Thread Content
The ability of a material to resist the penetration of a hard object into its surface is called hardness. The most common method for testing the hardness of steel is to file the edge of the workpiece with a file; the depth of the scratches left on its surface is used to determine its hardness level. This method is called the filing test, and it is not very scientific. Using a hardness tester for testing is more accurate; it is a common method for measuring hardness in modern tests. Common hardness testing methods include Brinell hardness, Rockwell hardness, and Vickers hardness, among others. Hardness is an important performance indicator for measuring the softness or hardness of metal materials. It can be understood as the material’s ability to resist elastic deformation, plastic deformation, or failure, or it can also be described as the material’s ability to resist residual deformation and reverse failure. Hardness is not a simple physical concept; rather, it is a comprehensive indicator of a material’s mechanical properties such as elasticity, plasticity, strength, and toughness. Hardness tests can be classified into various methods based on their testing procedures: static pressure methods (such as Brinell hardness, Rockwell hardness, Vickers hardness, etc.), scratch methods (such as Mohs hardness), rebound methods (such as Shore hardness), as well as microhardness and high-temperature hardness tests. Brinell hardness is denoted as HB (HBS/HBW) (refer to GB/T231‑1984). In production, the Brinell method is commonly used to determine the hardness of hardened materials resulting from rejection, normalizing, and quenching and tempering, as well as of blanks or semi-finished products made of cast iron, non-ferrous metals, and low-alloy structural steel. Rockwell hardness can be divided into four types: HRA, HRB, HRC, and HRD, each with different measurement ranges and application areas. In general production, HRC is the most commonly used. The indentation is small, allowing the measurement of the hardness of thin materials, hard materials, and finished parts. Vickers hardness is denoted as HV (refer to GB/T4340-1999), and it is used to measure extremely thin specimens. 1. Hardness of steel: The code for metal hardness is H. Depending on the hardness testing method, common designations include Brinell (HB), Rockwell (HRC), Vickers (HV), and Leeb (HL) hardnesses, among which HB and HRC are the most commonly used. HB has a wide range of applications, while HRC is suitable for materials with high surface hardness, such as those with heat-treated hardness. The difference between the two lies in the probe used by the hardness tester: the probe of a Brinell hardness tester is a steel ball, while that of a Rockwell hardness tester is a diamond. HV – Suitable for microscopic analysis. Vickers hardness (HV) is determined by using a diamond square pyramid indenter with a load of 120 kg or less and an apex angle of 136° to press into the surface of the material; the Vickers hardness value is then obtained by dividing the surface area of the indentation left on the material by the load applied. The HL handheld hardness tester is easy to use; it generates a rebound effect after the impact head strikes the surface being tested for hardness ； The hardness is calculated using the ratio of the rebound velocity to the impact velocity at a distance of 1 mm from the sample surface; the formula is: Rockwell hardness HL = 1000 × VB (rebound velocity) / VA (impact velocity). The hardness values obtained using a portable Rockwell hardness tester in Rockwell (HL) scale can be converted to Brinell (HB), Rockwell (HRC), Vickers (HV), and Shore (HS) hardnesses. Or, using the Rockwell principle, hardness values can be directly measured using Brinell (HB), Rockwell (HRC), Vickers (HV), Rockwell (HL), and Shore (HS) scales. 2. HB - Brinell hardness ； Brinell hardness (HB) is generally used for softer materials, such as non-ferrous metals, and steel before heat treatment or after annealing. Rockwell hardness (HRC) is generally used for materials with high hardness, such as those that have undergone heat treatment. Brinell hardness (HB) is determined by applying a test load of a certain magnitude to force a hardened steel ball or cemented carbide ball of a specified diameter into the surface of the metal being tested; after holding this pressure for a defined period, the load is removed and the diameter of the indentation left on the surface is measured. The Brinell hardness value is the quotient of the load divided by the surface area of the indented sphere. Generally, a hardened steel ball of a certain size (with a diameter of usually 10 mm) is pressed into the surface of the material under a specific load (usually 3000 kg). This pressure is maintained for a certain period of time; after the load is removed, the ratio of the load to the area of the indentation gives the Brinell hardness value (HB), which is expressed in kilogram-force per mm2 (N/mm2). 3. Rockwell hardness is a hardness value determined by the depth of plastic deformation of the indentation. 0.002 millimeters is used as a unit of hardness. When HB>450 or the sample is too small, the Brinell hardness test cannot be used; instead, the Rockwell hardness measurement should be employed. It involves using a diamond cone with an apex angle of 120° or steel balls with diameters of 1.59 and 3.18 mm to press against the surface of the material under test; the hardness of the material is determined from the depth of the indentation. Depending on the hardness of the test material, it is expressed using three different scales: HRA: This is the hardness value obtained using a load of 60 kg and a diamond cone indenter, and it is used for materials with extremely high hardness (such as cemented carbides). HRB: It is the hardness obtained using a load of 100 kg and hardened steel balls with a diameter of 1.58 mm; it is used for materials with lower hardness (such as annealed steel, cast iron, etc.). HRC: It is the hardness determined using a 150 kg load and a diamond cone indenter, and it is used for materials with very high hardness (such as quenched steel). Additionally: 1. HRC refers to the Rockwell hardness scale C. 2. Both HRC and HB are widely used in production. 3. The applicable range for HRC is from 20 to 67, which corresponds to HB values of 225 to 650. If the hardness is above this range, the Rockwell hardness scale A, HRA, is used. If the hardness is below this range, the Rockwell hardness B scale HRB is used. The upper limit for Fabric type hardness is HB650; it must not exceed this value. 4. The indenter for the C scale of the Rockwell hardness tester is a diamond cone with an apex angle of 120 degrees; the testing load is a fixed value, which is 150 kilogram-force according to Chinese standards. The indenter of a Brinell hardness tester is a hardened steel ball (HBS) or a cemented carbide ball (HBW), and the testing load varies depending on the ball diameter, ranging from 3000 to 31.25 kilogram-force. 5. The Rockwell hardness indentation is very small, and the measurement values are localized; therefore, it is necessary to measure several points and take their average. It is suitable for finished products and thin sheets, and falls under the category of non-destructive testing. Brinell hardness produces larger indentations and yields accurate measurement values; it is not suitable for finished products and thin sheets, and is generally not classified as a type of non-destructive testing. 6. The hardness value of Rockwell hardness is an irrational number without any units. (Therefore, it is incorrect to refer to the Lo hardness as being a certain degree.) ) The hardness value of Fabric type hardness is expressed in units, and it has a certain approximate relationship with tensile strength. 7. Rockwell hardness is displayed directly on the dial or in numerical form; it is easy to operate, fast and intuitive, making it suitable for mass production. Brinell hardness requires measuring the indentation diameter under a microscope and then consulting a table or performing calculations, which is a rather cumbersome process. 8. Under certain conditions, HB and HRC can be interchanged using tables. Its mental calculation formula can be roughly remembered as: 1HRC≈1/10HB. The hardness test is the simplest and most straightforward method among mechanical property tests. In order to use hardness testing as a substitute for certain mechanical property tests, it is necessary in production to have a relatively accurate conversion relationship between hardness and strength. Practice has shown that there is an approximate corresponding relationship among various hardness values of metal materials, as well as between hardness values and strength values. Since the hardness value is determined by the resistance to initial plastic deformation and the resistance to further plastic deformation, the higher the strength of a material, the greater its resistance to plastic deformation, and thus the higher its hardness value