Hardness Measurement

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. 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. 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. 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. 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. 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.