Many part-quality and production factors play a part in determining whether to use vacuum carburizing or conventional carburizing, and should be seriously evaluated prior to selection of a specific type of equipment.

Development Of Vacuum Carburizing

In the 1970s, prototype systems were developed in the U.S. for carburizing below atmospheric pressure, using hydrocarbons such as methane and propane as carburizing agents with oil as a quenching medium. Difficulties with these systems were experienced in terms of carbon control, extensive fixturing, uniformity of carburizing and sooting. Furthermore, because they used oil as a quenching medium, they did not offer an advantage over conventional carburizing systems, and hence, never became production viable.

In the 1980s, development of vacuum carburizing was again undertaken. Carburizing was conducted in vacuum using hydrocarbons such as propane, methane and acetylene, and quenching was carried out using high-pressure nitrogen. The major deficiency of these systems was that since carburizing and quenching were conducted in the same chamber, the chamber designed for carburization was not suitable for quenching, and vice versa. Furthermore, since the entire chamber and load had to be quenched, quenching speeds were too slow and only a limited amount of materials and loads could be processed.

In the early 1990s, European vacuum furnace manufacturers took on the challenge of developing low-pressure carburizing systems. Based on past experience, they realized these systems must have the ability to carburize and quench in separate chambers to effectively carry out each process. A discussion of the mechanism of carbon implantation is presented here before discussing the development of vacuum system design.

Fig 1 Summary of selected vacuum carburizing processes

Vacuum carburizing mechanisms

Basically, two vacuum carburizing processes are vacuum and plasma carburizing (Fig. 1). Vacuum carburizing uses a carburizing gas such as propane or acetylene and plasma carburizing uses methane as the carbon source.

Two mechanisms used for vacuum carburizing gas pulse and pressure pulse. In the gas-pulse process, a treatment pressure of less than 20 mm of Hg using a carrier gas (nitrogen) is established in the work chamber. During the carburizing pulse, carburizing gas replaces the carrier gas and it is switched back to carrier gas in the pulse pause. Suitable case depth is achieved using several carrier-gas and carburizing-gas pulses.

Whereas the pressure remains the same in the gas-pulse process, the pressure is varied between 0.1 and 20 mm of Hg in the pressure-pulse method. During the pressure pulse, carburizing gas is introduced and chamber pressure increases to 20 mm, and after a certain time, the chamber is evacuated to a pressure of 0.1 mm. Optimal case depth is achieved by alternating the pulses of atmosphere and evacuation. This method is extremely efficient and can be used for densely packed loads, parts with blind holes and complicated geometric shapes.

Fig 2 High-pressure gas-quenching process matrix. Material, part dimension and hardness specification determine quenching process parameters.

High-Pressure Gas Quenching

High-pressure gas quenching uses an inert gas in the pressure range of 10 to 20 bar. Velocity and flow direction of the quenching gas are extremely important to achieve optimal quenching. Because of the physical qualities of the gases (helium, for example), higher quenching velocities can be obtained using the same motor horsepower. Depending on the material to be hardened and its chemical composition, the ideal parameter combination may be selected from the quenching gas, quenching pressure, flow velocity and directional control of flow. Figure 2 shows a process matrix for high-pressure gas quenching with examples of materials hardened according to the indicated process parameter.

Fig 3 Scheme of the multipurpose chamber furnace

Why vacuum carburize?

Compared with conventional carburizing plus oil quench, which is suitable for a majority of products, vacuum carburizing plus high-pressure gas quenching shows its full potential in applications including:

  • Parts with blind holes, such as fuel-injection system components
  • Parts with complicated shapes, such as precision gears and other drivetrain components, which may distort excessively in the conventional carburizing and quench process
  • Parts where it is possible to minimize or eliminate subsequent operations such as grinding, shot peening, machining and other finishing procedures
  • Parts requiring press quenching, such as bearing races, rings and pinions and other drivetrain components

Fig 4 Charging sequence in Type VZKQ multipurpose chamber furnace

Vacuum system design

The development in 1993 of the cold chamber as a separate quenching module led to the development of special multichamber furnace systems for hardening and case hardening with high-pressure gas quenching of low-alloy steels and case-hardening steels.

Available heat-treating systems depending on the production volume, part configuration and process flexibility include two-chamber, continuous pusher multichamber and linked single-chamber [ModulTherm(r)] furnaces. Two chamber furnace

In a two-chamber furnace, heating and quenching are done in separate chambers, which can be designed to achieve optimal results for carburizing and quenching (Fig. 3 and 4). System design does not have to be compromised to meet the dual capability of carburizing and quenching, as in a single-chamber furnace. Furthermore, quenching in a cold chamber eliminates having to quench the entire heating chamber, which allows achieving a very high cooling rate (similar to agitated quench oil).

Work is loaded in the quench chamber and moved to the heating and carburizing section. An optional circulating fan accomplishes rapid, uniform heating by convection below the radiation temperature range. After evacuating the heating chamber, work is heated up to carburizing temperature in an inert atmosphere, after which a carburizing agent is introduced and parts are carburized using a pulse or pressure carburizing method.

After carburizing, the temperature is lowered to the diffusion temperature and the load is transferred to the quench chamber wherein a recirculating fan increases quenching gas (nitrogen or helium) pressure to 20 atmospheres. The heated gas is cooled using an integral rib-type, water-cooled heat exchanger located either above or below the load, and the gas can be exhausted to atmosphere or recovered for subsequent use. Work is removed from the chamber after quenching and another load put in.

System operation including monitoring, recipe, batch log management, alarms and other records required for production and quality controls is controlled using a PC. A PLC is used to control the movement of the workload within the system. The entire process takes place automatically.

These systems are suitable for low production and plants where multiple parts with varying cycles and production rates need to be processed. However, since each system requires its own quench system, they may not be as cost-effective as continuous systems.

Fig 5 Vacuum carburizing plus high-pressure gas quenching process in a sequential multichamber furnace
Continuous pusher multichamber furnace

In this type of furnace, the charge is loaded on one side of the furnace, carburized, diffused, cooled down to hardening temperature and quenched in the opposite side of the furnace (Fig. 5). The sequential multichamber furnace is divided into separate zones including charge vestibule and heating, carburizing, diffusion and quenching chambers, separated by doors or insulation slide bars.

A charge is loaded into the charge vestibule whose function essentially is to remove the oxygen from the work using a vacuum pump. The charge then moves into a three-zone heating chamber equipped with ceramic insulation and metallic heating elements. The three load positions provide gradual heating with very little distortion. To ensure rapid, uniform heating with minimal thermal stress, heating is carried out by convective means in a nitrogen atmosphere.

After reaching the preselected carburizing temperature, the charge is transported into the carburizing chamber, where the carburizing process is carried out using a gas-pulse or pressure-pulse mechanism. Work stays in the chamber until the desired case depth and carbon profile is achieved. The carburizing and heating chamber/diffusion chambers are separated by vacuum-tight slide bars, which are insulated with graphic hard felt and heated with graphite heating elements.

The two-zone diffusion chamber is divided using a thermal slide bar, which allows different charge temperatures without interfering in either zone. Diffusion takes place under a low partial-pressure nitrogen atmosphere, which suppresses sublimation of alloying constituents from the part's surface.

Upon completion of the thermochemical treatment, parts are transported for quenching in a cold chamber via high-pressure gas quenching using the parameters stored in the recipe. The cooling gas flows through the charge from top to bottom. The design of the modular quenching chamber and the selection of quenching medium and pressure are based on part cross section, chemical composition and shape. The basic design comprises the quenching chamber with fan and a heat exchanger. The maximum design comprises two fans and two heat exchangers, one on top and one below the charge. The quenching gas is either nitrogen or helium. The lower atomic weight of helium allows higher flow velocities (up to 60% higher) using the same motor power. Helium's physical properties are such that it provides better heat treat transfer, and consequently, allows higher quenching speeds.

Fig 6 Transport scheme of the ModulTherm system
Linked single-chamber furnace

Linked single-chamber furnaces were developed to achieve flexibility of production and metallurgical processes. Two system types available are System I and System II.

System I has treatment chambers located on both sides of a vacuum tunnel that contains a transfer mechanism to move the charge from the charge position to the treatment chamber and from the treatment chamber to the high pressure quench. Since the systems depend on the one shuttle mechanism, a shuttle malfunction causes a shutdown of the entire cell. To address the problem, a second-generation system (System II, or ModulThermR) was developed.

ModulTherm has treatment chambers located along a track. High-pressure gas quench and transfer chambers and a shuttle transfer system travel in front of treatment chambers (Fig. 6 and 7). After the carburize-diffuse-austenitize loop is completed, the shuttle system positions itself in front of the treatment chamber and transfers work from the furnace to the high-pressure quench. After quenching, the work is delivered to the unload table.

Fig 7 ModulTherm system

The shuttle module construction guarantees the same transport times when the charge is moved from treatment chamber to quenching chamber, which is very important for achieving reproducible part hardness. The high-pressure quench is equipped with recirculating fans, water-cooled heat exchangers and a baffle system to direct the flow of nitrogen or helium quenching gas (Fig. 8). System design can include the capability of reversible flow of quenching medium and soft start for motors to optimize power factor and variable flow rate to optimize dimensional control.

Fig 8 Modular concept of high-pressure gas quenching chamber

Treatment chambers (cold-wall furnaces equipped with graphite hard felt insulation and graphite heating elements) work independently from each other. An optional convective heating system provides a highly uniform heating process, which is suitable for annealing and high-temperature tempering.

Due to the high cost of helium, a gas-recovery system to reuse helium is essential. After quenching, the quenching gas is stress relieved in a buffer tank. A suitable multistep compressor pumps the helium back into the high-pressure storage tank after pressure equalization has been established. When atmospheric pressure is reached in the quenching chamber, the quenching gas is pumped down via a vacuum pump to a pressure of approximately 0.1 mmabs. A helium loss of about 300 liters per quenching cycle is expected due to the volume of the quenching chamber. To prevent saturation of helium with oxygen, the recovery system has a helium-purification system.

The ModulThermR system can also be designed to have multiple quench capability by incorporating a specially designed ModMobile transfer system. The workload is transported under vacuum in a heated car to the appropriate quench system, which can be high-pressure gas, oil or salt, depending upon metallurgical specifications.