Quick Brinell hardness number - obtained from the Brinell hardness test using the equation below
Hardness test - measures the resistance of a material to penetration by a sharp object. Common tests include, but are not limited to, the Brinell test, Rockwell test, Knoop test, and Vickers test
Equation The following is the equation for the Brinell hardness number:
HB =
F
0.5πD (D −
√
D2 − Di2
)
where F is the applied load in kilograms, D is the diameter of the indentor in millimeters, and Di is the diameter of the impression in millimeters.
Details
The hardness test measures the resistance to penetration of the surface of a material by a hard object. A variety of hardness tests have been devised, but the most commonly used are the Rockwell test and the Brinell test.
With relation to the Brinell hardness test, a hard steel sphere (usually 10 mm in diameter) is forced into the surface of the material. The diameter of the impression, typically 2 to 6 mm, is measured and the Brinell hardness number (abbreviated as HB or BHN) is calculated from the above equation. A Brinell hardness number can be obtained in just a few minutes with virtually no preparation of the specimen and without destroying the component, yet it provides a close approximation of the tensile strength.
Ball
Brale
Brinell Test
Rockwell Test
The Rockwell hardness test uses a small-diameter steel ball for soft materials and a diamond cone, or Brale, for harder materials. The depth of penetration of the indentor is automatically measured by the testing machine and converted to a Rockwell hardness number (HR). Several variations of the Rockwell test are used. A Rockwell C (HRC) test is used for hard steels, whereas a Rockwell F (HRF) test might be used for aluminum.
Hardness numbers are used primarily as a basis for comparison of materials, specifications for manufacturing and heat treatment, quality control, and correlation with other properties of materials. Hardness correlates well with wear resistance. A material used to crush or grind ore should be very hard to assure that the material is not eroded or abraded by the hard feed materials. Similarly, gear teeth in the transmission or the drive system of a vehicle should be hard enough that the teeth do not wear out. Typically, it is found that polymer materials are exceptionally soft, metals have an intermediate hardness, and ceramics are exceptionally hard.
Also, the Vickers (HV) and Knoop (HK) tests are microhardness test; they form such small indentations that a microscope is required to obtain the measurement.
Rockwell Hardness
Rockwell hardness tests are described by ASTM standard hardness method E-18 and measurements are quickly and easily made, they have good reproducibility, and the test machine for them is easy to use. In fact, the hardness number is read directly from a dial. Rockwell hardness scales are designated as A, B, C,..., etc. The indenters are described as a diamond, a 1/16-in-diameter ball, and a diamond for scales A, B, and C, respectively, where the load applied is either 60, 100, or 150 kg. Thus the Rockwell B scale, designated as RB, uses a 100-kg load and a No. 2 indenter, which is a 1/16-in-diameter ball. The Rockwell C scale RC uses a diamond cone, which is the No. 1 indenter, and a load of 150 kg. Hardness numbers so obtained are relative. Therefore a hardness RC = 50 has meaning only in relation to another hardness number using the same scale.
Brinell Hardness
The Brinell hardness is another test in very general use. In testing, the indenting tool through which force is applied is a ball and the hardness number HB is found as a number equal to the applied load divided by the spherical surface area of the indentation. Thus the units of HB are the same as those of stress, though they are seldom used. Brinell hardness testing takes more time, since HB must be computed from the test data. The primary advantage of both methods is that they are nondestructive in most cases. Both are empirically and directly related to the ultimate strength of the material tested. This means that the strength of parts could, if desired, be tested part by part during manufacture.
For steels the relationship between the minimum ultimate strength and the Brinell hardness number for 200 ≤ HB ≤ 450 is found to be:
Su = 0.495HB kpsi = 3.41HB MPa
Similar relationships for cast iron can be derived from data supplied by Krause. Data from 72 tests of gray iron produced by one foundry and poured in two sizes of test bars are reported in graph form. The minimum strength, as defined by the ASTM, is found from these data to be:
Eq1 Su = (0.23HB − 12.5) kpsi = (1.58HB − 86) MPa
Walton shows a chart from which the SAE minimum strength can be obtained. The result is:
Su = (0.2375HB − 16) kpsi
which is more conservative than the values obtained from Eq1.
Related ▪ L - Brinell Rockwell Hardness Conversion Charts: link1, link2
