Influence of K2ZrF6 and SiO2 on refining ability of flux for manufacturing bimetallic castings

Bimetallic material is considered as an advanced functional material due to the unique physical and mechanical properties varied over the layers. Formation of bimetallic castings with steel base greatly depends on diffusion coupling with the second material working layer at elevated temperatures. The common technological problem is to remove the oxide films from the solid steel surface while its heating prior pouring melt of the working layer. This work studies the protective refining fluxes matching the selected requirements of good wetting the surfaces of solidified metal, effective protection against oxidation in the working temperature range, easy separation after pouring liquid metal and high refining capacity for oxides. The most effective fluxes based on Na2B4O7 and B2O3 compounds were used. To improve their ability to enhance wetting, work of adhesion and reduce surface decarbonisation addition of K2ZrF6 and SiO2 to the flux in the amount of 3–4 wt.% was proposed and investigated. It was established that such additions increased wettability up to 9 and 20%, respectively and reduced the average rate of decarburization in the temperature range from 800 to 1000 °С on 57 and 37%. The complete reduction of iron from scale on the steel surface was observed, while in the case of Na2B4O7 — B2O3 system it achieved 30-40% only. The mechanism explaining such a result is proposed to be due to the prevailing effect of zirconium in protecting the surface of the steel base from the oxidation and decarburization. The important result of the study is the recommendation of optimal flux composition (wt.%): Na2B4O7 — from 60 to 80; B2O3 — from 10 to 30; K2ZrF6 — from 3 to 4; SiO2 from 3 to 4. Thus, such flux could improve the production of steel based bimetallic castings and increase their properties.