Rockwell Hardness Tester

1.     What is the background of Rockwell hardness testers development?

Rockwell hardness tester, the world’s first tester designed on the principle of the Rockwell scale, was co-invented by Hugh M. Rockwell (1890–1957) and Stanley P. Rockwell (1886–1940) in 1914. The patent for this instrument was granted in the year 1919. At first, this instrument used only the single side contact to inspect the hardness of metals. There was no need for sample support, and the test accuracy was in line with the standardized ISO6508, not below desktop Rockwell hardness tester. Later, the co-inventors improved its design in the years 1919 and 1921 by setting the foundation of the modern Rockwell hardness tester.

In the year 1930, C.H Wilson had modernized the design to make it more complete. In recent times, the Rockwell Hardness Tester was produced with touch screen control, numerical display, and curve display, as well as automatic printing test data in China. This simple operational testing method takes a rapid and direct measurement from a display screen, dial indicator, or optical projection screen. Like the Vickers hardness test and Brinell hardness test, this Rockwell type has become the most common method but comparatively considered as easier to perform.

2.     What are the types of Rockwell hardness testers?

There are varied types of Rockwell Hardness Testers, according to the varied factors listed below:

1. Application: General Rockwell Hardness Tester, Surface General Rockwell Hardness Tester, Comprehensive General Rockwell Hardness Tester
2. Operation: Electric Rockwell Hardness Tester, Manual Rockwell Hardness Tester
3. Display mode: Pointer type Rockwell Hardness Tester, Digital type Rockwell Hardness Tester
4. Reference platform: Manual platform Rockwell Hardness Tester, machine head mobile automatic Rockwell Hardness Tester, platform mobile automatic Rockwell Hardness Tester
5. Way of loading: motor and sensor composed of closed-loop loading, weight through the lever principle loading

3.     What are the scales of different Rockwell hardness testers?

There are four common types of Rockwell hardness tester with varied scales as listed below:

•   Ordinary Rockwell hardness tester: HRA, HRB, HRC
•   Plastic Rockwell hardness tester: HRE, HRL, HRM
•   Superficial Rockwell hardness tester: 15N, 30N, 45N, 15T, 30T, 45T, 15W, 30W, 45W, 15X, 30X, 45X, 15Y, 30Y, 45Y
•   Twin Rockwell hardness tester (also named as Rockwell & superficial Rockwell hardness tester): HRA, HRB, HRC,15N, 15T, 15W, 15X, 15Y, 30N, 30T, 30W, 30X, 30Y, 45N, 45T, 45W, 45X, 45Y

4.     What are the characteristics of the Rockwell hardness test?

•       The Rockwell testing method can statically test the hardness of a test specimen in the macro range (test force >= 49.03 N), precisely with a test force of 29.42 to 1471 N using the standardized procedures such as ISO 6508 and ASTM E18.
•   This Rockwell hardness test is easier and more accurate compared to the other types of hardness testing methods.
•   This method can be used on all metals and to some degree for plastics, except in the scenario where the structure or surface conditions of the test metal would bring too many variations, where the indentations are too large, or where the sample shape or size prohibits its use.
•       It is a differential-depth method that measures the residual depth of indentation left by the indenter to detect the hardness value.
•       Indenter shape and material type: Depending on the specific Rockwell method employed, the indenter is either a hard metal ball (with varied diameters based on the method) or a diamond cone (with a cone angle of 120°). On the contrary, the size of the indentation can be measured in the Vickers, Brinell, and Knoop optical test methods.
•       The deeper the penetration level of a defined indenter at a defined test force into the workpiece (specimen) surface, the softer will be the tested material.
•   The Rockwell Hardness Scale helps in ranking the hardness of materials; hence used extensively in the fields of metallurgy and engineering. Every Rockwell scale uses a varied load weight to inspect the material. The results of the test are shown as a code that includes HR with the scale letter, i.e. HRA or HRB.
•   This testing requires only the least quantity of sample preparation so that it can easily and instantly measure the hardness in any industry across the world.
•   The major benefit of the Rockwell hardness testing is its maximum speed and the ability to exhibit hardness values directly after passing through the material.
•   Rockwell hardness testing methods are exceptionally useful due to their simplicity; they remove the extensive calculations used by other methods. This Rockwell test is widely used because of its complete reliability of results with the exceedingly little indentation to the material.
•   In general, the sample material to be tested should be a minimum of ten times the depth of the indentation. If a thin or brittle material needs to be tested, it is better to use a superficial Rockwell scale. It creates a shallower indentation and uses a lower load to prevent destructive materials like ceramics and sheet metal.

5.     How to evaluate hardness using Rockwell hardness tester?

The Rockwell hardness is detected by the use of a primary test force (minor load or preload), followed by an extra load (major load) to reach the aggregate necessary test load, ultimately returning to the same primary test force (preload or minor load). The first minor load shows the zero or reference position. When the major load is applied and held for a fixed amount of time (dwell time), it allows elastic recovery. Next, the major load is eliminated, while still supporting the minor load to show the shift from zero or reference position in establishing a Rockwell hardness value.

The Rockwell scale can be compared with the Mohs Test and the Brinell scale.

The Brinell scale was developed by the Swedish engineer named Johann A. Brinell in the year 1910, and this scale operates as follows:

A load is applied to a strengthened steel ball that stays on a flat metal surface expected to be evaluated. Next, the diameter of the dent that creates is also assessed.

According to Sizes.com, the hardness level can be measured with this formula: “The Brinell number which shows the metal’s hardness is determined by the load on the ball in kilograms divided by the spherical surface area of the dent in square millimeters.”

1. Load an initial force: In the Rockwell hardness test, the initial test force will be 10 kgf (98 N; 22 lbf); whereas in superficial Rockwell hardness test, the initial test force will be 3 kgf (29 N; 6.6 lbf).
2. Load main load: reference below form or table ‘Scales and values’.
3. Get away from the main load for a ‘dwell time’ appropriate for indentation to come to a standstill.
4. Release load: The Rockwell value will typically get displayed on a screen or dial automatically.

Test Result Illustration:

A = Depth reached by the indenter after applying preload (minor load)  

B = Position of indenter during Total load, Minor, and Major loads  

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

D = Distance measurement showing the difference between preload and major load position. This distance helps in calculating the Rockwell Hardness Number.

Varied indenters such as conical diamond with a round tip for harder metals as well as the ball indenters ranging with a diameter from 1/16” to ½” for softer materials can be used.

While selecting a Rockwell scale, one should select the scale that indicates the largest load as well as the largest indenter that can be possible without surpassing the already defined operation and accounting conditions that may affect the test result. These conditions involve test specimens below the least thickness for the depth of indentation, a test impression that comes too close to the edge of the specimen or another impression, or evaluation on cylindrical specimens.

Moreover, the test axis must remain within 2-degrees of perpendicular to confirm precise loading. There should be no deflection of the tester or test sample during the loading step due to conditions like dirt below the test specimen or on the elevating screw. It is essential to keep the surface finish clean and remove the decarburization from heat treatment. 

Sheet metal can be too thin as well as soft for Rockwell scale testing without exceeding the least thickness requirements as well as potentially indenting the test anvil. Here, a diamond anvil consistently influences the result.

There is one more special case in evaluating the cold-rolled sheet metal. The hardening can generate a gradient of hardness through the test sample, so any test can measure the average of the hardness over the depth of the indentation effect. But in this case, any Rockwell testing method proves to be doubtful. There is often a history of evaluation using a specific scale on a specific material that can be used and interpreted functionally by the operators.

6. How to read the Rockwell hardness value?

A Rockwell hardness value includes three components:

•       A numeric hardness values
•       The two letters ‘HR,’ that stands for ‘Hardness according to Rockwell’
•       The Rockwell scale designation, which defines the combination of major load (overall test force) and the indenter type used in the subsequent Rockwell method.

7. What are the advantages and disadvantages of the Rockwell hardness testing method?

The Rockwell hardness testing method has the following advantages:

• There is no need for specimen preparation such as separation, grinding, and embedding.

• The hardness value can be read directly and there is no need for optical evaluation (diagonal measurement as in the optical methods)

• This is a rapid (short test cycle) and inexpensive process compared to the Brinell, Vickers, and Knoop machines. They need not be equipped with complicated optical systems.

• It is non-destructive testing where the test specimen can be used for other purposes.

The Rockwell hardness testing method has the following disadvantages:

• This method is not the most accurate hardness testing method all the time. Even a slight inaccuracy in measuring the depth difference may end with a significant error in the calculated hardness value.

• The test location should be totally free of all types of contamination like oil, scale, or foreign bodies to achieve a significant test result.

• The indenter has unidentified effects on the test results, especially if the indenter is worn and the tip is no more appropriately acute. Use only the certified and calibrated indenters to reduce such effects.

• With rising hardness, it becomes more difficult to differentiate between the materials.

8. What are the requirements for Rockwell hardness tester samples?

•       Rockwell hardness testing is performed only on cylindrical or flat samples.
•       Cutting and/or machining are required to obtain the appropriately sized specimens from complex-shaped components.
•       The samples should have smooth parallel surfaces, free of coatings, gross, and scale contamination.
•       The minimum sample size depends on the sample hardness and test scale. Larger or thicker flat samples should have a minimum of 6 in. (150 mm).
•       The cylindrical samples should remain at least 1/8 in. (3 mm) in diameter, with a minimum thickness of 0.006 in. (150 µm).
•       The specific finishing depends on the test scale and material.

9. What are the points to be considered while using Rockwell hardness tester?

The below listed five steps should be followed mandatorily before using the Rockwell hardness tester:

•       Inject a little quantity of oil regularly into the contact surface between the screw and the handwheel.
•       Wipe off the top surface of the screw and the ending surface of the worktable before using the tester
•       Inspect the precision of the hardness tester with the standard hardness block regularly. Never allow the test on the supporting surface.
•       If the support surface of the standard hard block has burr, polish it with an oilstone.
•       While testing in varied positions, drag the hard block on the worktable and never take it away from the worktable

10. What is the procedure of error analysis in the Rockwell hardness test?

•       Varied finishing surfaces have varied effects on the Rockwell hardness test. The lesser the surface finishing, the higher the hardness test, and vice versa. The hardness level is lower for the rough surface with tool marks. In contrast, if the tempered parts increase the temperature tempering, there will be knife marks on the surface after transformation. If the anti-tempering ability is low, the hardness value will also be lesser. When testing parts goes below surface finishing score 7, they must be ground finely with a waste grinding wheel, followed by smooth grinding with a file or after polishing with a fine hand grinding wheel, and cleaned by wiping well.
•       The surface of heat treatment parts is tested with salt and sand while loading. The parts may slip if there is an oily, diamond head pushed into the role of lubrication, thereby reducing the friction and increasing the pressure depth. These are the two causes of the lower hardness value. The hardness of the loose layer reduces as well as the dense layer of the oxide layer rises in the inspected parts. Hence in order to measure the hardness, the parts must be removed and wiped well without any dirt.
•       The error of the hardness test for spherical, bevel (or taper), and cylindrical parts is higher than the plane parts. If the INDENTER is pressed into the surface of the part, the resistance around the INDENTER will become smaller than that of the plane. The lesser the radius of curvature and the more the slope, the reduction of the hardness value will be significantly greater. Even the diamond indenters are also prone to damage. Hence the head and the worktable are concentric for these types of parts while designing a dedicated worktable.