Fatigue behavior of MAG welds of thermo-mechanically processed 700MC ultra high strength steel

Abstract

The present work analyzes the weldability of 8 mm thick plates of commercially available modern high strength steel with minimum yield strength of 700 MPa. The weldability is improved by modeling the welding conditions to evaluate and support the extensive analysis of the mechanical and the metallurgical properties of the welds. Materials were welded by a two-pass MAG process with four different heat inputs. All the welds and respective base materials underwent fatigue tests, tensile tests and hardness measurements. In tensile tests, the strains were measured using digital image correlation (DIC) equipment. Fatigue fracture surfaces were investigated for the highest and lowest loads tested using scanning electron microscopy. The failures in tensile and in fatigue tests occurred in the recrystallized HAZ. DIC measurements revealed that in tensile tests the strain localized in the domain of minimum hardness. The accumulative effect of both welding passes lowered the strength and hardness at the root side, making the root side more sensitive to weakening, as shown by hardness profiles. In addition, increasing the heat input lowered the fatigue strength of the welds. The welds had 150–300 MPa lower maximum fatigue strength (R=0.1) than the base material.