In-situ observation on the influences of Ti on phase transformation of weld metal processed by high-heat input welding

The in-situ observation of the phase transformation processes of weld metal during high-heat input welding were carried out by a high temperature laser confocal microscope. The influences of Ti content on the phase transformation process were investigated. It was found that the Ti inclusions could act as the nucleation sites for alpha ->gamma transformation during the heating stage of welding thermo cycle and inhibit the growth of austenite grains. The number of inclusions was increased with increasing Ti content. During the cooling stage of welding thermo cycle, the inclusions could induce the nucleation of acicular ferrites when the Ti content was below 0.078%. With increasing Ti content, more acicular ferrites collided with each other and restricted their further growth. When the Ti content was increased up to 0.115%, a proportion of Ti atoms were dissolved in the matrix, which increased the hardenability and thus generated the lath bainite microstructure instead of acicular ferrite.