A MELTING RATE AND TEMPERATURE DISTRIBUTION MODEL FOR SHIELDED METAL ARC-WELDING ELECTRODES

A mathematical model of the shielded metal are electrode has been deve loped to predict its temperature distribution and melting rates. The h eat transfer equations of the melting electrode were solved: Conductio n along the electrode, heat delivered to the end of the solid electrod e from the melt, the heat of decomposition of calcium carbonate in the covering and Joule heat generation in the electrode were considered. The ther mal properties of the core rod were allowed to vary with temp erature. The covering was modeled as a thermal mass (constant heat cap acity) with the same temperature distribution as the core rod. The rat e of heat transfer from the melt was set proportional to the current w ith the constant of proportionality determined by matching model predi ctions of the melting rate to experimental results. A finite differenc e technique was used to solve the final set of nonlinear equations. Ex periments were conducted on E7018, 354-mm-long, 3.2-mm-diameter commer cial electrodes to determine melting rates and, using thermocouples, t emperature profiles. The model was able to predict the consumed length within 11 mm for currents ranging from 117.5 to 160 A. Ar the recomme nded welding current of 130 A, the rms difference in the predicted tem perature between the model and experiment was 26 degrees C (79 degrees F). At the ex- tremes of the range in current for practical welding, 110 and 150 A, the rms differences between the model and the experimen ts were 108 degrees C (226 degrees F) and 150 degrees C (302 degrees F ), respectively. The model predicted that at 130 A, the CaCO3 in the c overing starts to be prematurely consumed when the are reaches the pos ition of about 170 mm from the holder; at 140 mm from the holder, all of the CaCO3 is predicted to be consumed prematurely.