Role of Titanium and Boron with Respect to Refinement of Microstructure

Study on Notch Toughness of Weld Metal in Large Current MIG Arc Welding (Report 2)

Abstract

In the previous paper, the authors reported the fact that, when using the titanium and boron bearing consumable wire in the large current MIG arc welding, oxygen generated by the decomposition of CO₂ in the shielding gas significantly affected both the microstructure and the notch toughness of weld metal. Besides, majority of both titanium and boron was revealed to exist in weld metal as the oxide inclusions in higher level of oxygen content.
This paper describes the metallurgical studies to aim to determine the inherent and complex effects of titanium, boron and oxygen on the refinement of microstructure of the weld metal. The main test-results are as follows:
(1) Titanium may serve to refine the microstructure of weld metal as the oxide inclusions. That is, γ boundaries can become pinned by these oxide inclusions in the δ to γ transformation stage, resulting into refinement of r grain. In addition, these oxide inclusions can become the nucleation sites of ferrite in the r to a transformation stage, resulting into fine acicular ferrite structure.
(2) The oxidizing of boron is retarded by titanium, and boron as solute atom still remains in the higher range of oxygen.
(3) The solute boron segregated in γ grain boundaries may suppress the nucleation and growth of proeutectoid ferrite.
(4) From the observation by electron microscopy and the analysis by energy dispersive spectroscopy, the fine dispersed inclusions in the matrix structure of weld metal were identified with the compound oxides of Ti, Mn, Al and Si.