What is metal hardness?
Hardness refers to the ability of a material to resist local deformation, especially plastic deformation, indentations or scratches.
It is an indicator of the hardness of the material.
Types of metal hardness
According to the different test methods, there are three types of hardness.
① Scratch hardness
It is mainly used to compare the hardness of different minerals. The method is to choose a stick with a hard end and a soft end, and slide the material to be tested along the rod, and determine the softness and hardness of the material to be tested according to the location of the scratch. Qualitatively speaking, the scratches made by hard objects are long and the scratches made by soft objects are short.
② Indentation hardness
Mainly used for metal materials, the method is to press the specified indenter into the material under test with a certain load, and compare the softness and hardness of the material under test with the local plastic deformation of the material surface. Due to different indenters, loads, and load durations, there are a variety of indentation hardness, mainly Brinell hardness, Rockwell hardness, Vickers hardness, and micro-hardness.
③ Rebound hardness
Mainly used for metal materials. The method is to make a special small hammer drop freely from a certain height to impact the sample of the tested material, and the hardness of the material is determined by the amount of strain energy (measured by the rebound height of the hammer) stored in (and then release) the sample during the impact.
Common HV = HB = HRC hardness comparison table
Hardness comparison table of common ferrous metals
(Approximate conversion of intensity)
Classification of hardness
HRC / HB approximate conversion tips
- When the hardness is higher than 20HRC, 1HRC≈10HB,
- When the hardness is lower than 20HRC, 1HRC≈11.5HB.
Notes: For cutting processing, 1HRC≈10HB can be basically converted (the hardness of the workpiece material has a fluctuation range)
The most common Brinell, Rockwell, and Vickers hardnesses of metal materials are all indentation hardness. The hardness value indicates the ability of the surface of a material to resist plastic deformation caused by the intrusion of another object. When measuring the hardness by the back-jumping method, the hardness value represents the size of the metal’s elastic deformation function.
Commonly used hardness
A hardened steel ball or hard alloy ball with a diameter of D is used as the indenter, and the corresponding test force F is pressed into the surface of the test piece. After the prescribed holding time, the test force is removed to obtain an indentation of diameter d. Divide the test force by the indentation surface area. The value obtained is the Brinell hardness value, and the symbol is expressed by HBS or HBW.
The difference between HBS and HBW is the indenter. HBS indicates that the indenter is a hardened steel ball, which is used to determine materials with a Brinell hardness value below 450, such as mild steel, gray cast iron and non-ferrous metals.
HBW indicates that the indenter is a hard alloy, which is used to measure materials with a Brinell hardness value below 650.
The same test block, when the other experimental conditions are exactly the same, the results of the two tests are different, the HBW value is often greater than the HBS value, and there is no quantitative law to follow.
After 2003, China has adopted international standards equivalently, eliminating steel ball indenters and using hard alloy ball heads.
Therefore, HBS was discontinued, and all Brinell hardness symbols were represented by HBW.
Many times Brinell hardness is only expressed by HB, which means HBW. However, HBS is still seen in the literature.
The Brinell hardness measurement method is suitable for cast iron, non-ferrous alloys, various annealed as well as quenched and tempered steels. It is not suitable to measure samples or workpieces that are too hard, too small, too thin, and do not allow large indentations on the surface.
A diamond cone with a cone apex angle of 120 ° or Ø1.588mm and Ø3.176mm hardened steel ball is used as the indenter and used in conjunction with the load. Under the initial load of 10kgf and the total load (that is, the initial load plus main load) of 60, 100 or 150kgf force, press the sample. After the total load, the hardness is expressed by the difference between the indentation depth when the main load is removed while the main load is retained and the indentation depth under the initial load.
The Rockwell hardness test uses three test forces and three indenters, which have a total of nine combinations, corresponding to nine scales of Rockwell hardness. The application of these 9 rulers covers almost all commonly used metal materials.
There are three commonly used HRA, HRB and HRC, of which HRC is the most widely used.
Table of commonly used Rockwell hardness test specifications
|Hardness symbol||Indenter type||Total test forceF/N（kgf）||Hardness range||Applications|
|HRA||120°diamond cone||588.4(60)||20~88||Hard alloy, carbide, shallow case hardening steel and etc.|
|HRB||Ø1.588mm Quenched steel ball||980.7(100)||20~100||Annealed or normalized steel, aluminum alloy, copper alloy, cast iron|
|HRC||120°diamond cone||1471(150)||20~70||Hardened steel, quenched and tempered steel, deep case hardening steel|
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 is reduced, 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. Therefore, the HRA scale should be used instead.
The Rockwell hardness test is simple, fast, and has small indentation. It can test the surface of finished products and harder and thinner work pieces. Because the indentation is small, the hardness value fluctuates greatly for materials with uneven structure and hardness, and the accuracy is not as high as the Brinell hardness.
Rockwell hardness is used to determine the hardness of steel, non-ferrous metals, cemented carbide, and the like.
The Vickers hardness measurement principle is similar to Brinell hardness. A diamond quadrangular pyramid indenter with an opposite angle of 136 ° was used to press the surface of the material with a specified test force F. The test force is removed after holding for a specified time, and the hardness value is expressed by the average pressure on the unit surface area of the regular quadrangular pyramid indentation, and the symbol is HV.
The Vickers hardness measurement range is large, and it can measure materials with a hardness in the range of 10 ~ 1000HV, and the indentation is small. It is generally used to measure thin materials and surface hardened layers with carburizing and nitriding.
A certain mass of impact body equipped with tungsten carbide ball head is used to impact the surface of the test piece under a certain force, and then rebound. Due to different material hardness, the rebound speed after 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. Then it is converted into Leeb hardness value by the electronic circuit, and the symbol is 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. Whether it is large, heavy or complex geometries, it can be easily inspected.
Another advantage of Leeb hardness is that the surface of the product is lightly damaged and sometimes used for a non-destructive test. It has a unique hardness test in all directions, narrow space and special parts.
- Micro Vickers hardness testing machine
- Vickers hardness testing machine
- Rockwell hardness testing machine
- Portable Leeb hardness tester
FAQs about metal hardness
How to measure hardness of metal?
Brinell hardness method, Rockwell hardness method and Vickers hardness method are the most commonly used methods to measure the hardness of metal.
How to increase metal hardness?
The strength of metal materials can be improved in the following five ways:
1) Carry out heat treatment process, and carry out heat treatment, quenching, tempering, normalizing, etc. according to the required performance and structure.
2) Shot peening on the surface can also improve the strength.
3) Grain boundary strengthening.
4) Dislocation strengthening.
5) Some compounds precipitated by deformation and aging can improve the strength.
At what temperature does metal lose hardness
At more than 200 degrees below zero, ordinary steel is as fragile as shrimps. Low temperature reduces the activity force between metal molecules and increases the rigidity of matter.
Simply put, the more it can not bear deformation, so it is easy to break under external force.
However, whether it will become brittle depends on the situation. For example, there is no low-temperature ductile brittle transition for face centered cubic materials.
What determines the hardness of a metal?
1). The hardness of metal depends on the strength of metal bond in metal;
2). The smaller the radius of metal atom, the stronger the metal bond and the greater the hardness;
3). The more valence electrons in the metal per unit volume, the stronger the metal bond and the greater the hardness. Generally speaking, for the main group metals, from left to right, in the same period, the atomic radius decreases and the number of valence electrons increases, so the hardness increases;
4). From top to bottom, with the same main group, the atomic radius increases and the number of valence electrons is the same, so the hardness decreases;
5). For transition elements, the radius decreases from left to right, but the number of valence electrons first increases and then decreases. Therefore, the law may not be as obvious as the main family. However, elements with high hardness are generally in the middle of the transition elements with the same period.