Hardness is not a basic physical property, but a property of a material. It is defined as resistance to the recess and is determined by measuring the permanent depth of the recess. More simply, when a constant force (load) and a given indicator are used, the smaller the recess, the harder the material becomes.

The indentation hardness value is obtained by measuring the depth or area of ​​the indentation using one of the 12 different test methods. Learn more about hardness testing basics here.

The Rockwell hardness test method, as defined in ASTM E-18, is the most widely used hardness test method. You must make a copy of this standard, and read and understand the standard before entering the Rockwell test.

Rockwell testing is generally easier and more accurate than other types of hardness testing methods. The Rockwell test method is used on all metals, except where the test metal structure or surface conditions bring too much variation; the indentation would be too large for the application; or the use of sample size or sample shape is prohibited.

The Rockwell method measures the depth of permanent indentation produced by a force / load on an input. First, a preliminary test force (commonly called a boot or small load) is applied to a sample using a diamond or ball recess. This preload breaks off the surface to reduce the effects of the surface finish. After holding the pre-test force for a defined dwell time, the bottom depth of the recess is measured.

After boot, call the overhead, main load to reach the required total load. This force is held for a predetermined time (residence time) to provide elastic recovery. This main load is then returned to the preload. After holding the pre-test force for a defined dwell time, the final recess depth is measured. The Rockwell hardness value is derived from the difference in baseline and final depth measurements. This distance is converted to a hardness number. Pre-test power is removed and the indicator is removed from the test sample.

Preliminary test loads (preloads) range from 3 kgf (used on the “Superficial” Rockwell scale) to 10 kgf (used on the “Normal” Rockwell scale). The total test forces range from 15 kgf to 150 kgf (superficial and regular) to 500 to 3000 kgf (macro hardness).

Test Method Illustration

A = The depth reached by the indenter after the preloading (small load)

B = Indicator position during total load, Minor plus Large loads

C = End position reached by the indenter after elastic recovery of the sample material

D = Distance measurement representing the difference between the preload and the main load position. This distance is used to calculate the Rockwell Hardness Number.

Rockwell Hardness Test Loads

Various indentations can be used: for harder materials For hard metals rounded tapered diamond 1 / 16 For softer materials with a diameter ranging from ”to ½”.

When selecting the Rockwell scale, a general guideline is to select the scale that indicates the largest possible load and the largest possible input, without exceeding the defined operating conditions and taking into account situations that may affect the test result. These conditions include test specimens that are below the minimum thickness for the indentation depth; a test impression that is too close to the edge of the sample or other representation; or testing on cylindrical samples.

In addition, the test axis should be 2 degrees vertically to ensure precise loading; There must be no deviation of the loading sample or test device due to dirt under the test specimen or test screw or conditions in the lifting screw. It is important to keep the surface clean and to remove decarburization from the heat treatment.

Sheet metal may be too thin and too soft for testing on a given Rockwell scale without exceeding the minimum thickness requirements and potentially entering the test anvil. In this case, a diamond anvil may be used to ensure a consistent effect of the result.

Another special case of testing cold rolled sheet is that work hardening can form a hardness gradient throughout the sample, so that any test measures the average of the hardness over the depth of the hardening effect. In this case, there is a test history that uses a specific scale on a particular material that any Rockwell test result will be suspicious of, often operators can interpret and use functionally.

For more information on Rockwell hardness testing, please contact EUROLAB Laboratory.