A little over one year ago, Aero Gear purchased a new – and very yellow – vacuum carburizing furnace in order to gain more control over case depth, hardness, and distortion compared to traditional atmosphere carburizing. Unlike atmosphere carburizing, our new furnace operates at pressures near the 1 micron range (that is 99.9999% of a perfect vacuum). What makes this technology so exciting is the ability to precisely control heat treatment parameters, and since its arrival in the beginning of July 2013, our team of engineers has been developing processes which remain consistent and repeatable. As a matter of fact, we have carburized nearly 6,000 pieces without a single metallurgical rejection.
Comparing Atmosphere and Vacuum Carburizing Results
Most of our precision gears are made out of the industry standard AISI 9310 and Pyrowear 53 steels. These low carbon steels were designed to be carburized in atmosphere furnaces, but we have seen that they vacuum carburize with more uniform case depth and hardness. The ability to drive carbon deeper into the surface also enables the gear to have much higher case hardness and compressive stresses which signify a stronger, tougher part. In addition, higher case hardness increases wear and pitting resistance. To prove the merits of vacuum carburizing, we performed a residual stress profile using x-ray diffraction on as-carburized samples of a gear made of AISI 9310 steel. The results are below:
Residual compressive stress in the case is 19% greater at the flank and 1260 MPa in the root when comparing vacuum vs. atmosphere carburized samples. Surface hardness for the atmosphere carburized sample is 62 HRC (Flank), 59 HRC (Root). Surface hardness for the vacuum carburized sample is 64 HRC (Flank), 62 HRC (Root).
One of the key reasons Aero Gear has invested in vacuum carburization is more distortion control. The distortion can be very detrimental to a gear, regardless of how robust the carburizing process may be. Tooth spacing error, for instance, can lead to uneven grinding and the risk of removing too much case from one side of the tooth than the other. We use our on-site CMM (Coordinate Measuring Machine) to inspect all features of a gear, and using this data our engineers can tailor processes and fixturing on an individual part number basis. Parts carburized in our vacuum furnace distort less and have better case uniformity throughout the gear with more predictability.
The addition of the vacuum furnace has allowed us to work on the development of heat treatment of new materials with our customers such as Ferrium C64. Aero Gear is currently developing heat treating and machining processes for Ferrium C64. This new material developed by Questek Innovations uses an M2C carbide precipitate to achieve hardness (where M is metal; C is carbon). Unlike AISI 9310 and Pyrowear 53, which use an epsilon carbide, the M2C carbide is much more effective and requires less carbon to achieve hardness. Precise carbon and temperature control is required for Ferrium C64 in order to avoid complex carbide (M6C, M7C3, M23C6, etc.) formation which could lead to crack propagation and reduced toughness during operation. Aero Gear’s engineers have developed processes to achieve optimum hardness and core properties while maintaining a carbide free microstructure. In addition, we have been developing machining processes to deal with the increased core hardness of Ferrium C64. Due to the significant amount of alloying constituents, machining parameters must be changed in order to compensate for the increased hardness and toughness. Furthermore, the capability to 2-bar quench allows us to reduce distortion, reducing machining time during further processing.
We are continually learning and advancing our ability to successfully heat treat and machine complex geometries. We have accomplished so much more than we expected with our vacuum furnace but we still have so much more to develop and learn. Our expectations for the next year are even greater than last. We are on a path to be the experts in manufacturing vacuum carburized/hardened ground gears in the aerospace industry.
Look for our next update in the near future!