BES - Fundamentals of Welding and Joining.

Fluid flow and Coarsening of Oxide Inclusions in Steel Weld.

The oxide inclusion characteristics such as, size, number density, composition, and oxidation sequence control the microstructure development in steel welds. In the previous work at ORNL, an inclusion model was developed to describe the inclusion formation as a function of weld metal composition and process parameters. In the present investigation, the effect of fluid flow on the inclusion growth was considered. The fluid flow conditions during welding were approximately simulated using a thermo-mechanical simulator. The results of this investigation showed that small inclusions in the liquid steel collided and coalesced into larger inclusions. This type of inclusion growth was found to be rapid than coarsening by diffusional growth. The calculations from theoretical model of collision and coalescence agreed with the experimental observations. The rate of collision and coalescence of inclusions was found to be proportional to the gradient of fluid flow velocity, residence time, and the size.

Significance:

Since the rate of inclusion growth is now linked to the fluid flow conditions in the liquid steel, the inclusion model can be extended to a wide range of welding processes. This research can be used to understand the removal of inclusions from the weld and may be used as a tool to control the inclusion size and number density in high-oxygen welds. Moreover, partial elimination of large oxide inclusions from the steel weld, after the deoxidation process, may prove to be advantages to ultra-low-carbon bainitic steel welds.

The research was conducted by S. S. Babu, S. A. David and J. M. Vitek. The research was sponsored by the Division of Materials Sciences, Office of Basic Energy Sciences of the U. S. Department of Energy.