What are the disadvantages of Vickers hardness tester

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release time : 2020-06-01 15:13:11

  1. What are the disadvantages of Vickers hardness tester


The hardness of a metal refers to the resistance to plastic deformation caused by the intrusion of foreign objects within the local volume of the metal surface. The hardness test method is simple and easy, without damaging the parts. The actual commonly used hardness test methods are: Brinell hardness, Rockwell hardness and Vickers hardness. The three hardness test values have a rough conversion relationship, and the conversion relationship table is generally available on the manual of machinery, mechanical technology or metal materials [Brinell hardness HB]

Brinell hardness is to press the direct D steel ball into the metal surface with a force of load P, and hold it for a certain period of time. The force per unit area is used as the hardness value of the metal, called Brinell hardness, and is recorded as HB.

HB = P / AB = P / (πDh) = 2P / (πD (D-SQD (D2-d2)))

Unit: P-kgf, D, h-mm

For steel, the generally selected steel ball D is 10mm, the load P is 3000kgf, and the pressing time is 10 seconds. The diameter d of the test should be in the range of 0.25D-0.6D. The upper limit of Brinell hardness is HB450, suitable for measuring the hardness of annealing, normalizing, quenched and tempered steel, cast iron and non-ferrous metals.

[Rockwell Hardness HR]

Rockwell hardness is the most commonly used method of hardness measurement in industrial production. Because the operation is simple and fast, the hardness value can be read directly without damaging the surface of the workpiece, and the measurable hardness range is wide. However, Rockwell hardness also has some shortcomings, such as small indentation, segregation of the material and uneven structure, the measurement results have a large degree of separation and poor reproducibility.

Rockwell hardness (HR) is also used to test hardness by indentation. It is used to measure the depth of the depression to represent the hardness value. Rocker hardness test indenters are divided into hard and soft. Does the hard indenter have an apex angle of 120? ? The diamond cone is used for hard materials such as hardened steel. HRA takes a load test of 60kgf, the effective range of hardness is> 70, suitable for cemented carbide, surface quenching layer and carburized layer; HRC takes a load test of 150kgf, the effective range of hardness is 20-67 (equivalent to HB230-700), Suitable for hardened steel and quenched and tempered steel.

The soft indenter is made of steel balls with a diameter of 1.588mm (1/16 "). It is used in annealed steel, non-ferrous metals, etc. Expressed in HRB, the effective range of hardness is 25-100 (equivalent to HB60-230).

The colors of the three Rockwell hardness scales on the dial are specified, HRA and HRC are black scales, and HRB are red scales.

[Vickers hardness HV]

Vickers hardness is also measured using the unit stress on the indentation area as the hardness value. The indenter used for Vickers hardness is the angle of the cone surface is 136? ? Diamond square cone.

During the test, a square indentation was pressed on the test surface of the sample under the load P. The average length d of the two diagonals of the indentation is measured to calculate the indentation area AV, and the hardness of the sample is expressed by the value of P / AV, and is expressed by HV.

HV = P / AV = 1.8544P / d2

The size of the load P can be selected according to the different samples, generally 5-100kgf


2, the unit of Vickers hardness tester



hardness

Hardness indicates the ability of a material to resist the pressing of hard objects into its surface. It is one of the important performance indexes of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indexes are Brinell hardness, Rockwell hardness and Vickers hardness.

1. Brinell hardness (HB)

Press a certain size (generally 10mm) hardened steel ball into the surface of the material with a certain load (generally 3000kg) for a period of time. After the load is removed, the ratio of the load to the indentation area is the Brinell hardness value ( HB), the unit is kilogram force / mm2 (N / mm2).

2. Rockwell hardness (HR)

When HB> 450 or the sample is too small, the Brinell hardness test cannot be used and the Rockwell hardness measurement is used instead. It uses a diamond cone with an apex angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18mm to be pressed into the surface of the material to be measured under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the different hardness of the test material, it is expressed in three different scales:

HRA: It is the hardness obtained by using a 60kg load and a diamond cone indenter. It is used for materials with extremely high hardness (such as cemented carbide, etc.).

HRB: It is a hardened steel ball with a load of 100kg and a diameter of 1.58mm. The hardness obtained is used for materials with lower hardness (such as annealed steel, cast iron, etc.).

HRC: It is the hardness obtained by using 150kg load and diamond cone indenter. It is used for materials with high hardness (such as hardened steel, etc.).

3 Vickers hardness (HV)

Press a load within 120kg and a diamond square cone indenter with an apex angle of 136 ° into the material surface, and divide the surface area of the material indentation pit by the load value, which is the Vickers hardness HV value (kgf / mm2).

In addition, the hardness has the following representation and test methods

Shore hardness tester-Shore hardness tester

The Shore hardness tester is a rubber type A hardness tester, which is a hand-held hardness tester that can accurately measure the Shore hardness of rubber (plastic) products. It measures the penetration of a specified pressure needle under specified pressure and time conditions. Use ideal test equipment on site. It has the advantages of easy portability, beautiful appearance and light weight.

Mohs hardness-Mohs "scale of hardness

Description: A standard representing the hardness of minerals. It was first proposed by the German mineralogist Frederich Mohs in 1824. The scratch method is used to scratch the surface of the tested mineral with a pyramid-shaped diamond drill needle, and it is customary to use Mohs hardness in mineralogy or gemology. The measured depth of scratches is divided into ten levels to indicate hardness: talc 1 (minimum hardness), gypsum 2, gypsum 3, calcite 3, fluorite 4, and apatite 5, orthoclase (feldspar; orthoclase; periclase) 6, quartz (quartz) 7, topaz (topaz) 8, corundum (corundum) 9, diamond (diamond) 10. Mohs hardness is also used to indicate the hardness of other solid materials.

Barcol hardness-Barcol scale of hardness

Barcol hardness (barcol hardness for short), first proposed by the American Barber-Colman company, is a kind of hardness class widely used in modern times in the world, with a specific indenter pressed into the sample under the pressure of a standard spring To characterize the hardness of the sample with the depth of the indentation. Barcol hardness tester (Barcol hardness tester) is a special testing tool for measuring the hardness of softer metals such as FRP products, reinforced or non-reinforced hard plastics, aluminum and aluminum alloys, brass and copper (especially suitable for FRP products), Has been recognized by most countries or international organizations. The American Society for Testing Materials (ASTM), Japan Industrial Standards (JIS), China and other countries have successively formulated national standards for the "Test Method for Measuring the Hardness of Glass Fiber Reinforced Plastics (GRP) with the Bacol Hardness Tester".

Shore hardness-Shore scleroscope hardness

Description: Referred to as HS. A standard indicating the hardness of a material. It was first proposed by the Englishman Albert F. Shore. The elastic rebound method is used to drop the striker from a certain height onto the surface of the material to be rebounded. The striker is a small cone with a tip, and diamond tips are often set on the tip. The hardness measured by the measured rebound height of the striker. The Shore hardness tester is suitable for measuring the Shore hardness values of ferrous and non-ferrous metals. Used to measure the hardness of rubber, plastics, metal materials, etc. It is often referred to as Shore hardness in the rubber and plastic industries


3. Working principle of Vickers hardness tester



The principle of Vickers hardness tester:

A regular quadrangular pyramid diamond indenter is used to press into the surface of the sample under the test force. After a specified time, the test force is removed and the diagonal length of the indentation on the sample surface is measured.

The quotient of the test force divided by the indentation surface area is the Vickers hardness value. The Vickers hardness value is calculated according to formula (3-1):

HV = constant * test force / indentation surface area ≈0.1891 F / d2 ………… (3-1)

In the formula:

HV ———— Vickers hardness symbol;

F ------ test force, N;

d ———— The arithmetic average of the two diagonals d1, d2 of the indentation, mm In practice, the Vickers hardness value is obtained by looking up the table based on the diagonal length d.

National standards stipulate that the diagonal length of the Vickers hardness indentation is 0.020 ~ 1.400mm.


5, the difference between Vickers hardness tester and Brinell, Rockwell



Hardness tester expert Zhao Tong answers your question and hopes to help you.

Vickers hardness tester, Brinell hardness tester and Rockwell hardness tester are the three most commonly used hardness testers.

Here are the differences:

1. Such as: Rockwell hardness tester RTD700, the principle is to measure the depth.

Advantages: easy and fast operation, direct digital display; it is the easiest to use in the three hardness testers.

Disadvantages: can not measure too thin workpieces below 0.2mm

2. Such as: Brinell hardness tester WHB-3000, optical principle, measuring the diameter of circular indentation.

Features: large indentation and high precision;

Disadvantages: optical principle, need to use the eye to measure the indentation diameter through the eyepiece.

However, you can choose the CCD hardness image processing system (the indentation image is projected on the computer)

3. For example, Vickers hardness tester VTD552, optical principle, measure the diagonal length of diamond indentation.

Features: Large indentation, high precision, and can measure very thin workpieces, which is the widest measurement range among the three machines.

Disadvantages: optical principle, need to use the eye to measure the indentation diameter through the eyepiece.

However, you can choose the CCD hardness image processing system (the indentation image is projected on the computer)


6, Vickers hardness tester Theoretical difference in the use of strength when measuring hardness:



The premise mentioned below is under the load that your workpiece can bear (will not be penetrated). If you hit with a 300-gram load, you will see that the indentation will be smaller than that with 1000 grams. In the future, the larger the indentation, the clearer the measurement error. But if the indentation you made with 1000 grams is too large and exceeds the visible range (you can't see the full picture), then you can't measure it. So how much load to choose, to ensure a few points:

1. The workpiece will not be penetrated

2. The indentation made should be within the visible range

3. Ensure that the larger the indentation in the visible range, the higher the accuracy.

I don't know if you have installed CCD and software on this Vickers hardness tester. If you install CCD and software, the measurement accuracy will be much higher, and the error will be within 10 degrees.

You can Baidu following Suzhou Oka Optics


Related concepts



diagonal

Diagonal is a geometric term that refers to a line segment connecting any two non-adjacent vertices of a polygon, or a line segment connecting any two vertices of a polyhedron that are not on the same plane. In addition, in algebra, the n-order determinant, the numbers from the upper left to the lower right are classified as the main diagonal, and the numbers from the lower left to the upper right are classified as the secondary diagonal. The term "diagonal" comes from the relationship between "horn" and "horn" in ancient Greek, and was later drawn into Latin ("slash").



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