Effect of Weld Cooling Rate on Inclusion Formation in Low Alloy Steel Welds

Technical Accomplishments

Oxide inclusion characteristics such as, size, number density, composition, and oxidation sequence control the microstructure development in low-alloy steel welds. In previous work at ORNL, an inclusion model was developed to describe the inclusion formation as a function of weld metal composition and process parameters. Further, the effect of fluid flow on inclusion coarsening was investigated and the results showed rapid coarsening of inclusions due to fluid-flow velocity gradients.

In the present work, the effect of widely varying weld metal cooling rate on inclusion formation was considered. Inclusion formation in autogenous laser, electron beam, and submerged-arc welds was investigated. Loss of oxygen from the weld metal was observed during electron and laser beam welding due to the low pressure and helium shielding, respectively. Rapid weld cooling rates and changes in oxygen concentration resulted in different inclusion characteristics in these welds. In spite of high cooling rates in laser welding, the inclusions were heterogeneous in nature [see Fig. 1]. The inclusion number density in the electron and laser beam welds was higher than in submerged-arc welds due to increased nucleation rate induced by large undercooling. The inclusion diameter and volume fractions were lower than in the submerged-arc welds. The inclusions in electron and laser beam welds were rich in aluminum and titanium compared to submerged-arc welds. The results compared well with the trends predicted by the inclusion model that was developed earlier. The inclusions that form during rapid weld cooling were found to nucleate acicular ferrite platelets as shown in Fig. 2. However, the extent of acicular ferrite formation was reduced by martensite formation due to rapid weld cooling.
 
Figure 1. Transmission electron micrograph shows the inhomogeneous nature of inclusions. Figure 2. Micrograph showing the nucleation of acicular ferrite platelet on an inclusion in the laser weld.

Significance:

The present work, in conjunction with earlier work by the authors, has shown that the phenomenological model for inclusion formation can be extended to a wide range of weld cooling conditions. It can be applied to processes ranging from low heat input laser welding to high heat input submerged arc welding. In addition, the present research has shown the importance of gas-metal reaction and undercooling on the inclusion formation characteristics. The research can be applied as a tool to enhance the formation of the acicular ferrite phase, which promotes an optimum combination of strength and toughness in welds.

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