Page 188 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

Welded Joint Design and Production 173

3.1 Structural Steels for Welded
Construction

3.1.1 Introduction
When selecting steels for structural applications, engineers usually
select the specific steel based upon the mechanical property of strength,
whether yield or tensile. These mechanical properties, along with the
modulus of elasticity, in general satisfy the requirements for structural
considerations. Additional requirements, such as notch toughness
(typically measured by the Charpy V-notch specimen), may be specified
to minimize brittle fracture, especially for dynamically loaded structures.
With the single exception of weathering steel for uncoated applications
exposed to atmospheric conditions, the chemical composition generally
is of little concern from a structural point of view.
In terms of weldability, the chemistry of the steel is at least as
important as, and arguably more important than, the mechanical
properties. During the thermal cutting processes associated with con-
struction, as well as when it is arc-welded, steel is subject to a variety
of thermal cycles which can alter its mechanical properties in the area
immediately adjacent to the weld metal. This area is known as the
heat-affected zone and is, by definition, base metal that has been ther-
mally affected by the welding or cutting process. While this region is
generally small (typically 2 to 3 mm wide), it may exhibit different
strength properties, and the toughness in this region may be dramati-
cally altered. Finally, the base metal composition may have a signifi-
cant effect on the weld metal composition, particularly for single-pass
welds and when welding procedures are used that result in deep pene-
tration. The chemistry of the base material is of the utmost impor-
tance in determining the suitability of a steel for welded construction.
The mechanical properties of steel cannot be overlooked as they relate
to welding, however. The strength of the steel will, in many cases,
determine the required strength level of the weld deposit.
Although most steels can be welded, they are not all welded with
the same degree of ease. Weldability is the term used to describe how
readily the steel can be welded. For new construction, it is always
advisable to select steels with good weldability since this will
inevitably lead to both high-quality and economical construction.
Steels with reduced weldability may require specialized electrodes,
techniques, preheat and postheat treatments, and joint designs.
While materials that are difficult to weld are successfully fabricated
every day, the use of these materials is generally inappropriate for
new construction given the variety of readily available steels with
excellent weldability. Occasionally, the engineer is faced with the sit-
uation of welding on material with less than optimum weldability,
such as when welding on existing structures is necessary. Under these

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