Low Pressure Carburization Process Improvement for a Ring Gear

Low pressure carburization (LPC) processes are becoming more widespread throughout industry due to the reduced cycle times and the control over the carbon profile through the case. Unlike gas carburization, which utilizes a constant carbon potential to maintain the available carbon on the part surface at a specific value, LPC utilizes boost and diffuse steps. A boost step involves the temporary addition of a carbon carrying gas to the furnace, usually acetylene, to increase the surface carbon to the saturation limit of austenite. If not properly controlled, the carbon available for diffusion can well exceed the saturation limit, creating unfavorable carburizing conditions. After a requisite amount of time has passed, generally one to several minutes, the carbon carrying gas is evacuated from the chamber. The carbon that was deposited on the surface during the boost step is then allowed to diffuse into the part, reducing the surface carbon. These two steps are then repeated until the required case depth and carbon profile are achieved.

For steel alloys that do not contain a significant amount of strong carbide forming elements, this process can be relatively easy to control. However, with the advent of high strength steels for the aerospace industry, most of which contain substantial amounts of strong carbide forming elements, such as chromium, molybdenum, and vanadium, the LPC process can be extremely challenging to control. These elements bond with the carbon deposited on the surface to form primary carbides, which if not properly dissolved can damage fatigue performance. Furthermore, long boost times, as well as very short diffuse times, can lead to the thermodynamic stabilization of these primary carbides.

While Fick’s Second Law describes the diffusion of carbon through a low alloy steel with reasonable accuracy, the same is not true of medium and high alloy steels. This is due to the presence of primary carbides forming and dissolving during the LPC process. When the boost step is occurring, the primary carbides take carbon away from the austenite solution, allowing more carbon to enter the solid solution from the surface. During the diffuse step, as the carbon that is in solid solution diffuses into the part, reducing the carbon in austenite, the primary carbides can dissolve to provide more carbon to the solid-state solution. If the primary carbides are not allowed to fully dissolve or shrink to a significantly small size before the next boost step begins, they will continue to grow. In order to properly predict the carbon profile of medium and high alloy steels, the primary carbide formation and dissolution must be considered. The heat treatment simulation software DANTE has this capability.

The following is a case study for the redesign of an LPC schedule for a ring gear using the DANTE heat treatment simulation software. The original LPC schedule, consisting of six boost-diffuse steps, was producing too many primary carbides during the process, and consequently, rolling contact fatigue performance was diminished.