The microstructural characteristic of HSLA welds containing different amounts of titanium were evaluated carefully. It was observed that the microstructure of welds consisted of ferrite with mixed morphologies, and small amounts of pearlite and martensite-austenite micro-constituents. Because of insufficient time for diffusion of carbon, formation of pearlite lamellae could not be completed in the weld region. Martensite was formed from carbon enrichment of austenite during nucleation and growth of acicular ferrite and bainitic ferrite. While coarse manganese sulfide particles had weak interface strength with matrix and formed some micro-fissures; increasing titanium amount of welds dispersed different modified morphologies of inclusions in the microstructure. Several types of precipitates including spherical, cubical, fine and irregular titanium carbonitride were found in the microstructure and they were favorite sites for nucleation of acicular ferrite. The size of cubical/spherical and fine precipitates was measured between 70-100 and 10-20 nm, respectively. Low energy interface of inclusion/matrix and the depletion of carbon in austenite were proposed as the nucleation mechanisms of acicular ferrite from nano-sized inclusions.

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