Hardness Comparison and Relationship with Tensile Strength

Hardness is a measure of a material's resistance to local deformation, particularly plastic deformation, indentation, or scratching, and is an indicator of the material's softness or hardness. The measurement methods for hardness mainly include indentation, rebound, and scratch methods. Among them, HRC, HV, and HB are three commonly used hardness indicators, representing Rockwell hardness on the C scale, Vickers hardness, and Brinell hardness, respectively. Here is an introduction to these three types of hardness, their application scenarios, and their relationship with tensile strength:

• Definition: In the Rockwell hardness test, a diamond cone indenter is used to measure the depth of plastic deformation of the indentation to determine the hardness value.
• Application Scenario: Mainly used for measuring harder materials, such as heat-treated steel, bearing steel, tool steel, etc.
• Relationship with Tensile Strength: When the hardness of steel is below 500HB, the tensile strength is directly proportional to the hardness, i.e., [text{Tensile Strength(kg/mm²)}=3.2timestext{HRC}].

• Definition: Vickers hardness uses a diamond square pyramid indenter with a relative face angle of 136°, pressing into the material surface with a specified test force, and the hardness value is represented by the average pressure on the unit surface area of the square pyramid indentation.
• Application Scenario: Suitable for measuring various materials, especially thinner materials and surface hardening layers, such as carburized and nitrided layers.
• Relationship with Tensile Strength: There is a certain corresponding relationship between hardness value and tensile strength, but this relationship is not valid in all scenarios, especially under different heat treatment conditions.

• Definition: Brinell hardness uses a hardened steel ball or tungsten carbide ball of a certain diameter to press into the surface of the metal to be tested with a certain test load, measuring the diameter of the indentation on the surface, and calculating the ratio of the spherical surface area of the indentation to the load.
• Application Scenario: Generally used when the material is softer, such as non-ferrous metals, steel before heat treatment, or steel after annealing.
• Relationship with Tensile Strength: When the hardness of steel is below 500HB, the tensile strength is directly proportional to the hardness, i.e., [text{Tensile Strength(kg/mm²)}=frac{1}{3}timestext{HB}].

There is an approximate corresponding relationship between hardness values and tensile strength values. This is because the hardness value is determined by the initial plastic deformation resistance and the continued plastic deformation resistance. The higher the strength of the material, the higher the plastic deformation resistance, and the higher the hardness value. However, this relationship may vary under different heat treatment conditions, especially in the low-temperature tempering state, where the distribution of tensile strength values is very scattered, making it difficult to accurately determine.

In summary, HRC, HV, and HB are three commonly used methods for measuring material hardness, each applicable to different materials and scenarios, and they have a certain relationship with the material's tensile strength. In practical applications, the appropriate hardness test method should be chosen based on the characteristics of the material and the testing requirements.