Rockwell hardness test

1. Rockwell hardness test


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Rockwell hardness test
History
Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball
bearing company and he wanted a fast non-destructive way to determine if the heat treatment
process they were doing on the bearing races was successful.
Types of Rockwell test
There are two types of Rockwell tests:
1. Rockwell: the minor load is 10 kgf, the major load is 60, 100, or 150 kgf.
2. Superficial Rockwell: the minor load is 3 kgf and major loads are 15, 30, or 45 kgf.
In both tests, the indenter may be either a diamond cone or tungsten carbide ball, depending
upon the characteristics of the material being tested.
Theory related to Rockwell hardness
Rockwell hardness values are expressed as a combination of a hardness number and a scale
symbol representing the indenter and the minor and major loads. The hardness number is
expressed by the symbol HR and the scale designation.
The Rockwell hardness test is defined in ASTM E 18 and several other standards. Rockwell
hardness testing differs from Brinell testing in that the Rockwell hardness number is based on
the difference of indenter depth from two load applications. Initially a minor load is applied,
and a zero datum is established. A major load is then applied for a specified period of time,
causing an additional penetration depth beyond the zero datum point previously established by
the minor load. After the specified dwell time for the major load, it is removed while still
keeping the minor load applied. The resulting Rockwell number represents the difference in
depth from the zero datum position as a result of the application of the major load. The entire
procedure requires only 5 to 10 s.
In this hardness and differential depth are inversely proportional to each other. Higher the depth
and low would be the hardness.
Hardness and depth are related as
??????????????????=
??????−ℎ
0.002

Where

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K = constant, for C scale it is 0.2 and for B scale it is 0.26
h = depth of impression in mm
Hence the formulae for
HRc = 100 – e where e = h/0.2
And for
HRB = 130 – e where e h/0.26
Scales and values
The most common indenter type is a diamond cone ground at 120 degrees for testing hardened
steels and carbides. Softer materials are typically tested using tungsten carbide balls ranging in
diameters from 1/16 in up to 1/2 in. The combination of indenter and test force make up the
Rockwell scale. These combinations make up 30 different scales and are expressed as the actual
hardness number followed by the letters HR and then the respective scale.
There are several alternative scales, the most commonly used being the "B" and "C" scales.
Both express hardness as an arbitrary dimensionless number

Table 1 scale and indenter of Rockwell hardness
Scale Abbreviation Load Indenter Use
A HRA 50 kgf 120° diamond cone Tungsten carbide
B HRB 100
kgf
1
⁄16-inch-diameter (1.588 mm)
steel sphere
Aluminium, brass, and
soft steels
C HRC 150
kgf
120° diamond cone Harder steels >B100
D HRD 100
kgf
120° diamond cone
E HRE 100
kgf
1
⁄8-inch-diameter (3.175 mm)
steel sphere

F HRF 60 kgf
1
⁄16-inch-diameter (1.588 mm)
steel sphere

G HRG 150
kgf
1
⁄16-inch-diameter (1.588 mm)
steel sphere

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Figure 1 brale indenter
Procedure
We were provided steel sample and their heat treatment had already done. Roughly polished
the surface and checked the sample either it is suitable for test or not. We used the brale
indenter. Placed the sample on anvil and applied the minor load of 10kg. Set the zero datum.
Applied the load of 45 kg and allowed the needle to settle. And got the result.
Calculation
1
st
readings
HRc = 38
2
nd
reading
HRc = 93
3
rd
reading
HRc = 91
Result
Acceptable range for Rockwell hardness test on C scale is 19 – 55 beyond this range results
are suspected. In our case the material was soft so it should not be tested on C scale.
Advantages of Rockwell hardness
There are several reasons for the popularity of the Rockwell test.
 The test itself is very rapid. On a manually operated unit, a Rockwell test takes only
five to ten seconds, depending upon the size and hardness of the specimen, as well as
pre-load and dwell time.
 The indentation is extremely small and usually does not need to be removed by
machining, making this a non-destructive test. A

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 The Rockwell test is applicable to a wide range of part sizes. Sheet metal as thin as
0.006 inch can be tested on the Rockwell superficial tester, and as long as the surface
area is large enough, there is no actual limitation to the size of your specimen.
 The Rockwell test is based on measurement of the depth of penetration with the
hardness number read directly from the dial gauge or digital display that is part of every
tester.
 The Rockwell hardness test is an empirical indentation hardness test that can provide
useful information about metallic materials. This information may correlate to tensile
strength, wear resistance, ductility, and other physical characteristics of metallic
materials, and may be useful in quality control and selection of materials.
 Rockwell hardness tests are considered satisfactory for acceptance testing of
commercial shipments, and have been used extensively in industry for this purpose.
 Rockwell hardness testing at a specific location on a part may not represent the physical
characteristics of the whole part or end product.

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References
 ASM metal handbook volume 8 mechanical testing and evaluation
 Mechanical metallurgy by dieter
 Fundamental of Rockwell www.wilsoninstrument.com
 http://www.wilsonhardness.com/resources/testmethods/rockwelltesting.asx