For the forging and extrusion industries, there are two temperature measurements that should be made. The first is the temperature of the billet as it is being heated – to make sure that the heating system heats it to the correct temperature value, and the second is the confirmation of the billet temperature just before it enters the forging or extrusion press. In practice, most plants use only one pyrometer to make both temperature readings, but in reality these are two different measurements and require the use of different steel infrared temperature sensors.
Why Billet Temperature Matters
Billet Heating: Most plants measure the billet temperature after it exits the billet furnace. However, the temperature of each billet varies making this measurement too late for real-time, precise feedback control. In order to actually control the temperature of each billet during the heating process, it is necessary to measure the billet while it is still inside the furnace. This technique is the only way to control the temperature of each billet to reach the exact process temperature.
Billet Confirmation: Process upsets and delays can result in a billet sitting and waiting before being loaded into the forging or extrusion press. If the delay is long and variable then the billet temperature can be low and variable. The lower temperature caused by the billet sitting can result in poor quality, excessive wear, process upsets, and in some cases damaged equipment.
The typical practice is to measure the billet temperature immediately after it exits the furnace. However, the corners of the billet typically heat faster and hotter than the bulk metal temperature. These hot corners can introduce a variable offset in pyrometer readings. But, if the temperature reading is taken 20-30 seconds after the billet exits the furnace, the corners have enough time to cool and the interference is eliminated. This typical practice for billet measurement is too late for feedback control and too early for billet confirmation. Moreover, the point of measurement coincides with the period at which the billet surface temperature is least uniform and most variable.
Billet Heating: Measure the billet while it is still inside the furnace. For the induction
heating process, the best configuration is a fiber-optic pyrometer that can view between the coil windings away from the end of the billet. When it is not practical to view between the coil windings the best practice is to aim the pyrometer to view the center of the billet face. Steel billets inside a flame-fired furnace have a tendency to produce surface oxides and scale. The steel billet should be measured inside the furnace because the heat prevents excessive cooling of the surface oxides and scale.
If a billet must be measured after it exits an induction heating coil it is best to make the measurement 20-30 seconds after exiting the furnace. This wait time will allow the corners to cool. If measuring immediately after exiting the induction coils, the timing of the measurement and signal conditioning settings should be adjusted to eliminate or minimize the interference from the hot corners. For a flame-fired furnace where the measurement of a steel billet must be taken outside the furnace, a relatively large spot size is necessary to allow the pyrometer to view around the colder spots of scale.
Billet Confirmation: Measure the billet temperature as close to the entry of the forging or extrusion press as is practical. For steel billets, configure the pyrometers to view a relatively large area (2 to 5in) to allow the pyrometers to view around the colder spots of scale. Aluminum and Copper billets require a full field of view for accurate measurements.
For billet-heating applications, SW, TC, DW or MW technologies are appropriate. The most appropriate technology is dependent on the measured material.
SW technology is recommended when measuring the temperature of a copper, steel or titanium billet inside a flame-fired furnace. When the billet is inside the furnace, the emissivity error is proportional to the difference in temperature between the measured object and the background. Therefore, the hot furnace walls minimize any sensitivity to emissivity variation, and the SW technologybillet_blog.jpg provides the greatest accuracy.
TC technology is recommended for steel or titanium billets that are free of significant levels of surface oxidation and scale when measured in or after an induction heating furnace or after exiting a flame-fired furnace.
DW technology is recommended for steel or titanium billets when misalignment, surface oxidation or scale are significant application issues when measured in or after an induction heating furnace or after exiting a flame-fired furnace.
MW technology is recommended for non-greybody billet materials such as aluminum and copper when measured in or after an induction heating furnace or after exiting a flame-fired furnace. The MW pyrometer technology may be used inside a flame-fired furnace only in the final zone(s) and in conjunction with a cool sight tube extending close to the surface of the billet in order to shield the pyrometer from flames and hot-wall reflections.
Heating Aluminum Billets above 500°C / 930°F: Above 500°C / 930°F the alloying elements migrate freely within the body of the aluminum billet, and, over time, they accumulate on the surface. This causes the emissive nature of the surface of the aluminum billet to vary significantly over time. If using the MW technology, the pyrometer reading may be tuned to be true; however, for consistent temperature readings, it is important for the amount of time the aluminum is above 500°C / 930°F to be reasonably consistent from billet to billet. Typically, billets that are heated above 500°C / 930°F for an excessive period of time are discarded due to the change in the composition of the material as the alloying elements migrate out of suspension.
For further Williamson recommendations for billet heating measurements please click the below link.