In the manufacture of metal and/or ceramic parts, it is common to form the starting material into the desired shape of a part and then to heat treat the part. The starting material is typically blended and/or coated with a lubricant or binder and may include optional additives as well.
For example, if the starting material is a powder the lubricant is added principally to increase the bulk density of the uncompacted powder. In addition, the lubricant allows a reduction in the pressure used to compact the powder to its specified density and shape. Still further, the force required to remove the compacted part from the mold used in the compacting process can also be reduced. Thus, the use of lubricants in the compacting of powders to form preheated parts is highly recommended.
In another example, solid strips of starting material (e.g. metals or metal alloys) may be coated with one or more lubricants prior to or during rolling to reduce the thickness of the strip.
Despite the advantages of using lubricants, there is a significant disadvantage associated with lubricants. Most lubricants have a decomposition temperature below the typical temperatures used to heat treat the part. Accordingly, the lubricants decompose into undesirable by-products including carbon or soot. This process results in an unattractive surface finish on the heat treated part and a coating of soot in the heat treating zone of the furnace.
There have been efforts to avoid and/or eliminate the presence of the decomposition products of lubricants, although such efforts have not been commercially successful. For example, Sidney G. Roberts, U.S. Pat. No. 4,104,061, discloses a process of removing gaseous or volatile contaminants from metal powders or compacts by employing a vacuum treatment followed by back filling with an innocuous depurative gas capable of forming a solid reaction product with the metal or the impurities in the metal. The gaseous reaction product is then removed.
John Blachford, U.S. Pat. No, 4,106,932, is directed to a new type of lubricant composition in the form of discrete pressure-rupturable microcapsules composed of a core containing a liquid lubricant surrounded by a shell of a degradable polymeric material and optionally a solid lubricant.
George M. Brasel, U.S. Pat. No. 5,059,387, discloses the use of a lubricant having as its primary constituent a thermosetting condensation resin.
Despite these efforts, the problems associated with the decomposition of lubricants in the heat treating zone and the deleterious affects on both the heat treated part and the walls of the furnace remain. It would therefore be a significant advance in the art of heat treating parts to effectively and economically conduct the heat treating process only after the lubricants have been substantially removed from the part prior to heat treating. In meeting this objective, it would be desirable to monitor the concentration of the lubricants in and/or on the part and only when the concentration is reduced to a predetermined value would the heat treating process commence. In this way, the level of the lubricants in the heat treating zone is minimized resulting in a desirable product and substantially reduced soot levels in the heat treating section of the furnace.