The working principle, classification and selection of indenter and test force of Rockwell hardness tester

1. The working principle of Rockwell hardness tester

Under specified conditions, press the indenter (diamond cone, steel ball or cemented carbide ball) into the surface of the sample in 2 steps. After the main test force is removed, the residual depth h of the indentation is measured under the initial test force. The residual depth of indentation h represents the level of hardness.
The principle of Rockwell hardness test is shown in the figure:


1—Indentation depth under the initial test force F0; 2—Indentation depth under the total test force F0+F1; 3—The elastic recovery depth after removing the main test force F1; 4—The residual indentation depth h; 5 —Sample surface; 6—Measurement reference surface; 7—Rockwell hardness value of indenter position is calculated by the following formula:

N-constant, for A, C, D, N, T scales, N=100; for other scales, N=130;
h-residual indentation depth, mm;
S-constant, for Rockwell hardness, S=0.002mm, for surface Rockwell hardness, S=0.001mm. The depth of indentation corresponding to each Rockwell hardness unit is 0.002mm for Rockwell hardness and 0.001mm for surface Rockwell hardness. The shallower the indentation, the higher the hardness.

2. Rockwell hardness testers are generally divided into functions and test ranges

Standard Rockwell hardness tester, surface Rockwell hardness tester, double Rockwell hardness tester, plastic Rockwell hardness tester and full Rockwell hardness tester. The most widely used in daily tests are standard Rockwell hardness testers and surface Rockwell hardness testers.

3. Standard Rockwell and surface Rockwell hardness scale, experimental force, indenter, hardness range and material comparison table

Rockwell hardness scale	hardness symbol	Indenter type	Initial test force F0 (N)	Main test force F1 (N)	Total test force F0+ F1 (N)	Scope of application	Measuring material
A	HRA	120°diamond cone	98.07	490.3	588.4	20～88HRA	Hard alloy Very thin and hard steel Hardened steel with a thin surface hardening layer
B	HRB	1.5875mm steel ball	98.07	882.6	980.7	20～100HRB	Low-carbon steel Soft metal Copper alloy Aluminum alloy Malleable iron
C	HRC	120°diamond cone	98.07	1373	1471	20～70HRC	General steel Harder castings Pearlite Malleable iron Hardened + tempered alloy steel
D	HRD	120°diamond cone	98.07	882.6	980.7	40～77HRD	Thin steel Case hardened steel Pearlite Malleable iron
E	HRE	3.175mm steel ball	98.07	882.6	980.7	70～100HRE	cast iron Smaller aluminum alloy magnesium alloy Bearing alloy
F	HRF	1.5875mm steel ball	98.07	490.3	588.4	60～100HRF	Non-ferrous metals with lower hardness (annealed copper alloy) Soft thin alloy plate
G	HRG	1.5875mm steel ball	98.07	1373	1471	30～94HRG	Malleable iron Copper-nickel-zinc Copper-nickel alloy
H	HRH	3.175mm steel ball	98.07	490.3	588.4	80～100HRH	Non-ferrous metals and light metal images with very low hardness (aluminum, zinc, lead)
K	HRK	3.175mm steel ball	98.07	1373	1471	40～100HRK	Bearing alloys and softer metals or thin materials

Surface Rockwell Hardness scale	hardness symbol	Indenter type	Initial test force F0 (N)	Main test force F1 (N)	Total test force F0+ F1 (N)	Scope of application
15N	HR15N	120° Diamond cone	29.42	117.7	147.1	70～94HR15N
30N	HR30N			264.8	294.2	42～86HR30N
45N	HR45N			411.9	441.3	20～77HR45N
15T	HR15T	1.5875mm Steel ball	29.42	117.7	147.1	67～93HR15T
30T	HR30T			264.8	294.2	29～82HR30T
45T	HR45T			411.9	441.3	10～72HR45T

4. The relationship between Rockwell hardness and the minimum thickness of the sample:

The minimum thickness of the test piece should be greater than 10 times the depth of the indentation. After the test, there should be no visible deformation marks on the back of the test piece.

Five, the minimum distance between two points tested in the Rockwell hardness test

In the Rockwell hardness test, keeping the distance between the indentations or the distance between the indentations and the edge of the pattern appropriately is very important for obtaining correct experimental results.
In ISO 6508-1, the requirements for the indentation spacing between the Rockwell hardness test and the surface Rockwell hardness test are the same:
The distance between the centers of two adjacent indentations should be at least 4 times the diameter of the indentation, but not less than 2mm;
The distance between the center of any indentation and the edge of the pattern should be 2.5 times the diameter of the indentation, but not less than 1mm.

Six, Rockwell hardness test requirements for the surface of the sample
There are two requirements for the surface of Rockwell hardness samples. One is to ensure that the test surface is flat as much as possible. This is mainly for processed samples. Because the hardness measurement principle of Rockwell hardness tester is calculated by measuring the depth of indentation Hardness value, so the ideal surface is flat. However, for many workpieces and semi-finished products, it may be a curved surface. The Rockwell hardness value measured on the curved surface has a larger error than that on a flat surface. In this respect, the curved surface correction value itself is approximate. On the other hand, the axis of the indenter should be tested at the highest point of the curved surface, and the deviation of the alignment is often the cause of the large error of the surface hardness value. Another requirement is the surface roughness. A sample surface that is too rough will lower the measured Rockwell hardness and increase the data dispersion. Therefore, the standard stipulates that the surface roughness (Ra) is not greater than 0.8um, especially for Machined specimens. In the Rockwell hardness measurement of some workpieces, the surface roughness (Ra) cannot reach 0.8um because the surface cannot be processed. At this time, attention should be paid to the accuracy of the test results.
The increase in the surface roughness of the pattern indicates that the microscopic surface unevenness becomes larger. Under the same test force, the indenter has a greater penetration depth, so the Rockwell hardness value reflected on the rougher pattern is lower.

Seven, the expression of Rockwell hardness test results

60HRC: 60-hardness value is 60; HR-Rockwell hardness; C-Rockwell C scale
Rockwell hardness is represented by hardness number, symbol HR, used scale and ball indenter symbol (steel ball is S, cemented carbide ball is W).
Example: 60HRBW means the Rockwell hardness value measured on scale B with a cemented carbide ball indenter is 60.