GB/T 33362-2016Metallic materials -- Conversion of hardness values (English PDF)

3.1 General

Conversions between different hardness values or between hardness values and tensile strength values have uncertainties, that shall be taken into account. Extensive research indicates that, regardless of how meticulously tests are conducted, it is difficult to establish a universally applicable conversion relationship between hardness values obtained via different methods. This is due to the complex relationship -- between the formation of an indentation in the material and the material's elastic and plastic deformation. Consequently, the closer the elastic properties of the material being tested are to those of the material used to establish the conversion relationship, the greater the equivalence of that relationship. Similarly, the greater the similarity in the indentation process (i.e., minimal differences in the force application process and test parameters), the greater the equivalence of the conversion relationship. Therefore, the reliability of conversions between hardness values and tensile strength values is the lowest.

Note. In many cases, yield strength or specified plastic extension strength provides information regarding the material's elastic properties.

The optimal test method shall be selected for hardness testing.

It shall be noted that the result of each hardness test applies only to the specific indentation area tested. For example, as the distance from the surface increases, Brinell and Vickers hardness test values, as well as tensile strength values, deviate from converted values due to changes in the material's elongation in that region. Indentations of different geometries are affected differently; consequently, even for the same specimen, conversion results between different hardness scales will not be consistent.

Hardness values shall only be converted, when the specified test methods cannot be used, for instance, if no suitable hardness tester is available or the required specimen cannot be obtained. Figures 3 and 4 assist in selecting the appropriate test method.

The average value derived from at least three hardness readings shall be used as the initial value for conversion.

To ensure acceptable measurement accuracy, the specimen surface shall be sufficiently smooth; the required degree of smoothness depends on the selected hardness test method, the test force, the appropriate specimen surface preparation method, for example, from machining (suitable for macro-hardness) to polishing (suitable for low test forces and micro-hardness).

The uncertainty of the converted values in these tables accounts for both the confidence interval of the hardness conversion curve, as derived from regression analysis, and the uncertainty of the original hardness or tensile strength test values. The confidence interval of the regression function is a parameter independent of the user; it is calculated based on the hardness function.

The uncertainty of the original test values is influenced by factors such as the repeatability of the hardness tester, specimen surface quality, uniformity of specimen hardness, the number of indentations made, that is, it depends on the test conditions under which the original values were obtained. This standard lists hardness values for various scales, and in some cases, corresponding tensile strength values enabling hardness conversion for different material types.

If the conversion tables are used solely for hardness comparison without performing actual hardness testing, the uncertainty of the converted value is reduced to that of the hardness conversion curve's confidence interval; in this scenario, it does not matter which value serves as the initial value and which as the converted value.

The supplier and purchaser may agree upon acceptable converted values and permissible deviation ranges. In such cases, the average of five measurement points shall be used as the initial test value.

3.2 Converted values

3.2.1 Limits of error

Depending on actual test conditions, pairs of test/converted values (e.g., HV/HRC, HRC/HV, HRA/HRN, HB/Rm) can be found in the tables provided in Appendixes A ~ G. This article outlines the fundamental principles to consider when selecting a hardness test method.

The following example illustrates the situation regarding hardness values with associated errors, when performing hardness conversions using Table C.2.

Given hardness value. (300 ± 30) HV Expected scale. HRC ......