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Microhardness Testing

Microhardness - 501 Stainless Steel Nitride Case DepthView Larger Image
Microhardness - Carburized Steel

The microstructure of this carburized steel was delineated using a 2% Nital etch. The depth of the carburized case (dark) was measured using 500 gram Knoop indentations.

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Microhardness testing is performed to determine a material's true hardness and is conducted using a precision diamond indenter called, "Knoop or Vickers". The microhardness microscope allows accurate determination of case hardness and depth and is used to certify conformance to hardness requirements for carburizing, nitriding, plasma nitriding, induction, flame and many other surface hardening processes.

In cases where a surface hardened microstructure appears abnormal, microhardness testing can be used to identifying soft ferrite (a decarborized layer) from hard carbides.

Terminology

The following terms relate to microhardness testing:

Case
The case is the outer surface hardened by thermal allotropic transformation.
Effective Case Depth (ECD)
The case depth ending at 50HRC.
Microhardness
The hardness of a material determined by indenting the surface with a Vickers (DPH) or Knoop (KHN) indenter using a very light load. Measurement of the resulting indentation requires the use ofa special measuring microscope. Microhardness testing is used to accurately measure the effective case depth in case hardening heat treatments.
Total Case Depth (TCD)
The case depth ending within one (1) HRC of core hardness.
  

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