There is renewed interest in understanding the pros and cons of atmosphere and vacuum carburizing as companies consider which technology to invest in for the future. Today, this interest is being fueled by the desire to substitute high gas-pressure quenching technology for oil-quenching technology in a number of applications. Distortion issues and alternative manufacturing methods are driving this agenda. Now that vacuum carburizing is once again being viewed as a cost-effective alternative to atmosphere carburizing, the question arises, what benefits does each technology offer?
Today, as shown in Fig. 1, atmosphere carburizing is the de facto industry standard. This method of carburizing represents a good compromise between cost and performance. Future growth, as indicated by Fig. 2, suggests that the greatest potential will come in vacuum carburizing. This method of carburizing offers proven metallurgical and environmental benefits.
It is unfortunate that the technologies are viewed as competitors instead of complementing one another. An existing "atmosphere versus vacuum" mentality has fostered an "us versus them" mentality, which has hurt both technologies. This resulted from the need in the 1960s for better control of the atmosphere carburizing process. One series of investigations into a solution to this problem led to the development of vacuum carburizing. It was immediately viewed as an alternative to atmosphere carburizing, promoting enhanced metallurgical properties and shorter cycle times. It was marketed as a panacea for all the problems of atmosphere carburizing, and, thus, initiated competition between the two technologies. Had vacuum carburizing proved to be a robust technology at that time, it is generally believed that a significant portion of today's installed equipment base would use this technology. Its failure to achieve commercial success can be directly related to reliability and cost. For example, creation of soot was the bane of vacuum carburizing. But perhaps a more accurate explanation lies in the fact that the equipment designs and process parameters were not optimized, and the technology's capabilities were oversold to the heat-treating community.
So which technology will succeed in the future? Not surprisingly, both. And both will be needed to meet the challenges posed by more stringent performance requirements to be met for a new generation of materials and manufacturing methods.
Atmosphere carburizing is the most common and accepted method of case hardening in use in the heat-treating industry today. Case hardening is used to produce a hard, wear-resistant surface layer, or case, on top of a ductile, shock-resistant interior, or core, on a variety of (primarily) ferrous materials.
Atmosphere carburizing is an empirically based, time-proven process in which a carbon-rich atmosphere surrounding a workload is used to chemically react with the surface of the parts to allow an adequate quantity of carbon to be absorbed at the surface and diffuse into the material.
Advantages of atmosphere carburizing include:
- The lowest initial capital equipment investment cost.
- Adequate process control; that is, all of the process variables are understood, and reliable control devices are available to provide a measure of process repeatability.
- Capability of high volume output using a wide variety of equipment styles, types and workload sizes. Furnace types include box, pit, mechanized box (integral- and sealed-quench furnaces), pusher, conveyor (mesh belt and cast link belt), shaker hearth, rotary hearth, rotary drum (rotary retort) and carbottom.
- Full automation capability with recipe and part-number control of heat treat cycles.
- Well-understood process problems allowing troubleshooting based on an established theoretical and empirical knowledge base.
The last point is very important. Often in the real world, cost or other considerations mean that problems cannot be avoided, but it is the ability to quickly and easily address the issues that arise that dictates the success of a given technology. This certainly is one of biggest advantages of atmosphere carburizing.
Disadvantages of atmosphere carburizing include:
- The need to "condition" equipment if idled or shut down prior to processing work.
- A requirement of knowledge through empirically gained experience to achieve repeatable results. This is due to a wide variability in the type of equipment, its operation, maintenance and constantly changing process conditions.
- The need for large material allowances for post-processing operations due to accuracy and finish requirements. Case depths typically are specified in wide ranges (e.g., 0.030 to 0.050 in., or 0.75 to 1.25 mm) to compensate for cycle-induced variability.
- Case depth quality issues; the best part of the case often is lost due to the amount of stock removal required.
- The need to constantly monitor environmental pollution issues including air quality (for potentially hazardous gases such as CO and NOx), water quality (for contamination concerns such as oil, minerals, etc.), waste disposal (quench oils) and safety issues (e.g., fire from combustible gases and quench oils, hot contact surfaces and pinch points)
Vacuum carburizing characteristics
Vacuum carburizing is a proven method of pure carburizing and pure diffusion in which carbon penetrates into the surface of the steel being processed without interference from external influences such as gas chemistry or surface contaminants.
Advantages of vacuum carburizing include:
- Easy integration into manufacturing. The process is clean, safe, simple to operate and easy to maintain. Also, working conditions are excellent (that is, there are no open flames, heat and pollution).
- Full automation capability using recipe or part-number control of heat treating cycles.
- Capability of higher temperatures and flexible cycles due to the type of equipment and the nature of the process.
- Precise process control achieved using computer simulations, which allow adjustments to established cycles.
- Consumption of energy by the equipment and process only when needed due to the nature of the vacuum operation.
Disadvantages of vacuum carburizing include:
- Higher initial capital equipment cost than atmosphere carburizing equipment.
- Empirical process control, which requires processing loads to determine optimum settings or to fine tune simulator.
- Formation of soot and tar, which occur due to the type, pressure and quantity of hydrocarbon gas introduced.
It is important to note that research during the past six years has succeeded in finding combinations of pressure, gas type and flow parameters to minimize soot and tar formation as a concern in the vacuum carburizing process.
Comparison of atmosphere and vacuum carburizing
Comparisons of atmosphere and vacuum technology in the past usually were made in an attempt to promote one technology over the other. Table 1 presents the relative merits of each process using important technical criteria, along with the appropriate cautions, which offers an unbiased comparison of the two processes.
The choice of the best carburizing technology-atmosphere or vacuum-will be made as it always has, on the relative merits of each technology for use in a specific application. However, the gap is narrowing and becoming less distinct as the advantages of each technology is being used to solve specific product performance and manufacturing needs.
Sensors, process simulators and carburizing controls technology now are available for both atmosphere and vacuum carburizing. Real-time control schemes are under development.
Fully automated systems consisting of all atmosphere furnaces, all vacuum equipment and combinations of both are being sold. Change cars for automated furnace lines are being designed to load and unload atmosphere or vacuum equipment.
The trend today is toward combination technologies, using either atmosphere or vacuum carburizing, either oil or high-pressure gas quenching. Even traditional high-volume atmosphere pusher carburizing furnaces are being offered with high-pressure gas quenching options.
Tomorrow will see modularity and "plug-and-play" technology to meet the rapid response demands of manufacturing in the 21st century.
For more information: Daniel Herring, "The Heat Treat Doctor," is president of The Herring Group Inc., PO Box 884, Elmhurst, IL 60126- 0884; tel: 630-834-3017; fax: 630-834-3117; e-mail: dherring @heat-treat-doctor.com; Internet: www.heat-treat-doctor.com