Hardness of metal materialsHardness refers to the ability of a material to resist local deformation, especially plastic deformation, indentation or scratch. It is an indicator to measure the hardness of a material. According to different test methods, hardness is divided into three types.
① Scratch hardness. It is mainly used to compare the hardness and softness of different minerals. The method is to select a rod with one end hard and the other end soft, and scratch the material to be tested along the rod, and determine the hardness and softness of the material to be tested according to the position of the scratch. Qualitatively speaking, the scratch made by a hard object is long, and the scratch made by a soft object is short.
② Indentation hardness. It is mainly used for metal materials. The method is to press the specified indenter into the material to be tested with a certain load, and compare the hardness and softness of the material to be tested by the size of the local plastic deformation on the surface of the material. Due to the differences in indenters, loads and load duration, there are many types of indentation hardness, mainly Brinell hardness, Rockwell hardness, Vickers hardness and microhardness.
③ Rebound hardness. It is mainly used for metal materials. The method is to make a special small hammer fall freely from a certain height to impact the sample of the material being tested, and the hardness of the material is determined by the amount of strain energy stored (and then released) by the sample during the impact (measured by the rebound height of the small hammer).
The most common Brinell hardness, Rockwell hardness and Vickers hardness of metal materials belong to indentation hardness. The hardness value indicates the ability of the material surface to resist plastic deformation caused by another object when it is pressed into the surface. The rebound method (Shore, Leeb) measures hardness, and the hardness value represents the size of the metal's elastic deformation function.
Brinell Hardness Brinell Hardness uses a hardened steel ball or carbide ball with a diameter of D as an indenter, and presses it into the surface of the specimen with the corresponding test force F. After the specified holding time, the test force is removed to obtain an indentation with a diameter of d. The test force is divided by the surface area of the indentation, and the value obtained is the Brinell hardness value, and the symbol is HBS or HBW.
The difference between HBS and HBW is the difference in the indenter. HBS means that the indenter is a hardened steel ball, which is used to measure materials with a Brinell hardness value below 450, such as mild steel, gray cast iron and non-ferrous metals. HBW means that the indenter is a carbide, which is used to measure materials with a Brinell hardness value below 650.
For the same test block, when other test conditions are exactly the same, the two test results are different, and the HBW value is often greater than the HBS value, and there is no quantitative rule to follow.
After 2003, my country has equivalently adopted international standards, canceled the steel ball indenter, and adopted carbide ball heads. Therefore, HBS is no longer used, and HBW is used to represent the Brinell hardness symbol. In many cases, Brinell hardness is only represented by HB, which refers to HBW. However, HBS is still seen in literature papers.
The Brinell hardness measurement method is suitable for cast iron, non-ferrous alloys, various annealed and tempered steels. It is not suitable for measuring samples or workpieces that are too hard, too small, too thin, and do not allow large indentations on the surface. Rockwell Hardness uses a diamond cone with a cone apex angle of 120° or a Ø1.588mm and Ø3.176mm hardened steel ball as an indenter and load. The specimen is pressed into the test piece under the action of an initial load of 10kgf and a total load of 60, 100 or 150kgf (i.e., initial load plus main load). After the total load is applied, the hardness is expressed by the difference between the indentation depth when the main load is removed but the main load is retained and the indentation depth under the action of the initial load. The Rockwell hardness test uses three test forces and three indenters, which have a total of 9 combinations, corresponding to the 9 scales of Rockwell hardness. The application of these 9 scales covers almost all commonly used metal materials. The commonly used ones are HRA, HRB and HRC, among which HRC is the most widely used. The use range of HRC scale is 20~70HRC. When the hardness value is less than 20HRC, because the conical part of the indenter is pressed too much, the sensitivity decreases, and the HRB scale should be used instead; when the hardness of the sample is greater than 67HRC, 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, so the HRA scale should generally be used instead.
The Rockwell hardness test is simple, quick, and has a small indentation. It can test the surface of finished products and harder and thinner workpieces. Due to the small indentation, the hardness value fluctuates greatly for materials with uneven structure and hardness, and the accuracy is not as high as Brinell hardness. Rockwell hardness is used to measure the hardness of steel, non-ferrous metals, cemented carbide, etc.
Vickers Hardness Vickers Hardness The measurement principle of Vickers hardness is similar to Brinell hardness. A diamond regular tetrahedron indenter with a relative angle of 136° is used to press into the surface of the material with a specified test force F. The test force is removed after maintaining it for a specified time. The hardness value is expressed by the average pressure per unit surface area of the regular tetrahedron indentation, and the symbol is HV. Vickers hardness has a wide measurement range and can measure materials with a hardness range of 10~1000HV. It has a small indentation and is generally used to measure thinner materials and surface hardening layers such as carburizing and nitriding.
Leeb Hardness uses a certain mass of impact body equipped with a tungsten carbide ball head to impact the surface of the test piece under a certain force, and then rebound. Due to the different hardness of the materials, the rebound speed after the impact is also different. Permanent magnetic material is installed on the impact device. When the impact body moves up and down, its peripheral coil will induce an electromagnetic signal proportional to the speed, which is then converted into a Leeb hardness value through an electronic circuit, and the symbol is marked as HL.
The Leeb hardness tester does not require a workbench. Its hardness sensor is as small as a pen and can be directly operated by hand. It can be easily detected regardless of whether it is a large, heavy workpiece or a workpiece with complex geometric dimensions.
Another advantage of Leeb hardness is that it causes little damage to the surface of the product and can sometimes be used as a non-destructive test; it is unique in hardness testing in all directions, narrow spaces and special parts.
