Effective Case Depth Measurement
Case-hardening occurs when the surface layer hardness of a ferrous metal is increased, forming what is known as a "case" layer over the remaining, unaffected "core" material. For rotating components involved in industries such as automation and defense, case hardening is desirable. It serves as a protective shell that is both stronger and more wear resistant than that of the original metal, prolonging part lifespan and preventing operation malfunctions during service. Case hardening is a cost-effective means of producing parts that have increased wear resistance on contact surfaces, while maintaining a ductile and tough inner core. Various methods, including carburizing, nitriding, and flame hardening, may be used to produce a case layer. Case hardening may also form inadvertently as a result of exposure of the part to elevated temperatures in specific environments in service, thereby negatively affecting its function.
Case depth measurements are conducted to verify the extent of case hardening, using Microindentation hardness testing and/or optical microscopy measurements. Total case depth is the total distance that carbon, nitrogen, or both have diffused inward from the surface of the part. It is the depth ta which the microstructure and hardness is no longer discernible from the core material. The case is not visible in the as-received condition; therefore, an etchant is applied to detect the contrasting case and core microstructures. Then a Vickers hardness traverse is performed to measure the changes in hardness from the part surface inward. Effect case depth is the distance from a part's surface to the point where a specified hardness is measured, typically 50 HRC. The required effective case depth will vary based on the application and customer specifications.
Common specifications associated with this testing are:
ASTM E92 or ASTM E384, which govern the standard test methods for Vickers hardness and Knoop hardness measurements of metallic materials.
SAE J423, which governs methods of measuring case depths
ASTM E140, which provides the standard hardness conversion tables for metals relationship among Brinell, Vickers, Rockwell, Superficial, Knoop, Scleroscope, and Leeb Hardness.
Entry Originally Posted April 2021