Micro-indentation hardness methods such as HV and HK are essential for certain situations because of the need to take multiple measurements within a small volume of material. Figure 1: Schematic of micro-hardness indentation trace to measure case depth in a surface-hardened steel. The hardness conversion tables and equations in the standards generally correlate empirically derived numerical scales for specific materials and are not direct conversions of physical property relationships. While the conversion from one scale to another is straightforward, the converted hardness values need to be considered in the context of expected results from the thermal treatment process. įurthermore, ASTM E384 confirms that there is no generally accepted method for precise conversion to other hardness scales. “Conversion of hardness values should be used only when it is impossible to test the material under the conditions specified, and when conversion is made it should be done with discretion and under controlled conditions.”. However, the guidance in the ASTM E140 standard regarding conversion from one scale to another, states the following: #Vickers to rockwell conversion iso#Conversion of hardness numbers from one scale to another is generally accomplished by reviewing the tables and equations available in externally published standards, such as ASTM E140 or ISO 18265. Hardness numbers may be calculated as a function of either a projected area or as the contact surface area of the indentation depending on the scale used. For an indentation hardness method, the reported hardness numbers are based on the load on the indenter, the geometry of the indenter tip, and the measurements of the permanent plastically deformed impression in the sample. Numerical hardness values are unique to each hardness scale and method. Furthermore, conversion between different methods is not mathematically exact. While hardness numbers have repeatable correlation to material strength, efforts to reduce hardness testing to a universal, fundamental physical test have not been successful. There are many hardness test methods that are performed by an array of techniques, from comparative references to scratch testing and indentation methods. Methods of measuring hardness have been documented and in use for more than two centuries. In this article, we explore the impact of converting from micro- to macro-indentation hardness methods - i.e., Vickers (HV) and Knoop (HK) to Rockwell C (HRC) - in case-hardened steels. This discussion delves into the practical implications of hardness conversion. The variation in conversion results from different hardness scales can make the difference between accepting and rejecting a production batch of components. However, hardness data conversions from one scale to another may not always yield the expected results. The process wherein heat-treatment practitioners measure hardness by one method and convert the results to a different scale is generally known and a common practice. The use of hardness testing as a quality control method to check the outcome of a thermal treatment process is a common application of this measurement technique.
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