Rockwell Hardness

Rockwell hardness test method is Proposed by S.P. Rockwell in 1919. The indenter used for Rockwell hardness is a diamond cone with a cone angle of 120 ° or a steel ball with a diameter of 1/16 inch (1 inch equals 25.4 mm), and the indentation depth is used as the basis for the calibration of the hardness value. During the measurement, the total test force is applied twice, including the preliminary test force and the additional test force (total force minus the preliminary force). Generally, the preliminary force is 10 kgf. After adding to the total force, the additional test force is removed and the indentation depth at this time is used to measure the hardness of the material. The Rockwell hardness is recorded as HR, The measured value is written after HB. The formula for calculating Rockwell hardness is:

1). h is premanent indention depth (permanent depth of indentation under preliminary test force after removal of additional test force)

2). k is the specified constant (K = 0.20 (mm) corresponding to diamond cone indenter, = 0.26 (mm) corresponding to the steel ball indenter)

3). 0.002 (mm) Indentation depth per Rockwell hardness unit

In order to adapt to a very wide measurement range, two methods can be used to change the test force and replace the indenter. Different test force and indenters constitute different Rockwell hardness scales. There are three commonly used scales: A, B, and C. scale B is used for medium hardness metal materials, such as annealed mild steel and medium carbon steel, brass, bronze and hard aluminum alloy, the indenter is a steel ball with a diameter of 1/16 inch, and the test force is 100 kgf, the scale range is from HRB0 to HRB100. When the hardness is higher than HRB100, the steel ball may be crushed. The scale C is used for materials with a hardness higher than HRB100, such as quenched steel, various quenched and tempered alloy steels. The indenter is a diamond cone with a vertex angle of 120 °, the test force is 150 kgf, the scale C is used from HRC20 to HRC70, and scales B and C are standard scales for Rockwell hardness. Scale A is used for tungsten/cemented carbide and other hard materials, and also used for hardened thin steel strips. Because the large test force is easy to damage the diamond indenter, the load is changed to 60 kgf. Scale A is the only scale of all Rockwell hardness scales that can be used in a wide range of hardnesses such as annealed brass to carbide.

The Rockwell hardness test uses three test forces and three indenters, which have a total of 9 combinations, corresponding to 9 cales of Rockwell hardness. The application of these 9 scales covers almost all commonly used metallic materials. The most commonly used scales are HRC, HRB and HRF. Among them, HRC scale are used to test hardened steel, tempered steel, quenched and tempered steel and some stainless steel, this is the most commonly used hardness test method in the metalworking industry. HRB scale is used to test various annealed steels, normalized steels, mild steels, some stainless steels and harder copper alloys. HRF scale are used to test pure copper, softer copper alloys and hard aluminum alloys. Although HRA scale can also be used for most ferrous metals, the practical application is generally limited to testing hard alloy and thin hard steel strip materials.

The surface Rockwell hardness test uses three test forces and two indenters, which have a total of 6 combinations, corresponding to 6 scales of surface Rockwell hardness. The surface Rockwell hardness test is a supplement to the Rockwell hardness test. When the Rockwell hardness test is applied, when the material is thin, the sample is small, the surface hardened layer is shallow, or the test surface coating, The surface Rockwell hardness test should be used instead. At this time, using the same indenter as the Rockwell hardness test and using a test force that is only a fraction of the Rockwell hardness test, an effective hardness test result can be obtained on the above sample. Surface Rockwell hardness scale N is suitable for materials similar to Rockwell hardness HRC, HRA and HRD test. Scale T is suitable for materials similar to Rockwell hardness HRB, HRF and HRG test.

The use range of scale HRC is HRC 20 ~ 70, when the hardness value is less than HRC20, the scale HRC should be used instead asthe conical part of the indenter is pressed too much, the sensitivity is reduced. Although the upper limit of the HRC scale is specified as 70HRC, when the hardness of the specimen is greater than HRC67, the pressure on the tip of the indenter is too large, the diamond is easily damaged, and the life of the indenter will be greatly shortened, therefore, the HRA scale should generally be used instead.

The use range of scale HRA is HRA 20~88, the following conversion relationship can be obtained from the American standard ASTME140:

27HRA≈30HRB

60HRA≈100HRB≈20HRC

85.6HRA≈68HRC

It can be seen that the test range of HRA scale covers the hardness range from soft steel (HRB), hard steel (HRC) to hard alloy. However, in fact HRA scale are rarely used for testing soft steel, it is mainly for testing thin hard steel plates, deep carburized steels and hard alloys. In terms of hard alloys, due to technological progress, the hardness of some materials has reached 93-94HRA, which has exceeded the standard. It has become customary for engineering to exceed HRA's high-end measurement range. HRA scale have a special purpose. When using a Rockwell hardness tester to test steel samples, if you do not know whether the sample is soft steel or hard steel, you can test it with an HRA scale first. If the hardness value is less than HRA60, you can use the HRB scale instead. If the hardness value is more than HRA60, you can use HRC scale instead.

The use range of the HRB scale isHRB 20 ~ 100. When the hardness value is lower than HRB20, the HRF scale should be instead due to the excessive penetration depth of the steel ball, the creep of the metal is intensified, the deformation time of the sample under the test force is prolonged, and the accuracy of the test value is reduced. When the hardness value is greater than HRB100, the HRC scale should be instead due to the steel ball is pressed too lightly, both sensitivity and accuracy decrease. When using the HRB scale to test steel specimens, it is particularly worth noting that when the specimen is not known in advance whether it is soft steel or hard steel, the HRB scale must never be used for testing, because the steel ball may be deformed and ball indenter will be damaged if it is applied to test quenched steel, this is the main reason for the damage of the steel ball indenter. In this case, you should first use a diamond indenter and test with an HRA scale, and then decide whether to use HRB or HRC.

The use range of HRF scale is HRF 60 ~ 100, It is a good detection method for testing pure copper and softer copper alloy materials. However, in China, its application is limited due to shortage of standard hardness blocks.

The HRG scale is suitable for materials with an HRB value close to 100. For materials such as beryllium bronze, phosphor bronze, and malleable cast iron with hardness ranges between the high end of HRB scale and the low end of HRC scale, if you use the HRG scale, you can greatly improve the sensitivity and test accuracy.