The temperature of molten iron is measured several times in foundries, as it is one of the most important physical parameters. The accuracy, reproducibility and reliability of these molten metal temperature measurements influence the quality of the end product as well as the productivity of the plant.
Here, Rudi Bittniok of Foseco gives an overview of the actual systems in use, and the new trends and developments - mainly for iron automotive foundries where automatic or semiautomatic pouring with a pouring furnace or tundish are used.
In hand moulding foundries, there is really only one way to measure the temperature and that is by means of a disposable dip lance.
This ‘one-shot’ solution is also used in automotive foundries at different stages of the production process. A basic overview of the typical ‘one-shot’ process follows.
One-shot lances are the most common way to measure the temperature at different stages of the liquid metal transport. Multiple use lances are also available. These systems are widely in use and easy to handle. There is no special equipment needed and the investment costs are quite low.
In the future disposable lances will always be found in foundries, especially at the melt deck, where continuous or optical systems are impossible to install.
However, there are also some disadvantages with using dip lances as they create a lot of waste and the measurement is dis-continuous.
There is also a health and safety aspect because the worker is directly involved in the temperature measurement process and splashes can be dangerous.
In addition, the inherent variability of this manual process means that temperatures are recorded at different locations and depths within the liquid metal. This makes it impossible to establish meaningful temperature trend data.
Continuous Dip Measurement
The ACCUMETRIX* temperature measurement system is a continuous dip measurement system placed in the runner of the pouring furnace or in the unheated tundish, close to the stopper. ACCUMETRIX guarantees safe working conditions as the probe is securely installed. No worker is required until the end of the shift where the thermocouple should be removed for the change of the protection tube.
The measurement principle uses the phenomena of the electromotive force (EMF) that is generated at the heated joint of two dissimilar conductive materials, known as the Seebeck effect, the magnitude of which is temperature specific.
The most widely used thermocouples for measuring temperature between 1300-1500°C are based on platinum-rhodium (Pt/Rh) alloys of which there are three internationally recognised versions - R, S and B.
ACCUMETRIX also results in less waste for the foundry.
For the past couple of years, special software has been available to monitor the results and store the data in a SQL database
The ACCUMETRIX temperature measurement system comprises:
• An inner long-lasting thermocouple type B
• A VISO refractory protector tube
• A support for the VISO refractory protector tube
• Cables and connectors
• A signal converter
• Software to store, interpret and report the results
Getting a real continuous system where the temperature of the melt is measured offsets the negative impact of certain investment costs compared to disposable lances.
It is necessary to have a certain bath level of approximately 150mm to ensure the best monitoring results. Sometimes it is not possible to maintain this - especially in pouring furnaces, where the metal bath level is changing for different reasons during the process.
Until the minimum metal height has been reached, the response time can be approximately on to two minutes but at least, the foundry worker can be sure to get a trend in the temperature profile.
The comparison between ACCUMETRIX and the disposable dip probes shows the enhanced ability of continuous systems to detect hot spots or cold spots. However, dip probes are still useful to compare results or for calibration purposes.
The system is a reliable solution for iron foundries casting ductile iron. In unheated pouring ladles (tundishes) the system works especially well and multiple lengths of protection tubes and thermocouples can be provided.
Case Study - MAT Iron Foundry, Germany
The MAT iron foundry in Ueckermünde, Germany had problems with excessive temperature probe waste, a risk of metal splash injuries with spot measurement systems and their automotive customers required evidence of temperature measurements.
The requirements of MAT were to improve a safe working environment, the elimination of disposable temperature probe waste and to deliver a continuous temperature measurement.
Foseco offered MAT the ACCUMETRIX system including the long life thermocouple, the hot zone cable and VISO thermocouple protection tube.
With the system the in line metal temperature is now under control. The system reduced the risk of metal splashes and included an installation of a pouring temperature database, which helped to reduce temperature related defects.
MAT also achieved the environmental benefits of reducing energy consumption and eliminated the waste of disposable temperature probes.
Optical Measurement Systems
Another possibility to measure the temperature in iron foundries is the continuous way by using an optical pyrometer. The system is called TEMPSTREAM.
Here, the measurement is directly in the pouring stream. The system captures the energy volume disposed by the pouring stream and two points are targeted.
It has to be considered that the accuracy of the pyrometer is always 1°C but that does not mean that the value of temperature taken is also +/- 1° because certain influences are disturbing the process (all types of dust which bring a shade to the measurements; dust on the lens, bad focus, slag on the nozzle, bad stream etc.). This means the value is a relative number and comparable measurements with dip probes are necessary from time to time. Also the system has to be frequently calibrated.
A picture with a thermal camera shows a further difficulty of an optical measurement system, as the temperature is never homogeneous. Therefore, the real capture temperature is changing, sometimes by 60°C! So process software is needed to compensate for this variability.
The TEMPSTREAM software stores ten measurements per second. For example: with a pouring time of ten seconds 100 values will be achieved which will be analysed to an ideal value.
In grey iron foundries, the TEMPSTREAM system is considered a reliable solution.
New Trends – ACCUOPTIX*
ACCUOPTIX* is also an optical continuous temperature measurement system.
For the moment it has only been used in continuous steel plants, but a version for testing in iron foundries will be available in the near future.
The new ACCUOPTIX sensor integrates a long-life optical detector with an advanced electronic interface to overcome the aforementioned described disadvantages. The associated refractory tube ensures, by its design, an optimum optical measurement and easy handling. Due to its remote installation, the optical sensor is not exposed to heat radiation and not subject to damage in case of accidental breakage of the refractory tube.
The principle of measurement of the ACCUOPTIX infrared sensor is based on the black body theory.
A black body is an ideal body that absorbs all the electromagnetic radiation received without diffusing or reflecting it. The black body can be defined as the ‘perfect absorber’ and hence ‘perfect emitter’ of electromagnetic energy.
To understand the nature of electromagnetic energy, we imagine an ideally dark room in which we place a steel rod to be heated up. During the heating-up procedure the steel rod will first become red and continue to become white with increasing temperature. Simultaneously the rod emits electromagnetic radiations relative to the colour. These radiations are proportional to the temperature of the body following Planck’s law.
The same effect is used by the infrared continuous temperature measurement. The refractory tube is considered as a black body. When immersed into the liquid steel, the wall of the refractory tube is heated up and will emit electromagnetic radiation. An optical lens assembly will capture this radiation inside the tube and transmit it to a pyrometer.
Liquid steel/iron heats the refractory tube causing the internal cavity of the tube to behave as a black body radiator. The photons that escape the cavity are absorbed by the photodiode to create a current in an external circuit
The value of this photocurrent corresponds directly to the received electromagnetic radiation and is proportional to the temperature of the steel.
The ACCUOPTIX optical based system is supplied with a high precision calibration device. It allows a fast and precise calibration of the complete measuring chain of the sensor directly on the casting-floor with a fully automatic process.
The systematic error of the measuring chain of the system corresponds to the error of the calibration device, and its instrumentation. The errors of the optical electronics are not considered in the systematic error. These errors exist, but they are totally eliminated during the calibration process by the calibration device. It guarantees systematic error of +/- 2.2°C. The instrumentation is a signal converter with a systematic error of +/-1.5°C. For ACCUOPTIX optical based continuous temperature measurement, a systematic error of +/-2.7°C is calculated.
There are different ways of measuring the temperature in iron foundries, which is an important factor to determine the casting quality.
This article has given an overview of the actual methods in use and Foseco can provide systems for all four methods. Sooner or later a foundry needs to monitor its temperature in a continuous way because of customer demands and the significant improvement in health and safety.
Therefore, the ‘one-shot’ dip probes will be replaced by continuous systems such as ACCUMETRIX or TEMPSTREAM.
Further new developments of the patented ACCUOPTIX system could also be interesting for automotive iron foundries.
Contact: Paul Jeffs, UK technical manager, Vesuvius UK Limited - Foseco Foundry Division, Tamworth, Staffordshire B78 3TL UK. Tel: +44 (0) 1827 289999, email: firstname.lastname@example.org web: www.foseco.com
* ACCUMETRIX and ACCUOPTIX are Trade Marks of the Vesuvius Group, registered in certain countries, used under licence.
For copies of the figures that accompany this article refer to the full printed version in the November 2016 issue of Foundry Trade Journal.