Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
is small relative to the whole assembly, so that as the weld and HAZ cool and
contract they are constrained by the surrounding unaffected material. Large
stresses develop, leading to local plastic deformation and, when all the structure
returns to room temperature, tensile residual stresses remain in the weld. The
magnitude of these stresses can be reduced by preheating the structure or the
stresses relieved by a post-weld heat-treatment, neither process being
straightforward for large structures.
These internal stresses can lead to cracking in the weld deposit and HAZ
through several mechanisms:
(a) Solidification cracking. This occurs in the weld deposit during cooling
and is found at the weld centre line or between columnar grains.
(b) Hydrogen-induced cracking in steel. Atomic hydrogen can be introduced
into the weld during the welding process, its principal origin being moisture
in the electrode fluxes employed, although hydrocarbons on the plate
being welded is another possible source. Hydrogen can give rise to socalled
cold cracking in the HAZ (underbead; root crack) or in the weld
metal itself and is the most serious and least understood of all weldcracking
problems. The solubility of hydrogen in austenite is much higher
than in ferrite or martensite, so that if a steel transforms from austenite
on cooling, it will be highly supersaturated with respect to hydrogen.
Under these conditions, hydrogen diffuses to discontinuities in the metal
such as grain boundaries and nonmetallic inclusions where it recombines
to form hydrogen gas as microscopic bubbles that can develop into
cracks.
(c) Liquation cracking. This occurs in the HAZ near the fusion line and is
associated with the segregation of impurities such as sulphur and
phosphorus to melted grain boundaries during welding. On cooling,
these segregants tend to form films of intermetallic compounds and,
with the development of high residual stresses, these impurity-weakened
boundaries tend to rupture.
(d) Lamellar tearing. This occurs just outside the HAZ, and is commonly
observed when a weld runs parallel to the surface of a plate. During the
rolling of steel plate, flattened stringers of MnS or oxide–silicate phase
are formed in the plane of the plate along the rolling direction. The
orientation of the residual stress in such a weld is such that the weak
inclusion/matrix interface can decohere and thus nucleate a crack.
3.3.4 Adhesive bonding
Adhesive bonding is brought about by applying adhesive to prepared surfaces,
which are then brought together. Heat may be applied to encourage adhesive
setting, although the temperatures employed are usually too low to affect the