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What is hardness?
A measure of resistance to deformation or penetration.
Why is hardness important in the study of metallurgy?
because hardness relates to several other key properties of metal, especially, strength, brittleness, and ductility. By measuring the hardness of metal, you are also indirectly measuring brittleness, strength, and ductility.
In a hardness tester using penetration as a technique, the hardness is proportional to what?
size of penetration
What are the two basic categories of hardness testing methods?
1. Penetration hardness: Very accurate. A penetrator on the machine is forced against a metal sample. The size of the indenture is measured, and then the measurement is converted to a hardness number.
2. Scratch: Very fast and crude. The metal sample is scratched by the edge of a tool or object. No numerical value is calculated. The sample is either called "hard" or "soft" depending on whether or not a scratch results.
Why must conversion charts or mathematical formulas be used to convert from the units of one hardness testing method to the other?
because no single hardness testing units is used universally as the main unit of measure. Instead, each type of machine tends to have its own units.
Name the nine most common methods of testing hardness.
1. Rockwell hardness
2. Rockwell superficial hardness
3. Brinell hardness
4. Vickers microhardness
5. Knoop microhardness
6. Shore Scleroscope hardness
7. Sonodur hardness
8. Mohs hardness
9. File hardness
Describe the Rockwell hardness testing method
It is the most widely used of all hardness testing methods:
1. This method eliminates the effects of small surface imperfections such as surface scale, surface roughness, lack of flatness, and carbon layer on the sample before testing. It does this by using a preliminary minor load to pierce through any imperfections. During this first step, the anvil is raised manually until a minor load of about 10 kg is applied causing the penetrator to penetrate slightly into the sample.
2. After the minor load is applied, the major load (60 kg, 100 kg, or 150 kg) is applied by actuating a handle or lever on the front of the machine. The penetrator moves deeper into the sample.
3. The hardness value is read directly from either a digital readout or from a rotary dial on the machine. The scale reads in Rockwell hardness values which are based on the depth of penetration.
What are the three different penetrators used for Rockwell testing?
1. A 1/8" dia tunsten-carbide ball.
2. A 1/16" dia tungsten-carbide ball.
3. A conical-shaped, diamond-point penetrator
What is tungsten-carbide?
a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through a process called sintering. Tungsten carbide is approximately twice as stiff as steel, and is double the density of steel
How many scales are there in the Rockwell system?
Describe the nine scales used in the Rockwell system.
Which Rockwell scale is used for the hardest metals?
For testing steel, which two scales are the most used for testing hardness?
Rc and the Rb. The Rc scale is used to test the hard steels and the Rockwell B scale is used to test the softer, low-carbon steels, aluminum, and other soft nonferrous materials.
What is the hardest Rockwell scale value for steel that can be obtained?
65 Rc (Rockwell C scale)
What are the advantages of Rockwell hardness testing?
1. The minor load greatly reduces or eliminates the effect of surface imperfections.
2. Human error is reduced because the hardness value can be read directly from a scale.
Describe the Rockwell superficial hardness testing method.
It tests the hardness closer to the outside surface of the metal. In order to do this, the testing method uses smaller loads than the regular Rockwell testing method. The three common loads are 15 kg, 30 kg, and 45 kg. The same diamond-point penetrator and 1/16" ball used in the Rockwell testing method is also used in the Rockwell superficial testing method.
There are two choices of penetrators and three choices of loading, so there are six total combinations that represent scales available for use on the Rockwell superficial tester.
Describe the six total scales that are used for the Rockwell superficial testing.
They are 15N, 30N, 45N, 15T, 30T, and 45T. Each numerical value equals the load in kg. The N indicates that the diamond-point penetrator was used, and T indicates that the ball point penetrator was used.
For example, a 55-15N reading would mean that the test measures to 55 Rockwell units from a load of 15 kg applied with a diamond-tip penetrator.
What are the advantages of the Rockwell superficial hardness testing?
1. Thin materials can be tested
2. Hardness near the surface can be tested
3. Case-hardened surfaces can be tested
The Brinell hardness testing method was proposed by who?
Johan August Brinell, in 1900. This was the first quantitative method for the hardness of metals.
Describe the Brinell testing method
Similar to Rockwell testing method except it uses a tungsten-carbide ball shaped penetrator only as the penetrating device. The ball is approximately 10 mm in diameter.
After the indent is made, the dia of it is measured using a small microscope with a calibrated lens and this measurement is then converted to a hardness value using BHN (Brinell Hardness Number) as the unit of measurement.
What is the load used with Brinell testing?
Generally, the force used in the test is 3000 kg. Sometimes, a second BHN scale is used in conjuction with a 500 kg force. This scale is used primarily for softer and thinner samples.
For making Brinell tests, the load should be applied for a minimum of _____________ seconds.
What are the pros and cons of the Brinell method?
1. Very accurate hardness measuring method for soft materials.
1. Generally limited to testing softer metals as too much force is required with a ball type penetrator to make a measureable dent on a very hard surface.
2. The ball makes a relatively large indenture in the surface, compared to other testing methods, therefore often rendering the sample as scrap material after testing.
3. Because of the large indenture, Brinell testing method cannot be used on thin materials.
What is the formula for finding the BHN when using the Brinell testing method?
What are microhardness testers?
Testers that use small loads and sharply pointed penetrators, resulting in very small indentations. Often used on parts which need to be used again rather than be scrapped.
What are the two most common microhardness testing methods?
1. Vickers microhardness testing
2. Knoop microhardness testing
What are some examples of applications of microhardness testing?
1. thin plates
2. metal foils
3. fine wire
4. glass (may fracture otherwise)
5. ceramics (may fracture otherwise)
6. hardness of coatings
7. case-hardened parts
Microtesters are often used to test areas smaller than the size of what? These testers are more often found where than where else?
a crystal or grain/research labs than on a manufacturing floor
Describe the Vickers microhardness testing method
Similar to Rockwell tester except it uses a diamond shape, the load is less, and it uses different units of measure. The process is:
1. A diamond-shape penetrator penetrates the sample as the anvil is raised.
2. The contact pressure is increased until 50 kg is reached.
3. A diamond shaped indenture results
4. The diagonal of the diamond shaped (upside down pyramid shaped) indenture is measured.
5. This measurement is converted to a DPH (Diamond Pyramid Hardness) value
What is the load used for Vickers testing?
What does DPH stand for?
diamond pyramid hardness value
What is the formula used to calculate DPH when using the Vickers testing method?
What are the advantages of the Vickers testing method?
1. can be used on harder materials because the pointed penetrator can probe into a hard surface more easily than a ball-type penetrator can.
2. can be used on smaller areas
3. requires a smaller load
Describe the Knoop microhardness testing method.
Uses an even smaller load than the Vickers testing method. A load of less than 4 kg is used and a load of merely 25 grams can be used for extremely small areas. The Knoop penetrator does not have a square base like the Vickers method. It has a diamond cross section known as an "elongated pyramid." The ratio of the diagonals is 7 to 1. The process of testing the hardness of a sample is the same as with the Vickers method.
What is the load used with Knoop microhardness testing?
A load of less than 4 kg is used and a load of merely 25 grams can be used for extremely small areas.
Most new microhardness testers are equipped for both what?
Vickers and Knoop testing
Describe the Shore Sclerescope hardness testing method.
The surface of a sample is not penetrated. Rather, a hammer ball is dropped onto the sample, and the hardness is proportional to the bounce of the object.
1. The sample to be tested rests on an anvil
2. A small metal ball is dropped from a height of 10 inches.
3. The hammer strikes the test sample and rebounds.
4. The height of the first bounce is monitored and this height is converted into hardness value in units of Shore.
This testing methods assumes that the higher the ball bounces, the harder the material is.
The hammer ball of a Shore Sclerescope wieghs how much?
40 grains (less than .001 pounds, or less than 3 grams)
The Shore Sclerescope requires better what for accuracy than other hardness testing methods?
better surface quality of the sample; smooth, flat, clean, and horizontal.
Describe the Sonodur hardness testing method
This method is based on the way hardness affects the natural resonant frequency of a piece of metal.
A diamond-shaped magnetostrictive rod (.75 mm in diameter) is pressed against the sample to be tested. An electrical coil is used to vibrate the rod, and the frequency at which it vibrates the greatest is determined in the electronic portion of the machine. This frequency is known as the resonant frequency. The harder the material, the higher the resonant frequency. The resonant frequency value is then converted to a hardness number.
The Sonodur machine reads in ___________ units.
What are the advantages of the Sonodur method?
1. The testing device is small and portable
2. It gives a very quick response and does not damage the specimen
3. Is considered very accurate
What is the order of the stones in the mohs scale?
Describe the Mohs test method
Developed in 1812 by Friedrich Mohs and is based on concepts described by Pliny the Elder around AD 77.
The Mohs scale places minerals in a hardness range based on the ability of harder minerals to scratch softer ones. Early scientists realized a need to compare the hardness of minerals. Mohs selected ten minerals of varying hardness. The softest mineral, talc, was assigned a hardness value of 1 and the hardest mineral, diamond, was assigned a hardness value of 10. The other minerals were assigned values of 2 through 9.
To use Mohs scale, the material sample is struck with one of the minerals. If the sample is scratched, it is softer than the mineral. This procedure is continued until a mineral that does not scratch the sample is found.
This method remains useful, especially for field work in mineralogy.
Describe the file hardness testing method
Fast, simple, and convenient-but inaccurate. The inspector takes a file in one hand and the test material in the other. An attempt is made to scratch the test material by scraping it once with the edge of the file. If the material does not scratch, it is file hard.
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