Vacuum Carburizing: A Technology Whose Time Has Come
Vacuum carburizing (also called low-pressure carburizing) has come of age, and today is a viable alternative to atmosphere carburizing for high-volume, low-cost, environmentally friendly heat treatment. Flexible process cycles and cellular designs, the result of which is high up-time productivity, made this technology renaissance practical.
Carburizing diversity: gas selection
Today, propane (C3H8) and acetylene (C2H2) are the most common hydrocarbon gases used to supply a source of carbon to the work. Other less common carbon enrichment sources can be used under special circumstances and include natural gas (CH4), ethylene (C2H6), and more recently, cyclohexane (C6H12), a liquid. Although the supply of carbon comes from a variety of sources, and the delivery methods vary, the common element is that the gas must dissociate at the surface of the work to obtain the highest possible carbon transfer ratio, typically 40% or greater.
Quenching diversity: quench selection
Dimensional change in components is a complex subject, with both the heating and quenching cycles capable of producing component variation. The amount and type of size variation is dependent on the mass and geometry of the part, the workload configuration and prior history. Dimensional change is to a large extent influenced by the quenching method as well as the medium used (e.g., air, gas, oil, polymer, salt, water and brine). In vacuum carburizing/carbonitriding, the quenching choice most often is high-pressure gas, which is well known for its reduction of distortion to key parts. Distortion results are reproducible throughout the load and from load to load. Therefore, pre heat-treatment machining operations can further minimize distortion in tandem with high-pressure gas quenching techniques. Of critical importance is that the transformation achieves both surface and core hardness with optimized microstructure.
High-pressure gas quenching
A number of different gases can be used as quench media including argon, helium, hydrogen, nitrogen and various gas blends. Quenching of workloads (Fig. 1) using nitrogen is both economical and highly efficient provided the quenching chamber is constructed in such a way as to take advantage of the properties of this gas. Nitrogen is the most common gas used today up to 20 bar pressure in 85 to 95% of the installed systems throughout the world.
In addition to the normal quench variables involved with using high gas pressure quenching, special consideration must be given to options that add flexibility, such as flow contouring via variable speed drives and optimized gas flow patterns via baffling so as to avoid turbulence and excessive pressure drops. Even the availability and control of cooling water (capacity, temperature and flow rate) must be considered. Future developments in gas blending technology will further enhance performance results. With all of these factors in balance, the number of successful applications is almost limitless.
Vacuum furnaces have been provided with oil quenching capability for many years. Today, designs incorporating vacuum-sealed doors allow the use of heated oil in the 130 to 375 F (55 to 190 C) range. Large capacity tanks with multiple agitators, directional flow baffles and heat exchangers ensure that oil circulation is uniform throughout the charge area and that the oil-temperature rise on quenching a full load is kept under 40 F (4 C). Oil quenching of large production workloads (Figs. 2 and 3) is an established quenching method, which generally results in some part distortion that must be compensated for by post heat treatment operations. In certain instances, oil quenching is the only reliable method of producing acceptable results, especially for large cross-section parts and low-alloy grades.
Vacuum carburizing equipment comes in a variety of configurations including single cell and dual-cell systems (Fig. 4) as well as multiple (3 to 10) cell systems (Fig. 5). High volume productivity requires innovative designs incorporating both flexibility of processing as well as system expandability. The ability to mix and match cells, or chambers, according to their specific function (vacuum carburize/carbonitride, preheat, reheat, anneal, degas and slow cool) is key to process flexibility and increased productivity today, and will be in the future. This advantage has led to many high volume production installations in North America and throughout the world. Horizontal or vertical load orientations are viable options.
Vacuum carburizing technology is being applied to a broad diversity of product applications in highly demanding industries. The following examples illustrate improvements in distortion control, elimination of press and plug quenching, achievement of tight case depth uniformity (including parts with deep blind holes) and fully martensitic microstructures with a variety of materials.
For example, automotive ring gears of AISI 5120 (Fig. 6) are vacuum carburized to a 0.025 in. (0.635 mm) effective case depth and high-pressure gas quenching using 17 bar nitrogen.
This process resulted in a reduction in distortion (Fig. 7) over conventional processing methods.
Other common applications besides automotive applications include heavy truck rings and pinions, aerospace transmission gears, off-highway heavy-vehicle transmission gears, and various industrial products, such as hydraulic pump cams. Typical load weights vary from 500 to 1,000 lb (225 to 455 kg). Commonly carburized materials, quenching methods and core hardness values for section thickness up to 1 in. (25 mm) are summarized in Table 2.
If required, selected surfaces can be coated with stop-off paint or be copper plated to prevent carburization. Today, most carburizing temperatures are between 1700 and 1800 F (930 and (980 C) with cycle times producing effective case depths in the range of 0.010 to 0.080 in. (0.25 to 2.05 mm) or greater. In certain instances, vacuum carburizing eliminates the need for slow cooling, reheating and subsequent press or plug quenching.
Future diversity - Where do we go from here?
Worldwide acceptance of vacuum carburizing technology has been responsible for the installation of over 500 carburizing cells throughout North America, Asia and Europe. Most of these designs are multicell furnaces for automotive and industrial companies who required high volume production with high up-time productivity. Continued innovations in both the vacuum carburizing process as well as equipment design are necessary and have lead to the success of this mature technology. Vacuum carburizing is now the preferred choice of most users. In other words, when given the choice, vacuum carburizing is preferred and gas-quench technology is particularly appealing if the required core hardness can be achieved. If necessary, slight changes to the material chemistry, or the ability to specify material in the upper end of the hardenability band can be considered to improve the core hardness.
One example of the continued development effort is the design of a gas-fired processing cell (Fig. 12) as a means of improved energy efficiency and to provide an alternative energy source.
Simplicity and modularity of design as well as a reduction in process optimization has significantly reduced the need for maintenance. Today, the use of advanced control systems allows automatic equipment and process diagnostics. Remote monitoring and the ability to download the latest upgrades (Fig. 13) make resolution of many problems virtually instantaneous.
Equipment standardization is key to developing a planned preventative maintenance program reactive in real time now that cause-and-effect models and component mean time-between-failure (MTBF) data have been completely determined.
SIDEBAR: Vacuum carburizing process basics
A workload is introduced into a cell (or vacuum chamber) of the carburizing furnace (see figure), which is then evacuated to a pressure below 2,000 microns (2.7 mbar). Upon reaching the required vacuum level, the workload is either heated in place (single-cell furnace design) or transferred to a heating/carburizing cell (multiple-cell furnace design). Upon reaching the austenitizing temperature, parts are soaked until all areas of the load reach thermal equilibrium. The vacuum carburizing or carbonitriding process begins at that time.
A hydrocarbon gas, typically propane or acetylene, is introduced at a pressure between 1 and 20 torr (1.3 and 26.6 mbar). The amount of hydrocarbon addition is predetermined by recipe selection, with 25 to 150 cf/cycle (1to 4 m3/cycle) being a typical consumption. Nitrogen often is added during the diffusion stage to maintain a set pressure level. The hydrocarbon gas is introduced and evacuated in a pulsed manner. After carburization, the workload is quenched either in the same chamber or rapidly (under 20 seconds) transferred to a quench cell to be slow cooled, oil or high-pressure gas quenched. A typical cycle is shown in the table.