Method for Accelerating Atmosphere Carburizing
Invention is defined as the act of creating something new or original, often in the form of a process or device. It is commonly thought to be 1% inspiration and 99% perspiration. Such is the case (pardon the pun) for one individual that The Doctor met nearly 40 years ago, who was absolutely convinced he had found a better method of atmosphere carburizing. His success is both a testament to the human spirit and a triumph of unwavering belief in an idea (combined with a great deal of persistence and hard work). The time has come to report on what appears to be a revolutionary breakthrough in atmosphere carburizing. Let’s learn more.
Heavy Carbon Company LLC (Pittsford, Mich.) and its founder/inventor, George Barbour, have been granted a patent (Fig. 1) for a technology and device that allows the user to carburize faster – over the limit of saturation of carbon in austenite – without forming excessive carbides or sooting up the furnace. In addition, the system can be easily retrofitted to any atmosphere-carburizing furnace.
In two separate and in-depth case studies, one of which will be reported in detail in an upcoming article in Industrial Heating, The Doctor has witnessed the invention at work. Testing was done at Euclid Heat Treating Company (Euclid, Ohio), a commercial heat treater running a typical batch IQ furnace, and then compared with in-house processing using conventional endothermic-gas carburizing. The metallurgical analysis results of these side-by-side comparison tests along with the tangible benefits to end users are impressive.
Oerlikon Fairfield (Lafayette, Ind.) is one company that has explored this technology and found it meets their expectations. The Endocarb system, as it is called, allows for rapid yet precise variation of the carbon potential in the furnace at critical times, thereby shortening the overall carburizing time. This system differs from conventional carburizing in several key ways:
- The device is the source of endothermic gas and can be mounted on the carburizer, simplifying gas piping and avoiding transmission issues. The close proximity of the carrier gas also minimizes the risk of leaks and decreases the amount of upkeep necessary to maintain the system. The system can easily be set up so that the furnace runs at reduced endothermic-gas flowrates.
- At 925°C (1700°F) the process can be set to run at a carbon potential of 1.5%, which is significantly higher than conventional values often in the range of 1.05-1.10% but certainly no higher than 1.2%.
- The increased carbon potential decreases the run time for the load. For a larger 320-kg (700-pound) bull gear (Fig. 2), the carburizing time to achieve a 2.3-mm (0.090-inch) effective case was decreased by nearly 18%.
- While conventional carburizing maintains a constant carbon potential during the boost phase, the Endocarb system uses “carbon cycling” (i.e., the process starts at a much higher potential, then is rapidly lowered well below the initial setpoint). This cycling continues throughout the run. In addition, at certain points, air is pumped into the furnace and a “controlled burnout” performed. As a result, atmosphere is rejuvenated and the furnace itself remains clean and soot-free.
In order to determine the quality and potential cost savings of this technology, tests comparing it against conventional carburizing were deemed absolutely necessary. Oerlikon Fairfield decided to conduct such an investigation to determine whether the time savings claimed could be achieved without sacrificing metallurgical quality or the performance characteristics of their gears.
Two identical gear sets were chosen for the trials involving six different materials (8620, 4320, 8822, 4820, 3310 and 4817) and eight different part numbers. Metallurgical results were compared to both internal and customer specifications and included checks of surface and core hardness (flank and root), effective case depth (tip, flank, root), case and core microstructure, carbide morphology, surface carbon content, retained-austenite percentages, NMTP percentages, decarburization and IGO/IGA.
For testing purposes, the austenitizing and carburizing temperatures were held constant for both trials. The parts were furnace-cooled after carburizing from both processes, then reheated and quenched in the same furnace to eliminate any difference related to the quenching process.
A Glimpse at the Test Results
Several of the important benchmarks were consistency of surface hardness (Table 1) and effective case depth (50 HRC). Samples were evaluated independently by all parties involved and the writer.
Table 1. Surface hardness comparison
The bottom line is that the parts in both trials met all specifications and few, if any, significant differences were observed, indicating that this new technology performed in a virtually identical way to the existing technology.
Microstructural evaluation (Fig. 3) confirmed that high-quality parts were produced in both tests, indicating that the Endocarb system had a distinct advantage based on shorter cycle time and a more cost-effective process.
It should be noted that the technology has been investigated by other interested companies. Although the results of these trials have not been made public, similar results in terms of quality and cycle-time savings have been observed.
As the readers of this column know, we do not endorse companies or their products per se, but rather focus on heat-treating processes, methods and equipment innovations that advance the state-of-the-art. This is one such instance. Look for the full article in the April 2018 issue of Industrial Heating, which will provide a more in-depth analysis of the technology, summarize all test results and present a business case for implementing the system. Trust, me, you will like what you see.