The Use of Conventional Furnaces in the Forging Industry
The forging industry is diverse and, as such, requires a variety of different furnaces, atmosphere-generating systems and quenching systems to accommodate its many needs. This is true not only of the forging process itself but also in the manufacture of forging dies needed to produce forgings.
CAR FURNACESOne of the most versatile types of furnaces widely applied in the forging industry is the car furnace. The car furnace is so attractive because it can be designed to accommodate almost any physical size or weight of products for processes such as forging, stress relieving, annealing, normalizing, tempering and drawing.
The load patterns are very forgiving and the versatility of the car furnace includes the potential heat sources such as oil, natural gas, propane or electricity. Some car furnaces can be designed with a single device to move the car in and out. The door is affixed to the car itself, which means you have a very simple operation.
These furnaces are designed to a capacity of 100,000 tons or more in temperature ranges up to 2400°F. Most furnaces in the forging industry are gas-fired, and a number of them use fiber linings for the insulation system. These are quite forgiving because they have the ability to change temperatures quickly and are not as affected by problems associated with a brick-lined furnace. The main reason for the popularity of car furnaces is that they are simple to build, operate and maintain. Properly maintained, they will give years of trouble-free service (Figure 1).
BOX FURNACESThe second most common furnace type is the box furnace. Like the car furnace, a substantial amount of versatility is inherent with this design. The major disadvantage of the box furnace is the access to the hearth. Quite often this disadvantage is overcome by the addition of a material-handling system that allows you to load the furnace with a forklift or rollers, including roller rail or roller hearth. This eliminates the major drawback with these furnaces, but it does, however, introduce an additional material-handling device. If you use the roller rail or roller hearth, unfortunately those devices are in the heat.
ROTARY HEARTH FURNACESLike most other industries, the forging industry needs automation. However, a major shift in a furnace’s design is necessary for that automation to occur. The rotary hearth furnace is one type that has a great deal of flexibility with regard to processes used in the forging industry such as billet heating, normalizing, hardening, carburizing and carbonitriding. It does, however, have limitations with regard to what you can place on the hearth. Rotary hearth furnaces are commonly purchased and centered around a single-sized product, whether it’s a billet, forged product such as a gear or a rough forging. Process flexibility is the primary advantage of a rotary hearth furnace, and the ability to automate comes a close second.
The cost of automation is generally coupled with versatility. This means that as the level of automation goes up, the price goes up but system versatility goes down. The process flexibility carries over into the use of atmospheres in the rotary hearth furnace. The mere fact that this furnace can be quite tight allows you to operate with almost any atmosphere-controlled process.
The accompanying illustration shows a rotary hearth system layout using the automation alluded to earlier. With a robot in the center of this layout, parts can be taken from the load station, positioned in the furnace, heated, quench pressed, placed into the subzero treatment center and then returned to an unload position to go on to further processing. This system can be programmed with the “lights out” operation. While the process flexibility is quite high, the ability to use different sized parts that are larger or smaller than the original design is quite limited.
Until now, we have talked mostly of batch systems or relatively low-production continuous systems that had either high volumes with limited automation or high automation with limited volumes. The roller hearth furnace is a way to handle large volumes of forgings with some consistency in the process and still have a high degree of automation.
Continuous workflow at these high production rates does come with a price, however. The price is that, generally, smaller parts are batched prior to being able to use a roller hearth furnace. Batching leads you to the implementation of baskets or trays in such a way that the material handling system sees a consistent pattern with the tray or basket regardless of the variations in the load.
The temperature range, as with the car furnace, is quite wide. These furnaces will range in operation up to 2200°F and can be used for all the processes one might need in a forging operation. The forge shop, depending upon the needs in a particular plant, determines the automation and the control complexity or simplicity. Once the product leaves the forging press, it is quite common that there will be no human interface until after the product has been delivered from the heat-treating system.
Recipes can be set up in the control system so that variations in a plant can be applied quickly and easily by simply selecting a number. This data can also be collected and stored for process improvement or tracking of a particular product in the production process (Figure 2).
BATCH INTEGRAL QUENCH FURNACESGenerally, the roller hearth deals with high volumes (5,000-10,000 lb/hr is not uncommon). However, in the 1,000–3,000 lb/hr range, a commercially viable approach is to use batch-type integral quench furnaces. These have an advantage similar to that of the roller hearth. Batching is required for the utilization of this system, but the integral-quench system has a high degree of operational flexibility. It can be coupled with washers and tempering furnaces so that you have a complete operational system with great production-capacity flexibility. This comes from the fact that furnaces can be added to the batching process or alternatively shut down or idled depending on current capacity needs.
These systems are flexible also in that they can utilize different quenching media. By having multiple furnaces with integral quench tanks, those quenching media can be in the same system. As production capacities vary, simply idling one or more of the furnaces in the system can change your production output. The key to any integral-quench system is the material-handling system with which it is coupled. This is true of a number of the other furnaces we’ve already talked about, including the rotary hearth and the roller hearth (Figure 3).
MATERIAL HANDLINGMaterial-handling systems are something most full-service industrial heating companies need to supply out of necessity for proper operation of an industrial heating system. If you’re using trays or baskets, their handling becomes a critical piece of any heat-treating system. If you’re handling the product itself, the material-handing system is equally critical.
Among those that supply industrial heating equipment, most supply conveyors, cross transfers or load cars to move material from one section of the heating system to the other. Industrial heating companies often manufacture their own lines of material-handling products. More specialized systems (such as material-handling robots) can be purchased from other vendors as needed. In any case, at our company we feel it is absolutely necessary that our software engineers design material-handling software operating in conjunction with our furnaces. This assures the interfacing is done properly, and our customers will receive production systems that meet their long-term needs.
PROTECTIVE ATMOSPHERES AND VACUUMSProtective atmospheres are critical to a number of processes used in the forging industry, despite the fact that a substantial number of furnaces use air as their atmosphere. We manufacture several different kinds of protective-atmosphere generating systems, including endothermic or Endogas, exothermic or Exogas, or dissociated ammonia or Ammogas. The selection of one of these generators is dependent on the process, and any good furnace vendor can help you with proper selection. These generators use various feedstocks and can use different methods of heating depending on the particular customer’s requirements. Control of the generator in the production of gas, as well as the control of the atmosphere once in the furnace, is critically important to a successful industrial heating system.
So far, we’ve been discussing the preheating or thermal processing of forgings themselves. But through our line of vacuum furnaces we are also involved in the preparation and thermal treatment of forging dies. The furnace shown here is one of our high-pressure gas quenching systems. It is built to meet stringent industry specifications.
This is a batch-type furnace, so the versatility previously described for a batch furnace is applicable to this furnace also. Generally, the need for large volumes of dies in tooling is not present in most forging operations. Therefore, the batch furnace lends itself very well to those heat-treating needs. Precision control of the system allows for hands-off operation, and several options are available for heat treating a variety of different die steels used for industry (Figure 4).