INFLUENCE OF VIBRATION ON THE RESIDUAL STRESSES AND MICROSTRUCTURE OF STEEL WELDMENTS

Abstract

Today welding has provided ample freedom to the modern designer to develop metallic products in which optimum mechanical properties, lightness and aesthetics are harmoniously blended. The major drawback of the welding process is the introduction of deleterious residual stresses into many metal structures and an unfavourable grain structure and grain size of the weldments and heat affected zone. Hence, whenever technically and economically possible a welded structure is submitted to a treatment aimed at eliminating, or at least at reducing, the residual stresses resulting from welding. Mechanical and thermo - mechanical methods are used to add favourable stresses and thereby remove distortions and residual stresses. In the, present work the influence of induced vibrations on residual stresses in the weld metal has been studied. The . evaluation has been based on cyclic low vibrational loading. Vibrations were imparted to weld coupons in two sets of conditions. In one case the weld coupon was vibrated during welding, and in hot condition followed by vibrations imparted in cold conditions. In the other case the vibrational treatement was given during welding and in hot condition only. Centre hole drilling technique was employed for measurement of residual stresses. it was observed that the magnitude of stress relaxation obtained as a result of hot vibrational treatment was of the order .of 22%. On the contrary, an additional increase of only.13% in stress relaxation was achieved corresponding to a 4.6 times increase in the vibrational loading in cold condition. It may be clarified here that hot vibrational treatment preceeds the cold vibrational treatment. iv Results of the above experiments indicate that vibrational stress treatment is an effective method of relaxing residual stresses in low carbon structural steels. Low frequency vibrational treatment is effective for relief of residual stresses both during and after welding. The relaxation being more. pronounced when treatment was given during welding and in hot condition as compared to cold - stress relaxation. It has been found that magnitude of the vibrational stresses required to achieve a greater degree of stress relaxation depends on the magnitude of the residual stresses originally present in the weld. Amount of stress relaxation achieved is also directly proportional to the amplitude of vibrational stresses. Evaluation of the effect of vibration on grain refinement of weldments has also been done as per ASTM E 112 and ASTM E 562 with different sets of vibration parameters. It has been found that introduction of mechanical vibrations into the weld pool in the frequency range 30 to 60 Hz is found to result in grain refinement with favourable microstructure. The change is more prominent in the 30 Hz frequency range where amplitude of vibration is 0.49 mm as compared to 0.15 mm amplitude in case of 60 Hz vibrations. The combined effect of vibration during solidification and in hot condition has found to improve the mechanical properties and hardness of the weld material marginally, whereas there is a considerable improvement in the toughness properties of weld metal.