of welding. Excessively high heat input can negatively
affect HAZ properties by causing softening in these areas. Excessively
low heat input can result in hardening of the HAZ.
Weld metal properties may be negatively affected by extremely
high heat input welding procedures, causing a decrease in both the
yield strength and tensile strength, as well as the notch toughness of
the weld deposit. Excessively low heat input may result in high-
strength weld metal and may also decrease the notch toughness of
the weld deposit. Optimum mechanical properties are generally
obtained from both the weld metal and the HAZ if the heat input is
maintained in the 30- to 80-kJ/in range.
Post-Northridge evaluation of fractured connections has revealed that
excessively high heat input welding procedures were commonly used,
confirmed by the presence of very large weld beads that, in some cases,
exceeded the maximum limits prescribed by the D1.1 code. These may
have had some corollary effects on weld metal and HAZ properties.
Material properties and connection design. While a welded structure
acts as a one-piece unit, the material properties are not isotropic
throughout all zones. When high demands are placed upon connections,
each series of material properties must behave as expected.

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