The vacuum carburizing process has consistently succeeded in delivering superior part quality over the past few decades.
Additionally, the use of vacuum technology for carburizing has always shown the most potential for enhancing the manufacturing process by reducing the number of manufacturing steps and the processing time required to produce a part.¹ Increased productivity helps achieve these savings, resulting in lower part cost and reduced total cost of ownership.
Optimizing the case-depth uniformity of parts during the vaccum carburizing process also helps achieving lower part costs. For this, the carburizing process and influencing parameters, such as the quality of thermal processing equipment utilized, uniformity of heating and positioning of the parts within the load, all need to be analyzed and optimized.
However, the first step in doing so is to understanding the emergence of vacuum carburizing and the advantages it provides for producing gears of high quality.
The Evolution of Low-Pressure Vacuum Carburizing
The development of a low-pressure carburizing technology that was fully competitive with gas carburizing began in the 1960s. While, at the time, low-pressure carburizing provided numerous benefits with respect to component quality, process time and minimized fluid burnoff and heat emissions, it still had an increased amount of soot forming in the furnace.
Additionally, there were high maintenance requirements when propane was used as a carburizing gas with relatively high partial pressures. In the mid-nineties, though, acetylene was discovered to have greater qualities as a reactive gas in vacuum carburizing (AvaC®).2,3 Table 1 shows this historical development of vacuum carburizing technology.
Table 1. Historical development of vacuum carburizing
||2000 & beyond
|Process Development Begins
||Process Introduction to Industry
||Process Limitations Uncovered
||Process Solutions Found
||AvaC® Production Carburizing
|R&D activities focus on finding alternatives to atmosphere gas carburizing.
||Ultra-high-pressure carburizing techniques developed using natural gas, 100-percent methane and propane.
||R&D activities focus on methods to reduce carburizing pressure, as well as investigating gas pressure quenching as an alternative to oil quenching.
||R&D activities focus on finding a solution to excessive soot and tar formation by using acetylene and equipment designed specifically for low-pressure carburizing.
||Combination of low-pressure carburizing equipment designs using acetylene achieve production vacuum carburizing with 95+ percent up-time reliability.
|First vacuum carburizing patents issued.
||Various vacuum carburizing method patents issued.
||High maintenance and low up-time due to excessive soot from propane use halts commercialization.
||Patents issued on use of low-pressure carburizing with acetylene.
||Production loads are heavy, dense and include all types of part geometry in all industries.
|Production loads are light, open and simple geometry.
||Production loads are heavier, denser and include both simple and complex geometries.
||Lower carburizing pressures and various gas introduction methods are adopted to attempt to reduce soot formation.
||Combination of low-pressure carburizing with acetylene as the carburizing gas eliminates soot and tar formation (a concern in vacuum carburizing).
||Modular-designed batch, semi-continuous and continuous vacuum carburizing furnaces become integrated into manufacturing and become a viable alternative to the use of atmosphere furnaces.
|Limitations of existing vacuum equipment identified.
||Equipment limitations improve with the introduction of new vacuum integral oil quench batch equipment.
||Plasma carburizing becomes a popular alternative to vacuum carburizing.
||Industry confidence and process credibility concerns addressed.
||Changes in material chemistry make gas quenching an economical alternative to oil quenching.
In all, what the history of vacuum carburizing demonstrates is that for the technology to succeed, an effective combination of equipment design, such as Ipsen's Turbo²Treater® vacuum furnace and process development (AvaC) had to be found.
The Role of Low-Pressure Vacuum Carburizing
When it comes to low-pressure carburizing in vacuum furnaces, the goal is to carburize all parts within a load uniformly, to the same case depth and to the same surface carbon content.
Low-pressure vacuum carburizing is marked by its potential to provide precise process control which, in turn, helps result in a reduction in manufacturing and maintenance costs, process repeatability and uniform part microstructures.
It must be remembered that the vacuum heat-treating systems used play a vital role in the ability of vacuum carburizing in order to achieve such precise process control. An example of such equipment is Ipsen's Turbo²Treater vacuum furnace, which features uniform cooling and heating of the parts and high quench speeds. All of which are essential factors for consistently achieving improved part quality and repeatability.
Ipsen's patented acetylene vacuum carburizing (AvaC) process is one of the several processes offered by the Turbo²Treater vacuum furnace. The AvaC process lends itself extremely well to being used in combination with high-pressure gas quenching, and it is best suited for integration into production lines.
Why AvaC? A Process Overview
It has been discovered that the AvaC process produces twice the carbon availability as compared to traditional carburizing agents, resulting in exceptional carbon transfer into the parts. AvaC is also capable of creating an oxidation-free surface microstructure while allowing even carburization of complex-geometry components.
Wherever possible, AvaC is used in combination with dry, high-pressure gas quenching as the hardening step. This provides the industry with an environmentally friendly, safe, clean and flexible case hardening process. This process, when compared with oil quenching, is also capable of reducing distortion and improving case-depth uniformity.
Turbo²Treater – Efficiency in Power
Ipsen's Turbo²Treater furnace continues to set new standards in efficiency, versatility and quality. The Turbo²Treater, re-engineered for ease of installation and global operation, offers the latest technology and technical solutions (Figure 1).
With over 200 furnaces sold all over the world, the Turbo²Treater’s cost-effective, reliable design and standardized production process allow Ipsen to provide rapid delivery times and pass on savings directly to customers.
Figure 1. Ipsen's Turbo²Treater vacuum furnace
The Turbo²Treater offers quench pressures of up to 12 bar and convection-assisted heating that speeds up cycle times and increases temperature uniformity,. It is also considered to be ideal for hardening low-alloyed materials.
In addition, its alternating directional flow, i.e. top to bottom or bottom to top, brings about an increase in quench uniformity and minimizes part distortion. The Turbo²Treater furnace also features an isothermal hold operation build-in with software that provides automatic temperature regulation and helps control distortion.
Understanding the AvaC Process
The AvaC process involves alternate injections of acetylene (boost) and a neutral gas, such as nitrogen, for diffusion. During the boost injection, acetylene will only dissociate when in contact with metallic surfaces, enabling uniform carburization. At the same time, it almost completely prevents the tar and soot formation problem, that occurs in earlier propane carburizers.
High carbon availability is one of the most significant advantages of this process. This helps ensure extremely homogenous carburizing, even for very high load densities and complex geometries. Overall, AvaC is a fairly diverse process, capable of processing parts with complex and simple geometries, wrought and powder metal materials, variations in section size, dense loading arrangements and deep, medium and shallow case-depth requirements.
As shown in Figure 2, after the carburizing temperature is reached, the first carburizing step is initiated by injecting acetylene into the furnace to pressures between 3 and 5 Torr. The carbon transfer is so effective that the limit of carbon solubility in austenite is reached after only a few minutes. Therefore, it becomes necessary to stop the first carburizing step after a relatively short time by interrupting the gas supply and evacuating the furnace chamber.
Figure 2. Typical cycle with temperature and pressure curve
The first diffusion step is initiated by deactivation of the boost event and evacuating the furnace chamber. During this segment, the carbon transferred into the material, as well as the surface carbon content, decrease until the preferred surface carbon content is reached.
Further carburizing and diffusion steps may need to follow based on the specified material case depth. Once the specified case depth has been obtained, the next step is quenching. This typically involves decreasing the load temperature and quenching the load in the same chamber.
Finally, control of the AvaC process for low-pressure carburizing involves an understanding of the variables that influence carbon transfer and diffusion. These include time (total boost or carburizing time, total diffusion time and the number/duration of carburization and diffusion steps); gas parameters (type, pressure and flow rate) and temperature.
Depending on the part surface area and geometry, the parameters listed above are determined as constants resulting in uniform carburization.
Investigating the AvaC Process and its Effect on Case-Depth Uniformity
The most noteworthy benefit of the AvaC process can be found when the different hydrocarbon gases for low-pressure carburizing are compared for their penetration power into small-diameter, long blind holes. This test involved samples with blind holes of 0.11 inches (.28 mm) in diameter and 3.55 inches (90 mm) in length (Figure 3).
Figure 3. Example of blind hole
In this case, the test cycle used was carburizing at 1,650 °F (899 °C) (3 Torr pressure) and fast cooling in 2-bar nitrogen, followed by re-hardening from 1,580 °F (860 °C) using a nitrogen quench at 5 bar. The surface hardness was measured within the blind hole at different distances from the opening, after sectioning the round bar sample of 5115 steel.
Figure 4 shows the results of these surface hardness measurements, clearly indicating that the carburizing power of propane and ethylene is enough to only carburize the initial 0.23 inches (5.8 mm) of the blind hole.
It was determined that the carburizing fell off rather significantly up to a 1-inch (25.4 mm) hole depth. The surface of the hole surface was completely uncarburized after 1 inch of hole depth (25.4 mm).
Figure 4. Surface hardness results
In contrast, vacuum carburizing with acetylene results in a complete carburizing effect along the entire length of the bore, all the way to the bottom of the 3.55-inch (90 mm) blind hole. The structure of the carburized case is completely free of any intergranular (internal) oxidation as the only atmosphere that comes into contact with the parts during the carburizing process is the hydrocarbon acetylene. As one can see, the acetylene has a completely different carburizing capability than that of ethylene or propane.
Integration of the AvaC Process and Cutting-Edge Technology
As discussed above, the AvaC process is best suited for integration into production lines. Therefore, this Ipsen-developed process is capable of offering several advantages to furnace users, including:
- Higher temperatures
- High carbon transfer rate
- High efficency due to low gas consumption
- Even carburizing process, even for complex geometries
- Enhanced part quality with part-to-part and load-to-load repeatability
- No intergranular oxidation (IGO), thermal radiation, flames or conditioning of the furnace (for the AvaC process)
- Decreased cycle times due to increased carbon diffusion rates and higher carburizing temperatures
Advanced Vacuum Furnace Design: Advantages for AvaC Applications
While the AvaC process offers numerous advantages, it is also essential that the vacuum furnace used in combination with the AvaC process has the ability to deliver optimum efficiency and optimize case-depth uniformity during the carburizing process. For example, Ipsen's Turbo²Treater furnace features a mass flow controller that is designed to be compatible with acetylene gas.
Additionally, the heat treating system should be able to provide a precise, uniform gas flow to ensure a fully optimized vacuum carburizing process. The Turbo²Treater vacuum furnace is able to achieve this because of its vessel housing, which includes a vacuum penetration point, i.e, a multi-point fuel injection delivery system that is located between the hot zone and the cold wall tank.
With injector nozzles penetrating the hot zone, this manifold system is able to precisely meter and deliver a continuous flow of process gas, such as acetylene, from outside the hot zone to inside. This lowers the risk of hot zone contamination. It also prevents the process gas from collecting on the cold wall and provides a uniform gas flow, which helps ensure consistent part quality and case-depth uniformities.
VacuProf® – Expert Controls System for Intuitive User Operation
All treatment processes in the Turbo²Treater furnace are controlled by Ipsen's proprietary software, VacuProf® (Figure 5). Through its use, vacuum furnace users can achieve manufacturing time and cost savings, improved operational reliability and increased quality consistency.
Figure 5. VacuProf control screen
Moreover, all the data generated by the process is transported to the computer system where it is then available for specific processing and conversion, e.g, for logging, archiving, visualization and reporting of errors and thresholds.
Proven and dependable, the VacuProf controls system is a powerful tool that provides:
- Built-in online manual, parts list and historical records
- Easy-to-use interface with consistent user prompts and color menus
- Remote diagnostics and monitoring of the cycle
- Extensive data collection and the ability to print or download reports
- Repeatable results with programmable recipes and optimization of processes
- Sophisticated alarm package that monitors the furnace and suggests preventative maintenance
VacuProf Expert Software
This software is unique to the market and allows a user without any special prior knowledge to select the correct process for the type of steel to be treated. The user simply has to enter the characteristics of the load geometry, steel and a few other details, such as desired hardness or the quenching and heating characteristics.
A possible heat treatment recipe for the entered material will then be recommended by the VacuProf Expert software.
Advanced Controls Software: Applications of AvaC Simulation Software
Available through the VacuProf controls system, complete heat treatment cycle programs with heating, treatment and quenching segments are developed with the help of AvaC Simulation software, shown in Figure 6.
The program simulates low-pressure vacuum carburizing cycles and calculates carbon profiles that are dependent on the case depth, surface carbon content and temperature. The calculations are based on the carbon transfer characteristics of acetylene gas.
Figure 6. AvaC Simulation software tab
Development of Superior Quenching Capabilities
After the specified case depth has been obtained through vacuum carburizing, the next step applied is typically quenching. The goals for quenching with reduced distortion can generally be defined as follows:
- Material and part-adapted timing to control the quench intensity
- Uniform heat extraction on every part within one load
- Uniform heat extraction over the whole surface of the part
Today's requests of adapting the quenching intensity of quench systems to the needs of varied components, particularly hardenability and minimization of distortion, have also resulted in the increased production of quality components. As such, the Turbo²Treater furnace was also designed with these requirements in mind.
Enhanced Gas Flow
Utilizing a high-volume flow with vertical gas quenching, the Turbo²Treater furnace will be able to reach pressuresof up to 8 bar when using argon, or 12 bar when using nitrogen. By default, the quench direction is from top to bottom; however it can be reversed to flow from bottom to top and back.
It is also possible for furnace users to set the interval at which direction changes need to occur based on load temperature and time, thus enabling a more uniform quench and part quality.
A main feature of the Turbo²Treater that improves its quenching capabilities, though, is its utilization of the Coanda Effect. This effect is where the flow follows a curved surface. The Coanda Effect, integrated into the design of the hot zone's baffle, makes it possible for the gas to enter the hot zone in a highly uniform manner. This ensures all the parts are hit with the same amount of quench gas, and that a uniform part quality is achieved throughout the load.
The Turbo²Treater furnace's square hot zone is also covered with a special high-performance carbon fiber composite (CFC), used in the form of either laminated CFC or as CFC with foil coating, that can withstand temperatures of up to 3,600 °F (2,000 °C).
This additional covering of the hot zone is considered to be beneficial for high-pressure gas quenching as it protects the hot zone from the high-velocity gas stream. This helps extend the hot zone's service life, thus reducing maintenance costs and subsequent servicing.
Past experience shows there is significant potential for optimizing, as well as producing, a more uniform quench. The implementation of such features has produced a more uniform hardening of parts, particularly for gear components, with an advanced microstructure and reduced distortion. Additionally, using the AvaC process in tandem with high-pressure gas quenching can optimize the manufacturing process by decreasing both the processing time and the number of manufacturing steps needed to heat treat.
The thermal processing industry has to heat treat a wide range of parts in terms of geometries, load sizes, materials and more. Therefore it is essential to achieve precise process control. By employing vacuum carburizing with acetylene, the industry can continually experience uniform part microstructures, reduced manufacturing costs and process repeatability. They can also control and optimize case-depth uniformity, even for dense loads and complex geometries.
However, the need for an improved carburizing process does not stop there. As demonstrated by the history of vacuum carburizing, the need to produce parts with reduced distortion and enhanced case-depth uniformity required the development of Ipsen's patented AvaC process.
It also made it necessary to produce a heat treatment furnace, such as Ipsen's Turbo²Treater vacuum furnace, that meets the industry's diverse process demands. In the end, by refining the vacuum carburizing process, the end user will be able to optimize the manufacturing process which eventually results in the production of high-quality gears with lower cost for every single part.
 Herring, D. H., Applying Just-In-Time Manufacturing Techniques to Heat Treating, Advanced Materials and Processes, 1994.
 EU Patent EP 0 818 555 dated 28 March 1996, JH Corporation, Japan.
 EU Patent EP 0 882 811 dated 3 June 1997, Ipsen International GmbH, Kleve, Germany.
This information has been sourced, reviewed and adapted from materials provided by Ipsen.
For more information on this source, please visit Ipsen.