EFFECT OF NIOBIUM ON HAZ TOUGHNESS OF HSLA STEELS

Experimental low alloy steels containing different levels of C, Nb and Mn were employed, to each of which a simulated thermal cycle was applied to produce CGHAZ microstructure. It was found that Nb has some beneficial effects, such as retardation of GB ferrite and enhancement of intragranular ferrite formation in low and medium carbon steels. For high carbon steels, it enhances the carbides precipitation and as a result the MA area fraction decreases. The initiation site of brittle fracture was found to be dependent on the microstructure. Three categories of microstructure are described; namely grain boundary ferrite with Widmnastatten ferrite plus areas of degenerated pearlite, which is denoted as category I; grain boundary ferrite plus intragranular acicular ferrite which is called category II; and finally the lath structure which is called category III. The brittle fracture was observed to be initiated at the pearlite colonies in category I, to be associated with the grain boundary ferrite in category II, and to be initiated at the intersection of the bainitic packets in category III. The area fraction of MA increases by Nb addition for low and medium Carbon steels but for high Carbon steels, MA area fraction shows a slight increase due to carbides precipitation.