The acceptance of low-pressure (vacuum) carburizing (LPC) technology in manufacturing is due in large part to the ease of recipe development and modification made possible by the use of simulators designed specifically for this purpose. Let’s learn more.

Simulator Basics

Today’s LPC simulators are highly accurate, often being able to develop the proper recipe without test loads for most common steels (this is especially useful for the commercial heat-treatment industry). Custom steels or chemistry modifications often require only one or two test loads prior to starting production runs. Inputs for simulator calculations usually include material type, part geometry, carburizing temperature, hardening temperature, target effective case depth, surface area of the load and targeted final surface carbon content.

Based on this input data, a carburizing recipe is established complete with the number and duration of hydrocarbon gas injection steps during the boost stage and the number and duration of evacuation or gas dilution steps during the diffusion stage. Gas flowrates are precisely determined and controlled via mass flowmeters. The program output is displayed graphically, illustrating the carbon profile as a function of depth as well as the surface carbon concentration at various steps in the process (Fig. 1).

Fig. 1. Typical output screen from an LPC simulator (courtesy of SECO/Vacuum Technologies)

Material selection is typically done either from a lookup table of common steels (U.S. and/or European grades) or through the input of a custom chemistry. This chemical composition data (often in conjunction with Jominy data) is also used by some simulator packages to help determine proper quench response. These process simulators can also provide anticipated hardness profiles as a function of depth from the surface. It should be noted that when a carbon profile is converted into a hardness profile, part geometry and quenching variables may skew the final result.

One of the simplest ways in which to determine the effectiveness of a particular manufacturer’s simulator is to review various material choices available and ask if the output recipe has been confirmed by extensive empirical trials. In other words, have loads been run and results analyzed via testing or just through scientific calculation?

Part geometry is usually limited to simple shapes (cylindrical, spherical, rectangular, etc.) rather than asking the user to input sophisticated geometries. On the other hand, calculation of load surface area must be more accurate (within about ±10%) so that the proper amount of hydrocarbon gas is added. Surface carbon values are lower than their atmosphere counterparts – 0.70-0.75% C being typical.

Outputs for simulator calculations usually include recipe/program, carbon profile, carbon flux/mass flow and surface carbon content.

While the final surface carbon content always corresponds to the setpoint value, it is important to recognize that most simulators estimate the carburizing case depth within about ±5% of this value. For this reason, an initial trial run is always recommended. It should also be noted that these simulators have now become more and more pre-cise and should be able to produce specification requirements with little testing. Once a process has been success-fully simulated, repeatability of results is almost a foregone conclusion (provided one has control of the equip-ment-induced variability). 

Most simulator packages are run remotely on an office computer, and process recipes are uploaded to the furnace with a few keystrokes. They can also be manually input through a setup Wizard following instructions and dropdown menus (Fig. 2).

Fig. 2. Recipe input – material selection (courtesy of ALD Vacuum Technologies, GmbH)

Tweaking the Cycle

In some instances, the data obtained by running test parts indicates that the recipe needs to be modified to fine-tune the results. Common reasons for this include issues with:

• Carbide formation and/or carbide necklacing. This indicates that either the surface carbon content is too high during the boost portions of the cycle, the diffusion time was insufficient, the soak time at temperature was too short or the final hardening temperature was too low. Possible recipe changes include either shortening the boost times or reducing the number of boost steps. Additionally, adding more diffusion steps or increasing soak times and/or temperature will eliminate the problem. 

• Retained austenite. One possible reason for this is that the surface carbon content of the steel in question is too high, so lengthening the last diffusion step will help reduce the final surface carbon value. Another cause is direct quenching from carburizing temperature, so reducing the temperature before quenching is recommended.  

• Carbon content too high/low. The surface area of the load should be carefully calculated to see if the gas flowrate value is too high/low. Some manufacturers offer a limited number of flowrates (e.g., high, normal, low) that are not ideally matched to the actual surface area present, resulting in a higher/lower surface carbon content than anticipated. 

• Surface pitting. This indicates that the surface carbon was too low during processing and should be increased. Pitting on gear teeth is a common example. 

• Over-carburizing. In areas where the geometry influences carbon absorption (such as the tips of gear teeth), reducing the flowrate during the boost stage or adding more but shorter-duration boost steps helps minimize this effect. 

• Part distortion. There are numerous factors that influence part distortion, including the type and hardenability of the steel, the type of quench medium (oil or high-pressure gas) and the final hardening temperature, to name a few. One wants to select a material with the lowest hardenability that will achieve the required mechanical properties, including surface and core hardness. Quenching from a lower temperature may also help, and the recipe should be adjusted so that there is enough time for the core/surface temperature of the parts (load) to equalize before quenching.

Universal LPC Simulation Software

Until recently, LPC simulators were captive to the type of equipment being purchased. The principal limitation was that (as designed) the software would work only on a particular manufacturer’s furnaces running one type of hydro-carbon gas in a defined pressure range. In essence, the OEMs had proprietary software designed to work only with their systems. Today, at least one manufacturer has developed LPC simulation software that will produce recipes for all types of carburizing furnaces running all of the common hydrocarbon gas mixtures in use (e.g., acetylene, acetylene + hydrogen, actelyene + nitrogen and acetylene + ethylene + hydrogen).

Summary

The inherent advantage of low-pressure carburizing is its consistency, repeatability and flexibility. As more and more LPC furnaces find their way into the heat-treat shop, the need for and convenience of a universal simulator will become paramount. One should expect to find, as a minimum, the following features on any LPC simulator:

• An extensive materials database
• The capability of creating and exporting a heat-treatment recipe (i.e., a sequence of cycle steps) showing the carbon profile (including surface, intermediate and limit carbon) as well as deviation from set values
• Save and load recipe functions
• The ability to edit recipes, add or delete alloys, add custom chemistries and change individual elements in the standard materials list (such as carbon content)
• Graphic displays for carbon and/or hardness profiles, carbon percentage at various stages in the process, and carbon flux and/or mass flow in addition to the process recipe inputs. For ease of use, a zoom function, scalable axes, historical record retrieval and the ability to save data to external files should be included. 

If you are or will be using LPC in the future, be sure the equipment has or comes with one of these state-of-the-art devices. 

References

1. SIM-Vac® Vacuum Carburizing Simulation Software Manual, SECO/Vacuum Technologies, 2018
2. VC-Sim® Recipe Simulation Manual, ALD Vacuum Technologies GmbH, 2018
3. Wolowiec-Korecka, Emilia, “Principles of Heat Treatment Process Modeling,” ASM International, Proceedings of the 28th ASM Heat Treating Society Conference, 2015
4. Kula, Piotr, Emilia Wolowiec, Konrad Dybowski, Robert Pietraski, Radomir Atraszkiewicz and Sylwester Paweta, “Carburizing: Deep, Case Structure and Process Technology,” Encyclopedia of Iron, Steel and Their Alloys, Rafael Colas and George E. Totten (Eds.), Taylor & Francis, 2015
5. Wolowiec, Emilia, Piotr Kula, Lukasz Kolodziejczyk, Konrad Dybowski and Maciej Korecki, “Mathematical Modeling of the Vacuum Carburizing Process,” ASM International, Proceedings of the Thermal Process Modeling and Computer Simulation Conference, 2014
6. Kula, P., R. Atraszkiewicz and E. Stanczyk-Wolowiec, “Modern Gas-Quenching Chambers Supported by SimVaCPlus® Hardness Application,” Industrial Heating, March 2008
7. Bill Gornicki, ALD Vacuum Systems, Inc., technical contributions and private correspondence
8. Dr. Maciej Korecki, SECO/WARWICK S.A., technical and editorial contributions and private correspondence