Testing and Specifying Mechanical Properties
Hardness and Tensile Strength
The most common mechanical properties specified are hardness and/or tensile strength. These are controlled by annealing, cold working or hardening and tempering, and always specified in light of the application requirements. As a guide for testing and specifying, it should be noted that the relationship between tensile strength and hardness is similar for all high carbon steels.
Hardness or tensile strength should be specified as a range of minimum-maximum values to avoid costly overspecification. This range can be based on tests of hardness or tensile strength or can be deduced from desired performance characteristics.
When performance characteristics are used to determine hardness, design stresses, tensile strength or temper they should be explicitly noted so that the supplier and user can double-check the specification.
By analyzing design stresses in a strip steel application, design engineers can estimate the required yield strength and then calculate the ultimate tensile strength for the material. For most hardened and tempered high carbon strip, minimum yield strength is approximately 85% of the ultimate tensile strength.
Whether hardness specifications are based on calculated mechanical properties or on test results, an understanding of common hardness tests can help designers, engineers and purchasers specify hardness accurately.
Specification Based on Tests
Strip steel specifications should be based on tests whenever samples are available. Rockwell hardness tests are the most widely used in the U.S. for several reasons, including speed and the relatively low cost of the equipment. In a Rockwell test, the material is simply placed on an anvil and subjected to the pressure of either a diamond cone or a ball penetrator, first under the weight of a relatively light, or "minor" load, and then subsequently under the greater weight of a "major" load. Exact load weights involved determine the specific scale. Test values directly indicate the difference in depth of penetration under the weight of the minor and major loads.
Two types of Rockwell testers are used for different strip thickness ranges, and each type, using a diamond cone penetrator, provides measurements in three hardness scales, which are differentiated by the mass of the major load. (The diamond cone penetrator scales are used whenever hardened and tempered steel or other hard materials are to be tested.)
The standard tester is armed with relatively massive major loads of 150 kg, 100kg, and 60kg for the Rockwell C, D, and A scales, respectively; the minor load in all three cases is 10kg. The superficial tester makes use of major loads of 45 kg, 30 kg, and 15 kg for the Rockwell 45N, 30N, and 15N scales, respectively; the minor load for each of those three scales is 3kg.
The Diamond Pyramid Hardness (DPH) test uses even lighter loads - 2 kg, 1 kg, and 1/2 kg - to measure the hardness of the very thinnest gauges of strip steel. Each test involves only one load; there is no minor load. Diagonal dimensions, rather than the depth of the indentation are measured, and a single scale is used for all penetrator loads.
To guarantee accuracy, sensitivity, and repeatability of the measured values, it is essential to select the test and scale appropriate to the thickness of the strip to be tested. Refer to Table 10 for hardness scale vs. thickness of strip requirements.
Normally, a hardness test should be carried out with the largest recommended load allowed on the penetrator to minimize the test's sensitivity to surface variations and imperfections, such as machining marks and decarburization. Scales based on lighter loads should be avoided.
If, however, the strip is too thin to support the penetrator load, the impression of the penetrator into the strip will result in a bump or bulge on the underside of the strip. The formation of such a bulge is known as the "anvil effect" and usually indicates an inaccurate hardness reading.
Furthermore, with extremely thin strip, there is the danger that the penetrator might actually pierce the strip and dent the anvil itself, giving an improper reading and impairing the accuracy of the tester in future use.
Stacking several strips of the same material is not recommended. The cushion-like spring action between the strips will prevent an accurate reading.
Tensile strength tests
Tensile strength testing involves pulling a sample in tension until it breaks. It is more time-consuming than hardness testing, but has the important advantage of always using the same unit of measurement (psi), regardless of strip thickness.
Because tensile strength and hardness are related, the specifier can cite just tensile strength, or convert to a hardness value by using appropriate conversion charts included in this brochure.
Conversion to other hardness scales on tensile values according to ASTM E-18
There is no general method for accurately converting the Rockwell hardness numbers on one scale to hardness numbers on another Rockwell Scale, or to other types of hardness numbers, or to tensile strength values. Such conversions are, at best, approximations and therefore should be avoided, except for special cases where a reliable basis for the approximate conversion has been obtained by comparison tests.
Many steel buyers have learned through experience what specific Rockwell C-scale values are associated with their own applications. Other scales, such as Rockwell 15N or 30N, are less well understood, and thus not as readily associated with product applications. As a result, specifiers may cite a Rockwell C-scale hardness, regardless of the strip thickness, in order to obtain a certain temper quality.
For example, a user may specify Rc48 hardness, even though the requested strip is only 0.020 inches (0.508 mm) thick, just to indicate that the strip should be given a full temper. This practice can cause misunderstandings if the specifier fails to explain that it is not the tested C-scale hardness itself that is desired, but the performance associated with it.
Even more misleading is improper use of hardness conversion charts. A person without expert knowledge of the lighter hardness scales should never convert a C-scale value to a lighter scale for an equivalent temper. Even when a selected chart lists values that convert accurately in terms of hardness, the converted values may not yield equivalent strip characteristics. To provide equal temper and spring characteristics in different thicknesses of a given steel, hardness and tensile strength must be increased when strip thickness is decreased.
A Rc48 hardness, for example, will be a full temper in strip more than 0.035 in. (0.89 mm) thick, but the equivalent hardness (by conversion) of R30N 66.5 will be less than a full temper in strip that is 0.025 in. (0.63 mm) thick.
Those who wish to specify a Rockwell 15N or 30N hardness offering spring characteristics equivalent to those of a known C-scale hardness can use the comparison charts shown in the Equivalent Hardness Numbers and Tensile Strengths section of this web site.