In Pursuit of Purity, Technology Goes with the Flow
Molten metal demands a lot of attention, to ensure purity while it is in the furnace and stability as its filling the mold. In between time, when the furnace is tapped or poured out, the metal must be monitored to ensure its quality is not degraded – and there is technology to assist with that.
Liquid metal tapping may involve various metals and alloys (steel, iron, aluminum, copper, brass) for operations of varying scales. Accurate measurements of liquid metal temperature are required to ensure the quality of the final product.
Non-contact temperature measurements
Thermocouples often take these measurements, typically by dipping thermocouples that contact the liquid metal. It is impossible to measure the metal stream, so the reading must be taken in the ladle or converter, where the metal runs out into the mold or another vessel.
Dipping the thermocouple into the metal delivers an average temperature for the liquid metal within seconds, and at its dipping depth. It dips into the metal, avoiding any surface deposits that could affect the measurement.
However, a manual measurement requires the operator to be close to the metal stream, which is a safety hazard in that hot and smoky atmosphere. Also, thermocouple readings are likely to be inconsistent – two or three different thermocouples are likely to produce two or three different measurements, so accuracy is an issue. Cost is another, as the dipping tip for each thermocouple must be replaced after one or two measurements.
Non-contact measurement of the liquid metal can be done using a hand-held portable pyrometer, a stationary pyrometer, or a thermal imager, taken on the surface of the metal or of the metal pouring/tapping stream. However, oxygen in the atmosphere may lead to an oxide layer forming on the liquid metal surface, creating slag/dross. This affects the emissivity of the metal, making it more difficult to achieve an accurate reading.
Additionally, as the slag layer becomes thicker, the surface temperature decreases compared to the liquid metal temperature.
So, for non-contact temperature measurements it is important to maintain a view of the metal surface where it is nearly (or completely) free of metal oxide.
Automated or manual tapping processes where liquid metal is poured into molds is such an application. A thermal imager or pyrometer is able to measure the temperature of the liquid tapping stream, which typically has only a thin oxide layer (a “pouring skin”) on its surface. This ensures the temperature being measured is that of the liquid metal, as it runs into the mold to create the final product. It is the latest and most exact point to measure the temperature of the liquid metal in the tapping/pouring process, before it solidifies, so it provides the best, 24/7 assessment of the quality and metallurgical properties of the metal product.