# Difference Of HRA, HRB And HRC In Rockwell Hardness

A, B and C in HRA, HRB and HRC in Rockwell hardness are three different standards, called scale A, scale B and scale C. Rockwell hardness test is one of several common indentation hardness tests used today. The initial pressure of the three scales is 98 07N (10 kgf), and finally calculate the hardness value according to the indentation depth. Scale A uses a sphere-conical diamond indenter and then pressurizes to 588 4N (60kgf); Scale B uses a steel ball with a diameter of 1. 588 mm (1/16 inch) as the indenter, and then pressurizes it to 980 7N (100kgf); Scale C uses the same spherical cone diamond as scale A as the indenter, but the force after pressurization is 1471N (150kgf).

Therefore, scale B is suitable for relatively soft materials, while scale C is suitable for harder materials. Practice has proved that there is an approximate corresponding relationship between various hardness values of metal materials and between hardness values and strength values. Because the hardness value is determined by the initial plastic deformation resistance and the continuous plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value. However, the conversion relationship of various materials is not the same – hardness conversion formula:

## Hardness Conversion Formula

1. Shore hardness (HS)=Brinell hardness (BHN)/10+12
2. Shore hardness (HS)=Rockwell hardness (HRC)+15
3. Brinell hardness (BIHN)=Rockwell hardness (HV)
4. Rockwell hardness (HRC)=Brinell hardness (BHN)/10-3 hardness measurement range:

HS<100HB<500HRC<70HV<1300 (80~88) HRA, (85~95) HRB, (20~70) HRC Rockwell hardness in HRA, HRB, HRC, etc. A, B, C are three different standards, called scale A, scale B, and scale C. Rockwell hardness test is one of several common indentation hardness tests used today. The initial pressure of the three scales is 98.07N (10 kgf). Finally, the hardness value is calculated according to the indentation depth. Scale A uses a sphere-conical diamond indenter and then pressurizes to 588 4N (60kgf); Scale B uses a steel ball with a diameter of 1. 588 mm (1/16 inch) as the indenter, and then pressurizes it to 980 7N (100kgf); Scale C uses the same spherical cone diamond as scale A as the indenter, but the force after pressurization is 1471N (150kgf). Therefore, scale B is suitable for relatively soft materials, while scale C is suitable for harder materials. Practice has proved that there is a difference between various hardness values, hardness values and strength values of metal materials

There is an approximate corresponding relationship. Because the hardness value is determined by the initial plastic deformation resistance and the continuous plastic deformation resistance, the higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value. However, the conversion relationship of various materials is not uniform. Hardness indicates the ability of a material to resist hard objects pressing into its surface. It is one of the important performance indicators of metal materials. The higher the hardness, the better the wear resistance. The commonly used hardness indexes include Brinell hardness, Rockwell hardness and Vickers hardness.

## Brinell Hardness, Rockwell Hardness And Vickers Hardness

1. Brinell hardness (HB) Press a hardened steel ball of a certain size (generally 10mm in diameter) into the surface of the material with a certain load (generally 3000kg), and keep it for a period of time. After unloading, the ratio of the load to the indentation area is the Brinell hardness value (HB), in kilogram force/mm2 (N/mm2).
2. Rockwell hardness (HR) When HB>450 or the sample is too small, the Brinell hardness test cannot be used instead of Rockwell hardness measurement. It is to press a diamond cone with a top angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18 mm into the surface of the tested material under a certain load, and calculate the hardness of the material from the depth of the indentation. According to the hardness of the test material, there are three different scales:

HRA: It is the hardness obtained by using 60kg load and diamond cone indenter, and is used for materials with extremely high hardness (such as cemented carbide).

HRB: The hardness obtained by using 100kg load and 1.58mm diameter hardened steel ball is used for materials with low hardness (such as annealed steel, cast iron, etc.).

HRC: is the hardness obtained by using 150kg load and diamond cone press, which is used for materials with high hardness (such as quenched steel, etc.).

1. Vickers hardness (HV) is pressed into the material surface with a load of less than 120kg and a diamond square cone indenter with a top angle of 136 °. The Vickers hardness HV value (kgf/mm2) is obtained by dividing the surface product of the material indentation pit by the load value. “HK=139.54 * P/1.2. In the formula: HK – Knoop hardness, Mpa; P – load, kg; L – diagonal length of the pit, mm. The European countries adopt Vickers hardness, while the United States adopts Wupu hardness. Megapascals (MPa) is the legal unit of measurement of microhardness, while kg/mm2 is the commonly used unit of hardness calculation. The conversion formula between them is 1kg/mm2=9 80665Mpa.

The specific difference and change of hardness between Rockwell hardness (HIRC) and Brinell hardness (HB) is a performance index to measure the hardness and softness of materials. There are many methods of hardness test, the principle is different, and the measured hardness value and meaning are also different. The most commonly used is the static load indentation hardness test, that is, Brinell hardness HB, Rockwell hardness HRA, HRB, HRC, Vickers hardness HV, rubber and plastic Shore hardness HA, HD, and other hardness values indicate the ability of the material surface to resist the indentation of hard objects. The Leeb hardness HI and Shore hardness HS belong to the concave jump hardness test, and their values represent the elastic deformation work of the metal. Therefore, hardness is not a simple physical quantity, but a comprehensive performance index reflecting the elasticity, plasticity, strength and workability of materials.

## Hardness Test and Application

1. HRC means Rockwell hardness C scale.
2. HRC and HB are widely used in production.
3. HRC application range HRC 20~67, equivalent to HB225 — 650. If the hardness is higher than this range, Rockwell hardness A scale HRA is used. If the hardness is lower than this range, Rockwell hardness B scale HRB shall be used. The upper limit of cloth hardness HB650 cannot be higher than this value.
4. The indenter of the C scale of Rockwell hardness tester is a diamond cone with a top angle of 120 degrees. The test load is – the determined value. The Chinese standard is 150 kg force. The indenter of Brinell hardness tester is hardened steel ball HBS or cemented carbide ball HBW. The test load varies with the ball diameter, from 3000 to 31.25 kgf.
5. Rockwell hardness indentation is very small, and the measured value is local. It is necessary to measure the number of points to find the average value. It is applicable to finished products and thin films, and belongs to the category of nondestructive testing. The cloth hardness indentation is large, and the measured value is accurate. It is not applicable to finished products and thin films, and generally does not belong to the category of nondestructive testing.
6. The hardness value of Rockwell hardness is an unknown number and has no unit. Therefore, it is incorrect to call Rockwell hardness how many degrees. The hardness value of cloth hardness has units and has a definite approximate relationship with tensile strength.
7. Rockwell hardness can be displayed directly on the dial, and can also be displayed digitally. It is easy to operate, fast and intuitive, and suitable for mass production. Cloth hardness requires a microscope to measure the indentation diameter, and then check the table or calculate, which is complicated.
8. Under certain conditions, HB and HRC can be checked and exchanged. Hardness test is the simplest and easiest test method in mechanical property test. In order to replace some mechanical property tests with hardness tests, a more accurate conversion relationship between hardness and strength is needed in production. Practice has proved that the hardness values and strength values of metal materials are similar.