Development and characterization of boron-free glazes for Al2O3-C stopper materials – A thermodynamic and DoE approach

Al 2 O 3 -C stoppers ensure the production of semi-finished or cast steel parts by enabling the control of the steel flow from the tundish or ladle. To minimize their oxidation during pre-heating as well as the interaction with molten steel (and slag), they are protected by boron-containing outer ceramic glazes and self-glazes. To develop equivalent boron-free stoppers, several self-glaze-forming agents of the system CaO-Al 2 O 3 -MgO-SiO 2 -Na 2 O-K 2 O were explored during this study. Thermodynamic calculations revealed that the addition of a mixture of CaSiO 3 , MgO, and Na 2 CO 3 to Al 2 O 3 -C would lead to a proper self-glaze forming behaviour that provides an effective oxidation resistance. Design of experiments was employed to optimize cold isostatically pressed batches carbonized at 1000 °C regarding their oxidation resistance at 1300 °C as well as the refractoriness under load behaviour. As a result, at least two Al 2 O 3 -C stopper compositions (Na 2 CO 3 :MgO:CaSiO 3 = 1.5:2:2 and 1.5:2:0.5) were identified that lead to a glaze formation that stops the oxidation even up to 1600 °C. While an amount of 2–3.5 wt.-% Na 2 CO 3 and CaSiO 3 showed a significant effect on the oxidation resistance, an amount of 2.0 wt.-% of MgO is necessary to ensure a high refractoriness as well. SEM/EDX and EBSD investigations revealed that the additives formed an amorphous glaze without attacking the Al 2 O 3 component. The physical and thermo-mechanical properties of the newly developed batches fulfil the requirements for an application as stopper materials. Batch B1 shows an open porosity of 13.5 %, a cold modulus of rupture of 8.4 MPa, an hot modulus of rupture of 4.7 MPa, and an elasticity of 15.32 GPa.