Failures in service rarely, if ever,
occur in properly made welds of adequate design. If a fracture
occurs, it is initiated at a notchlike defect. Notches occur for
various reasons.
The toe of a weld may form a natural
notch. The weld may contain flaws that act as notches. A welding-arc
strike in the base metal may have an embrittling effect, especially
if weld metal is not deposited.
A crack started at such notches will
propagate along a path determined by local stresses and notch
toughness of adjacent material.
Preheating before welding minimizes the
risk of brittle failure. Its primary effect initially is to reduce
the temperature gradient between the weld and adjoining base metal.
Thus, there is less likelihood of
cracking during cooling and there is an opportunity for entrapped
hydrogen, a possible source of embrittlement, to escape. A consequent
effect of preheating is improved ductility and notch toughness of
base and weld metals, and lower transition temperature of weld.
Rapid cooling of a weld can have an
adverse effect. One reason that arc strikes that do not deposit weld
metal are dangerous is that the heated metal cools very fast. This
causes severe embrittlement.
Such arc strikes should be completely
removed. The material should be preheated, to prevent local
hardening, and weld metal should be deposited to fill the depression.
Welding processes that deposit weld
metal low in hydrogen and have suitable moisture control often can
eliminate the need for preheat. Such processes include use of
low-hydrogen electrodes and inert-arc and submerged-arc welding.
Pronounced segregation in base metal
may cause welds to crack under certain fabricating conditions. These
include use of high-heat-input electrodes and deposition of large
beads at slow speeds, as in automatic welding.
Cracking due to segregation, however,
is rare for the degree of segregation normally occurring in hot
rolled carbon-steel plates. Welds sometimes are peened to prevent
cracking or distortion, although special welding sequences and
procedures may be more effective.
Specifications often prohibit peening
of the first and last weld passes. Peening of the first pass may
crack or punch through the weld.
Peening of the last pass makes
inspection for cracks difficult. Peening considerably reduces
toughness and impact properties of the weld metal. The adverse
effects, however, are eliminated by the covering weld layer (last
pass).
(M. E. Shank, Control of Steel
Construction to Avoid Brittle Failure, Welding Research Council, New
York; R. D. Stout and W. D. Doty, Weldability of Steels, Welding
Research Council, New York.)
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