STUDIES ON FATIGUE BEHAVIOUR OF RESISTANCE SPOT WELDED Mn-Cr-Mo DUAL-PHASE STEEL

Abstract

The fatigue test of resistance spot welded trot roiled Mn-Cr-Mo dual-phase steel, of sheet thickness 3.3 mm, has been carried out by INSTRON universal testing machine under load control mode. The fatigue behaviour of the dual-phase steel has been studied by varying weld time from 30 to 50 cycle keeping electrode force and holding. time constant at 565 Kg and 48 cycle respectively. It has been.observed that fatigue life increase with increase in weld time for a given value of maximum load. This is attributed to variation in nugget diameter, which is found to -increase with increase in weld time from 30 to 50 cycle. The fatigue cracks are found to propagate from weld centre due to presence of discontinuity flaws at the central region. The microstructure of the weld has also been studied and it has been found that ferrite at the weld centre coarsens from accicular to bulky nature as the weld time increase from 30 to 50 cycle. Further, the effect of corrosion on fatigue life of resistsance spot weld is investigated. The fatigue life of weld is found to decrease logarithmically by _30-40% with increase in corrosion time from 7 to 21 days primarily because of pit formation at the weld boundary.