Using hot-box temperature-profiling systems to monitor product temperatures in furnaces has become accepted in many areas of the heat-treatment and allied industries. Twenty years ago, the concept of putting an electronic data logger into a furnace with products was seen as radical, but use of these systems is now commonplace in applications such as aluminum brazing, furnace surveying and even steel slab reheating.
In most applications, furnaces can easily accommodate the monitoring system as well as the product, but a great deal of resourcefulness is required to enable the monitoring operation to be carried out in some. Homogenizing aluminum logs in a walking-beam furnace is one of these.
The Heat-Treatment Process
After casting, aluminum logs undergo a homogenizing heat-treatment process to ensure uniform distribution of the alloying elements within the structure of the log. This involves heating the log at a controlled rate, soaking at temperature for a specific period of time and cooling at a specific rate.
Log Temperature Measurement
When setting furnace conditions for new production batches, monitoring the actual product temperature of the logs throughout the furnace is vital to optimizing the operation of the furnace – maximizing throughput while ensuring the correct metallurgical structure of the product.
Measurement of the product temperature is not considered a problem when the operation is carried out in a batch furnace because thermocouples can be inserted at the required depth in a test log in any part of the load and led out through an aperture to a data logger outside the furnace. Since the log does not move during the process, the temperature data can be collected with relatively few problems.
When homogenizing is carried out in a continuous process (such as a walking-beam furnace), however, monitoring the product temperature from a data logger external to the furnace is not possible because the logs generally travel in various directions as they enter, move through the hot zone and exit the furnace. Also, the logs can slowly rotate due to the action of the walking beam. These factors make external monitoring with long thermocouples impractical during normal production.
The solution is to use a hot-box temperature-monitoring system where a thermal barrier can be attached to the log protecting a data logger as it gathers temperature data from thermocouples set within the test product. In this way, the product temperature profile can be accurately monitored as the test system travels through the process during a normal production run. After the system exits the furnace and data is downloaded from the logger, software capable of carrying out a fast and accurate analysis of the process is the final important element of this type of system.
Designing a hot-box system to operate attached to logs in a walking-beam furnace is not without difficulty. Here are several design challenges.
- There are generally size restrictions at the entrance and exit of the walking-beam furnace, so the thermal barrier of the hot-box system must be designed to be the same diameter (or preferably smaller than the log) or it may impede its travel through the furnace.
- Total time in the process can be around 10 hours or more at temperatures approaching 1100°F (593°C). Designing a system to withstand these conditions, while working within the product-diameter restrictions, can be difficult because normal thermal-barrier design (to insulate the data logger) will not be able to withstand the heat of the furnace over such a long period of time.
- The interface between the log and the thermal barrier must be well designed to ensure the system and log do not part company in the furnace.
- The aluminum logs can be up to 26 feet long, and thermocouples feeding back data from one end of the log to the data logger must be kept within the product boundary to ensure they do not snag during the process.
Hot-box monitoring systems normally operate using a two-stage insulation package consisting of a highly effective insulation layer around a phase-change heat sink to protect the data logger. The phase-change medium is generally a type of eutectic salt that absorbs energy during the phase-change period, changing from a solid to a liquid state. Where thermal-barrier size is restricted and processes involve long duration/high temperature, however, evaporating water is used as a phase-change medium, changing from liquid to gas (steam) as it evaporates and prolongs the period it can remain in the furnace.
Designing the barrier with enough thermal capacity to get the test log through the furnace before the water has fully evaporated is the key to successfully monitoring this process. Therefore, the amount of water within the thermal barrier has to be maximized. The barrier design must also allow the steam to evaporate while not losing any water as the barrier rotates. For this type of system, it is also necessary for the data logger to be able to operate at 212°F since it is surrounded by boiling water. This requires careful selection of the electronic components and circuit design.
Good communication with the customer at design stage is essential because, prior to trial, the test log must be machined to accept both the system and the thermocouples, keeping both within the boundaries of the product.
Having established the diameter range of the logs and the process parameters, the size of the system (length and diameter) can be calculated, and a piece of the log equal to the length of the thermal barrier can be cut off and discarded. The end of the log is then machined to accept the holder section of the thermal barrier where it can be firmly secured. A slot is machined longitudinally along the log to guide the thermocouples to holes drilled at right angles to the correct measuring depth. This ensures that both the thermocouples and the hot-box system are kept within the boundaries of the product.
When this is complete, the thermocouples are positioned, the data logger is reset and placed in the thermal barrier fixed to the test log, and the trial is ready to run.
Measuring core temperatures while products rotate in a furnace can present many challenges, but with careful design of the monitoring system, it can be achieved and valuable temperature data obtained. In a recent commissioning of a hot-box system at a major German casting plant, a manufacturer was able to accurately monitor the temperature profile of the test log in all three stages of the heat-treatment process. Following this trial, they were able to substantially reduce the time the products were spending in the soaking zone of the furnace with no loss of product quality.
Monitoring Temperatures in Other Aerospace Applications
In the aerospace industry, temperature monitoring is also carried out in aluminum mills when rolling plates from slabs and then heat treating (T6) after the rolling operation. Structural areas of the wing and fuselage are manufactured from the aluminum-alloy plates.
Prior to the hot-rolling operation, slabs (or ingots) are reheated in a continuous pusher furnace to temperatures of around 1020°F (550°C). Using a hot-box temperature profiling system rigidly attached to the slab eliminates the need to trail thermocouples through the furnace and allows actual temperature data to be collected from deep within the test slab as it passes through the reheat furnace. Accurately measuring slab temperatures helps to ensure that the correct thermal balance is achieved efficiently throughout the product thickness. Non-homogeneous temperatures can cause variation in downstream processing and final product quality, ultimately leading to energy wastage, higher costs and rejections.
Modern monitoring systems use high-accuracy, multichannel data loggers that measure temperature at up to 20 critical points. The data logger is protected from the heat of the furnace by using an evaporative hot-box thermal barrier where water is allowed to evaporate during the process. Design of the thermal barrier is critical to prevent water loss during charge and discharge from the furnace as the test slab and hot-box measuring system rotate through 180 degrees.
In the subsequent heat-treatment operation (solution heat treatment), the rolled plates are soaked at temperatures of around 975°F (525°C), then directly spray quenched with high-pressure water jets. Temperature monitoring in this operation is desirable not only to gather vital data from the product but also to survey the furnace to the AMS 2750E specification required in the aerospace industry.
The difficulties to overcome in this process include: the clearance for the thermal barrier is generally limited in the furnace and spray quench, and an electronic instrument passing through a furnace and full water quench can be prone to leakage. Careful design of the thermal barrier with a sacrificial outer insulation layer shielding a sealed evaporative barrier together with a waterproof data logger means the process can be monitored without breaking production, saving many hours of furnace time compared to the traditional trailing-thermocouple method.