Temperature Control in Heat Treating,Temperature Control in Heat Treating,Temperature Control in Heat Treating
TEMPERATURE CONTROL is an integral part of heat-treating operations, which generally consist of three separate
functions: material movement, the application of energy, and the supervision of process conditions. In a typical heat-treat
operation, work is moved into a furnace, heated according to a time-temperature program, cooled or quenched, and finally
moved out of the furnace or quench vessel. The temperature, and frequently the atmosphere, must be controlled precisely
in order to achieve the desired metallurgical results.
Temperature Control Loops
Temperature instrumentation and control systems include temperature sensors, controllers, final control elements for
controlling energy flow, measurement instruments, and set-point programmers. Temperature control also requires an
understanding of the heat transfer mechanisms of conduction, convection, and radiation.
In recent years the separate functions of material movement, application of energy, and supervision of process conditions
have been handled in the following way: programmable logic controllers (PLCs) have been used to handle material
movement; analog variables (energy, temperature, pressure, flow, vacuum, percent carbon, and so on) have been
controlled by single-loop controllers and set-point programmers; process supervision or data management has been done
by strip chart recorders or data loggers. This separation of functions requires communication among the logic controllers,
loop controllers, and data-acquisition equipment. Finally, when a supervisory computer is required to handle such
functions as scheduling, parts purchasing, and statistical process control, a further layer of interfaces is required to tie the
whole set of instrumentation together.

A basic control loop (Fig. 1) includes a temperature sensor, controller, and final control element. The heat process
temperature is detected by a temperature sensor that generates a signal proportional to the process temperature. This
actual temperature is compared to the desired temperature determined by the controller set point. Based on this
comparison, the controller develops an output signal that adjusts the final control element regulating heat flow to the heattreating
process.

Sensor Signal Transmitter. The basic control loop may incorporate a temperature transmitter to amplify the
temperature sensor signals. This amplification is desirable when the control instrument is remotely located from the
temperature sensor. The amplification also helps to avoid interaction between two or more instruments using a common
sensor. For example, the temperature signal may be transmitted to a remote central control room where it is connected to
several controllers, a recorder, a data-acquisition system, or a digital computer.
Auxiliary devices used with the basic control loop include measurement instruments with set-point programming (Fig.
2). The measurement instrument monitors the same temperature sensor as that used by the controller. The set-point
programmer automatically varies the controller set point to provide a temperature cycle or temperature program in
accordance with an established plan.

The reduced cost and more compact packaging of digital support hardware has spawned the integration of temperature
control with auxiliary functions such as process documentation and workpiece flow. These process management systems
can (Fig. 2) integrate the functions of temperature measurement, loop control, variable set-point control, logic (parts
movement) control, data acquisition, data management, results display, and report writing, all in a package no more costly
than a multipoint recorder and a few single-loop controllers.

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