Temperature measurement and control of the hottest

2022-08-07
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Temperature measurement and control of precision casting successful precision casting manufacturers know the importance of process control for producing high-quality castings. The key variables in the casting process include mold temperature, mold insulation characteristics, cycle time and operator methods, but the most critical process variable is metal temperature. In the precision casting process, there are many major difficulties in the non-contact measurement of metal temperature. However, a recently developed device can provide quantitative feedback of real-time accuracy, revealing potential problems

the importance of temperature

in the precision casting process, especially in the "equiaxed" process, the metal temperature is the dominant factor. Therefore, it also has a direct impact on many quality characteristics. If the measurement and control are not proper, the difference of metal temperature will affect the size of finished castings, grain size, porosity (surface and interior), mechanical properties, product quality (i.e. the tendency of hot tearing), and the fullness of thin-walled parts

therefore, improving metal temperature measurement and control will improve quality and productivity, reduce maintenance and labor costs, and reduce testing costs and compensation costs

difficulty of temperature measurement

precision casting, especially precision casting using induction melting equipment, generally uses some type of non-contact infrared radiation thermocouple or pyrometer as the primary or secondary means of metal temperature measurement. People who use conventional pyrometers may not understand the potential error sources of their measurements, but simply pay attention to the "accuracy" technical conditions of the instrument, so they are often misled. These precision technical conditions are only ideal targets in the laboratory environment. Some situations in the real world can lead to surprisingly high measurement error values, including (but not limited to) the following:

1, unknown/changing emissivity - a variety of alloys, disturbance effect, temperature and wavelength dependence, and composition changes during processing, all of which play a role in the unpredictability of emissivity

2. Steam emission: for high-pressure melting (close to and above atmospheric pressure), the gas overflowing from the melting pool or crucible will increase or decrease the thermal radiation, resulting in errors

3. Observation hole obstacle: for most instruments, any weakening of signal will cause the decrease of temperature indication value; Dirt on the observation window affects the accuracy of most pyrometers

4. Observation window glass material: not all glasses have the same transmission performance; Some glasses are "gray", while the transmissivity of other glasses changes with wavelength. This will disable the conventional pyrometer

5. Calibration: the industry standard is to calibrate once a year. However, the drift and failure of the instrument have their own schedule. The ideal approach is to calibrate all optical elements used in the factory (observation glass or observation glass)

6. Instrument calibration: Aiming through the lens requires that the two optical paths overlap accurately, which will affect the conventional pyrometers of all levels

these difficulties are unique to optical temperature measurement. At the same time, there are process related difficulties, which complicate the temperature measurement of any type of instrument, including:

1. Acceptable range of process variables: unless the whole melting furnace is in a stable state (usually, this is unrealistic), there will be a range of temperature in the casting process. It is very important that this temperature range can ensure the quality of products

2. Signal processing capability: each analog to digital or digital to analog conversion between measuring instruments and control equipment is a potential error source, and the wide analog range leads to the lack of precision

3. Melting technology: poor melting technology will lead to excessive boiling of high steam pressure elements, disturbance of molten pool surface or formation of reaction products, all of which will cause errors of conventional pyrometers

4. Matching among ingots, crucibles and coils: these three components of the melting system are important for the melting cycle characteristics. Improper matching will cause slow and uneven melting, local overheating or sputtering. The above (3) selection of static verification points is also the source of error of conventional pyrometers

high temperature spectrometer for problem solving

high temperature measurement technology has its inherent advantages: no pollution, no poisoning when removing the sensor; Easy to install and use; Continuous measurement is available; No consumable materials; Catastrophic failures (loss of measurement function) are extremely rare. Now, the progress of high temperature measurement science has solved various problems related to the real world in use. High temperature spectrometer is a new instrument. It is an expert system multi wavelength pyrometer, which has good ability in solving these problems

in addition to providing excellent real-world accuracy, the high temperature energy spectrometer has many other advantages: it can provide real-time quality readings and tolerances (i.e. uncertainty in measurement) for each measurement; It can also provide signal strength, comparison between target and ideal target under the same temperature and state. These two functions can provide valuable information about the raw material and process status, help to ensure the correct alloy composition and show whether the alloy material has been boiled and evaporated. Obviously, users can apply this information to some more advanced fields

in various applications, the high temperature spectrometer has solved the difficulty of non-contact temperature measurement

1. Emissivity: emissivity will change with each batch of material samples, which is a correlation between theoretical calculation in high temperature measurement and material behavior in the real world. For the precision casting industry, the emissivity of metals changes greatly. The emissivity of any sample depends on the composition, the history of mechanical and thermal properties, the wavelength at which the measurement is made, and the temperature itself. Analysts believe that the relative error of temperature is directly proportional to the relative error of emissivity, that is:

where: t is the temperature, which is the emissivity, Δ T and Δ Is the respective error. For precision casting, the emissivity of liquid metal is often in the range of 0.15 ~ 0.30, and the small emissivity in the denominator will have a great impact on the temperature error

a casting workshop may provide parts made of 20 or 30 different alloy elements. The quantification of the impact of small changes in alloy materials on metal emissivity has not been carried out on a large scale. Therefore, there is no manual to query the emissivity of precision casting alloys. The similarity of components cannot be used to estimate the emissivity, and a small amount of additives can greatly change the emissivity. As shown in Figure 1, the composition difference of the emissivity of the two alloys shown in the figure is 2% of the atomic weight of the added elements. The resulting difference in emissivity causes a pyrometer "calibrated" against an alloy to produce a reading error of several hundred degrees. Large errors will cause process chaos and make the smelting furnace stop production for several days. Otherwise, the service life of the equipment will be affected.

the high temperature spectrometer is a pyrometer that can accurately measure without any information in advance. No matter what the emissivity is, it is not limited by the environment. Figure 2 shows the temperature and emissivity recorded by far high temperature spectrometer for monitoring nickel base precision casting alloy. It can be seen from the figure that each change of the power setting value causes a rapid spike increase in emissivity, which is caused by the disturbance caused by the electromagnetic stirring of molten material, which will strengthen the emissivity. Liquid movement forms a small cavity, which increases absorption and emission due to multiple reflections. Secondly, when the melt is cooled, the emissivity undergoes a stepwise change: at about 1:15, the incidence decreases by more than 10%, from 0.245 to 0.220

this effect is consistent with the boiling evaporation of alloy materials. When this change occurs, the temperature remains constant. Finally, the melt freezes and the emissivity changes dramatically from 0.22 to 0.60. The slowly decreasing temperature and the slowly increasing emissivity at the same time indicate that the process of metal hardening experiences a slurry state, rather than a sudden change in phase state like water turning into ice. Figure 3 shows the same process as Figure 2, but this time, the output of a conventional pyrometer is added. In addition to the large temperature error, it should be noted that the conventional pyrometer cannot measure during power-off cooling. The pyrometer reported an increase in temperature between 1:35 and 1:50. This is a false condition caused by the increase of emissivity during metal cooling

in the actual operation process, the huge temperature error caused by incorrect emissivity not only affects the product quality, but also has some obvious consequences, such as the waste of power, the extension of cycle time and the intensification of wear of refractory materials. Figure 4 shows the temperature and emissivity measured by the conventional pyrometer in four consecutive casting cycles. The two tracing curves in Figure 5 show the temperature and emissivity measured by the high temperature spectrometer in four consecutive casting cycles. The peak temperature is not without special repeatability. It can be seen that there are many quite large peaks in the emissivity in Figure 4, indicating that there is a particularly large disturbance. The spike was caused by the severe electromagnetic stirring announced by versalis company in Italy and Yulex company engaged in agricultural biomaterials in the United States on January 28, 2013. The process is as follows: the disturbance in the molten material strengthens the emissivity, which is interpreted as an over temperature value by the conventional pyrometer; Then, as a response to the phenomenon, the controller cuts off the power supply; After the power supply is cut off, the disturbance subsides. Then, the conventional pyrometer detects that the temperature is too low, and the power supply is switched on again. The resulting current surge excites the molten material, and the periodic cycle begins. The severe disturbance causes the erosion of refractory materials, resulting in impurities in the product

comparing this behavior in Figure 4 with that in Figure 5 using a high-temperature spectrometer, the emissivity reading is the same, and the temperature increase is the same, but the temperature range is low. The reason is that the high temperature spectrometer is used for accurate measurement, so now the process is accurately realizing the setting temperature. It can be seen from the figure that the temperature tracing track smoothly reaches the set point and accurately controls it until the end of each cycle, all of which are completed by the same controller and control algorithm. At the same time, it is very obvious that the spikes indicating the disturbance of molten material are also greatly reduced, and the power supply disconnection and disturbance caused by repeated disturbance conditions are eliminated. In the process of full power heating, there are still some disturbances caused by electromagnetic stirring. However, although the emissivity changes, because the temperature is accurately controlled, it can reach the set point smoothly, and the disturbance will disappear

the advantages of the improved control are as follows: the evaporation capacity at high temperature is reduced, and the product quality is improved; As the erosion of refractory materials is reduced, inclusions are reduced; As the casting cycle reaches the actual set point rather than the false high value, the casting cycle is accelerated and the output is increased; As the erosion of refractory materials is reduced, the maintenance cost is reduced and the power cost is reduced

2. Steam emission: it is well known that in the process of processing, metal will be lost due to evaporation, and the resulting metal steam, together with the exhaust gas from crucible, sensitive element or other smelting furnace equipment, will selectively absorb certain

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