Induction Heat Treating : Induction Hardening and Tempering in a Con-trolled Atmosphere Within a Cellular Manufacturing Environment
Cellular manufacturing has become popular for certain types of manufacturers. It has been found to be an efficient way to arrange machines such that a product can be processed progressively from one workstation to another, thus eliminating the need to wait for an entire batch to be completed or requiring additional handling between operations. Depending on the type of product manufactured, cells may be dedicated to a process, a sub-component or an entire product. Some of the benefits of the cellular concept are:
The challenge posed by our customer was to integrate induction hardening into a cell for the production of a hand tool. The tool is manufactured from a medium-carbon alloy steel (4037) requiring a tempered hardness of 41 to 46 Rockwell "C" (Rc). After hardening, the part must be able to be chromium plated without further cleanup.
Research & DevelopmentPrior to designing the induction-hardening system, a developmental effort was undertaken. The key was to determine the hardening and tempering parameters (temperature and time) necessary to meet the required material properties. These pa-rameters, as well as part configuration, were also considerations in the induction-coil design. The additional requirement that the part needs to be chrome plated without further cleanup after heat treat meant that hardening must occur in a controlled atmosphere. One of the necessary tests was to cross-section actual parts to be assured that there was no decarburization, and that the microstructure was fully-tempered martensite.
Induction Harden and TemperInduction-hardening applications are often selective or "case." This application was through-hardened as would be typical of parts hardened in a conventional batch furnace. As in any hardening process, the part needs to be heated to austenitizing temperature and then quenched. Induction hardening typically requires higher temperature than a furnace due to the speed of heating. As with hardening, tempering is also a function of time and temperature, and induction tempering generally uses a shorter time and higher temper temperature to achieve results similar to those of a furnace temper.
Another key parameter of the hardening process is the atmosphere. As previously mentioned, the atmosphere needs to be protective enough to produce decarb-free and scale-free parts in preparation for subsequent plating. Several atmospheres were considered and three were tested: argon, a 5% hydrogen/95% nitrogen mix and nitrogen. The mixed atmosphere produced parts with a bluish tint while the argon and nitrogen atmospheres both produced "clean" parts. Nitrogen was chosen because it was most economical.
The design of the system to inject the gas, to contain it for minimal loss and to prevent oxygen entrainment was more challenging. Figure 1 shows the design of the atmospheric feed system. A stainless hood was created to cover the machine, and the bottom of the hood was submerged in water as a seal. The work-piece entrance and exit were gasketed with rubber to both prevent loss of nitrogen and entrainment of oxygen.
Quenching is the final consideration in the hardening system. Quenching is done with water, which is fed into the quench zone of the machine. The spray set-up is shown in Fig. 2. The water was supplied from a quench tank, which was part of a contained system and was circulated through filtration. Water temperature was maintained at 85 to 90°F. The user can set the temperature, and a sensor provides feedback to the PLC and the display. The PLC controls a regulating valve to a chiller which maintains the temperature.
Machine DesignBased on a number of considerations, such as part configuration, cell layout, efficient use of floor space, etc., it was decided that the machine should be circular. Figure 3 shows this circular configuration and identifies the 12 position index table. As you can see, there are four stations in the hardening zone with two dedicated to quench, three for tempering, and the last two for cooling and inspecting. The basic machine layout is shown in Fig. 4.
Given the layout, the next challenge was to design the coil to allow a continuous feed of parts. Since the parts were being fed through the coil, the coil was stationary. The heat input was altered by the strategic placement of concentrators, which provided for efficient heating. The hardening coil is shown in Fig. 5. Specially designed fixturing was required in order to package the coil in the machine.
Embedded Power SuppliesLike other design considerations of this system, the power supply required a compact configuration. The cell, and conse-quently this machine, needed to be built with mobility in mind as the cell may need to be moved to a different manufacturing plant. Considering all factors, it was decided to locate the power supplies in the same enclosure as the machine controls and interface them directly with the PLC. Through calculation and testing, the power supply for hardening was determined to be 30 KW and 30 - 50 KHz and 10 KW with 10 - 30 KHz for tempering. Both power supplies are in the PLC system and are shown in Fig. 6. One of the monitored process parameters is the energy usage.
In addition to energy usage, the quench temperature and quench flow are also monitored. A PLC was installed with a touch screen interface, which allows for the entry of the recipe for the specific part. Although the machine was designed with one part in mind, it has changeable and keyed tooling that allow it to be quite easily set up for other parts.
Pre-Production Product TestingTesting was a two-fold process. The first testing stage was run to determine the optimum settings for both hardening and tempering. This involved running a series of parts with five different input settings for hardening and tempering. A sample was retained from each test run for metallurgical testing. Hardness and decarb were checked on each part. Based on these test results, the optimum process for hardening and tempering was determined. It was also determined that using a nitrogen atmosphere resulted in parts which required no post-HT cleaning or machining.
Using the newly established process, a production runoff test was performed, which involved the random sampling of five parts every 20 minutes with each part being tested for hardness in nine locations. Again, decarb was also checked on sectioned parts and no decarburization occurred. Sampling included a total of 40 parts run at production rate, over a period of about three hours, with no downtime. Testing showed that the machine could produce 43.4 Rc mean with a standard deviation of ½ point, and the unit performed to the customer's up time and SPC requirements for hardness.
Efficiency of the OperationThe induction-hardening operation was found to be both energy and design efficient. The system is energy efficient while run-ning and no power is used when the system stops. There is instant shutdown and restart capability and the process is very controllable.
The key design efficiency for this type of process is the short cycle time to delivery of completed parts. This is possible due to the scale-free nature of the finished part. Our customer also found less physical distortion as compared with running the same parts in a basket of a batch furnace.
SummaryWhile the design of a custom induction-hardening system within a manufacturing cell is challenging, the benefits are many. Benefits include:
The flexibility of induction-hardening systems allows uniquely configured machines to be designed. These machines can be built for a specific part or to be reconfigured to multiple part geometries. They can also be configured to operate in a cellular manufacturing environment with limited available space. As a result, the system is simple and yet sophisticated. IH
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: induction hardening, cellular manufacturing, PLC, controlled atmosphere, austenitize, quench