– Melting processes
Melting processes in metallurgy are high-temperature processes, and in those areas exposed to thermal stresses, materials are required that can reliably withstand application temperatures from 600 to around 2000 °C over economically viable periods.
For special applications in the NF metal industry, components made of oxide and non-oxide ceramics are used, which, besides their suitability for high-temperature processes, offer other application-specific properties. The following examples make this clear.
- Platinum and platinum alloy melts on a purity level of 99.9 % and higher are usually produced in vacuum at temperatures up to 2000 °C by means of inductive heating. The oxide ceramic containers used for this purpose are exposed to high thermal and thermomechanical stresses. Moreover, a high purity level is required from these materials as during the melting process the low oxygen partial pressure can lead to the reduction of accessory substances of the container material and consequently to their diffusion into the metal melt. These so-called platinum poisons like, for example, Si, Al, C, P and S react with the platinum to low-melting phases or, on later use in air, to oxides with the result of material embrittlement. Both lead to impaired quality of the material during its production or use.
For such precious metal melts, today mainly a high-grade oxide ceramic crucible and plate material made of high-purity cubic ZrO2, usually doped with CaO, is used, doping with other oxides effecting increased strength and accordingly increased resistance of the material to thermal shock. As platinum and its alloys do not wet the ceramic, the ceramic material does not have to be dense-sintered.
- During the processing of aluminium melts, oxide and non-oxide materials are used in positions where special requirements for thermal stability, thermal shock resistance, surface properties and corrosion resistance have to be met. In direct contact with aluminium melts, the use of components made of Al2TiO5 (ATI) or Si3N4 is suitable as these material types are not wetted by the aluminium melts and also bring extremely high resistance to thermal shock and high temperature gradients.
- The high vapour pressure of magnesium already just above its melting point means that during the melting process special protective measures are needed to limit and avoid evaporation losses. The inert gases used for this purpose like, for example, SF6, SO2 or CO2, are damaging to the environment and sometimes toxic. One measure that at least can effect a drastic reduction in evaporation losses is the use of oxide ceramic hollow parts that cover the Mg melt. According to the results of a research project, ATI ceramic is also suitable as a material for this application.
- In the production of melts made of titanium, on account of the pronounced reactivity of this material, the container materials have to meet special requirements. A container material generally suitable for such melts is Y2O3 Selected typical properties of such a ceramic are listed in the following.
- Purity: >99.9 %
- Melting temperature: 2450 °C
- Stable in oxidizing and reducing conditions to over 2000 °C
- Low wettability when exposed to pure titanium melts
The quality of a steel melt can be evaluated based on measurement of oxygen activity. The short-term recording of the measured values necessary for evaluation can be performed with the help of electrochemical sensors that are immersed in the melt. The solid electrolyte made of ZrO2 ceramic primarily used in the form of a small-size tube usually a consists of a high-purity, dense-sintered Mg-PSZ type that thanks to its high thermal and thermomechanical stability enables sufficiently long measurement times.
Also during the carburization of steel, such sensor elements are used in this application for monitoring atmospheres containing CO, H2, N2 and gaseous hydrocarbons. The solid electrolyte consists in this application generally of a Y-PSZ type. It is often operated as a tube closed at one end with lengths to the order of 800 mm in long-term use in the furnace atmosphere.
Such sensors are often a key measurement and control instrument in the production of copper with purities well over 99.5 %.
The materials of the oxide ceramic sensor elements made of Mg-PSZ and Y-PSZ generally exhibit the properties listed as a guide in the following:
- Purity >99.5 %
- Fe content:
- Density: >98 % of the theoretical value
- High thermal stability
- High thermal shock resistance
- High resistance to deformation even at high temperature
- He leak rate: <10-10 mbar*L/s
- Mean bending strength: >350 MPa
- Modulus of elasticity: 200 GPa
- Specific electrical resistance in the range 104 Ω*cm at 500 °C and 25 Ω*cm at 1000 °C
- Surfaces suitable fort hick film technology
- Realization of substance-to-substance high-vacuum-tight bonds with metals