Overview of basic knowledge of hardness
In the vast field of materials science, hardness is a crucial concept. It is like an inherent characteristic of materials, profoundly influencing the performance of materials in various application scenarios. Simply put, hardness reflects the ability of a material to locally resist the indentation of a hard object into its surface. This ability is like a defense line of the material, determining whether it can maintain its own shape and performance under external pressure.
From a more macroscopic perspective, hardness is an indicator that measures the local resistance of a solid to the intrusion of external objects. The hardness of different materials varies, just as different warriors have different defensive capabilities. In order to accurately compare the hardness of various materials, people have stipulated different testing methods, thus giving rise to different hardness standards. However, it should be noted that the mechanical meanings of various hardness standards are different, and they cannot be directly converted into each other. Nevertheless, we can compare them through experiments to understand the performance of different materials under different hardness standards.
Detailed description of the classification of hardness
Scratch hardness
Scratch hardness is mainly used to compare the hardness of different minerals. Imagine that we have a rod with one end hard and the other end soft in our hands, just like a special ruler. When we draw the material to be tested across this rod, it's like a subtle contest. Based on the position where the scratch appears, we can determine the hardness of the material to be tested. Qualitatively speaking, if a hard object is drawn across, the scratch it leaves will be relatively long, which is like a powerful force leaving a more obvious mark on the rod; while the scratch made by a soft object is relatively short, just like a gentle touch leaving only a shallow mark.
Indentation hardness
Indentation hardness is mainly applied to metallic materials. Its testing method is to press a specified indenter into the material to be tested with a certain load, and compare the hardness of the tested material by observing the size of the local plastic deformation on the material surface. Due to the differences in the indenter, load, and load duration, there are various types of indentation hardness. Among them, Brinell hardness, Rockwell hardness, Vickers hardness, and microhardness are some of the more common ones. Different hardness types are suitable for different materials and testing scenarios. They are like different tools, each with its own unique purpose.
Rebound hardness
Rebound hardness is also mainly used for metallic materials. The specific testing method is to let a specially made small hammer freely fall from a certain height to impact the specimen of the material to be tested. During this process, the specimen will store strain energy and then release it. We measure the amount of strain energy stored and released by the specimen during the impact process through the rebound height of the small hammer, and then determine the hardness of the material. This testing method is like a transfer and feedback of energy, revealing the hardness secret of the material through the rebound situation of the small hammer.
Multiple ways of expressing metal hardness
Leeb hardness (HL)
The code for Leeb hardness is HL. It is measured using a portable hardness tester, and the operation is very convenient. Its measurement principle is to utilize the rebound after the impact ball head impacts the hardness surface. The hardness is calculated by the ratio of the rebound velocity to the impact velocity of the punch at a distance of 1 mm from the specimen surface. The specific formula is: Leeb hardness HL = 1000 × VB (rebound velocity) / VA (impact velocity). The portable Leeb hardness tester has powerful functions. After measuring with Leeb (HL), it can be converted into Brinell (HB), Rockwell (HRC), Vickers (HV), and Shore (HS) hardness. Of course, the Leeb principle can also be used to directly measure the hardness values in Brinell (HB), Rockwell (HRC), Vickers (HV), Leeb (HL), and Shore (HS), providing more options for our measurement work.
Rockwell hardness (HR)
Rockwell hardness can be divided into four types: HRA, HRB, HRC, and HRD. The measurement range and application range of each type are different. In general production, HRC is the most widely used. Rockwell hardness (HRC) is usually used for materials with higher hardness, such as materials after heat treatment. Its indentation is small, which enables it to measure the hardness of thinner materials, hard materials, and finished parts. The probe of the Rockwell hardness tester is made of diamond. The hardness value index is determined by the depth of plastic deformation of the indentation, with 0.002 mm as one hardness unit. When HB
Brinell hardness (HB)
Brinell hardness is expressed as HB[N(kgf/mm²)] (HBS\HBW). In production, the Brinell hardness test method is commonly used to measure the hardness of steel parts that have undergone annealing, normalizing and quenching and tempering, as well as the hardness of blanks or semi - finished products of cast iron, non - ferrous metals, low - alloy structural steels, etc. The measuring head of the Brinell hardness tester is a steel ball, which is generally used when the material is relatively soft, such as non - ferrous metals, and steel before heat treatment or after annealing. Its measurement method is to use a certain test load to press a hardened steel ball or a cemented carbide ball of a certain diameter into the surface of the metal to be tested. After maintaining the load for a specified time, the load is removed, and then the diameter of the indentation on the tested surface is measured. The Brinell hardness value is the quotient obtained by dividing the load by the spherical surface area of the indentation. Usually, a hardened steel ball of a certain size (generally 10mm in diameter) is pressed into the material surface with a certain load (generally 3000kg) and maintained for a period of time. After removing the load, the ratio of the load to the indentation area is the Brinell hardness value (HB), and the unit is kilogram - force/mm² (N/mm²).
Vickers hardness (HV)
Vickers hardness is denoted as HV, and it is mainly used to measure extremely thin specimens. This hardness measurement method has important applications in some fields with high precision requirements and thin specimens.
Shore hardness (HS)
Shore hardness is a standard for indicating the hardness of materials, first proposed by the British man Albert F. Shore. It uses the elastic rebound method. A striker is dropped from a certain height onto the surface of the material being tested, and the striker rebounds. The striker is a small cone with a pointed tip, which is often inlaid with diamond. We use the measured height of the striker's rebound to represent the hardness. The Shore hardness test is a dynamic force test. Compared with static force test methods such as Brinell, Rockwell, and Vickers, its accuracy is slightly lower. It is affected by factors such as the verticality during testing and the surface finish of the specimen, and the data dispersion is relatively large. The comparison of its test results is only limited to materials with the same elastic modulus. The Shore hardness tester has certain requirements for the thickness and weight of the specimen and is not suitable for thinner and smaller specimens. However, it is a portable hand - held instrument, convenient for on - site testing. It has a simple structure, is easy to operate, and has high testing efficiency. The Shore hardness tester is suitable for measuring the Shore hardness values of ferrous and non - ferrous metals, and is particularly suitable for medium - and large - sized workpieces in the metallurgy and heavy machinery industries, such as large components, castings, forgings, crankshafts, rolls, extra - large gears, machine tool guide rails and other workpieces. In the rubber and plastic industries, Shore hardness is often referred to as Shao's hardness.
Knoop hardness
Knoop hardness is a hardness value measured as an absolute value, and this value is mainly used in the field of processing. Generally speaking, the Knoop hardness of diamond is 7000 - 8000 kg/mm². The Knoop hardness test is mainly used in metallurgy and metallography research, and is especially suitable for testing hard and brittle materials. It is often used to test materials such as enamel, glass, artificial diamond, cermet, and minerals. It can also be used to measure the effective depth of the surface hardened layer, and to test the hardness of small parts, small areas, thin materials, thin wires, the area near the blade, electroplated layers, and dental materials. There is no dedicated hardness tester for the Knoop hardness test. Usually, a micro - Vickers hardness tester is shared. One only needs to replace the indenter and change the algorithm for calculating the hardness value. In the field of micro - hardness testing, the United States usually uses a Knoop indenter with a force of 100g, while Europe is accustomed to using a Vickers indenter with a larger force of 500g.
Mohs hardness
Mohs scale of hardness is a standard for indicating the hardness of minerals. It has a wide range of applications in the field of mineralogy, helping people to better understand and distinguish the hardness characteristics of different minerals.
Shore hardness
Shore hardness refers to the reading value measured by a Shore durometer, and its unit is "degree". Its description methods are divided into two types, A and D, representing different hardness ranges respectively. For materials with a hardness below 90 degrees, a Shore A durometer is used for testing and obtaining data, while for those with a hardness of 90 degrees and above, a Shore D durometer is used for testing and obtaining data. Generally speaking, for a rubber or plastic product, testers can make a pre - judgment before testing based on experience, so as to decide whether to use a Shore A durometer or a Shore D durometer for testing. For products with relatively high hand - feel elasticity or those that are relatively soft, such as glue bottles for stationery, TPU TPR plastic film bags, etc., testers can directly decide to use a Shore A durometer for testing; while for products with basically no hand - feel elasticity or those that are relatively hard, such as PC, ABS, PP, etc., a Shore D durometer can be used for testing. If the degree is Shore Axx, it indicates that the hardness is relatively low; if it is Shore Dxx, it indicates that the hardness is relatively high. It should be added that the unit of Shore hardness is not comprehensive enough. The unit expression of type A is HA, and that of type D is HD.
Conversion relationships between hardness values
Although different hardness standards cannot be directly converted, there are some empirical conversion formulas. For example, Shore hardness (HS) = Brinell hardness (BHN)/10 + 12; Shore hardness (HS) = Rockwell hardness (HRC) + 15; Rockwell hardness (HRC) = Brinell hardness (BHN)/10 - 3. However, these conversion formulas have certain limitations in the hardness measurement range. Generally speaking, HS..< 100, hb< 500, hrc< 70, hv< 1300. when performing hardness conversion, we need to ensure that the hardness values are within these ranges to guarantee the accuracy of the conversion results.
Other knowledge related to hardness
The meaning of HRC
HRC is the code for the Rockwell hardness scale C. It holds an important position in material hardness measurement and can accurately reflect the hardness characteristics of materials.
Applications of HRC and HB
Both HRC and HB are widely used in production. HRC is suitable for materials with higher hardness and can provide us with accurate hardness data to help us determine whether the materials meet the production requirements. On the other hand, HB is often used for hardness measurement of softer materials and plays an important role in the hardness testing of some semi-finished products and blanks.
Scope of application of HRC
The applicable range of HRC is generally between 20 and 67, which is equivalent to HB 225 - 650. If the hardness of the material is higher than this range, the Rockwell hardness A scale HRA needs to be used for measurement. Such a distinction can ensure that we select the most appropriate method in different hardness measurement scenarios, so as to obtain accurate hardness data.
Hardness scale selection and range limitation
In the field of material hardness testing, the selection of hardness scales is of crucial importance. When the hardness of the material is below a specific range, the Rockwell hardness B scale (HRB) becomes the appropriate choice. This selection is based on considerations of material properties and testing accuracy. Because within this hardness range, the HRB scale can more accurately reflect the hardness of the material, providing reliable data support for subsequent processing and use.
As for Brinell hardness, there is an upper limit, which is HB650. The Brinell hardness of a material cannot exceed this value, which is jointly determined by the principle of Brinell hardness testing, the indenter, the load and other factors. Once this upper limit is exceeded, it may lead to abnormal indentations, making it impossible to accurately measure the true hardness of the material, and may also damage the testing equipment.
Hardness tester indenter and test load
The indenter of the Rockwell hardness tester on the C scale is unique. It is a diamond cone with an apex angle of 120 degrees. This design allows the indenter to concentrate stress during the test, thus forming clear and regular indentations on the material surface. The test load is a determined value, which is 150 kilogram - force according to Chinese standards. This fixed load ensures the consistency and comparability of the test results, enabling the test data from different batches and different laboratories to be referenced to each other.
The indenters of the Brinell hardness tester are divided into two types: hardened steel balls (HBS) and cemented carbide balls (HBW). Different indenters are suitable for materials with different hardness ranges. Moreover, the test load is not a fixed value but varies with the ball diameter, ranging from 3000 to 31.25 kgf. This flexible load setting can meet the testing requirements of various materials with different hardness and sizes.
Indentation characteristics and scope of application
The indentation of Rockwell hardness is very small, and this characteristic makes the measured value have a certain locality. Due to the small indentation, it can only reflect the hardness of the local area of the material. To obtain more accurate results, it is necessary to measure multiple points and calculate the average value. Also because of the small indentation, the damage to the finished products and thin sheets is extremely small. Therefore, Rockwell hardness testing is suitable for finished products and thin sheets and is usually classified as a non-destructive testing method. This is crucial for materials and products that do not allow obvious damage.
The indentation of Brinell hardness is relatively large, which makes the measured value more accurate and can comprehensively reflect the hardness of a relatively large area of the material. However, precisely because of the large indentation, it is not suitable for the inspection of finished products and thin sheets, as the large indentation may damage the appearance and performance of the finished products and affect the integrity of the thin sheets. Therefore, Brinell hardness testing is generally not classified as a non-destructive testing method.
Characteristics of hardness values
The hardness value of Rockwell hardness is an unnamed number without a unit. This often leads to misunderstandings. It is incorrect to customarily say that the Rockwell hardness is so many degrees. The design of this unnamed number is to facilitate unification and comparison among different testing equipment and standards. It only represents the relative magnitude of the material's hardness and is not affected by unit conversion.
The hardness value of Brinell hardness has a unit and has a certain approximate relationship with the tensile strength. This characteristic makes Brinell hardness of great significance in the evaluation of the mechanical properties of materials. By measuring the Brinell hardness, the tensile strength of the material can be roughly inferred, providing a reference for the selection and design of materials.
Convenience of detection operation
Rockwell hardness testing has obvious advantages in operation. It can directly display the hardness value on the dial or through digital display, with convenient, fast and intuitive operation. This characteristic makes it very suitable for mass production scenarios. In large-scale production, it can quickly obtain hardness data, adjust the production process in a timely manner, and improve production efficiency.
Brinell hardness testing is relatively cumbersome. It requires using a microscope to measure the diameter of the indentation, and then determining the hardness value by looking up tables or performing calculations. This process is not only time-consuming but also requires a high level of skill from the operator. However, it is precisely this relatively complex operation that ensures the high precision of Brinell hardness testing.
Hardness value conversion relationship
Under certain conditions, HB (Brinell hardness) and HRC (Rockwell hardness scale C) can be interchanged by looking up tables. This provides convenience for data comparison and conversion between different hardness testing methods. In addition, there is also an approximate mental calculation formula that can help us quickly estimate, that is, 1 HRC ≈ 1/10 HB. Although this is only an approximate formula, in some occasions where high precision is not required, it can quickly give an approximate hardness conversion value, providing a certain reference for practical work.