Original Date: 05/12/1997
Revision Date: 01/18/2007
Best Practice : Carburizing Process Inspection
Cincinnati Milacron performs various in-house heat treatment processes for specified surface hardness requirements. Carburization, a primary heat treatment process, produces case-hardened surfaces with specific case depths and effective hardness values. Reliability measurements are obtained from a sample coupon (processed with the test load) and analyzed to verify the case depth and hardness value. Over the years, Cincinnati Milacron has transitioned through several coupon types including shim stock, test pins, fracture pins, and bored round stock. Although data obtained from these types focus on carbon profile and surface hardness, they do not accurately or efficiently measure effective case depth. Cincinnati Milacron has developed a unique coupon for in-process inspection which accurately measures effective case depth in the test load after carburizing.
Cincinnati Milacron uses continuous oxygen monitoring (rather than intermittent sampling) to achieve a more efficient and effective process. Continuous monitoring of the carburizing chamber atmosphere shortens the overall carburizing process and provides better control by boosting or employing an elevated carbon-rich atmosphere during the initial heating. Since 1993, Cincinnati Milacron has tailored its carburizing process with a standardized wedge- shaped coupon. The initial coupon (1" x 3/8" x 6" flat bar stock) has the same relative carbon content as the test load. After carburizing with the test load, the coupon is ground to a standard wedge profile. Depth measurements along the slope are marked, and hardness values are recorded at the specified effective case depth. If the wedge is beyond specification limits, the carbon profile is determined with a spectrometer. Then, the operator calculates the corrective heat treat cycle (e.g., rerun time, temperature, carburizing level) by entering the carbon profile information into a computer program.
By using ground wedge profiles for its in-process inspection, Cincinnati Milacron gains several improvements in accuracy and efficiency. The carburizing process checks and releases the test load in 2.5 hours, while prior methods required inspection times of 4 to 24 hours. Effective case depth is now measured directly, and only one coupon is required to obtain data for the effective case depth and the carburized case depth. All operations can be performed by the operator with a 98% initial success rate. With these improvements, Cincinnati Milacron has decreased cost during production and increased product reliability.
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