Assessment of mechanical properties in high-strength steel weld metals by means of phase transformation temperature

The evaluation of the mechanical properties of weld metals in structural steels is one of the most important factors of weld quality control. Improvements in methods of tensile strength assessment could facilitate the choice and development of suitable filler materials. For low-alloyed ferritic steel weld metals, the mechanical properties depend strongly on the cooling rate of the weld. Since the formation of ferrite and bainite is diffusion controlled, the solid state phase transformation temperatures are also influenced by the cooling rate. In this paper, experimental analyses of the mechanical properties of welded joints in high-strength structural steels are made taking into account the phase transformation temperature of the weld metal. Therefore, joint welds of quenched and tempered and TMCP high-strength structural steels in the yield strength range between 690 and 1100MPa were carried out using the gas metal arc welding process. Within the welding setup a pyrometer was used to acquire the cooling curve of the weld metal surface. This allowed to apply the single-sensor DTA method for the determination of phase transformation temperatures. A comparison with longitudinal tensile test results showed that a correlation between tensile strength and phase transformation start temperature exists for the welded joints under investigation. Even for yield strength and notch impact toughness, a relationship could be observed. For a reduced phase transformation temperature, higher yield strength and impact toughness were found.