Computerized Heat Treatment Planning System for Batch Furnaces
Center for Heat Treating Excellence (CHTE) is dedicated to the advancement of heat treating through collaborative research and development in accordance with the Heat Treating Technology Roadmap, the current R&D Plan and the Vision 2020 document. The activities at CHTE are clustered in three groupings: (1) projects currently underway; (2) ongoing DOE-funded projects; and (3) completed projects. A description of the research projects currently underway and completed projects can be found at www.wpi.edu/ academics/research/chte/, or www.wpi. edu/+mpi. Completed projects include:
- Solution Heat Treatment of Aluminum Alloys: Effect on Microstructure and Service Properties
- Quenching - Understanding, Control-ling and Optimizing the Process - I: Control of Distortion and Residual Stress in Heat Treated Components
- Development of an Analytical Tool for Part Load Design and Temperature Control Within Loaded Furnaces and Parts
- Computerized Heat Treatment Planning System for Batch Furnaces (CHT-bf)
CHT-bf is discussed in this article.
CHT-bf system description
Computerized Heat Treatment Planning System for Batch Furnaces (CHT-bf) is a software tool used to determine optimal part load design and furnace temperature control in heat-treating processes. For a given part geometry and furnace condition, and for a specified part load and thermal schedule, CHT-bf predicts the temperature profiles of parts at different furnace locations. Therefore, by changing the part load and thermal schedule, the heat treatment process can be optimized to ensure quality and shortened cycle time. The capabilities of this enabling tool have been validated through industrial trials and beta sites, with measured temperature data collected during production.
The objective of the project was to develop a software tool to predict the temperature distribution of parts in a batch furnace, when the furnace conditions and thermal schedule are specified, to optimize the workload and effectively control the furnace. A focus group comprising CHTE member company representatives and led by Max Hoetzl of Surface Combustion Inc. (Maumee, Ohio) guided the development of CHT-bf by addressing the industrial needs and providing production information. Surface Combustion also assisted by having the research team intern at its corporate laboratories, and provided some initial algorithms for the software program.
Before developing the software, visits by the research team to over 15 member companies (commercial heat treaters and furnace manufacturers) provided valuable information about real operating conditions and user needs, as well as user expectations of such a tool. The software was validated at various CHTE member facilities and several case studies were conducted, which served as the basis to add new functions and improve the graphical user interface in upgrading the software.
Background and mathematical models
During the heat treatment of components/parts, heat transfer involves radiation between furnace and parts and between adjacent parts, conduction inside a part and convection between the furnace and parts, as well as heat loss in the furnace. Three assumptions used to simplify the problem are:
- The furnace temperature is uniform.
- The furnace serves as a heating resource and heat storage.
- Atmosphere temperature is uniform and is the same as the furnace temperature.
The simulation system was developed with industrial application in mind. For instance, three-dimensional (3-D) solid geometrical models are not used in CHT-bf, because they would be extremely time consuming and would require finite element analysis and meshing of the part to be heat treated. Therefore, all calculations are based on a hybrid method of analytical methods and numerical calculation for predefined regular geometrical shapes.
Various modules of CHT-bf are used to calculate the two types of radiation heat transfer; that is, between the furnace and parts and between the parts themselves. Types of convection are natural and forced (with a circulation fan turned on). In the conduction model, parts are classified as thin-section (lumped capacitance) and heavy section. For thin sections, the temperature can be considered uniform during the entire process, while it is necessary to consider the conduction inside heavy section parts. The criterion for classification is based on the Biot number.
The furnace model contains PID (proportional, integral and derivative) control, available heat calculation for gas-fired furnaces, and heat terms such as heat input by the circulation fan, heat loss from the furnace door and walls, heat storage in the furnace walls and auxiliaries, and heat loss by cooling tubes for some special furnaces.
The furnace model, radiation, convection and conduction models are integrated under one main module, and the CHT-bf encapsulates these modules behind a powerful user-friendly graphical user interface.
CHT-bf is a Windows-based stand-alone software for simulating the heating of parts in a furnace. It contains a comprehensive database of more than 500 materials for parts being heat treated, together with furnace elements, several widely used furnaces, furnace atmospheres and fuels. Figure 1 shows the system structure of CHT-bf.
For example, workpiece materials include most of carbon and alloy steels, stainless steels and tool steels, as well as various metals like aluminum and titanium and their alloys. The basic shapes are classified into 14 categories. All commonly heat-treated parts can be simplified to fit in one of these categories, or a custom shape feature can be used to input surface area if a part does not fit. Furnace types include direct- and indirect-fired gas and electric furnaces, pit furnaces, vacuum furnaces and tempering furnaces. These data help the user to use the software without defining an exhaustive set of parameters.
A separate database management feature containing several user interface guides assists users to add new data into the database seamlessly (Fig. 2). Basically, there are several forms in which user specifies the name, material properties/other properties interactively.
CHT-bf contains several features for industrial applications including the capability to accurately predict furnace temperature profiles, to simulate various load patterns in the furnace, to calculate important heat treat terms and to predict fuel flow rate to determine fuel required as a function of time for better control of furnace performance (Fig. 3).
The temperature profile prediction feature (Fig. 4) allows the user to:
With the load-simulation feature (Fig. 5), the user can:
- Simulate arranged and complex random load patterns
- Simulate effects by varying the load pattern, thermal schedule and PID control
- Determine temperature profiles of the slowest and the fastest heating parts for a given load pattern.
The feature for calculation of heat-treat terms (Fig. 6) allows the user to:
- Predict the heat required for the load under different conditions
- Plot the heat stored in the furnace and the load as a function of time
- Calculate different heat losses from the furnace
The system is basically plug and play. Computer system hardware requirements are:
The current practice in the heat-treating industry is to specify the heat-treatment cycle based on experience and historical rules of thumb. This involves a great deal of guesswork, and is not an effective way to operate a modern facility. Many times, heat-treating cycles may be longer than necessary, and parts may be soaked for a longer time than required. Also, nonuniform heating of the parts inside the furnace (due to inefficient part loading) leads to reduced yields, which has a critical economic impact. CHT-bf is an enabling tool that allows the heat transfer to simulate heating of the parts inside the furnace, eliminates the guesswork, increases productivity, and ultimately increases the bottom line. With CHT-bf, the ritual enshrinement is eliminated, and a robust predictive tool guides the heat treater.
A heat-treating case study at American Heat Treating Inc. (Monroe, Conn.) illustrates some of the benefits of CHT-bf. Furnace specifications, part and part load are shown in Figs. 7 and 8, and results are shown in Fig. 9. CHT-bf helped the company to discover ways to reduce total cycle time and ensure even heat up rates. It also increased the rate of success in predicting actual cycle time allowing for more accurate quoting of new work. Reported benefits include a reduction in cycle time of more than 20% based on the system predictions. After adopting the new cycle, the predicted results were verified by placing thermocouples at various locations in the furnace. CHT-bf provided a better understanding of the fastest and the slowest heated parts for the current load pattern, according to the company.
A brazing case study was carried out at Bodycote (South Windsor, Conn.) to check results using different numbers of parts (12, 18 and 24) in a batch. Results are shown in Fig. 10. CHT-bf proved its strength in this case, where there is no historical data available to make a guess, and the parts were critical. The software allowed simulating results by changing multiple parameters until an optimum result was obtained. Predicted results were used to run the load and the results were accurate. This allowed the company to obtain a high throughput right from the first load.
CHTE developed a computerized heat-treating planning system for batch furnaces (CHT-bf) that can predict temperature versus time profiles for all types of workpieces. The system can determine the thermal historical of the fastest and slowest heated workpieces and, thus, optimize the load pattern, thermal schedule and furnace control parameters. The ability to use CHT-bf to optimize the heat-treating process and to shorten process cycles was validated by a number of case studies.
A similar system (CHT-cf) is under development for continuous furnaces. The first version will be released toward the end of 2003. It has been planned to expand the research for integrating with quenching process modeling, starting with gas quenching.