Carburization, Decarburization, Vickers Hardness Test and More
Microhardness testing is a method of determining a material’s hardness or resistance to penetration when test samples are very small or thin, or when small regions in a composite sample or plating are to be measured. The microhardness test can measure surface to core hardness on carburized or case-hardened parts (case depths), as well as surface conditions such as grinding burns, carburization or decarburization. Microhardness testing is performed to evaluate cold work, determine weld quality and determine mechanical properties of small parts.
Two microhardness test methods are the Knoop hardness test and Vickers hardness test. During microhardness testing, a Vickers (DPH) or Knoop (KHN) diamond indenter is pressed into the material’s surface with a penetrator and a light load of up to 1000 grams. The result of applying the load with a penetrator is an indent or permanent deformation of the material surface caused by the shape of the indenter.
Both the Knoop hardness test and Vickers hardness test methods use specific measurements from the indent, in conjunction with formulas, to calculate material hardness. Accurate measurement of the resulting indentation requires the use of a special microhardness testing microscope because the indents are so small. Conversions from microhardness test values to tensile strength and other hardness scales (e.g. Rockwell) are available for many metals and alloys.
Microhardness testing provides valuable information for the following purposes:
- Certify conformance to hardness requirements for carburizing, nitriding, plasma nitriding, induction, flame and many other surface hardening processes
- Measure the effective case depth in case hardening heat treatments
- Measure surface or core hardness of carburized parts, as well as surface conditions such as grinding burns or decarburization
- Provide precise and detailed information on surface features of materials that have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking
- Determine hardness of different micro-constituents within a structure, or measure steep hardness gradients such as those encountered in casehardening
The Test Process
Knoop – The Knoop hardness test is performed by applying controlled force of 1000 grams or less for a specific amount of time to an indenter in a rhombus-shape (elongated four-sided pyramid). The hardness of the material is determined by the depth to which the Knoop indenter penetrates. The impression is measured microscopically and, when combined with the amount of the test load, can be used to calculate the hardness value on the Knoop scale. Knoop hardness numbers are often cited in conjunction with specific load values.
Vickers – The Vickers hardness test can be performed on both the micro and macro scales (some Vickers testers have a maximum test load of up to 50 kilograms). Like Knoop microhardness testing, these tests are also performed by applying controlled pressure for a standard length of time, but with a square-based diamond pyramid indenter. The diagonal of the resulting indention is measured under a microscope, then this measurement and the test load are used in a specific formula to calculate the Vickers hardness value.
Microhardness testing performed at LTI is PRI/Nadcap and A2LA accredited and completed according to detailed procedures and applicable industry standards to assure reliable results. All results are documented in Certified Test Reports.
Microhardness Testing and Examinations
- Vickers Hardness Test – load weights up to 50 kilograms
- Knoop Hardness Test – load weights up to 1000 grams
- Surface Contamination
- Carburization & Decarburization
- Case Depth