November 2022 Volume 4

MATERIALS

Question: What are the most common types of heat-treating equipment used for forged products and what are the selection criteria for deciding which to use? This question fits in nicely with the theme of the current issue. It is also a very important topic for those of us who work with forgings to understand, whether you are considering purchasing new equipment for an internal operation or evaluating commercial vendor options. There are numerous options in the market for heat treating equipment that can be used to process forged products, but only certain types may be a good application for your specific part configuration. There is a lot to discuss on this matter, so this column will be the first in a series discussing heat treating equipment. In this column, we will go over some general concepts and then discuss two of the most common types of batch equipment. We will discuss many other types of equipment in later editions. Before we get into more detailed descriptions of different furnace types, it is important to make some general statements about this topic. First, process capability must be the primary and overriding factor in equipment selection for any heat-treating process. This means that the furnace type selected must be capable of producing parts within specification on ALL parts in each batch/run and also from one run to the next without unreasonable human process or quality monitoring (i.e., constant process changes or enhanced inspection solely for the sake of monitoring equipment quality). While certain equipment types may offer higher part throughput rates than others, we cannot consider anything that jeopardizes a capable process. Where these concerns are relevant, it will be noted. Assuming known quality outcomes, we can then discuss which furnace types are more efficient or offer better part throughput for different applications. The factors that will go into this evaluation are energy efficiency, labor input required, and general throughput capability. Ultimately, any effective heat-treating process for forgings will get evaluated for how many pounds per hour it can produce, so this will be a focus of our discussion.We also need to consider overall maintenance burden which relates directly to operating cost. This is usually directly related to the complexity of the furnace system. Finally, most types of furnaces discussed herein can be equipped using either a direct fired burner setup in which the burners fire directly into the heating chamber or radiant tube in which the burners fire into sealed alloy tubes. Direct fired systems offer a more efficient means of heating work and can operate over a wider temperature range (1400-2200°F typical) but will result in further scale/oxidation buildup on the parts. Furnaces equipped with radiant tube style burners offer the ability to run protective atmospheres in the heating chamber (typically endothermic gas based) which inhibit oxidation of the work pieces. While this Heat Treat Corner By Chuck Hartwig

is a less efficient method with a restricted operating temp range (1500-1850F typical), it may have specialty applications if the desire is to maximize quench severity (this topic is discussed in previous columns) or keep certain parts free from scale if they are exceptionally difficult to clean. Box Furnace A box furnace is likely the most common furnace for heat treating forged products. It is the simplest design of all furnace types and can usually be built by in-house engineering if desired. A box furnace is defined as a cubic heating zone with an access door on the front which opens vertically or horizontally directly to the heating chamber for loading/unloading. The workload rests atop either cast ceramic piers or alloy rails. A box furnace can be serviced by a lift truck, charge car (if equipped with alloy rail/grid system), or custom manipulator system. While box furnaces are subject to the overall throughput limits of batch processing, they have many advantages in terms of flexibility and can be acquired in any size imaginable so batch sizes can be quite large. Box furnaces can accurately operate over a wide range of temperatures. They can be used to run both very large individual work pieces and vast arrays of smaller forgings that can be stacked up on individual grid/basket layers. Additionally, multiple box furnaces can be assembled into one furnace line that feeds multiple quench systems. In this manner, several different alloys and processes can be run at one time within the heat treat cell because each box furnace can be serving a different quench system via external handling system. Box furnaces tend to be very reliable with low maintenance burden. They have the fewest moving parts of any furnace we will discuss, and the internal components tend to hold up much better over time than other types of heat-treating equipment, especially if the design incorporates ceramic fiber cast piers for the work supports in the furnace. Batch Integral Quench (IQ) Furnace The batch integral quench furnace is the workhorse unit of heat treating departments worldwide. These furnaces are comprised of a radiant tube fired heating chamber in the rear half and a sealed oil quench chamber in the front vestibule. All work is placed on a base grid that moves along roller rails in the furnace. These furnaces have a fair amount of process versatility in that they can run neutral hardening processes as well as carburizing. While it is unusual to carburize rough forgings, this flexibility is an attractive feature for commercial heat-treating operations which may run both forged and machined components. The transfer from heating chamber to

FIA MAGAZINE | NOVEMBER 2022 44