There are many designs of commercially manufactured Rockwell hardness testing machines. The testing machines discussed in this Guide and specified by the referenced test method standards are limited to only those types of machines capable of performing a true “Rockwell indentation hardness test." Sometimes a true Rockwell test cannot be performed due to the size of the part or its configuration. There are other devices and instruments on the market that can be used in many of these situations, which can also report a Rockwell hardness number. However, the measurement methods used by these devices are not in accordance with the Rockwell indentation hardness principle. These devices employ other test principles, such as striker rebound or eddy-current, and make measurements to which a Rockwell number is correlated. These devices may have some advantages, such as portability, but they cannot report a true Rockwell hardness number.
There have been many improvements in the designs of Rockwell hardness testers over the past 50 years. The most significant improvements have been in the manner in which the forces are applied, the manner in which the indentation depth is measured and the hardness value displayed, and in the automation of the testing machine’s operation. Remarkably, many of the older designs of Rockwell machines are still in use, so that a brief discussion of the differences may be beneficial.
Good Practice Recommendation
Not all Rockwell hardness machines are equal. All machines may be capable of performing a Rockwell hardness test in accordance with the requirements specified in test method standards, but some may be more suitable for your specific needs. When choosing a Rockwell hardness machine, consider factors such as: the accuracy and measurement repeatability that is required; whether versatility in the testing cycle may be required; the required speed of testing; the Rockwell scales that will be used; the required resolution of the hardness number; the size of material normally tested; and the accessories that may be needed.
Testing Precautions
When using devices that employ measurement methods other than the Rockwell indentation hardness principle, the type of measurement device that was used should be reported with the correlated Rockwell numbers. This information provides the user of the measurement data a better understanding of how the data was obtained.
3.3.1 Scales That Can Be Tested
Because the regular Rockwell and superficial Rockwell tests use distinctly different levels of force and two different resolutions of depth measurement most Rockwell machines in the past were designed to test only regular scales or superficial scales. This has become less true today as new machine development has produced many Rockwell machines designs that are capable of testing both regular and superficial scales, sometimes referred to as “twin testers" or “combination testers." These machines usually can test all of the different Rockwell scales, and, in some cases, they can also perform other types of hardness tests.
3.3.2 Force Application Mechanism
Since its development, the most common designs of Rockwell machines have applied the preliminary test force by compression of a helical spring, and have applied the total force by dead weights through a force multiplying lever system. With many years of usage, it is not unusual to find that in older machines the preliminary force springs and the knife-edges supporting the total force lever arms have become worn causing errors in the application of the forces.
With the advent of reliable electronically controlled feedback systems, new machine designs have been developed such as machines that apply the forces with a screw-driven device controlled by a load-cell to monitor the applied force. The new designs have the advantage that the testing cycle can be fully controllable, and errors associated with a lever arm or preliminary force spring are eliminated; however, different errors may be introduced associated with the load-cell or electronics. Lever-arm/spring design machines are continually being improved and are in common use today as reliable testing instruments, but the trend of many Rockwell machine manufacturers is towards developing load-cell design machines.
By varying either the preliminary force level or the total force level, different Rockwell hardness measurement values can be obtained for the same material. The reason for this is illustrated in Figure 2A, Figure 2B, Figure 3A, and Figure 3B, which are plots of Rockwell A scale (HRA) test data measured at NIST(14). Figure 2A illustrates the sequence of how the test forces are applied during the HRA test, with the resulting indentation depth shown in Figure 2B. Each figure shows two overlapping HRA tests; the solid line represents a test using the standard preliminary force of 98.07 N (10 kgf), and the dashed line represents a test where the preliminary force was increased to 103.95 N (10.6 kgf). The test having the higher preliminary force (dashed line) resulted in a slightly increased indentation depth at the first application of preliminary force. Changing the preliminary force level appears to have had negligible effect on the remaining part of the hardness test. Thus, an increase in the level of the preliminary force causes an increase in the indentation depth at the first application of preliminary force. This reduces the measurement value, Rockwell Hardness Testing Machine h, used for the calculation of the
Rockwell hardness number and results in a higher hardness value. For the same reasons, a decrease in the level of the preliminary force results in a lower hardness value,
Figure 3A and Figure 3B illustrate what occurs when the total force level is increased. The test having the higher total force (dashed line) resulted in an increased indentation depth at the application of total force. Following the application of total force, as the additional force is removed returning to the preliminary force level, most of the increased increment in indentation depth is maintained. The increased indentation depth enlarges the measurement value, h, and, thus, results in a lower hardness value. This is the opposite effect of that discussed previously (shown in Figure 2) for an increase in the preliminary force level. Additional tests have shown the two effects to be essentially independent of each other and, therefore, additive in their effect. The magnitudes of the effects that changes in the preliminary and total forces have on the Rockwell hardness measurement value are given in Appendix A for the Rockwell scales that use a diamond indenter and the Rockwell scales that use a 1.588 mm (1/16 in) diameter ball indenter. Also in Appendix A, data is presented illustrating the magnitude of measurement variation that can be obtained for the Rockwell scales that use a diamond indenter while maintaining the force levels within the ASTM and ISO tolerances. From this data it is seen that a variation of ± 0.5 Rockwell units can easily be achieved for some hardness levels simply by adjusting the force levels within the acceptable tolerance limits.
3.3.3 Depth Measurement; Hardness Value Calculation and Display
The dial indicating-gage was the original method used in Rockwell machines for measuring the indentation depth and for calculating and displaying the Rockwell hardness number. Due to the simplicity of its operation, it continues to be used in some of today’s Rockwell machine designs. The general principle of its operation is to mechanically measure the movement of the indenter through a multiplying lever system. The dial face is calibrated to indicate the Rockwell number corresponding to the displacement of the indenter. Usually, the dial divisions have represented whole Rockwell numbers, allowing an estimation of the hardness number to only ½ Rockwell unit. Over years of use, dial gages and lever systems often become worn or misaligned in many machines, adding a component of error to the Rockwell measurement.
Many Rockwell machines produced today use one of several different types of electronic or optical displacement-measuring instruments for directly measuring the depth of indentation. The signal from the measuring instrument is electronically converted to a Rockwell hardness number, which is displayed digitally, sometimes having a resolution of 0.01 Rockwell units. Typically, these new displacement-measuring instruments have a greater accuracy than most dial gage/lever systems, but as often happens with digital displays, showing a number with many decimal places may imply a greater accuracy than is possible with the instrumentation.
The formulas for calculating Rockwell hardness, as given in 2.4 above, directly relate the measured depth of the indenter to the Rockwell hardness number Consequently, an error in the depth measurement relates to an error in the hardness measurement result as: For regular Rockwell scales:0.002 mm error in depth =1 HR unit error. For superficial Rockwell scales: 0.001 mm error in depth =1 HR unit error. Both the ASTM(2) and ISO(4) standards specify that the depth measuring system have an accuracy of at least 0.5 Rockwell numbers.
3.3.4 Manual and Automatic Operation
For many years, most designs of Rockwell hardness machines required that the operator manually apply and remove the preliminary and total forces. This allowed the operator a great deal of control over the testing cycle; however, consistency in the testing cycle varied between operators. The manual operation also was considered to take too much time for production testing.
Eventually, motors were incorporated into Rockwell machine designs to provide an automated and repeatable testing cycle. Some machines were fully automated to drive the application of the forces at a higher rate than was typical for a person. The increased rate of testing is considered important for production testing, but the automated operation removes much of the control by the user. For many of the earlier automatic machines, the operator could not vary the testing cycle. This was good in one respect, it retained consistency from operator to operator; however, the testing cycle was usually set by the manufacturer to complete a test in a relatively short time, with fast force application rates and short dwell times. In following discussions of the Rockwell testing cycle, it will be shown that fast force application rates and short dwell times can lead to poor measurement repeatability.
Recognizing that many testing applications required better measurement repeatability, as well as control of the testing cycle due to varying material plasticity, manufacturers of automatic machines began modifying their designs to allow the operator to adjust the testing cycle. Many of today’s Rockwell machines can be set to a “standard" test cycle, while also allowing the testing cycle to be adjusted to better fit the users’ needs.
3.3.5 Test Material Support (Anvils)
One of the most important requirements for making a valid Rockwell hardness test is that the test material be well supported to prevent any movement during the test. Even the slightest movement can significantly alter the hardness result. If the test material moves during the test, the movement may be reflected as an error in the depth measurement. Bear in mind that for a Rockwell superficial test, an error in the depth measurement of one-hundredth of a millimeter will produce an error of 10 Rockwell points (see 3.3.3 above).
There are many types of material supports or anvils available for testing different shapes and sizes of test material. The test method standards provide some guidance for selecting an appropriate anvil. In general, flat material should be tested on a flat anvil. Material that is curved should be tested with the convex surface supported on a V-shaped or a double-roller style anvil. Small or thin samples, sheet metal, or parts that do not have flat under-surfaces should be tested on a spot anvil having a small, elevated, flat bearing surface. There are some Rockwell machine designs that apply a clamping force to the test material that is greater than the Rockwell test force. This type of machine is useful when testing larger parts.
Good Practice Recommendations
- Often overlooked sources of error in Rockwell testing are the anvil and anvil seat. A dirty anvil seat and almost any perceptible flaw on the anvil and anvil seat, such as scratches or indents, can significantly affect the hardness result. The anvils and the anvil seat should be routinely cleaned and inspected for damage and replaced or reground when damage occurs.
- When testing large samples of test material or material with a long shape that significantly overhangs the hardness machine’s anvil support, the material should be additionally supported using suitable outboard fixtures. Otherwise, the overhang may cause a cantilever or lateral force to be applied to the indenter, resulting in measurement error or damage to the indenter. These types of parts should not be supported by hand.
- It is very important that the method used to attach an anvil to a Rockwell machine prevents any rocking or other movement of the anvil during the test. Many Rockwell machine designs attach the anvil by inserting its base into a slip fitting. This design is suitable for most purposes, although for critical applications, it may be beneficial to rigidly affix the anvil to the testing machine.
- Each time an anvil is installed, regardless of its design, it must be adequately seated to the testing machine by making repeated hardness tests on a uniform piece of material, such as a test block. Repeat the tests until there is no increasing or decreasing trend in the measured hardness values.
- When testing curved parts, it is extremely important 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. The proper alignment of a V-shaped or Rockwell Hardness Testing Machine anvil may be checked by first making one Rockwell test on a cylindrical piece, then, after rotating the anvil 90 ° without moving the test piece, make a second test. If the second test falls exactly at the same location as the first test, the alignment of the indenter is likely satisfactory.
Testing Precaution
- The anvil must present the material test surface perpendicular to the indentation direction of the indenter. If the test surface is tested at an angle with respect to the indentation direction, the measurement will be adversely affected, usually lowering the measured value from the true hardness.
3.3.6 Hysteresis
Each time a Rockwell hardness test is made, the testing machine will undergo flexure in some of the machine components including the machine frame. If the flexure is not entirely elastic during the application and removal of the additional force, the testing machine may exhibit hysteresis in its flexure. Since the indenter-depth measurement systems of most Rockwell hardness machines are directly connected to the machine frame, any hysteresis would be reflected in the indenter-depth measurement system. A hysteresis effect can also occur in the indenter-depth measurement system itself as the direction of measurement reverses after applying the total force. In both cases, the hysteresis is likely to result in an offset or bias in the test result.
Testing Precautions
- Excessive hysteresis may indicate problems with the Rockwell machine caused by worn or dirty parts, such as in the depth measurement system, the elevating screw and anvil seat.
3.3.7 Repeatability
The repeatability of a hardness machine is its ability to obtain the same hardness measurement result on an ideally uniform material over a short period of time where the test conditions (including the operator) do not vary. Imagine a material that is perfectly uniform in hardness, which has been ideally prepared for Rockwell hardness testing. If a small number of Rockwell tests were made repeatedly on this material, it would be found that the measurement results were likely not identical, but rather they varied randomly over a range of values. The degree to which the measurement values agree provides an indication of the repeatability of the Rockwell hardness machine. As with most measuring devices, no matter how much effort is made to eliminate the sources of this random variability, it is impossible to do away with completely. Test Procedure machines exhibit some level of lack of repeatability, which sporadically adds error to measurement values. Whereas, errors in force, depth, and hysteresis are typically systematic errors that contribute to a bias in the hardness measurement, lack of repeatability is a randomly occurring error. The lack of repeatability will typically increase in instances such as when parts of the hardness machine are worn, when excessive friction is occurring during a test, or when the machine requires cleaning. The level of repeatability of a hardness machine often varies between different Rockwell scales due to variances such as the force levels and types of indenters. The repeatability may also vary at different hardness levels within the same scale due to the variations related to differing indentation depths.
The ASTM(2) and ISO (4) standards specify a method for assessing the lack of repeatability of a Rockwell machine, which involves making hardness measurements across the surface of reference test blocks (see 5.2.1). The acceptability of the testing machine is determined from the difference between the maximum and minimum measured hardness values. Satisfactory tolerances on this measure of repeatability vary from 1.0 to 2.0 Rockwell units for ASTM and from 1.2 to 6.6 units for ISO, depending on the Rockwell scale and hardness level.
3.3.8 Indenters
The indenter is a major contributor to Rockwell hardness measurement error. Both the spheroconical diamond indenter and the ball indenter have characteristics that can cause significant measurement biases. In fact, indenter measurement bias has often been used to offset other measurement errors associated with the hardness machine. Like hardness machines, the measurement performance of a Rockwell indenter is dependent on more than its physical parameters. Differences in indenter performance may also be due to the indenter’s manufacturing process. Two indenters with virtually the same shape may produce significantly differing hardness measurements. It is recommended that the indenters to be used be certified for performance with respect to a higher-level master indenter. In the past, an often-used procedure to certify Rockwell indenters was to make hardness tests on reference test blocks, and compare the measurement to the block value. When using this procedure, if the indenter performance did not agree with the block value, it was difficult to determine whether the source of the error was due to the indenter, the standardizing machine, the reference block values, or some combination of these variables.
The test method standards state acceptability tolerances for the performance of diamond indenters. ASTM(2)allows the performance to deviate from 0.5 to 1.0 Rockwell units from test block values, depending on the hardness level. ISO(4)allows the performance to deviate 0.8 Rockwell units from the performance of a reference indenter. There are currently no requirements for the performance of ball indenters in either ASTM or ISO standards. It should be noted that a Rockwell indenter has formally been referred to as a “penetrator" or “stylus."
There are several different designs currently used for the base (opposite end of the indentation tip) of Rockwell indenters because of the varying styles of indenter holders found on different manufacturer’s hardness machines. Indenters may be attached to machines using such methods as slip fittings, threaded fixtures, or with a collet fixture. Not all indenter designs can be used with all holder styles. Whatever method is used, it is imperative that there is no movement of the indenter in its holder during a test.
Good Practice Recommendations
- Indenters should be used that are certified to be within tolerances for both shape (geometry) and performance with respect to a reference indenter. This applies to all types of Rockwell indenters. In the past, it was common for diamond indenters to be certified for performance only.
- Only indenters should be used that have been verified for use with the particular Rockwell machine, such as during an indirect verification (see 5.2). In cases that other indenters must be used, they should be verified in some manner for use with the testing machine. The best verification method is to perform a full indirect verification of the applicable Rockwell scales using the indenter in question. Other verification techniques may also be appropriate.
- Periodically, indenters should be visually inspected for damage with the aid of adequate magnification (20X or higher).
- Every effort should be made to keep indenters clean, particularly the indenting portion and the surface that is seated against the testing machine. Indenters should be cleaned periodically in a manner that will not leave residue on the indenting portion of the tip.
- Each time an indenter is installed, regardless of its design, its seating surface must be adequately seated against the indenter holder by making repeated hardness tests on a uniform piece of material, such as a test block. Repeat the tests until there is no increasing or decreasing trend in the measured hardness values.
Test Procedure
Testing Precaution
- If an indenter is dropped or hit with the test piece or anvil, it is imperative that before using it further, it should be thoroughly inspected for damage and verified for performance for each Rockwell scale that is used. Performance verification is necessary because the measuring ability of an indenter, particularly a diamond indenter, can change significantly without any outward visible signs of damage.
3.3.8.1 Spheroconical Diamond Indenter
The Rockwell diamond indenter is used with the HRA, HRC, HRD, HR15N, HR30N, and HR45N scales. The diamond indenter scales are typically used when testing harder materials such as steel, tungsten, and cemented carbides. Diamond is needed for testing hard materials to ensure that the indenter itself does not deform during the indentation process. Any permanent deformation of the indenter would adversely affect the hardness measurement of the test material. A typical Rockwell diamond indenter consists of a metal holder into which a diamond tip is permanently attached. The diamond tip is specified by test method standards to have a spheroconical geometry with a 120 ° included cone angle and a 0.2 mm radius tip, with the cone and radial tip blending in a tangential manner as illustrated in Figure 4.
There are several error sources that can affect the measurement performance of the Rockwell diamond indenter. Some error sources are obvious, and others are difficult to determine. The most common error source is an incorrectly shaped spheroconical diamond tip. In the past, this commonly occurred because diamond is very difficult to machine into the spheroconical geometry, and, until recently, many indenter manufacturers did not have adequate instruments to accurately measure the diamond shape. Common practice in the manufacture of diamond indenters was to machine the diamond shape close to nominal, and then certify the indenter only by performance testing with little or no actual direct verification of its geometry. Increasingly, today’s manufacturers have developed the capabilities to accurately measure the indenter geometries and detect variations that are out of tolerance.
Form errors in the indenter shape often translate into significant errors in the hardness measurement. This is because a Rockwell hardness value is related to the volume of material displaced by the indenter during the application of the Rockwell test forces. The displaced volume is related to how deep the indenter penetrates the material. If two Rockwell tests are made using indenters having similar but slightly different geometries, essentially the same volume of material will be displaced, but the depth of indentation will vary, and, thus, the calculated Rockwell hardness value will be different.
Rockwell Hardness Testing Machine
If a series of Rockwell hardness tests is made on a number of materials ranging progressively from soft to hard, then as the material hardness increases, less of the diamond tip penetrates the material. Therefore, depending on the hardness of the test material, errors in the cone angle or tip radius will cause varying degrees of error in the hardness measurement. Because harder materials produce shallower penetration depths, the test material is primarily in contact with the radial tip, which will have the greater influence on measurement error. The cone angle will have a greater influence for softer materials exhibiting deeper indentations, since the test material is being displaced by more of the conical portion of the diamond.
Other sources of error include form error at the tangential blend, the surface roughness of the diamond, the alignment of the indenter axis with respect to the seating surface of the indenter to the test machine, a poorly machined seating surface, and hysteresis in the indenter itself as it is loaded and unloaded, possibly due to problems with the interface between the diamond and the metal portion of the indenter. Many of these indenter problems may produce measurement errors that will vary depending on the hardness scale used, the hardness level of the test material, or the type of test material. Consequently, Rockwell diamond indenters are sometimes certified for specific Rockwell scales.
Good Practice Recommendation
If possible, a diamond indenter should be chosen that is certified for each Rockwell scale that will be used or as many scales as possible. To obtain the highest accuracy, use of more than one diamond indenter may be desired, each certified for specific Rockwell scales. This allows an indenter to be chosen that may agree more closely with the performance of a reference indenter for a specific Rockwell scale, even though the performance is not as close (or possibly not acceptable) for other diamond scales. In the United States, Rockwell diamond indenters are sometimes designated as being a “C," “N," or “A" indenter. Usually, these designations mean the following: a “C" indenter is appropriate for use with the regular Rockwell scales (HRA, HRC, HRD), a “N" indenter is appropriate for the superficial Rockwell scales (HR15N, HR30N, HR45N), and an “A" indenter usually refers to being acceptable for testing carbides at the high end of the HRA scale. Be aware that the ISO test method requires that each diamond indenter be performance certified for all Rockwell scales requiring a diamond indenter.
3.3.8.2 Ball Indenters
Rockwell ball indenters are used with all Rockwell scales with the exception of the A, C, D, and N scales for which the diamond indenter is used. Typically, ball indenters are used when testing materials such as soft steels, copper alloys, aluminum alloys, and bearing metals. There are four standard sizes of ball indenters specified by ASTM(2)having diameters of 1.588 mm (1/16in),3.175 mm (1/8in), 6.350 mm (1/4in), and 12.70 mm (1/2in). The ISO(4)specifies only the 1.588 mm (1/16in) and 3.175 mm (1/8in) diameter balls. The choice of indenter size, and, thus, hardness scale, is largely based on the hardness and thickness of the test material. Generally, the ball size is increased for thinner and softer materials. A typical Rockwell ball indenter consists of a metal holder for the ball with a threaded cap to hold the ball in place.
Rockwell indenter balls can be made of either steel or tungsten carbide (WC). In the past, most Rockwell hardness testing with ball indenters has used steel balls, typically bearing balls; however, there is currently a general move towards the use of tungsten carbide balls. Presently in the year 2000, ASTM specifies steel balls as the standard indenter, and, until recently, ISO had required that Rockwell tests be performed using only steel balls but now allows the use of tungsten carbide balls. A problem with steel balls is that they tend to flatten over time at the contact point with the test specimen, particularly when testing harder materials. An indenter with a flattened ball will not penetrate as deeply into test materials, indicating an apparent higher hardness for the material. The tungsten carbide ball was introduced to help overcome this problem. The harder tungsten carbide is much less susceptible to flattening than steel balls.
Tests have indicated(14) that the use of tungsten carbide ball indenters may result in a lower hardness measurement than when a steel ball indenter is used. This may be partly due to differences in the compliance of the two ball materials. Fortunately, the publishers of the ISO standard also require that the measurement values be reported with a scale designation ending in the letter “S" when a steel ball is used or “W" when a tungsten carbide ball is used.
Rockwell Hardness Testing Machine
Although this designation differentiates between tests made with the two indenters, users of the measurement data must be aware that measurement differences may occur.
Good Practice Recommendation
When steel ball indenters are used, it is important that performance verification checks with reference test blocks be made frequently. This is because of the tendency of the steel ball to flatten over time, particularly when testing harder materials. Since the flattening may increase gradually, the performance of the indenter should be consistently monitored at a rate appropriate for the usage of the indenter and the hardness level of the material tested.
Testing Precaution
A steel ball can be flattened quickly if a test is mistakenly made on a material above the appropriate hardness range (over 100 HRB) or if the indenter is hit by the anvil or is used to test too thin material.
When testing very soft materials, it is important to ensure that the design of the indenter cap allows adequate protrusion of the ball. Otherwise, the cap may contact the test material, preventing full penetration into the test material, and result in an erroneously high hardness value. Be aware that it is possible for the cap to contact the test material without any physical indication on the surface of the test material.
