AvaC™ is a proven process for vacuum carburizing with acetylene. One of the most important advantages of this process is high carbon availability.

The AvaC™ process involves alternate injection of acetylene (boost) and a neutral gas like nitrogen for diffusion. During boost injection, acetylene will only dissociate in contact with metallic surfaces, thus allowing for uniform carburizing. At the same time, it almost totally eliminates the soot and tar formation problem known to occur from propane.

Process Description

As shown in Figure 1, once the carburizing temperature is reached, the first carburizing step is initiated by admitting acetylene into the furnace to pressures between 3 and 5 torr. Carbon transfer is so effective that the limit of carbon solubility in austenite is reached after only a few minutes. Therefore, the first carburizing step must be stopped after a relatively short time by interrupting the gas supply and evacuating the furnace chamber.

This initiates the second step, or the first diffusion segment. The carbon transferred into the material and the surface carbon content decreases until it reaches the desired surface content. Depending on the material case depth specified, further carburizing and diffusion steps follow. Once the specified case depth is obtained, direct hardening usually involves reducing the load temperature and quenching the load, either in the same chamber or in a separate chamber.

Controlling the Process

Control of the AvaC process is done via numerous physical parameters, which are gas temperature, gas flow, gas pressure, and the number and duration of carburizing and diffusion steps. The number and duration of the carburizing and diffusion steps must be determined in order to meet the case-depth specifications. A simulation program is used to keep pre-testing to a minimum. The module “AvaC-Simulation” creates low-pressure carburizing cycle programs (Fig. 2). The simulation program calculates carbon profiles dependent on the temperature, surface carbon content and case depth. The calculations are based on carbon-transfer characteristics of acetylene gas.

The most remarkable benefit to AvaC can be found when the different hydrocarbon gases for low-pressure carburizing are evaluated for their penetration power into small-diameter, long, blind holes. This aspect has been investigated for samples with blind holes of 0.011 inch in diameter and 3.55 inches in length (Fig. 3). The test cycle used in this case was a 10-minute pure boost carburizing at 1650°F (3-torr pressure) and fast cooling in 2-bar nitrogen, followed by rehardening at 1580°F, using a nitrogen quench at 5 bar. After sectioning the round bar sample of 5115 steel, the surface hardness was measured inside the blind hole at various distances from the opening.

The results of these surface-hardness measurements are shown in Figure 4. This clearly indicates that the carburizing power of propane and ethylene is only sufficient to carburize the initial 0.23 inch of the blind hole. It was determined that the carburizing fell off rather significantly up to 1.00 inch hole depth. After 1.00 inch, the hole surface was completely uncarburized.

In contrast, vacuum carburizing with acetylene results in a complete carburizing effect along the whole length of the bore, fully to the bottom of the 3.55-inch blind hole. The acetylene has a totally different carburizing capability than that of propane or ethylene.

Another feature/benefit is becoming more relevant during industrial utilization of this new technology and the desire of industries to move toward more “green” technologies. Despite the high carbon availability and the greater carburizing capability of acetylene, no soot or tar is produced.

Examples of AvaC Vacuum Carburizing

This new and cost-effective technology is yielding unexpected and superior results and is quickly being adopted across many industries.

The extreme uniformity of carburizing produced by acetylene carburizing of such components is shown. At the same time, the structure of the carburized case is totally free of any intergranular (internal) oxidation because the only atmosphere that comes into contact with the nozzles during the carburizing process is the hydrocarbon acetylene.

A wide range of materials and processing techniques in vacuum carburizing can be seen in the following examples. These examples demonstrate the diversity of the process using vacuum carburizing techniques on small and large components. Additionally, the examples selected had simple and complex geometry, were wrought and powder-metal materials, had critical distortion concerns, and required oil and high-pressure gas-quenching methods. Also considered were parts requiring dense loading arrangements; having variations in section size; and requiring shallow, medium and deep case-depth requirements. This variety underscores the type of products adaptable to the AvaC process in vacuum carburizing equipment.

Process Advantage

One of the most important process advantages, as illustrated in Figure 5, is high carbon availability, which ensures homogeneous carburizing even for complex geometries and very high load densities.

Pitch-to-root case ratio of 70% for atmosphere carburizing is improved to 85-90% with AvaC. Other advantages include:

• Shorter process times due to high carbon flux, high carburizing temperature and elimination of furnace conditioning
• Enhanced component quality due to elimination of internal oxidation and precise case uniformity
• Carburizing of complex geometry and dense loads
• Safe process due to the lack of flammable waste gases
• High furnace availability/reliability due to elimination of soot or tar formation
• Higher part-to-part, load-to-load repeatability over atmosphere technology

Process Advantage over Atmosphere Furnaces

The AvaC process provides the following features and benefits over conventional atmosphere furnaces:

• Better work environment with cold-wall design that provides lower shell temperature
• No costly exhaust hoods or stacks required
• Faster startups and shutdowns with no furnace idling over the weekends
• No endothermic gas generators required
• Gas-quench furnaces require less floor space and no post washing to remove quench oils
• No pits or special foundation requirements needed

For more information: Contact Aymeric Goldsteinas, product development manager, Ipsen, Inc., 984 Ipsen Rd., Cherry Valley, IL 61016; tel: 800-727-7625; fax: 815-332-4995; e-mail: aymeric.goldsteinas@ipsenusa.com; web: www.ipsenusa.com

Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: vacuum carburizing, acetylene, high-pressure gas quenching, case depth, endothermic

SIDEBAR: AvaC™ Furnace Configurations

The furnace can be provided in the following configurations and sizes.

Fig. 6. Single-chamber design (top)
Single-chamber workspace
24 inches wide x 24 inches high x 36 inches long
36 inches wide x 36 inches high x 48 inches long

Fig. 7. Two-chamber design: oil and/or gas (bottom)
Two-chamber workspace
24 inches wide x 24 inches high x 36 inches long
36 inches wide x 36 inches high x 48 inches long