3.4 Hardness
3.4.1 Hardness Measurement
The Brinell hardness number (BHN) was first introduced by J. A. Brinell of Sweden in 1900. It is calculated by Equation 3.24 based on the stress per unit surface area of indentation, as illustrated in Figure 3.16. Brinell hardness numbers were usually tabulated in reference charts before the test machine was computerized. The Brinell hardness test is not suitable for very thin specimens due to the depth of indentation impressed onto the part, or very hard materials because of the induced deformation of the tester itself.
BHN F
D D D Di
= π − −
2
2 2
( ) (3.24)
where F = indenting force, D = diameter of indenter ball, and Di = diameter of indention.
The Rockwell hardness test was first introduced to check if the bearing race of a ball bearing was properly heat treated. It is the most widely utilized hardness testing method in the United States today. Figure 3.17a shows a stand-alone Rockwell hardness tester; similar testers, though possibly of dif- ferent models and configurations, can be found in many laboratories. It has a control panel on the top and a load selection dial on the right side. The tester is equipped with an intuitive liquid crystal display (LCD) atop the control panel to show the test result and a Universal Serial Bus (USB) port at the upper right corner of the control panel for data transfer. Figure 3.17b illus- trates the further integration of modern computer and image technologies with the hardness measurement. It shows the indention on the screen and the data process of hardness measurements in the computer. The installed software allows automatic hardness measurement and digitalized image processing. It also makes the statistical analysis on hardness measurement, such as standard deviation, easy. The software enhances the data filing and exporting capacities as well.
Figure 3.18 illustrates the Rockwell hardness measurement process, where the indenter is first applied to the specimen surface with a minor load F1 of 10 kg to introduce an initial indention, e, and to establish a zero refer- ence position. The major load, F2, which may be either 60, 100, or 150 kg, is then applied for a specified dwell time on the surface. The major load is then released, leaving only the minor load on during the hardness reading. The Rockwell hardness number is a measurement of the indention depth, h, on the test specimen. It is worth noting that it is a linear measurement of inden- tation resistance with a different unit from stress. There are many scales in the Rockwell hardness readings, ranging from A through F, and continuing on. The two most commonly used are the B and C scales. The B scale is used for relatively soft materials such as aluminum alloys, and the C scale is for hard materials such as stainless steel. Each scale has 100 divisions and the hardness numbers are designated as RB or RC, respectively. If a material has a hardness number close to or above RB = 100, the C scale should be used.
Most materials have their Rockwell hardness numbers below RC = 70. The detailed test methods are explained in the following standards: ASTM E18
Indenting force F
D
Di
Indenter Specimen
FIgurE 3.16
Brinell hardness measurement.
(a)
(b) FIgurE 3.17
(a) Rockwell hardness test machine. (b) Computer and image technologies utilized in hardness measurement.
(ASTM, 2008) and ISO 6508 (ISO, 2005) for metallic materials, and ASTM D787 (ASTM, 2009) for plastic materials.
The Vickers pyramid hardness (VPH) test uses various diamond pyramid indenters. Therefore, Vickers hardness is also referred to as diamond pyra- mid hardness (DPH). Vickers and Brinell hardness numbers are both cal- culated based on the applied load per unit area of indentation. Their values are very close to each other at the low hardness range. The Vickers hard- ness number retains its accuracy at higher values up to 1,300 (about BHN 850), while the BHN shows noticeable deviation from the VPH number at hardness numbers higher than 500. This deviation is due to the induced deformation on the steel ball indenter used for the Brinell hardness test, as demonstrated in Figure 3.19, which shows the conversion among the five hardness scales: Rockwell A, Rockwell B, Rockwell C, Brinell, and Vickers for nickel alloys. Considering that the Rockwell hardness number is based on the indentation depth, while the Brinell and Vickers hardness numbers are based on the load per unit area, the conversion between them is indirect and just an approximation. This conversion should be avoided in reverse engineering analysis if possible. It is also worth noting that the complexity of elastic and plastic sample deformations during a hardness test and the defor- mation of the indenter itself make the reproduction of the same hardness test results virtually impossible, even using the same hardness scale, as shown in Figure 3.20 (Low, 2001). It plots the hardness data in consecutive tests for 10 days, all on the same sample and in the Rockwell C scale. Five consecu- tive tests were conducted each day; each individual test usually produced a slightly different result every day. Also shown is a line plot of the daily average, which varies from day to day as well. For a representative average hardness number, four or more tests on a sample are usually conducted for most hardness tests.
Vickers microhardness is most widely used for microhardness tests of thin coatings. The Shore scleroscope hardness test is a dynamic test that measures rebound height/energy as an indicator of surface hardness by dropping a test hammer onto the surface. The rebound height/energy is heavily depen- dent on the material elasticity; therefore, the Shore hardness should only be
e
h F1
F1 + F2
F1
FIgurE 3.18
Rockwell hardness measurement.
used to compare materials with similar elasticity. It has the potential to be used for hardness comparison between the surfaces that are hardened by a thin coating, like a hard-coating wear resistance surface, or case-hardened gear surfaces in reverse engineering analysis.
In light of the complexity and variance of hardness measurement, it is essential that the hardness numbers are measured on the same scale as specified in the reference material specification for direct comparison. If not
600
500
400
200 300
Hardness
Vickers Brinell Rockwell A Rockwell B Rockwell C
100
0
FIgurE 3.19 Hardness conversion.
45.6 45.65
45.55
45.45 45.5
45.35 45.4
Rockwell C Hardness
45.25 45.3
45.20 1 2 3 4 5 6
Sequence of Test
7 8 9 10 11
FIgurE 3.20
Scattering hardness test data.
feasible, only hardness numbers measured under comparable conditions can be converted to a same scale for comparison.