the target temperature, heat treat operators can take the guesswork out of determin- ing cycle time.
What’s the Point? Many factors contribute
Rounding Up the Results F
ollowing were the key results of an experimental study of a non-contact infrared load temperature control method for determining when varying load geometries are on heat in the
to the total energy consump- tion of a load during heat treatment, including process cycle times, load scheduling and furnace efficiency. The goal of direct load sens- ing, or On-heat Prediction through Aggressive Infrared Detection (OP-AID), was to decrease natural gas con- sumption during heat treat- ment by reducing process cycle times through improved process control. In addition to energy savings, tighter heat treatment process control and shorter heat treatment cycles could result in improved product quality and consistency, increased operational pro- ductivity and a reduction in greenhouse gas emissions. Fig. 1 shows typical temperature vs.
annealing process: • At the on-heat detection point determined by the method, the surface temperature for all trials was well within the maximum tolerance specified by the heat treater.
• At the on-heat detection point, the difference between the center temperature and target temperature was less than or equal to 20F for all trials except for the rod bundle, which had a center temperature that was 35F below target at steady-state.
• The method achieved time savings over the conventional hour-per-inch rule of 24 minutes for rectangular geometries, 158 minutes for a large diameter cylinder and 37 minutes for a rod bundle.
MC
time curves for the surface and center of a heat treatment load undergoing an annealing heating cycle. The load took approximately four hours to reach a uniform temperature at its surface and center. Once the load reached the fur- nace temperature set point, it was held at this target temperature (soaked) for a specified amount of time. Thus, the overall heat treatment cycle could be viewed in two main portions, the control ramp-up and the control soak. For most annealing heat treatments, the metallur- gical annealing reactions are completed
during the ramp-up time prior to estab- lishment of steady state conditions. In many cases, the control soak period is often built into a heat treatment cycle as a safety buffer to ensure that the entire load is uniformly heated to its required annealing temperature. In contemporary practice, these soak times far exceed the necessary time requirement to cause the desired metallurgical reactions. With a more accurate picture of the load temperature at the surface and center, the control ramp portion of the cycle can be more tightly controlled, and the safety buffer of the control soak can be reduced or eliminated. The challenge is to accurately and
repeatedly determine the precise time at which the heat treatment load is at temperature to minimize overall heat treatment cycle time. A control strategy was developed to analyze the rate of change of the surface
temperature of the load to infer when the center of the load had reached the target temperature. The basis for this method was that the rate of change of the surface temperature of the load ap- proaches zero as the load becomes uniformly heated throughout. The surface temperature
of the load was directly measured via an infrared sensor. Since the measure- ment of interest is the rate of change of the load surface temperature (rather than the load temperature itself), the absolute accuracy of the
infrared sensor measurement is less criti- cal. The relative measure of the rate of change in load surface temperature is far less affected by variables such as load emissivity and furnace wall reflectivity and therefore is a robust measurement system for load on-heat detection.
Does It Work? The OP-AID method was experi-
mentally evaluated in three trials with loads of varying geometries. All trials were conducted in a 311,000-cu.-in. gas fired batch furnace. The different load geometries tested were: • a 49.25 x 7.88 x 3-in. rectangle with approximate volume of 1,165 cu. in. (Fig. 2);
• an 8-in. diameter cylinder with ap- proximate volume of 1,700 cu. in. (Fig. 3);
• a 3-in. diameter bundle of 0.3125- in. diameter rods with approximate
Fig. 5. Shown is the thermocouple and infrared sensor measurement for the rectangular geometry.
48
Fig. 6. The infrared signal derivative was maintained between the control limits from 3,960 seconds into the process onwards.
MODERN CASTING / March 2010
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