Weldability of nitrogen-enhanced HSLA steels

Light microscopy, electron microscopy and Charpy-V-Notch (CVN) testing were used to characterize two Ti-V microalloyed steels which contained 130 and 30 ppm of nitrogen with respect to weldability. Particular emphasis has been placed on the understanding of the heat input-microstructure-toughness relationship in the high nitrogen HSLA steel. Within the 1 to 3 kJ/mm heat input range, the high nitrogen steel exhibited coarse grained heat affected zone (CGHAZ) toughness superior to that of the low nitrogen steel. The transition temperatures (measured at 50 ft-lb impact energy) were, in average, 15 to 60 C lower than those presented by the low nitrogen steel. Quantitative metallography results showed that high nitrogen steel CGHAZs have smaller austenite grain size and larger amount of grain boundary ferrite. Combining the microstructural information and toughness data, it was concluded that for the high nitrogen steel to exhibit superior properties than the low nitrogen steel, the final HAZ microstructure must contain at least 60% of lathy (bainitic) phases, balanced with ferritic products.