|
The Rockwell hardness test method procedure is described and specified
by the test method standards. To facilitate comparisons with other Rockwell
hardness data, the requirements of the standards should be adhered to. In
cases where the measurement of hardness is to meet a product or material
specification and must follow a particular test method standard document, the
test procedures must adhere to the requirements of the standard.
3.4.1 Set Appropriate Rockwell Scale
The Rockwell machine must be set up for testing the chosen Rockwell
scale, such that the appropriate indenter type and force levels are used. The
appropriate indenter and force levels, corresponding to each Rockwell scale,
are given in Figure 1 and by the test method standards.
Test Procedure
Good Practice Recommendations
• The user should confirm that the indenter chosen for testing has been
previously verified for use with the particular testing machine.
• Whenever the test forces, indenters or anvils are changed, a daily check
or verification (see 6.2) of the performance of the testing machine should
be performed using reference test blocks as described by the test method
standards. In cases where the anvil to be used cannot be used for testing
a test block (e.g., a V-anvil for testing round parts), then parts or test
specimens of known hardness that can be tested with the anvil should
be maintained by the user to perform the daily check. A daily verification
should be performed at least once each day of testing regardless of whether
the indenter, anvil, or forces are changed. The daily verification tests should
be performed after the indenter and/or anvil have been seated.
Testing Precautions
• Some older designs of Rockwell machines that apply the total force by
weights acting through a lever arm may require that the proper weights be
added or removed from a hanger rod. Be aware that, in some cases, the
weights have been calibrated for a specific hardness machine and may not
produce the correct forces on other machines.
• Care must be taken to not contact the indenter when installing or removing
an anvil. Many indenters are damaged in this way. If the anvil contacts
the indenter, the indenter should be inspected and performance verified
(see 6.2) prior to further testing.
3.4.2 Testing Cycle
The Rockwell testing cycle is the sequence of operations that the hardness
machine undergoes during a measurement. The testing cycle includes the rates
at which the forces are applied and the time periods that the forces are held
constant, referred to as dwell times. The Rockwell hardness testing cycle can
be separated into eight steps, as indicated in Figure 5. These steps fall into
two categories: (1) application or removal of test forces; and (2) dwell times.
Annex B provides expanded explanations of the individual effects that each of
the testing cycle steps has on the hardness result.
When used to test most materials, particularly metals, the Rockwell
hardness test is testing cycle dependent. By using different testing cycles,
the measurement will yield different hardness results. Because the Rockwell
test is testing cycle dependent, the hardness result is not complete unless the
testing cycle that was used is also known. This dilemma of obtaining different
hardness values for the same material is partially solved by adhering to test
method standards, which define tolerances on the testing cycle.
3.4.2.1 Application or Removal of Test Forces
The step in the Rockwell testing cycle where the preliminary force is increased
to the total force level (step 4 in Figure 5) has been shown
(15)
to significantly
affect the measured hardness value. By changing the rate that the force is
applied, particularly during the last part of the force application, a range of
hardness values can be obtained. The effect may be due either to rate
sensitivity of the material under test, or to the dynamics of the hardness tester,
or a combination of both. The magnitude of the rate effect is highly dependent
on the type and hardness of the test material. It is important that the test forces
are applied at rates in accordance with the test method standards. In both the
cases of too rapid loading or loading too slowly, the test measurement can be
adversely affected.
3.4.2.2 Dwell Times
Each of the three dwell time steps of the testing cycle affect the hardness
result because of creep or elastic recovery of the test material occurring during
these periods of constant force. The effects of the dwell times can be
summarized as:
1. Errors in the dwell time will produce the largest differences in hardness
measurement results when shorter dwell times are used. The user should
take this into account when choosing an appropriate test cycle. An increase
in testing speed may reduce the repeatability in measurement results.
2. In general, the Rockwell hardness number is most affected by the total
force dwell time, followed by the preliminary force dwell time, and then the
recovery dwell time. This depends somewhat on the hardness level of the
material.
Good Practice Recommendations
• When Rockwell hardness comparisons are to be made between two
laboratories, or two test machines, or even between two tests made on
the same hardness tester, the testing cycles that are used should agree
as closely as possible, particularly when short dwell times are used. How
close the test cycles should agree depends on the desired precision of the
hardness result. For example, in situations where the Rockwell hardness
measurement must only agree within several Rockwell hardness points,
perhaps any testing cycle within the specified ranges would be acceptable.
However, in cases where the results must have a close comparison, or there
is disagreement between laboratories, each Rockwell measurement should
be made using the same test cycle.
• When the testing machine design requires that the operator either fully or
partially perform the test procedure manually, the operator should make
every effort to operate the machine such that testing cycle requirements are
being met.
Testing Precaution
• In cases where the operator applies the preliminary force manually, such as
is common for older machines, the correct preliminary force level may be
overshot. The operator must not adjust back to the proper force. The error
to the measurement value has already occurred. In this situation, the test
should be stopped and a different location tested.
3.4.3 Seating the Anvil and Indenter
Prior to making Rockwell measurements, the hardness machine anvil and
indenter must be adequately seated. This may be accomplished by performing
standard Rockwell hardness tests on a material having a uniform hardness,
such as reference test blocks. The seating tests should be repeated until
the successive measurement values show no trend of increasing or
decreasing hardness.
3.4.4 Cleaning the Anvil and Indenter
The hardness machine anvil and indenter should be thoroughly cleaned per
manufacturer’s recommendations. In the absence of manufacturer’s cleaning
instructions, it is recommended that the anvil and indenter be cleaned with ethyl
alcohol and dried using a lint free cloth. Lastly, blow the surfaces clean of dust
using filtered air, such as from a commercial compressed air can or bottle.
Do not blow clean by mouth.
3.4.5 Placement and Removal of Test Material
Usually, material to be tested with a Rockwell hardness machine is placed
on the anvil by hand by the operator. In some cases, mechanical systems are
used to automatically place and remove samples. The contact area of the test
material and anvil must be clean without the presence of dust, dirt, or lubricant.
It is extremely important that the test material be well supported to prevent any
movement during the test.
Good Practice Recommendation
• When a spot anvil is used that is too small to support the test material
without assistance by the operator, the operator should carefully place
the test material onto the anvil so that it is flat against the anvil surface.
The operator should hold the material steady during the application of the
preliminary force, and release it just before the preliminary force is fully
applied. This type of testing requires a skilled operator that can perform
the test without applying any added force to the test from misalignment or
movement of the test sample.
Testing Precautions
• Care must be taken to not contact the indenter when placing the test
material on the support anvil and particularly when removing the test
material. Many indenters are damaged in this way. If the test material
contacts the indenter, the indenter should be inspected and performance
verified (see 6.2) prior to further testing.
Test Procedure
The test material must be placed on the anvil such that the anvil is not
scratched, indented, or damaged in any way.
3.4.6 Making the Measurement
As with most testing equipment and instrumentation, the operation of Rockwell
hardness machines varies from manufacturer to manufacturer and from model
to model. Depending on the machine model, the responsibility of the operator
can vary from manually applying and controlling each of the test forces to
simply pushing a button. The user should read and follow the recommended
operating procedures found in the manufacturer’s manual.
Testing Precautions
• The test material must not be held by hand during the testing process,
except as allowed when using the spot anvil (see above). Holding the test
material by hand can cause movement of the material during a test.
• During the testing process, the operator should avoid contact with the testing
machine, the test material, and the table or stand supporting the testing
machine, except when required to operate the machine. Contact can induce
shock and vibration that can affect the test.
• When testing curved parts, special care is needed to ensure that the
specimen support correctly aligns the part and prevents movement of the
part during a test.
3.4.7 Spacing of Indentations
As a Rockwell hardness measurement is being made, the material
deformation zone extends in all directions around the indentation. This
process typically increases the hardness of the deformation zone by inducing
residual stress and cold-working the deformed material. If a second indentation
is made near an existing indentation such that the deformation zone surrounding
the new indentation overlaps the hardened material surrounding the previous
indentation, then the apparent measured hardness likely will be erroneously
elevated. This effect is increased the closer two indentations are made to
each other until the indentations become so close that the wall of the original
indentation begins collapsing, likely lowering the apparent hardness.
The general rule as specified by the ASTM (2)test method standards is that the
distance between the centers of two indentations must be at least 3 times the
diameter of the indentation. The ISO test method standard
(3) specify that the distance be at least 4 times the diameter of the indentation (but not less than 2 mm). Although these are reasonable guidelines, tests have shown (15)That interaction with an adjacent indent can occur at these and greater distances.
Also, take into consideration that the effect will be multiplied by multiple
adjacent indents. The user should determine the appropriate distance for
the material to be tested.
3.4.8 Testing Curved Surfaces
Rockwell numbers obtained from measurements made on curved surfaces
must be corrected depending on the radius of curvature and whether the
surface is convex or concave. In the case of convex surfaces, such as the
outside of a cylinder, a correction value must be added to the test result to
increase the measured hardness value. This is because a convex surface
curves away from the indenter tip providing less surrounding material to
support the indenter than is the case for flat material. As a result, the indenter
penetrates the material more deeply and indicates a lower hardness than
the true value. Similarly, for concave surfaces, a correction value must be
subtracted from the test result to decrease the apparent hardness value. This
is because a concave surface curves towards the indenter tip, and provides
additional material to support the indenter than when testing flat material, and,
consequently, produces a shallower indentation and a higher hardness than
the true value. As the radius of curvature gets smaller, the error in the
measurement result becomes more pronounced requiring a larger correction
to be made.
The ASTM(2)and ISO(4)standards specify values for correcting tests made
on a few types of curved surfaces. The corrections given in test method
standards are to be considered approximations only. Both ASTM and ISO
give corrections for tests made on convex cylindrical surfaces. ISO also
provides limited corrections for testing on convex spherical surfaces. These
correction values are to be added to the measured hardness value to obtain an
approximation of the actual hardness of the material. If correction values for
concave surfaces are not available, the correction values given by the test
method standards for convex surfaces may be subtracted from the measured
value to provide a rough approximation of the material hardness. This
procedure for correcting tests on concave surfaces should only be used to
obtain an approximate value and not to meet a specification.
Good Practice Recommendation
• It is recommended that users develop their own correction values specific
for the type of material and radius of curvature that will be tested. This may
be done by testing samples of the same material in both the curved and flat
geometries, for example, by testing the curved surface and flat ends of a
cylinder. Be certain that the test surface conditions are the same for both
the curved and flat specimens.
Testing Precautions
• When testing curved parts, it is extremely important that care be taken to
ensure that the part is properly aligned such that the indentation is made
at the apex of a convex surface or at the bottom of a concave surface
(see 3.3.5). It is also extremely important to ensure that the part does not
move during testing.
• When applying correction values provided in the test method standards for
tests on curved surfaces, be certain that the corrections used are for the
same geometry as the test piece. Be aware that tests on surfaces that
curve in two axes, such as a sphere will require different corrections than
surfaces that curve in only one axis such as a cylinder.
• Depending on the hardness level, Rockwell tests should not be made on
curved surfaces below a certain radius of curvature due to the errors
associated with the large corrections that would be needed. ASTM and
ISO recommends that for Rockwell scales using a diamond indenter or a
1.588 mm (1/16 in) diameter ball indenter, regular Rockwell scale tests
should not be made on convex cylinders below 6.4 mm (1/4in) in diameter,
and superficial Rockwell scale tests should not be made on convex cylinders
below 3.2 mm (1/8in) in diameter. ISO also states that Rockwell tests on
the A, C, D, N, and T scales should not be made when the correction is
greater than three Rockwell units, and tests on the B scale should not be
made when the correction is greater than five Rockwell units.
3.4.9 Test Environment
The degree to which the testing environment affects the Rockwell hardness
test is generally difficult to quantify; however, three of the major environmental
factors that can contribute to measurement error are the testing temperature,
excessive vibration and general cleanliness.
Good Practice Recommendation
When choosing the location for installing a hardness machine, consider the
environmental conditions over the entire workday as well as seasonal changes
throughout the year.
3.4.9.1 Temperature
The test temperature can affect Rockwell hardness measurement results due
to two causes: (1) variations in the operation of the testing machine due to
temperature; and (2) temperature dependency of the test material. Variations
in the operation of the testing machine cannot be generalized for all Rockwell
testing machines. Because of the many designs of Rockwell hardness
machines having different principles of operation and instrumentation, it is
likely that each will have unique dependencies on temperature.
The temperature dependency of the test material will vary depending on
the type of material and the Rockwell scale that is used for testing. As an
indication of the typical magnitude of this effect, the following relationships
are provided. Yamamoto and Yano (16)determined that for their specific HRC
test blocks, the temperature dependence was -0.03 HRC/°C at 20 HRC,
-0.02 HRC/°C at 40 HRC and -0.01 HRC/°C at 60 HRC. W. Kersten (16)determined a similar relationship for the material he tested of -0.0185 HRC/°C,
independent of HRC level.
Good Practice Recommendations
• Placement of a Rockwell hardness machine in an area that will have to
operate over a wide range of temperatures should be avoided whenever
possible. To obtain the most repeatable results, the temperature of the
hardness machine and the test material should be maintained within a
narrow temperature range. The appropriate range is dependent on the
user’s needs. The test method standards state typical testing temperatures
within the range of 10 °C to 35 °C. The ISO test method standard requires
that a test temperature of (23 ± 5) °C be used when tests are carried out
under controlled conditions.
• For some industries, it is common for a Rockwell machine to be used in an
environment that is subject to wide temperature fluctuations. In these cases,
it is important to ensure that the Rockwell machine is capable of performing
within tolerances over the range of temperatures. This may be determined
by verifying the performance of the hardness machine with reference
blocks as the temperature of the testing environment changes. When
performing these verifications, it is desirable to separate any affect due
to the temperature dependency of the reference block material. To the
extent possible, prior to and during the verifications, the blocks should
be maintained near to the temperature at which they were calibrated.
However, condensation on the test block must be avoided.
• Although the hardness machine may operate satisfactorily over a wide
temperature range, the test material may also exhibit varying hardness
values at differing temperatures. Consequently, when the temperature
dependency of the test material is not known, it is recommended to report
the test temperature with the hardness measurement results when the
temperature is suspected to be a factor.
3.4.9.2 Vibration
The Rockwell test method standards warn the user to avoid making Rockwell
hardness measurements when the testing machine is subjected to excessive
vibration or shock. As with the other environmental factors, the degree to
which vibration may affect the hardness measurement is dependent on the
design of the testing machine.
Good Practice Recommendation
• Rockwell hardness machines should be placed on an isolated table or
workbench, which is not shared with other equipment.
• Testing locations susceptible to excessive vibration should be avoided such
as near machinery, near worker high traffic areas, on loading docks, or
adjacent to heavily traveled roads or railroad tracks.
3.4.9.3 Cleanliness
Many designs of Rockwell hardness machines are highly susceptible to
measurement errors when dust, dirt, or oil is deposited and accumulated on
machine components. A more critical problem can occur when these types
of contaminants adhere to the specimen support anvils, elevating screw, or,
in particular, to the indenter.
3.4.10 Reporting Results
Rockwell hardness numbers should be reported as required by the test method
standards using appropriate rounding techniques. The numeric value must be
followed by the symbol HR and the scale designation. For example, 64 HRC
represents a Rockwell hardness number of 64 on the Rockwell C scale, and
81 HR30N represents a Rockwell superficial hardness number of 81 on the
Rockwell 30N scale. The ISO test method standards state the additional
requirement that when a ball indenter is used, the scale designation is followed
by the letter “S" when using a steel ball and the letter “W" to indicate the use
of a tungsten carbide ball. For example, 72 HRBW represents a Rockwell
hardness number of 72 on the B scale measured using a tungsten carbide
ball indenter.
3.4.11 Conversion to Other Hardness Scales or Properties
There is no general method of accurately converting the Rockwell hardness
numbers determined on one scale to Rockwell hardness numbers on another
scale, or to other types of hardness numbers, or to tensile strength values.
Nevertheless, hardness conversion tables are published by ASTM
(17), in theliterature, and often by hardness equipment manufacturers. Such conversions
are, at best, approximations and, therefore, should be avoided except for
special cases where a reliable basis for the approximate conversion has been
obtained by comparison tests.
Back
| 
