STUDIES ON MECHANICAL PROPERTIES AND FRACTURE TOUGHNESS OF WELDED HIGH MARTENSITE DUAL PHASE STEELS FOR STRUCTURAL APPLICATIONS

Composite microstructures of ferrite and 30 to 76% martensite have been obtained by intermediate Quenching (IQ) heat treatment of a boron-vanadium micro-alloyed steel. Welding parameters were established for shielded metal are welding (SMAW) using bead on plate experiments. Test coupons were prepared as per AWS D1.1 (Structural welding code). The microstructural studies were correlated with microhardness, tensile, bend, impact and fracture toughness values. A Scanning Electron Microscope analysis of the broken welded tensile specimens and ½ CT specimens were carried out. The results indicate that as the percentage of martensite content increases in the welded specimens, there has been increase in the ultimate tensile strength, yield strength, percentage of elongation, impact and fracture toughness values. The fractographic observation of welded tensile specimens indicates predominantly cleavage to predominantly dimpled fracture surface. Where as, ½ CT welded specimens indicate a cleavage type of fracture in the fatigue pre- cracked regions and a dimpled type of fracture in the fast crack growth region due to tensile loading. Further it has been found that the microstructure of the base material has a bearing on the mechanical behaviour exhibited by the weldments. The mechanical properties of the welded joints were found to be comparable with mechanical properties of the base metal.