Hardness is a performance index to measure the hardness of a material. There are many methods for hardness testing, and the principles are different. The measured hardness values and meanings are not exactly the same. Commonly used are static load indentation hardness tests, namely Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC), and Vickers hardness (HV), whose values indicate the ability of the material surface to resist the indentation of hard objects. Leeb hardness (HL) and Shore hardness (HS) belong to rebound hardness tests, and their values represent the magnitude of the work of elastic deformation of the metal. Therefore, hardness is not a simple physical quantity, but a comprehensive performance index that reflects the elasticity, plasticity, strength and toughness of the material.
1. Hardness of steel
The code for metal hardness is H. According to different hardness test methods, Brinell (HB), Rockwell (HRC), Vickers (HV), Leeb (HL) hardness, etc. are conventionally expressed, among which HB and HRC are more commonly used.
HB has a wide range of applications, and HRC is suitable for materials with high surface hardness, such as heat treatment hardness. The difference between the two is that the probe of the hardness tester is different. The probe of the Brinell hardness tester is a steel ball, while the probe of the Rockwell hardness tester is a diamond.
HV-suitable for microscope analysis. Vickers hardness (HV) is pressed into the material surface with a load of less than 120kg and a diamond square cone indenter with a vertex angle of 136°. The surface area of the material indentation pit is divided by the load value, which is the Vickers hardness value (HV).
HL portable hardness tester is convenient for measurement. After the impact ball head impacts the hardness surface, it bounces; the hardness is calculated by the ratio of the rebound speed of the punch at 1mm from the sample surface to the impact speed. The formula is: Leeb hardness HL=1000×VB (rebound speed)/VA (impact speed).
2. HB - Brinell hardness
Brinell hardness (HB) is generally used when the material is softer, such as non-ferrous metals, steel before heat treatment or after annealing. Rockwell hardness (HRC) is generally used for materials with higher hardness, such as hardness after heat treatment, etc.
Brinell hardness (HB) is a test load of a certain size. A hardened steel ball or carbide ball of a certain diameter is pressed into the metal surface to be tested. The test load is kept for a specified time, and then the load is removed to measure the diameter of the indentation on the surface to be tested. The Brinell hardness value is the quotient obtained by dividing the load by the spherical surface area of the indentation. Generally, a hardened steel ball of a certain size (usually 10mm in diameter) is pressed into the surface of the material with a certain load (usually 3000kg) and kept for a period of time. After the load is removed, the ratio of the load to its indentation area is the Brinell hardness value (HB), and the unit is kilogram force/mm2 (N/mm2).
3. Rockwell hardness is a hardness value indicator determined by the depth of plastic deformation of the indentation.
0.002 mm is used as a hardness unit. When HB>450 or the sample is too small, the Brinell hardness test cannot be used and Rockwell hardness measurement is used instead. It uses a diamond cone with a vertex angle of 120° or a steel ball with a diameter of 1.59 or 3.18 mm to press into the surface of the material under test under a certain load, and the hardness of the material is calculated from the depth of the indentation. According to the hardness of the test material, it is expressed in three different scales:
HRA: It is the hardness obtained by using a 60kg load and a diamond cone indenter, which is used for materials with extremely high hardness (such as cemented carbide, etc.).
HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm, which is used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: It is the hardness obtained by using a 150kg load and a diamond cone indenter, which is used for materials with very high hardness (such as hardened steel, etc.).
Hardness test is a simple and easy test method in mechanical property test. In order to replace some mechanical property tests with hardness test, a more accurate conversion relationship between hardness and strength is required in production.
There is an approximate corresponding relationship between the various hardness values of metal materials and between the hardness value and the strength value. Because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance, the higher the material strength, the higher the plastic deformation resistance and the higher the hardness value.