Carbon Restore Heat Treat for Investment Castings
Certain processes, such as investment casting, create a decarburized layer on the surface of the part. Microstructurally decarburization is exhibited by ferrite from the surface of the part to some depth, depending on casting variables or prior thermal processes. Often the layer of ferrite is objectionable, due its low strength and wear properties. This condition can be remedied by the process of carbon restoration. This process is essentially carburizing the part such that the surface carbon potential matches the base carbon content of the specified material.
The process of carbon restoration requires an atmosphere furnace using endo-thermic gas as the medium to add carbon to the surface of the components or castings. It is essentially a carburizing heat treatment and thus it is critical that the furnace have a system that controls the carbon content of the furnace.
ThermTech has many options for processing parts needing carbon restoration. The sizes of ThermTech’s furnaces capable of carbon restoration range up to 36” x 72” x 48” and an 8000 lbs. maximum loading capacity. All of ThermTech’s furnaces are certified to AMS 2750 for pyrometry.
The critical variables for carbon restoration are the base carbon and chemistry of the material and the depth of decarburization. Once these variables are known a process can be derived for the proper time, temperature and carbon potential to yield a part free from partial or total decarburization.
Final hardness and mechanical properties are another consideration. Hardening and carbon restoration can often be achieved in the same cycle depending on alloy. If the parts require machining an annealed microstructure may be required. It is best to discuss the requirements for final properties with the heat treater prior to the submission of the purchase order so that the proper process or processes can be agreed upon.
The general process of carbon restoration applies to a large range of materials. That said the process parameters can vary greatly for each alloy and the final properties that are required for a given component. The most important aspects of carbon restoration are the correct carbon potential to diffuse carbon into the surface of the part and the proper time at heat to assure that the carbon content is uniform from the surface into the part and that the formerly low carbon zone has been totally restored.