TO STUDY THE WELDING BEHAVIOUR OF CaO-A1203 BASED FLUXES

Abstract

For flux shielded arc welding process, it is desirable to achieve the specified weld metal chemistry along with lower amount of dissolved oxygen. This is essential for attaining mechanical and chemical properties of weld metal in the required range. Specific electrode wire, flux combination is selected for welding of a given material as the dissolved oxygen affects the toughness of the we-ld metal for quality steels. Fluxes giving rise to lower oxygen content are invariably employed. In the present investigation of CaO-A1203 flux system has been selected, containing small amounts of MnO, Ti02 and Si02. To attain the desired melting point and melting range, necessary quantities of CaF2 have been incorporated based on the available information in the form of binary, ternary and quarternary oxide, fluoride phase diagrams. At the outset the prime criteria of selection of different flux composition has been the liquidus temperature. Flux compositions suitably designed were used to prepare fused fluxes by melting. Flux compositions were melted in small quantities in graphite. crucibles in induction melting furnace. The molten fluxes were quenched, crushed and graded in predecided size ranges. Later they were mixed in definite proportions to provide a predecided size consist. All the fluxes were prepared with same size consist. The basicity of the fluxes ranged between 1.55 to 4.11. The manufactured fluxes were then used for bead-on-plate welding, using plain carbon steel electrode wire and plain carbon steel base plate. Each flux was used to lay bead at four different current values namely 400,450,500 and 550, Amps. keeping voltage, welding speed, electrode extension and polarity constant. For each welding run the welding behaviour of flux was carefully observed and it was found that the welding behaviour improved as basicity of the flux is increased. Sample were prepared from each weld run and were subjected to microscopic and metallographic examination to determine depth of penetration and amount of metal deposition. For each weld run percentage dilution was determined and found that amount of dilution increases with increase in basicity and current. The weld metal was analysed for carbon, silicon and manganese content and compared with expected composition based on percentage dilution. It was found that for most of the fluxes there was significant loss in carbon and silicon, where as manganese was gained by the weld metal. Loss of carbon increases with increase in current, but decrease with increase in basicity. Where as Silicon content decreases with increase in current and flux basicity. Fluxes having basicity 3.53 and 4.11 resulted in minimum Silicon content in weld metal, where as they showed maximum gain in manganese content. The effect of CaF2 which varies between 25 to 33.14 wt. % has been found to be negligible on molten flux weld metal reactions.