The
paper reports the effect of pulsing parameters on the magnitude
of welding residual stresses by using gas tungsten arc welding
on type 304 austenitic stainless steel. The problems related
to residual stresses in weldments are of major concern, as
they may modify the resistance to brittle fracture, fatigue
strength and stress corrosion cracking of a welded part or
structure. The residual stress has been determined by hole
drilling strain gauge method of ASTM standard E-837. Optimization
analysis has been carried out to find out the optimum magnitude
of pulsing parameters, viz., amplitude ratio, pulse frequency,
duration ratio and root gap corresponding to the least residual
stresses. To reduce the number of samples to minimum, Taguchi
experimental design technique has been used and values of
the pulsing parameters at which the residual stress is minimal
have been obtained. The experimental results show that a greater
amplitude ratio and duration ratio can increase the magnitude
of residual stresses and greater pulse frequency can reduce
the magnitude of residual stresses.
The predecessor of the pulsed current Gas Tungsten Arc Welding (GTAW) process as it is known today was developed in the then Soviet Union by M.P. Zaytsw in 1953 (Becker and Adams, 1979). Pulsed GTA welding process is a welding technique in which the arc current alternates between two levels with heating and fusion taking place during periods of high current and cooling and solidification during low current periods. The pulsed current GTAW has many specific advantages such as enhanced arc stability, increased weld depth to width ratio, reduced hot cracking sensitivity, refined grain size and reduced porosity (Becker and Adams, 1979; and Cornu Jean, 1988).
During
welding, local non-uniform heating and subsequent cooling
cause the development of complex thermal strains and stresses
that finally lead to residual stresses and distortion. These
effects are usually detrimental directly or indirectly to
the integrity and service behavior of a welded structure.
In particular, high tensile stresses in the region near the
weld might promote brittle fracture, change the fatigue strength,
or lead under suitable conditions to stress corrosion cracking.
Compressive residual stresses combined with initial distortion
may reduce the buckling strength, where as excessive deformation
might directly prevent the structure from performing its intended
function (Murugan et al.,1996; and Masubuchi,1980). |
