Rockwell Hardness Testing Method

What Is the Rockwell Hardness Testing Method

The Rockwell hardness testing method is a hardness testing method based on the principle that, under a specific load, the depth to which a cone- or ball-shaped indenter penetrates the material being tested tells us about the material’s hardness.

Unlike the Brinell and Vickers methods, in the Rockwell hardness testing method hardness is not calculated from the ratio of load to indentation area.
Indentation depth is what’s taken into account.

Indentation depth and hardness value can be read from a depth-measuring instrument (a depth gauge). This makes it possible to obtain hardness readings quickly, and the process can be automated if desired.

Advantages and disadvantages of the Rockwell hardness testing method. Where it’s used (above)

The type of indenter used is indicated in the method’s designation. When a diamond indenter is used, the method is expressed as HRC and HRA. For a steel ball, HRB and HRF are used. The difference between these comes from the force values applied during the test. Accordingly;

HRC = 1373 N, HRA = 490 N, HRB = 883 N, HRF = 490 N.

In industry, the Rockwell C and B methods are used most often.
The RC method uses a diamond cone tip,
and the RB method uses a steel ball.

How Is the Rockwell Hardness Test Performed?

Rockwell Tester: This is the instrument used to measure Rockwell hardness. It measures hardness by determining how deep an indenter penetrates the sample.

For the Rockwell hardness test, the area of the surfaces to be tested should not exceed 25 cm². Surfaces should be polished. Surfaces should be checked for roughness or imperfections that could affect the hardness measurement.

The indenter used for the Rockwell hardness test is either a steel ball or a spherical-conical diamond tip.

A small preliminary load of 10 kgf is applied first. The indenter seats itself on the material and holds it in place. The dial’s pointer is set to zero on the black-numbered scale. Then the major load (F2) is applied. The major load is the total load applied. The depth measurement is based solely on the increase in depth caused by that increase in load.

After the major load has been applied and then removed, per standard procedure, the value shown by the pointer is read while the minor (preliminary) load is still applied. When a steel ball is used as the indenter, it’s typical for the major load to be 60 kgf or 100 kgf, though other loads can also be used if needed.

What Is the Rockwell Hardness Value?

The Rockwell hardness value is a number that is inversely proportional to the increase in indentation depth that occurs when the major load is applied, compared to the depth resulting from the minor (preliminary) load alone.

Calculating the Rockwell Hardness Value

In the RC method: a depth of 0.2 mm = “0” RC hardness,
“0” depth means 100 RC hardness.
0.002 mm depth = 1 RC
If, at the end of a measurement, the indentation depth is 0.1 mm, the hardness is 50 RC. If the depth is 0.02 mm, the object’s hardness is 0.02/0.002 = 10, so 100 − 10 = 90 RC.

Since the Rockwell hardness values corresponding to indentation-depth measurements are shown directly on the dial, hardness can be read directly.

No Rockwell hardness value is ever expressed with just a single number. The type of indenter and the amount of load used during the test must always be indicated with a symbol.
In every case the minor load is 10 kgf, and the dial is set after this minor load is applied.
The dial gauge is set to show zero
on the C scale, or 30 on the B scale.

For the Rockwell hardness test, using a ball-shaped indenter is not recommended on materials that give readings greater than 100 on any scale.
That’s because accuracy is lower in that case, and the ball flattens out. Using a conical diamond indenter is not recommended on materials that give readings smaller than 20.

Measurements should be made with the largest usable ball, because as the indenter’s size increases, measurement accuracy is lost.

Indenters:

The standard indenters used for the Rockwell hardness testing method are,

• a spherical-tipped conical diamond indenter, and
• steel balls with diameters of 1.588 mm (1/16 inch),
• 3.175 mm (1/8 inch),
• 6.35 mm (1/4 inch), and
• 12.7 mm (1/2 inch).

The diamond indenter consists of a cone with an included angle of 120 ± 0.5°, with its tip formed by a spherical section with a radius of 0.2 mm. The steel balls used must have a smooth surface. The balls’ diameter must not differ from the nominal value by more than ± 0.0035 mm. The tip’s surface should be checked periodically with a magnifying glass.

Such a check reveals whether a small piece has chipped off the diamond tip, or whether the ball-shaped tip has become flattened. If such a defect is found, the indenter should be replaced. The indenter must be free of foreign matter such as dust, dirt, or grease, as this affects the results.

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