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What are the disadvantages of Vickers hardness testers

Writer:Anonymous   2020-02-26 11:56:04


1, what are the disadvantages of Vickers hardness tester



The hardness of metal refers to the resistance of the metal surface to the plastic deformation caused by the intrusion of foreign objects. The higher the hardness, the stronger the metal's ability to resist plastic deformation, and the more difficult it is for the metal to deform plastically. The hardness test method is simple and easy, and does not damage the parts. There are three commonly used hardness test methods: Brinell hardness, Rockwell hardness, and Vickers hardness. The three kinds of hardness test values have a general conversion relationship. The manuals of machinery, mechanical technology or metal materials generally have conversion tables. [Brinell hardness HB]

The Brinell hardness is to press the steel ball of direct D into the metal surface with a load of P and hold it for a certain time. Measure the indentation diameter d on the metal surface, and calculate the indentation area AB based on this. The force per unit area is used as the hardness value of the metal, called Brinell hardness, and 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 obtained in the test should be in the range of 0.25D-0.6D. The upper limit of Brinell hardness is HB450, which is suitable for measuring the hardness of annealed, normalized, quenched and tempered steel, cast iron and non-ferrous metals.

[Rockwell hardness HR]

Rockwell hardness is the most commonly used hardness measurement method 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 range of measurable hardness is wide. However, Rockwell hardness also has some disadvantages. For example, due to the small indentation, segregation of the material and the uneven structure of the material, the measurement results have large resolution 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. The indenter used in Rockwell hardness test is divided into hard and soft. The hard indenter has a vertex angle of 120? ? Diamond cone for hard materials such as hardened steel. HRA has a load test of 60kgf, and the effective range of hardness is> 70, which is suitable for hard alloys, surface hardened layers and carburized layers. HRC has a load test of 150kgf, and the effective range of hardness is 20-67 (equivalent to HB230-700). Suitable for quenched and quenched and tempered steel.

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

The three Rockwell hardness scales are specified on the dial. HRA and HRC are black scales, and HRB is 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 a cone angle of 136? ? Diamond square cone.

During the test, a square indentation was formed on the test surface of the sample under the load P. The average length d of the two diagonal lines of the indentation was measured to calculate the indentation area AV, and the hardness of the sample was expressed by the value of P / AV, and 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, Vickers hardness tester unit



hardness

Hardness refers to the ability of a material to resist the pressing of a hard object into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the abrasion resistance. Common hardness indicators are Brinell hardness, Rockwell hardness, and Vickers hardness.

1. Brinell hardness (HB)

A certain load (usually 3000kg) is used to press a hardened steel ball of a certain size (generally 10mm in diameter) into the surface of the material for a period of time. After unloading, 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 adopted and the Rockwell hardness measurement is used instead. It is a diamond cone with a vertex angle of 120 ° or a steel ball with a diameter of 1.59 and 3.18mm. It is pressed into the surface of the material under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the hardness of the test material, it is expressed in three different scales:

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

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

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

3 Vickers hardness (HV)

A diamond square cone indenter with a load within 120kg and a vertex angle of 136 ° was pressed into the surface of the material, and the surface area of the material indentation pit was divided by the load value to obtain the Vickers hardness HV value (kgf / mm2).

In addition, the hardness is also expressed and tested as follows

Shore hardness tester

The Shore A hardness tester is a rubber type A hardness tester. It is a handheld hardness tester that can accurately determine the Shore hardness of rubber (plastic) products. It measures the penetration of a specified pressure needle at a specified pressure and time. Use ideal test equipment on site. It has the advantages of easy carrying, beautiful appearance and light weight.

Mohs "scale of hardness

Note: A standard for mineral hardness. It was first proposed by 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 pin. It is customary to use Mohs hardness in mineralogy or gemology. The measured depth of the scratch is divided into ten grades to represent the hardness: talc 1 (minimum hardness), gypsum 2, calcite 3, fluorite 4, apatite 5. Orthoclase (feldspar; orthoclase; periclase) 6, quartz (quartz) 7, topaz (topaz) 8, corundum (9), diamond (diamond) (10). Mohs hardness is also used to indicate the hardness of other solid materials.

Barcol scale of hardness

Barcol hardness (Barcol hardness for short), first proposed by American Barber-Colman company, is a kind of hardness category widely used in modern world. It uses a specific indenter to press the sample under the pressure of a standard spring The hardness of the sample is characterized by the depth of the indentation. Bacol hardness tester (Barcol hardness tester) is a special testing tool for measuring the hardness of FRP products, reinforced or non-reinforced hard plastics, aluminum and aluminum alloys, brass, copper and other soft metals (especially suitable for FRP products). Has been recognized by most countries or international organizations. The American Materials Testing Association (ASTM), Japanese Industrial Standards (JIS), China and other countries have successively formulated national standards for the "Test method for measuring the glass fiber reinforced plastic (GRP) hardness by Bacol hardness tester".

Shore hardness-Shore scleroscope hardness

Note: referred to as HS. A measure of the hardness of a material. First proposed by Englishman Albert F. Shore. The rebound rebound method is used to drop the striker from a certain height onto the surface of the material under test. A striker is a small cone with a tip that is often set with diamonds. The measured rebound height of the striker pin is used to indicate the hardness. The Shore hardness tester is suitable for determining the Shore hardness values of ferrous and non-ferrous metals. Used to measure the hardness of rubber, plastic, metal materials, etc. It is often called the Shore hardness in the rubber and plastic industries


3, Vickers hardness tester works



Principle of Vickers hardness tester:

A square pyramid diamond indenter is used, which is pressed into the surface of the sample under the action of the test force, and after the prescribed time is maintained, the test force is removed, and the diagonal length of the surface indentation 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 ———— Testing force, N;

d ———— The arithmetic mean value of the two diagonal lines d1 and d2 of the indentation. 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 Vickers hardness indentation ranges from 0.020 to 1.400mm.


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



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

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

Here's the difference:

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

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

Disadvantages: cannot 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 accuracy;

Disadvantages: optical principle, the diameter of the indentation needs to be measured through the eyepiece with the eye.

However, a CCD hardness image processing system can be selected (the indentation image is projected on a computer)

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

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

Disadvantages: optical principle, the diameter of the indentation needs to be measured through the eyepiece with the eye.

However, a CCD hardness image processing system can be selected (the indentation image is projected on a computer)


6, Vickers hardness tester, the theoretical difference in the use of force:



The prerequisites mentioned below are under the load that your workpiece can withstand (not to be penetrated). If you hit with a 300g load, you will see that the indentation will be smaller than that with 1000g. In the future, the larger the indentation, the clearer the measurement error will be. However, if the indentation you make with 1000 grams is too large and exceeds the visible range (you can't see the whole picture), then you can't measure it. So how much load to choose, make sure a few things:

1. The workpiece will not be penetrated

2. Make the indentation in the visible range

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

I do n’t know if your Vickers hardness tester is equipped with a CCD and software. If you add a CCD and software, the measurement accuracy will be much higher, and the error will be within 10 degrees.

You can Baidu following Suzhou Ouka 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 face. In addition, in algebra, the determinant of the nth order, 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 sub diagonal. The term "diagonal" is derived from the ancient Greek "horn" and the relationship between "horn" and was later pulled into Latin ("slash").