Microstructural evolution in low carbon steel Ti–B weld metals with several Al levels

Abstract

Microstructure development in welding material containing Ti and B has been in situ observed by using laser scanning confocal microscopy, with various cooling rates. The samples with different aluminum contents were prepared for the observation. In the sample with low Al/O ratio (0.48–0.73), an acicular ferrite microstructure transforming from intragranular nucleation site was in situ observed. On the other hand, with high Al/O ratio, the bainite microstructure originating from grain boundary of austenite was in situ observed. The transformation temperature decreased with increasing aluminum content. It was clear that the microstructure difference was due to the different inclusion in the matrix. Based on those observations, the chemical composition, cooling rate and microstructure were systematically related and the microstructural developments were summarized as a CCT diagram.

Keywords:

• acicular ferrite
• bainite
• in situ observation
• nucleation site
• weld metal