TO STUDY THE EFFECT OF HEAT INPUT ON THE MICROSTRUCTURE OF WELDMENT IN WELDING OF II.S.L.A. (MICRO ALLOYED) STEEL

Abstract

In the present investigation HSLA (microalloyed) steel plate of 6.25 mm thickness has been fusion welded by manual metal arc welding using 3.15 mm diameter low hydrogen basic electrode (AWS E7016) at voltage of 30V and currents of 150, 175, 200, 225 and .250 Amp. The net heat input generated for the above power ratings are 2800, 3000, 3150, 3280 and 3450 J/mm. A constant welding speed approximately 1.87 mm/sec. has been maintained. The welded specimens are subsequently prepared for metallographic examination and hardness testing and bend test. It is observed that with increase in net heat input the amount of weld metal deposition is increased e.g. for Hnet 2800, 3000, 3150, 3280 and 3450 J/mm the metal deposition rate (Md) is found to be 9.33, 11.23, 11.57, 12.52 and 12.73 respectively. The microstructure of the base metal has been found to be of typical low carbon steel having ferrite + pearlite. However the grain size of the present ' steel is relatively finer due to the presence of grain refiner Nb. In all the samples acicular structure and widmanstatten structure has been observed in the HAZ. Typical cast structure has been observed in the weld metal region of all the samples. With increase in net heat input grain coarsening is observed in the weld metal region and in the HAZ region. Coarser dendritic ferrite are clearly visible-in higher net heat input samples. Macrohardness values have been found to decrease from the weld metal .region to the base metal. The hardness of the weldmetal is always found to be more than that of the HAZ and hardness of the HAZ is found to be more than that of the base metal. With increase in net heat input the hardness of the we1dmetal and HAZ is found to be decreasing. (v) For Hnet 2800, 3000, 3150, 3280 and 3450 J/mm the peak hardness values in the weldmetal region are found to be 197, 193, 187, 184,. 179 VHN. The hardness of the base metal is 164 VHN. The hardness values at a distance of 4 mm from the weld centre line in the HAZ region have been found to be 190, 188, 182, 180 and 177 VHN, for the above mentioned heat inputs respectively. This has been explained on the basis, of grain coarsening. No welded sample has been fou