|
Problems developed due to nonuniform heating and cooling at the point of
welding give rise to uneven temperature distribution of the weld zone, Heat-Affected
Zone (HAZ) and base metal. This finally leads to incompatible strains, which in turn
result in residual stresses—the remains in the structure after it has cooled down to
ambient temperature. The strain and stresses transverse to the weld may lead to the
formation of hot cracks. High tensile residual stresses are known to promote fracture,
corrosion cracking and fatigue in the welded structure. On the other hand, compressive
residual stresses tend to reduce the buckling strength. Formation of residual stress in
weldment can mainly be attributed to three sources: nonuniform simultaneous heating
and cooling of the part during welding, variation of shrinkage due to variable
cooling rates in the different regions of the welding (surface cooling effects), and
the volumetric changes during metallurgical phase transformations.
Lyamin (1999) studied the influence of residual stresses on the crack resistance
of axi-symmetric diffusion welded metal ceramic joints. He found that the most
dangerous were tensile stresses in contrast to the compressive ones, which efficiently resist
the creation and development of crack like defects. Crack resistance of welded
joints depends on their types (compensated or non-compensated) and relative thickness
of the layers as well.
Ingateva et al. (1999) studied residual stresses in welding circular and oval
holes. The results of comparison of three variants of the form of holes in the pipeline
showed that the circular hole was the most efficient from the viewpoint of the
stress distribution. The process of cutting a hole of this type could be mechanized using
a special milling head. |
