The ASTM specifies thirty different Rockwell scales, each employing a different combination of test forces and indenter types, which allows the testing Test Procedure of most types of metallic materials and products. When Rockwell hardness is called out by a product standard or specification, the choice of scale is usually specified. In situations where the user must choose the appropriate Rockwell scale, there are several factors that should be considered. These include the type of test material, the test material thickness, the test material area or width, the test material homogeneity, and the limitations of each Rockwell scale.
3.1.1 Type of Test Material
Table 1 lists the typical types of materials that are suitable for testing on each of the thirty Rockwell hardness scales. When deciding on an appropriate Rockwell scale for a particular material, information in this table can assist the user in narrowing down the number of scales to choose from.
3.1.2 Test Material Thickness
As a Rockwell hardness measurement is being made, the material surrounding the indentation is plastically deformed with the deformation extending well below the indentation depth. If the deformation extends completely through the thickness of thin test material, then the deformed material will flow at the interface with the supporting anvil. This will influence the deformation process likely causing the test to give erroneous hardness results. Thus, the test material must have a sufficient thickness in order to obtain a valid Rockwell test value. Similarly, for products that are manufactured to a specific thickness, a Rockwell scale having the appropriate combination of test forces and indenter size must be chosen based on that thickness.
When the approximate hardness of the test material is known, the minimum thickness needed to obtain valid Rockwell measurements may be estimated from data tables and graphs available in the literature, such as in the ASTM standard(2). In general, the zone of deformation extends no more than 10 times the depth of indentation for a diamond indenter test and 15 times the depth of indentation for a ball indenter. As a rule, there should be no deformation on the support side of the test material following a Rockwell test, although such markings are not always indicative of a bad test.
- Testing of too thin material can damage a steel anvil by marring the surface or producing a small indentation. In either case, further testing should not continue with the damaged anvil.
- Stacking one or more additional layers of metallic material together cannot make up for an insufficient material thickness. The material flow between the layers will produce inaccurate measurements.
If the objective of the Rockwell test is to measure the hardness of a surface feature such as a case-hardened surface, the scale chosen should be based on the thickness of this surface feature.
3.1.3 Test Material Area (or Width)
In the same way that the deformation extends below an indentation, thus limiting the minimum material thickness, the deformation also extends outward through the material width. If a Rockwell measurement is made near the edge of the test material, the deformation surrounding the indentation may extend to the edge and push out the material, thus lowering the measured hardness value. This effect is more significant for softer materials. The general rule as specified by the test method standards is that the distance between the center of an indentation and the edge of the material must be at least 2½ times the diameter of the indentation. The ISO test method standard (3)also specifies that the distance must not be less than 1 mm. Therefore, in cases where Rockwell hardness testing is to be made on narrow width material or material having a small area size, a Rockwell scale must be chosen that produces indentations small enough to prevent this edge interaction.
3.1.4 Test Material Homogeneity
The size and location of metallurgical features in the test material should be considered when choosing the Rockwell scale. For materials that are not homogeneous, an appropriate Rockwell scale should be chosen that would produce a sufficiently large indentation to obtain a hardness value representative of the material as a whole. Also keep in mind that the area surrounding a Rockwell indentation also affects the test result (see above discussions). If the deformation zone surrounding a Rockwell indentation extends into adjacent regions of a differing hardness, such as the heat affected zone of a weld, the test measurement may be influenced. In such cases, a Rockwell scale should be chosen that uses test forces and indenters that produce a small enough indentation to avoid the influence of these areas.
3.1.5 Scale Limitations
Each Rockwell scale is an arbitrarily defined range of numbers from 0 to 100† covering a specific range of material hardness. Although, theoretically, the entire scale can be used for hardness testing, there are practical limitations on the range of testing for many of the Rockwell scales. At the low hardness end of the scales, these limits result from the indenter penetrating too deeply into the material, possibly causing contact with the indenter cap for ball indenters.
In the case of diamond indenters, the sensitivity of the test diminishes as the diamond indenter penetrates further down the conical portion of the diamond. At the high hardness end of the scales, these limits result from the likelihood of fracturing or significantly reducing the life of a diamond indenter. In the case of ball indenters, the sensitivity of the test diminishes, and there is increased possibility of flattening a steel indenter ball. The ISO standard (3)suggests thelimits given in Table 2 for some Rockwell scales.
When several Rockwell scales are acceptable for testing a material, generally, the most commonly used scale for the type of material to be tested should be chosen. In cases where this Rockwell scale is not appropriate for the particular application, the scale employing the highest forces may be the best choice. The highest force will produce the largest indentation covering more of the test material, and it will provide a Rockwell hardness value more representative of the material as a whole. Additionally, the highest test forces provide the most sensitivity in Rockwell hardness testing.
- In circumstances where the user wants to compare measurements with previously obtained Rockwell hardness data, the same scale should be chosen as was used for the previous testing as long as a valid test can be obtained. This is preferred to testing on one Rockwell scale and then converting the data to another Rockwell scale by way of conversion tables. Converted data is never as accurate as the original measurement.
- If the approximate hardness of a material is not known, a diamond indenter scale should be tried first. A diamond indenter is not likely to be damaged by penetrating too deeply into a soft material, where as a ball indenter may be flattened or damaged if the material is too hard.