Effect of titanium and nitrogen on the transformation characteristics of acicular ferrite in reheated C–Mn steel weld metals

Abstract

The nucleation and growth processes of acicular ferrite in C–Mn steel weld metals containing various levels of titanium and nitrogen have been studied using dilatometry combined with transmission electron microscopy associated with energy-dispersive X-ray analysis. The as-deposited weld metals were thermally cycled under continuous cooling conditions, and fully developed and partially transformed microstructures were studied. Results showed that austenite transformed to ferrite in titanium-free C–Mn steel weld metals over a comparatively large range, and the resulting microstructure was dominated by Widmanstatten ferrite. The addition of titanium, typically 410 ppm, to C–Mn steel weld metals was found to accelerate the transformation kinetics, reducing the range in transformation temperature and modifying the weld microstructure with Widmanstatten ferrite being replaced by intragranularly nucleated acicular ferrite. Subsequent additions of nitrogen of around 240 ppm to C–Mn and C–Mn–Ti inhibited the transformation, but this soluble nitrogen had a little effect on weld microstructure. It is concluded that the development of acicular ferrite in C–Mn–Ti–N steel weld metals is associated with TiO- and TiN-type inclusions which act as effective sites for nucleation of acicular ferrite. This finding illustrates the possibility of using nitrogen as a deliberate alloying addition under some circumstances.