Microstructural and Inclusion Characteristics of C–Mn Steel Welds at a Minimal Level of Titanium

To better determine the lower limit of titanium content for the formation of acicular ferrite, C–Mn steel welds varying the titanium concentration at a minimal level from nearly zero to about 30 ppm have been investigated. Optical and SEM observations of as-deposited weld microstructures showed that when the titanium content increased from 1 to 9 ppm or higher, a dramatic change in weld microstructure from one dominant with ferrite with second phase to one with acicular ferrite takes place, being attributed to a concurrent increase in nucleation potency of inclusions for acicular ferrite plates. These results suggest that the critical limit of titanium required is about 9 ppm or even lower for the present weld system. In TEM, the inclusions of titanium-containing welds were found to be partly covered with a titanium-rich phase, indicating that this phase plays an important role in promoting the nucleation ability of inclusions and thus in developing a weld microstructure composed of a high proportion of acicular ferrite.