Prediction of weld-metal composition during flux-shielded welding

Abstract

It is well known that the slag and metal do not reach equilibrium during fluxshielded welding, due to the short reaction times and large thermal gradients inherent in the process. Nonetheless, many investigators have attempted to define the effective equilibrium by empirical analysis of experimental data. The results of these analyses have generally been contradictory and, in most cases, have lacked a firm scientific basis.

In the present study, it was assumed that a thermodynamic equilibrium does exist, but that the approach to this equilibrium is controlled by kinetic processes. The equilibrium is further assumed to be related to the concentration of an element in the slag withrespect to the concentration of the element in the weld metal. It is thus possible to modify data from steelmaking slag-metal reactions for use in welding. In the present paper, the specific assumptions and the procedure for calculating the equilibrium composition of the slag and the metal are described. This procedure allows a priori prediction of the equilibrium from the initial composition of the electrode, the base plate, and the flux. The results are specific to particular flux systems (e.g., manganese silicate or alumino-silicate) and depend on the basicity of the flux as defined by a modified International Institute of Welding formula. The primary reactions of interest include oxygen, silicon, and manganese. Studies involving chromium are in progress. Slag-metal reactions involving nickel or molybdenum are thought to be less important.

The theoretical predictions have been tested experimentally by two techniques. In the first, a series of over one hundred welds, with initial manganese varying from 0.01 pct to 3.1 pct, silicon varying from 0.002 pct to 3.2 pct, and basicity index varying from 0.2 to 4.0, were compared with the predicted equilibria. Although equilibrium was rarely attained, the change in composition was always in the direction toward, rather than away from, equilibrium. This confirms the generality of the calculation.

Next, several specific fluxes were tested by varying the initial weld-pool composition. The weld-pool chemistry at which no increase or decrease in Mn or Si occurs during welding is defined as the experimental equilibrium composition, which is then compared with the predicted equilibrium. In all cases, the agreement was quite good. The difference between experiment and prediction is rarely greater than 0.1 pct for variations in Mn of 0 to 1.8 pct and in Si of 0.12 to 1.3 pct.