Hardness of materials type as Brinell Rockwell Vickers

 

 

 

Hardness of materials type as Brinell Rockwell Vickers

 

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Hardness of materials type as Brinell Rockwell Vickers

Hardness
Hardness is an engineering property that is related to the wear resistance of a material, its ability to abrade or indent another material, or its resistance to permanent or plastic deformation. The selection of the appropriate hardness test is dependent upon the relative hardness of the material being tested and the amount of damage that can be tolerated on the surface of the test specimen. Table 4.5 provides information relative to the most used hardness standards, each having application for certain types of materials. Hardness can be measured by three types of tests: (1) indentation—Brinell, Rockwell, Knoop, Durometer; (2) rebound or dynamic—Scleroscope; (3) scratch—Moh. These standards together with the materials to which they apply are as follows


1. Brinell—ferrous and nonferrous metals, carbon and graphite
2. Rockwell B—nonferrous metals or sheet metals
3. Rockwell C—ferrous metals
4. Rockwell M—thermoplastic and thermosetting polymers
5. Rockwell R—thermoplastics and thermosetting polymers
6. Knoop—hard materials produced in thin sections or small parts
7. Vickers diamond pyramid hardness—all metals


8. Durometer—rubber and rubber‑like materials
9. Scleroscope—primarily used for ferrous alloys
10. Moh—minerals
Table 4.5

We compare the characteristics of these various hardness tests in Fig. 4.23.
The Brinell hardness test is applicable to most metals and their alloys. This test provides a number related to the area of the permanent impression made by a ball indentor, usually 10 mm in diameter, pressed into the surface of the material under a specified load. The larger the Brinell number, the harder the material. The Brinell hardness number (BHN) is computed as follows:

where P is the applied load in kg, D is the diameter of ball in mm, d is the diameter of impression the ball makes in the material measured in mm and BHN is the Brinell hardness in kg/mm2.
Rockwell hardness is also an indentation test used on both metals and plastics. The Rockwell method makes a smaller indentation compared with the Brinell test, is more applicable to thin materials and is more rapid. The Rockwell number is derived from the net increase in depth of an impression of a standard indentor as the load is increased from a fixed minor load to a major load and then returned to the minor load. The Rockwell C test (using a brale as indentor and a major load of 150 kg) is used primarily for ferrous alloys, whereas the Rockwell B procedure (using as indentor a 1/16 in. ball and a major load of 100 kg) is used for softer or thinner alloys for which a minimum indentation is preferred. Rockwell M numbers are applied to hard plastics.
The Knoop and Vickers hardness tests measure the microhardness of small areas of a specimen. They are especially suited for measuring the hardness of very small parts, thin sections, and individual grains of a material, using a microscope equipped with a measuring eyepiece. Both tests impose a known load on a small region of the surface of a material for a specified time. In the case of the Knoop test, the indentor is a diamond with a length‑to‑width ratio of about 3:1, whereas the Vickers indentor is a square diamond pyramid with an apical angle of 136°. Both values of hardness are calculated by dividing the applied load by the projected area of the indentation. For the Vickers method, V = P/0.5393d2, where V is the Vickers hardness number, P is the imposed load (1 to 120 kg), and d the diagonal of indentation in mm.
The Shore Scleroscope is a rebound device that drops a ball of standard mass and dimensions through a given distance. The height of the rebound is measured. As the hardness of the test surface increases, the rebound height increases because less energy is lost in plastic deformation of the test surface.
The Durometer hardness test is used in conjunction with elastomers (rubber and rubberlike materials). Unlike other hardness tests, which measure plastic deformation, this test measures elastic deformation. Here an indentor of hardened steel is extended into the material being tested. A hardness value of 100 represents zero extension of the indentor; at zero reading, the indentor extends  in. beyond the presser foot that surrounds the area being measured.
Moh hardness values are used principally in the designation of hardness of minerals. The Moh scale is so arranged that each mineral will scratch the mineral of the next lower number. Ten selected minerals have been used in the development of this scale. The scale, along with the values for these ten minerals are: talc, 1; rock salt or gypsum, 2; calcite, 3; fluorite, 4; apatite, 5; feldspar, 6; quartz, 7; topaz, 8; corundum, 9; diamond, 10. According to the Moh hardness scale, a human fingernail has a hardness of 2, annealed copper 3, and martensite 7. The Moh scale has too few values to be of practical use in the metal‑working field, since four values cover the range of the softest to the hardest metals.
In view of the wide variation in the characteristics of engineering materials, one hardness test for all materials is not practical, but conversion from one hardness scale to another can be done in some cases. The Brinell and Rockwell B and S scales can be converted from one to the other. To approximate equivalent hardness numbers, the alignment chart shown in Table 4.5 can be used. This chart does not included a Durometer scale, since correlation between Durometer and Brinell values is inconsistent. The Durometer test measures the elastic properties of a material, whereas all other indentation hardness tests measure the plastic properties of a material.
4.4.4   Impact Properties
In some designs, dynamic forces are likely to cause failure. For example an alloy may be hard and have high compressive strength and yet be unable to withstand a sharp blow. In particular, low‑carbon steels are susceptible to brittle failure at certain temperatures (Fig. 4.24). Experience has demonstrated that the impact test is sensitive to the brittle behavior of such alloys. Most impact tests use a calibrated hammer to strike a notched or unnotched test specimen. In the former, the test result is strongly dependent on the base of the notch, where there is a large concentration of biaxial stresses that produce a fracture with little plastic flow. The impact test is particularly sensitive to internal stress producers such as inclusions, flake graphite, second phases, and internal cracks.
The results from an impact test are not easily expressed in terms of design requirements because it is not possible to determine the biaxial stress conditions at the notch. There also seems to be no general agreement on the interpretation or significance of the result. Nonetheless the impact test has proved especially useful in defining the temperature at which steel changes from brittle to ductile behavior. Low‑carbon steels are particularly susceptible to brittle failure in a cold environment such as the North Atlantic. There were cases of Liberty ships of World War II vintage splitting in two as a result of brittle behavior when traveling in heavy seas during the winter.
In a particular design having a notch or any abrupt change in cross section, the maximum stress occurs at this location and may exceed the stress computed by typical formulas based upon simplified assumptions in connection with stress distribution. The ratio of this maximum stress to the nominal stress is known as a stress concentration factor, usually denoted by K. Stress concentration factors may be determined experimentally or by calculations based on the theory of elasticity. Figure 4.24 illustrates stress concentration factors for fillets of various radius divided by the thickness of castings subjected to torsion, tension, and bending stresses.
Impact properties are an important consideration in developing tooling for many applications.  Processes that impart high impact loads may limit the range of appropriate tool materials since chipping or fracture may occur.  In stamping a punch that that removes material by driving through the sheet, sometimes thousands of times per minute, sees high impact loads and chipping may occur if a tool material is used that cannot absorb sufficient energy.  Cutting tools also can see impacts where the tool is periodically engaged and then disengaged in the cutting due to the geometry of the part being cut.  In both cases care must be taken in selecting the right material and material grade.

 

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Hardness of materials type as Brinell Rockwell Vickers