Volume 08 - Mechanical Testing and Evaluation Part 4 docx

number is based on the average of these two measurements Table provides a simple way to convert the indentation diameter to the Brinell hardness number The indentations produced in Brinell hardness tests may exhibit different surface characteristics In some instances there is a ridge around the indentation that extends above the surface of the workpiece In other instances the edge of the indentation is below the original surface Sometimes there is no difference at all The first phenomenon, called “ridging,” is illustrated in Fig 13(a) The second phenomenon, called “sinking,” is illustrated in Fig 13(b) An example of no difference is shown in Fig 13(c) Cold-worked metals and decarburized steels are those most likely to exhibit ridging Fully annealed metals and light case-hardened steels more often show sinking around the indentation Fig 13 Sectional views of Brinell indentations (a) Ridging-type Brinell impression (b) Sinking-type Brinell impression (c) Flat-type Brinell impression The Brinell hardness number is related to the surface area of the indentation This is obtained by measuring the diameter of the indentation, based on the assumption that it is the diameter with which the indenter was in actual contact However, when either ridging or sinking is encountered there is always some doubt as to the exact part of the visible indentation with which the actual contact was made When ridging is present, the apparent diameter of the indentation is greater than the true value, whereas the reverse is true when sinking occurs Because of the above conditions, measurements of indentation diameters require experience and some judgment on the part of the operator Experience can be gained by measuring calibration indents in the standardized test block Even when all precautions and limitations are observed, the Brinell indentations for some materials vary in shape For example, materials that have been subjected to unidirectional cold working often exhibit extreme elliptical indentations In such cases, where best possible accuracy is required, the indentation is measured in four directions approximately 45° apart, and the average of these four readings is used to determine the Brinell hardness number Other techniques such as Rockwell-type depth measurements are often used with highproduction equipment Semiautomatic Indent Measurements In an effort to reduce measurement errors, image analysis systems are available for the measurement of the indent area The systems normally consist of a solid-state camera mounted on a flexible probe, which is typically manually placed over the indent (Fig 14) A computer program then analyzes the indent and calculates the size and Brinell number The advantage of these systems is that they can reduce the errors associated with the optical measurements done by an operator The surface finish requirements are frequently higher as the computer can have difficulty measuring noncircular indents or jagged edges for which an experienced operator could make judgments and correct as needed Fig 14 Computerized Brinell hardness testing optical scanning system General Precautions and Limitations To avoid misapplication and errors in Brinell hardness testing, the fundamentals and limitations of the test must be thoroughly understood The following precautions should be observed before testing Thickness of the testpiece should be such that no bulge or other marking showing the effect of the load appears on the side of the piece opposite the impression The thickness of the specimen should be at least ten times the depth of the indentation Depth of indentation may be calculated from the formula: where P is load in kgf, D is ball diameter in mm, and HB is Brinell hardness number For example, a reading of 300 HB indicates: Therefore, the minimum thickness of the workpiece is 10 × 0.32 or 3.2 mm (0.125 in.) Table gives minimum thickness requirements Table Minimum thickness requirements for Brinell hardness tests Minimum thickness of specimen mm in 1.6 0.0625 3.2 0.125 Minimum hardness for which the Brinell test may be made safely 3000 kgf load 1500 kgf load 500 kgf load 602 301 100 301 150 50 4.8 0.1875 201 100 33 6.4 0.250 150 75 25 8.0 0.3125 120 60 20 9.6 0.375 100 50 17 Test surfaces that are flat give best results Curved test surfaces of less than 25 mm (1 in.) radius should not be tested Spacing of Indentations For accurate results, indentations must not be made near the edge of the workpiece Lack of sufficient supporting material on one side will result in abnormally large, unsymmetrical indentations In most instances the error in Brinell hardness number will not be significant if the distance from the center of the indentation to any edge of the workpiece is more than three times the diameter of the indentation Similarly, Brinell indentations must not be made too close to one another The first indentation may cause cold working of the surrounding area that could affect the subsequent test if made within this affected region It is generally agreed that the distance between centers of adjacent indentations should be at least three times the diameter of the indentation to eliminate significant errors Anviling The part must be anviled properly to minimize workpiece movement during the test and to position the test surface perpendicular to the test force within 2° Surface Finish The degree of accuracy attainable by the Brinell test can be greatly influenced by the surface finish of the workpiece The surface of the workpiece should be milled, ground, or polished so that the indentation is defined clearly enough to permit accurate measurement Care should be taken to avoid overheating or cold working the surface, as that may affect the hardness of the material In addition, for accurate results, the workpiece surface must be representative of the material Decarburization or any form of surface hardening must be removed prior to testing Testing Machines Various kinds of Brinell testers are available for laboratory, production, automatic, and portable testing These testers commonly use deadweight, hydraulic, pneumatic, elastic members (i.e., springs), or a closed-loop loadcell system to apply the test loads All testers must have a rigid frame to maintain the load and a means of controlling the rate of load application to avoid errors due to impact (500 kgf/s maximum) The loads must be consistently applied within 1.0% as indicated in ASTM E 10 In addition, the load must be applied so that the direction of load is perpendicular to the workpiece surface within 2° for best results Bench units for laboratory testing are available with deadweight loading and/or pneumatic loading Because of their high degree of accuracy, deadweight testers are most commonly used in laboratories and shops that low- to medium-rate production These units are constructed with weights connected mechanically to the Brinell ball indenter Minimum maintenance is required because there are few moving parts Figure 15(a) is an example of a motorized deadweight tester Fig 15 Bench-type Brinell testers (a) Motorized tester with deadweight loading Courtesy of Wilson Instruments (b) Brinell tester with combined deadweight loading and pneumatic operation Courtesy of NewAge Industries Bench units are also available with pneumatic load application or a combination of deadweight/pneumatic loading Figure 15(b) shows an example of the latter, where the load can be applied by release of deadweights or by pneumatic actuation In both deadweight and pneumatic bench units, the testpiece is placed on the anvil, which is raised by an elevating screw until the testpiece nearly touches the indenter ball Operator controls initiate the load, which is applied at a controlled rate and time duration by the test machine The testpiece is then removed from the anvil, and the indentation width is measured with a Brinell scope, typically at 20× power Testing with this type of apparatus is relatively slow and prone to operator influence on the test results Machines for Production Testing Hydraulic testers were developed to reduce testing time and operator fatigue in production operations Advantages of hydraulic testers include operating economy, simplicity of controls, and dependable accuracy The controls prevent the operator from applying the load too quickly and thus overloading The load is applied by a hydraulic cylinder and monitored by a pressure gage Normally the pressure can be adjusted to apply any load between 500 and 3000 kgf Hydraulic machines for production are available as bench-top or floor units (Fig 16) Fig 16 Hydraulic Brinell tester Courtesy of Wilson Instruments Automatic Testers Many types of automatic Brinell testers are currently available Most of these testers (such as the one shown in Fig 17) use a depth-measurement system to eliminate the time-consuming and operatorbiased measurement of the diameters All of these testers use a preliminary load (similar to the Rockwell principle) in conjunction with the standard Brinell loads Simple versions of this technique provide only comparative “go/no-go” hardness indications; more sophisticated models offer a microprocessor-controlled digital readout to convert the depth measurement to Brinell numbers Conversion from depth to diameter frequently varies for different materials and may require correlation studies to establish the proper relationship Fig 17 Automatic Brinell hardness tester with digital readout Courtesy of NewAge Industries These units can be fully automated to obtain production rates up to 600 tests per hour and can be incorporated into in-line production equipment The high-speed automatic testers typically comply with ASTM E 103, “Standard Method of Rapid Indentation Hardness Testing of Metallic Materials.” Portable Testing Machines The use of conventional hardness testers may occasionally be limited because the work must be brought to the machine and because the workpieces must be placed between the anvil and the indenter Portable Brinell testers that circumvent these limitations are available A typical portable instrument is shown in Fig 18 This type of tester weighs only about 11.4 kg (25 lb), so it can be easily transported to the workpieces Portable testers can accommodate a wider variety of workpieces than can the stationary types The tester attaches to the workpiece as would a C-clamp with the anvil on one side of the workpiece and the indenter on the other For very large parts, an encircling chain is used to hold the tester in place as pressure is applied Fig 18 Hydraulic, manually operated portable Brinell hardness tester Portable testers generally apply the load hydraulically, employing a spring-loaded relief valve The load is applied by operating the hydraulic pump until the relief valve opens momentarily With this type of tester, the hydraulic pressure should be applied three times when testing steel with a 3000 kgf load This is equivalent to a holding time of 15 s, as required by the more conventional method For other materials and loads, comparison tests should be made to determine the number of load applications required to give results equivalent to the conventional method A comparison-type tester that uses a calibrated shear pin is shown in 19Fig 19 In this method, a small pin of a known shear load is placed in the indenter assembly against the indenter (Fig 19b) Through hammer impact or static clamping load, the indenter is forced into the material only as far is it takes to shear the pin Excessive force is absorbed after shear by upward movement of the indenter into an empty cavity The resulting impression is measured by the conventional Brinell method This method does not comply with ASTM E 10 Fig 19 Pin Brinell hardness tester (a) Clamp loading tester (b) Schematic of pin Brinell principle Equipment Maintenance To maintain accurate results from Brinell testing, equipment must be calibrated and serviced regularly, especially when machines are exposed to shop environments The frequency of servicing depends on whether the testers are used in a production line or for making an occasional test However, it is important that they be serviced and calibrated on a regular basis Regular checking of the ball indenter for deformation is particularly important Indenters are susceptible to wear as well as to damage When an indenter becomes worn or damaged so that indentations no longer meet the standards, it must be replaced Under no circumstances should attempts be made to compensate for a worn or damaged indenter Verification of Loads, Indenters, and Microscopes As with any procedure that is dependent on several components, the accuracy of each must be verified to determine the accuracy of the result In the case of Brinell hardness testing, load, indenter, and microscope accuracies must lie within a specified tolerance to ensure accurate results Load Verification ASTM E 10 specifies that a Brinell hardness tester is acceptable for use over a load range within which the load error does not exceed ±1% Test loads should be checked by periodic calibration with a proving ring or load cell, the accuracy of which is traceable to the National Institute of Standards and Technology (NIST) Proving rings (see Fig 20) are an elastic calibration device that is placed on the anvil of the tester The deflection of the ring under the applied load is measured either by a micrometer screw and a vibrating reed or a reading dial gage The amount of elastic deflection is then converted into load in kilograms and compared with required accuracies Fig 20 Proving rings used for calibrating Brinell hardness testers Ball Indenter Verification The ball indenter must be accurate within ±0.0005 mm of its nominal diameter It is very difficult for the user to measure the ball in enough locations to guarantee the correct shape Therefore, a close visual inspection is normally done, and any sign of damage will require replacement A performance test (indirect verification) using test blocks is the best way to verify the ball When in doubt, the ball should be replaced with a new ball certified by the manufacturer to meet all of the requirements in ASTM E 10 Microscope Verification The measuring microscope or other device used for measuring the diameter of the impression should be verified at five intervals over the working range by the use of a scale of known accuracy such as a stage micrometer The adjustment of the micrometer microscope should be such that, throughout the range covered, the difference between the scale divisions of the microscope and of the calibrating scale does not exceed 0.01 mm Verification by Test Block (Indirect Verification) Standardized Brinell test blocks are available so that the accuracy of the Brinell hardness tester can be indirectly verified at the hardness level of the work being tested Commonly available hardnesses are: Test block material Hardness, HB Steel 500, 400, 350, 300, 250, 200 Brass 90 Aluminum 140 Good practice is to verify the tester throughout the hardness range encountered This ensures that all test parameters are within tolerance Application for Specific Materials As is true for other indentation methods of testing hardness, the most accurate results are obtained when testing homogeneous materials, regardless of the hardness range Steels Virtually all hardened-and-tempered or annealed steels within the range of hardness mentioned provide accurate results with the Brinell test However,a s a rule, case-hardened steels are totally unsuitable for Brinell testing In most instances, the surface hardness is above the practical range and is rarely thick enough to provide the required support for a Brinell test Thus, “cave in” results, and grossly inaccurate readings are obtained Cast Irons The large area of the test serves to average out the hardness difference between the iron and graphite particles present in most cast iron This averaging effect allows the Brinell test to serve as an excellent qualitycontrol tool Nonferrous metals (especially the wrought types) are generally amenable to Brinell testing, usually with the 500 kgf load, but occasionally with the 1500 kgf load Some high-strength alloys such as titanium- and nickel-base alloys that are phase-transformation- or age-hardened can utilize the 3000 kgf load In this situation, practical limits must be observed and some testing may be required to establish the optimal technique for testing a specific metal or alloy There are certain multiphase cast nonferrous alloys that are simply too soft for accurate Brinell testing Microhardness testing is then employed The lower limit of 16 HB with a 500 kgf load must always be observed Powder Metallurgy Parts Testing of P/M parts with a Brinell tester (or any sort of macro-hardness tester) involves the same problem as encountered with cast iron Instead of a soft graphite phase (some P/M parts also contain free graphite), P/M parts contain voids that may vary widely in size and number Light-load Brinell testing is sometimes used successfully for testing of P/M parts, but its only real value is as a quality-control tool in measuring the apparent hardness of P/M parts (see the article “Selection and Industrial Applications of Hardness Tests” for more information on P/M hardness testing.) Macroindentation Hardness Testing Edward L Tobolski, Wilson Instruments Division, Instron Corporation; Andrew Fee, Consultant Vickers Hardness Testing The Vickers hardness was first introduced in England in 1925 by R Smith and G Sandland (Ref 5) It was originally known as the 136° diamond pyramid hardness test because of the shape of the indenter The manufacture of the first tester was a company known as Vickers-Armstrong Limited, of Crayford, Kent, England As the test and the tester gained popularity, the name Vickers became the recognized designation for the test The Vickers test method is similar to the Brinell principle in that a defined shaped indenter is pressed into a material, the indenting force is removed, the resulting indentation diagonals are measured, and the hardness number is calculated by dividing the force by the surface area of the indentation Vickers testing is divided into two distinct types of hardness tests: macroindentation and microindentation tests These two types of tests are defined by the forces Microindentation Vickers (ASTM E 384) is from to 1000 gf and is covered in detail in the article “Microindentation Hardness Testing.” this section focuses on the macroindentation range with test forces from to 120 kgf as defined in ASTM E 92 Selected international standards for Vickers hardness testing are listed in Table Table Selected Vickers hardness testing standards Standard No ASTM E 92 BS EN ISO 65071 BS EN ISO 65072 BS EN ISO 65073 EN 23878 JIS B 7725 JIS B 7735 JIS Z 2244 JIS Z 2252 Title Standard Test Method for Vickers Hardness of Metallic Materials Metallic Materials—Vickers Hardness Test—Part 1: Test Method Metallic Materials—Vickers Hardness Test—Part 2: Verification of Testing Machines Metallic Materials—Vickers Hardness Test—Part 3: Calibration of Reference Blocks Hardmetals—Vickers Hardness Test Vickers Hardness—Verification of Testing Machines Vickers Hardness Test—Calibration of the Reference Blocks Vickers Hardness Test—Test Method Test Methods for Vickers Hardness at Elevated Temperatures Test Method As mentioned previously, the principle of the Vickers test is similar to the Brinell test, but the Vickers test is performed with different forces and indenters The square-base pyramidal diamond indenter is forced under a predetermined load ranging from to 120 kgf into the material to be tested After the forces have reached a static or equilibrium condition and further penetration ceases, the force remains applied for a specific time (10 to 15 s for normal test times) and is then removed The resulting unrecovered indentation diagonals are measured and averaged to give a value in millimeters These length measurements are used to calculated the Vickers hardness number (HV) The Vickers hardness number (formerly known as DPH for diamond pyramid hardness) is a number related to the applied force and the surface area of the measured unrecovered indentation produced by a square-base pyramidal diamond indenter The Vickers indenter has included face angles of 136° (Fig 21), and the Vickers hardness number (HV) is computer from the following equation: where P is the indentation load in kgf, and d is the mean diagonal of indentation, in mm This calculation of Vickers hardness can be done directly from this formula or from Table 10 (lookup table in ASTM E 92) This table contains calculated Vickers numbers for a kgf load, so that it is not necessary to calculate every test result For example, if the average measured diagonal length, d, is 0.0753 mm with a kgf load, then the Vickers number is: This value can be obtained directly from the lookup table For obtaining hardness numbers when other loads are used, simply multiply the number from the lookup table by the test load d/D Correction factor 0.169 0.910 0.179 0.905 0.189 0.900 0.200 0.895 Concave surface 0.009 1.005 0.017 1.020 0.025 1.015 0.034 1.020 0.042 1.025 0.050 1.030 0.058 1.035 0.066 1.040 0.074 1.045 0.082 1.050 0.089 1.055 0.097 1.060 0.104 1.065 0.112 1.070 d/D Correction factor 0.119 1.075 0.127 1.080 0.134 1.085 0.141 1.090 0.148 1.095 0.155 1.100 0.162 1.105 0.169 1.110 0.176 1.115 0.183 1.120 0.189 1.125 0.196 1.130 0.203 1.135 0.209 1.140 0.216 1.140 0.222 1.150 D, diameter of sphere in millimeters; d, mean diagonal of impression in millimeters Source: ASTM E 92 (Ref 3) Table Correction factors for use in Vickers hardness tests made on cylindrical surfaces One diagonal parallel to axis d/D Correction factor Convex surface 0.009 0.995 0.019 0.990 0.029 0.985 0.041 0.980 0.054 0.975 0.068 0.970 0.085 0.965 0.104 0.960 0.126 0.955 0.153 0.950 0.189 0.945 0.234 0.940 Concave surface 0.008 1.005 0.016 1.020 0.023 1.015 0.030 1.020 d/D Correction factor 0.036 1.025 0.042 1.030 0.048 1.035 0.053 1.040 0.058 1.045 0.063 1.050 0.067 1.055 0.071 1.060 0.076 1.065 0.079 1.070 0.083 1.075 0.087 1.080 0.090 1.085 0.093 1.090 0.097 1.095 0.100 1.100 0.103 1.105 0.105 1.110 0.108 1.115 d/D Correction factor 0.111 1.120 0.113 1.125 0.116 1.130 0.118 1.135 0.120 1.140 0.123 1.145 0.125 1.150 D, diameter of cylinder in millimeters; d, mean diagonal of impression in millimeters Source: ASTM E 92 (Ref 3) Example: Correction Factors for Vickers Hardness of a Convex Sphere The test conditions are: Diameter of sphere (D), mm 10 Vickers test load, kgf 10 Mean diagonal of indentation (d), mm 0.150 d/D 0.015 (i.e., 0.150/10) With a mean diagonal of 150 μm and a test load of 10 kgf, the Vickers hardness number for a flat surface is 824 (per ASTM E 92, Ref 3) From Table 5, the correction factor (by interpolation) for a convex surface is 0.983 The corrected hardness of the sphere is thus 824 × 0.983 = 810 HV10 Example: Correction Factors for Vickers Hardness of a Concave Cylinder (One Diagonal Parallel to Axis) The test conditions are: Diameter of cylinder (D), mm Vickers test load, kgf 30 Mean diagonal of indentation (d), mm 0.415 d/D 0.083 (i.e., 0.415/5) With a mean diagonal of 415 μm and a test load of 30 kgf, the Vickers hardness number for a flat surface is 323 (per ASTM E 92, Ref 3) From Table 7, the correction factor is 1.075 when d/D = 0.083 Thus, the hardness of the cylinder after correction is 323 × 1.075 = 347 HV30 Degree of Flatness An absolutely flat surface is the ideal condition for hardness testing, and some methods are more sensitive to this condition than are others To obtain accurate readings from Brinell, Rockwell, Scleroscope, and conventional microhardness testers, the surface being tested should be at least within or 3° of flatness—that is, close to 90° of the direction of travel of the indenter For example, when odd-shaped workpieces not have any surfaces parallel to the surface to be tested, it is often possible to provide adjustable fixtures, which can be tilted as required to allow a flat surface for testing This accommodation often is made with either the Brinell or the Rockwell tester In microhardness testing, securing and holding devices are used to attain a test surface that is sufficiently flat Similar approaches have been used for Brinell and Rockwell testing; frequently, devices are designed for specific workpieces Ultrasonic microhardness tests can be performed on surfaces that are not flat, however, because different principles are involved Surface Condition Surface condition is a term covering two different conditions, surface finish and surface composition, both of which can affect the selection of the optimal method and/or testing technique Surface Finish In general, the degree of surface smoothness required for accurate results is related directly to the size of the indenter Although the smoother finishes are highly desirable for any testing method, the Brinell test, which involves a large indenter, can be made and read with a reasonable degree of accuracy when the finish is comparable to finished-machined or rough-ground types In Rockwell testing, a finished ground surface is generally the minimum requirement, but polished surfaces are preferred In Vickers testing through microhardness testing (including Scleroscope), finish requirements are far more stringent By comparison, in microhardness testing with very light loads (less than 100 gf), the workpiece or specimen requires a surface finish equal to that used for microscopic examination at high magnification It is obvious that the degree of smoothness that can be obtained can have a profound effect on which test method is selected Surface Composition The other surface condition that can affect the selection of the hardness test method is surface composition (generally unique to steels) Decarburization, retained austenite, carburization, or other composition changes that result in a hard case are likely to influence selection In many instances, differences in surface conditions require the use of more than one method or scale Indent Location and Effects Location If an indentation is placed too close to the edge of a specimen the testpiece edge may bulge, causing a lower hardness value because of improper support in the test area To ensure an accurate test, the distance from the center of the indentation to the edge of the testpiece shall be at least 2.5 times the diameter of the indentation Therefore, when testing in a narrow area, the width of the test area must be at least five diameters when the indentation is made in the center The appropriate scale or test force must be selected for this minimum width Although the diameter of the indentation can be calculated, for practical purposes the minimum distance can be determined visually Effect of Indentation Marks An indentation hardness test cold works and/or work hardens the surrounding area If another indentation is made too close to this work-hardened area, the reading is usually higher in value than if placed outside the hardened area Generally, the softer the material, the more critical the spacing of the indentations A distance between the center of two adjacent indentations of at least three times the diameter of the indentations should be sufficient for most materials The presence (or absence) of test marks on a part can also be a factor in selecting a test procedure In most instances, the presence of Brinell impressions on workpieces such as forgings and castings is not objectionable On a finished part, however, a mark as large as a Brinell impression might be undesirable from an appearance standpoint, or in some instances, can interfere with its function There are notable cases where analysis of a service failure proved that a fracture was nucleated by a Brinell impression Rockwell indentation marks also can have a deleterious effect, although because the indentations are much smaller, the likelihood of damage is usually less than that caused by Brinell marks Generally, diamond indenter marks are not sufficient to impair the function of a part, except in the case of precision parts used for purposes such as in fuel control systems Rarely are marks left by Scleroscope or microhardness testers objectionable Production Rates The number of identical or similar parts being tested can also be a selection factor The Scleroscope lends itself to very rapid testing, when specific conditions exist, and is used frequently for high-production testing Likewise, under certain conditions, the ultrasonic hardness test can be used for microhardness testing of many identical parts As a rule, however, mass-production hardness testing is done with either the Brinell or the Rockwell tester Either instrument is available in partly or completely automated setups in which rejects are automatically separated References cited in this section “Standard Test Method for Rockwell Hardness of Plastics and Electrical Insulating Materials,” D 785-98, Annual Book of ASTM Standards, ASTM “Standard Test Method for Vickers Hardness of Metallic Materials,” E 92-82(1997)e2, Annual Book of ASTM Standards, ASTM Selection and Industrial Applications of Hardness Tests Andrew Fee, Consultant Accuracy and Frequency of Calibration Although the indentation-type test is a comparatively simple test to perform, reliable results depend a great deal on the accuracy of the equipment and the proper test method It is recommended the tester be checked each day that hardness tests are to be made and whenever the indenter, anvil, or test force is changed Standardized test blocks should be used to monitor the performance of the tester daily At least two test blocks should be used with hardness levels that bracket below and above the range of hardness levels that are normally tested Prior to doing any testing, it is good practice to ensure that the tester is operating according to manufacturer requirements and that the anvil and indenter are seated properly At least three hardness measurements should be made on any uniform specimen having a high hardness level in the scale to be verified The measurements should be continued until there is no trend (increasing or decreasing hardness) in the measurement values This technique implies that the tester's repeatability is consistent and that the indenter and anvil are seated adequately These results need not be recorded After the trial tests, at least three uniformly spaced hardness measurements should be made on each of the standardized test blocks If the average of the hardness measurements are within the tolerance marked on the blocks, the tester may be regarded as performing satisfactorily If not, an indirect verification should be performed In monitoring the tester in this manner it is recommended that these hardness measurements be recorded using acceptable statistical process control techniques, such as X-bar charts (measurement averages), R-charts (measurement ranges), gage repeatability and reproducibility (GRR) studies, and histograms (see the article “Gage Repeatability and Reproducibility in Hardness Testing” in this Volume) Most indentation-type testing should be carried out at a temperature within the limits of 10 to 35 °C (50–95 °F) If there is a possibility of hardness variation within these test-temperature limits, users may choose to control temperatures within a tighter range A range of 18 to 23 °C (64–81 °F) is recommended Tests performed outside this temperature range should be considered suspect NIST-Traceable Test Blocks Due to the empirical nature of hardness testing, the need for standardization of hardness values is an area of continued attention In many countries of Europe and Asia, for example, nationally traceable hardness standards have been around for many years Traceable standards can help resolve or reduce differences in test results between vendors and customers, who each rely on their test block for machine verification In 1990, after several meetings between the American Society for Testing and Materials (ASTM) and standards groups from Europe and Asia, the U.S government agreed to provide hardness standards for U.S manufacturers The reason for the change is that hardness, though based on traceable parameters, has had no absolute numbers For example, the loads on a tester can be verified with a traceable load cell, but the hardness values themselves are empirical; that is, hardness would not be directly traceable to any standard, national or otherwise In order to evaluate the magnitude of variation, commercially available test blocks were evaluated by the National Institute of Standards and Technology (NIST) A variation of 1.0 HRC was found to exist among test blocks supplied by domestic manufacturers A shift of almost 1.0 HRC also was realized versus standards from other countries This finding reinforced the need for standardization The hardness program at NIST involves traceable Standard Reference Material (SRM) blocks—or what industry refers to as “NIST-traceable test blocks.” The SRMs are calibrated at NIST by means of a dead-weight tester Only two of these machines exist in the world Other primary machines exist in other countries, but the only exact duplicate of the NIST machine is located at IMGC, which is the NIST equivalent in Italy NIST-traceable test blocks are available for three nominal ranges in the Rockwell C scale: • • • SRM 2810, “Rockwell C Scale Hardness—Low Range” (25 HRC nominal) SRM 2811, “Rockwell C Scale Hardness—Mid Range” (45 HRC nominal) SRM 2812, “Rockwell C Scale Hardness—High Range” (63 HRC nominal) The new NIST-traceable blocks, at a nominal size of 60 mm (2.36 in.) diameter and 15 mm (0.6 in.) thick, are larger than the typical Rockwell hardness test blocks They are made of steel in the appropriate Rockwell C range and have a polished mirrorlike surface Although most ASTM-type Rockwell C test blocks are labeled ±0.5 HRC on the high end (60 HRC range), the NIST blocks have much tighter tolerances (down to 0.1) Test locations are indicated on the block; associated hardness numbers and statistical information are listed on the certificate, enabling the user to find more than just the arithmetic mean of the hardness Secondary traceable standards are available from commercial test block manufacturers NIST standardized test blocks are based on methods (especially on the diamond indenter) that are more closely aligned with those of the national laboratories of other nations than with the values that were being used in North America The most dramatic change is tighter specification of indenter radius closer to the average ASTM-specified value of 200 μm This is slightly larger than previous standard indenter radius of 192 μm This change in indenter radius shifts values at the upper end of the Rockwell C scale (59–63 HRC), where values shifted upward by 0.5 to 0.8 points HRC From 46 to 58 HRC the shift was from 0.2 to 0.49 points, while the shift was insignificant below 46 HRC Selection and Industrial Applications of Hardness Tests Andrew Fee, Consultant Hardness Test Selection for Specific Materials Generally, the scale to be used for a specified material is indicated on engineering design drawings or in the test specifications However, at times the scale must be determined and selected to suit a given set of circumstances In general, the scale using a diamond indenter (Rockwell and Vickers) are used for testing hardened steels and alloys, while the ball indenters (Brinell and Rockwell) are used on more malleable materials Table is a general guide relating materials and scales for regular Rockwell testing As noted in Table 8, the Rockwell superficial scales (N and T) are used for testing similar material that may be too thin to accommodate the regular scales In microindentation hardness testing, the Knoop and Vickers diamond indenters are used for all testing Additional details about these indentation hardness test methods are given in separate articles in this Section of the Handbook Table Typical applications of regular Rockwell hardness scales Scale(a) B C Typical applications Copper alloys, soft steels, aluminum alloys, malleable iron Steel, hard cast irons, pearlitic malleable iron, titanium, deep case-hardened steel, and other materials harder than 100 HRB A Cemented carbides, thin steel, and shallow case-hardened steel D Thin steel and medium case-hardened steel and pearlitic malleable iron E Cast iron, aluminum and magnesium alloys, bearing metals F Annealed copper alloys, thin soft sheet metals G Phosphor bronze, beryllium copper, malleable irons Upper limit is 92 HRG to avoid flattening of ball H Aluminum, zinc, lead K, L, M, P, R, Bearing metals and other very soft or thin materials Use smallest ball and heaviest load that S, V not give anvil effect (a) The N scales of a superficial hardness tester are used for materials similar to those tested on the Rockwell C, A, and D scales but of thinner gage or case depth The T scales are used for materials similar to those tested on the Rockwell B, F, and G scales but of thinner gage When minute indentations are required, a superficial hardness tester should be used The W, X, and Y scales are used for very soft materials Conversion from one hardness scale to another also depends on the material being tested Therefore, this section provides some hardness conversion data for materials other than steel Hardness conversion tables for steel are included in the article “Hardness Conversions for Steels.” Steels Forgings, Castings, and Plate Products Annealed, hot-rolled, cold-finished, forged, or cast carbon and alloy steels usually are tested by the Brinell or Rockwell B method Because of the nature of forgings and most iron and steel castings, the Brinell test is the preferred test method; the larger Brinell indentation gives a better average value of the local surface and thus a truer homogentic hardness than would be expected with the Rockwell test Rockwell testing is used on specimens with fine grain composition or those that lack sufficient area to accommodate a Brinell test Although the Rockwell B scale (1.59 mm, or in., ball indenter) is used sometimes, the Rockwell E and K scales (3.175 mm, or in., ball indenter) are preferred because the larger indenter gives a better average reading The surface that is to be tested should be prepared, if needed, to allow for a well-defined indentation for accurate measurement Care should be taken to ensure that any surface preparation will not influence the condition of the surface by overheating and cold working To better correlate between Rockwell and Brinell values, it is suggested that three to five Rockwell tests be made and averaged to give a more representative hardness value because of the possible variations within the cast part Hard white iron castings and chilled rolls are usually tested using the Rockwell C and Vickers scales Hardened and Tempered Steels The hardness of quenched-and-tempered carbon, alloy, tool, and stainless steels is typically tested with a diamond indenter by Rockwell, Vickers, or microindentation techniques The Rockwell C test generally is used when conditions permit Rockwell C readings of less than 20 (or its equivalent in other scales) should not be considered valid, and some inaccuracy can be expected as the value drops below 30 HRC For hardenability testing, the Rockwell C scale is preferred (see the section “Hardenability Testing” in this article) Steel Sheet Depending on the thickness of the sheet, hardness specifications are usually given in the Rockwell B scale or a superficial Rockwell scale (HR30T or sometimes HR15T) Sheet metal is usually tested and controlled for its drawing and stamping capabilities with the Rockwell test A common industry description of the various sheet steel tempers is: Temper Hardness, HRB No Hard 90 ± No Half-hard 80 ± No Quarter-hard 70 ± No Soft 60 ± No Dead-soft 45 ± The verbal descriptions of the tempers involve wide tolerances, and a specification in the actual Rockwell hardness gives a more precise and defined tolerance for control of the end product Powder Metallurgy (P/M) Steels Because the density of P/M steels may vary from less than g/cm3 (0.25 lb/in.3) to a density approaching that of wrought steel (about 7.8 g/cm3, or 0.28 lb/in.3), the variation in hardness can vary widely Besides porosity, sintered P/M steels may also have inhomogeneous microstructures from graphite At least five consistent readings should be taken, in addition to any obviously high or low readings, which should be discarded The remaining five readings should be averaged Because of the variety of compositions and densities encountered in P/M materials, the recommendation for suitable test methods may require preliminary trials Generally, the Rockwell test, with its variety of scales, is the usual choice The Rockwell F, H, B, and the superficial T scales are generally used for hardness testing of P/M materials Heat treated P/M steels are sometimes tested in the Rockwell C scale (Table 9) Although not widely used, the Rockwell B scale may be combined with a carbide ball for testing hardened parts Data scattering is minimized with a Rockwell B 1.59 mm ( in.) diameter ball, and it is useful up to 120 HRB Table Common hardness scales used for P/M parts Heat treated Sintered hardness scale hardness scale Iron HRH, HRB HRB, HRC Iron-carbon HRB HRB, HRC Iron-nickel-carbon HRB HRC Prealloyed steel HRB HRC Bronze HRH … Brass HRH … Apparent Hardness In powder metallurgy there are generally two types of hardness specified—apparent hardness (macrohardness) and microhardness The microhardness is the hardness of each particle of material, and the apparent hardness is the hardness of the surface—bridging across many particles and the porosity, too Apparent hardness is typically measured according to Metal Powder Industries Federation (MPIF) Standard 43 (Ref 4) The procedure is relatively straightforward and quick The basics are: Material Obtain a sample part of adequate thickness and parallel configuration (or, for cylindrical parts, a correction factor may be used) The sample must be large enough so that the indenter marks from the hardness tester are at least three indenter diameters from any edge or previous impression Sand each face of the sample so that no burrs are present (burrs will cause erroneous readings), or be sure to use a holding fixture that avoids the burrs Take readings with a properly calibrated hardness tester Reject obvious outliers and report the average of at least five nonoutliers Typically, the outliers are on the low side The cause of these occasional low readings is a chance happening that the hardness indenter falls right into a pore Microhardness is usually measured according to MPIF Standard 51 (Ref 5) The determination of microhardness is significantly more difficult than measuring apparent hardness and requires specialized equipment that many P/M users not have on-site The procedure involves: Sectioning the part and making a polished mount for the evaluation Placing the mount in a special microhardness testing machine Under magnification, orienting the mount and making a diamond indenter mark precisely over a particle of the material Measuring the length of the penetration on the particle and converting this length to a hardness reading Microindentation hardness tests of porous materials can best be measured with Knoop or diamond pyramid hardness indenters at loads of 100 gf or greater In atomized irons, particles exhibit minimal porosity; consequently, the Knoop indenter is suitable because it makes a very shallow indentation and is not frequently disturbed by entering undisclosed pores Care should be taken in preparing the sample surface The diamond pyramid indenter is particularly well suited to irons that contain numerous fine internal pores Because of its greater depth of penetration, the diamond pyramid indenter frequently encounters hidden pores Microhardness testing and the measurement of effective case depth are covered by MPIF standard 51 (Ref 6) Cast Irons Accurate hardness values often are difficult to attain when the material has an inhomogeneous structure and composition This applies to the complex metal-carbon structure of cast irons Conventional hardness measurements of cast irons thus tend to be lower values than the hardness of the metal portion This discrepancy, which is more pronounced in gray iron than in ductile and malleable irons, occurs because conventional hardness readings are composite values that reflect the hardnesses of both the matrix metal and soft graphite Greater variations in hardness results may also occur from the inhomogeneous structure Therefore, a Brinell hardness test, by virtue of its indenter size, is preferred to provide more consistent average hardness values However, sometimes other scales may be required For example, when determining the hardness of small castings, it is often impossible to use a Brinell tester; a Rockwell tester must be used Fine grain structure, hard white-iron castings, and chilled rolls may also require the use of other scales, as previously noted in the section “Forgings, Castings, and Plate Products” in this article Conversions between different hardness scales have been developed for some types of cast irons For example, Fig shows conversions from Brinell to Rockwell B and G scales for malleable and pearlitic malleable irons, respectively Figure 6(b) shows Rockwell C equivalents for Brinell values of pearlitic malleable iron These conversions generally are accepted by producers of malleable iron Reliable hardness conversion for other types of cast irons, especially gray irons, is more difficult due to the variations in metallurgical conditions For example, Fig shows the relationship between observed Rockwell C readings and those converted from microhardness values for five gray irons of different carbon equivalents The wide variation illustrates the need to know the carbon equivalent of the iron being tested before a conversion chart can be developed For white iron, conversions are shown in Table 10 Table 10 Approximate equivalent hardness numbers of alloyed white irons Vickers hardnessNo., HV50 Brinell hardness No.(a), HBW 1000 (903) 980 (886) 960 (868) 940 (850) 920 (833) 900 (815) 880 (798) 860 (780) 840 (762) 820 (745) 800 (727) 780 (710) 760 (692) 740 (674) 720 (657) 700 (639) 680 621 660 604 640 586 620 569 600 551 580 533 560 516 540 498 520 481 500 463 480 445 460 428 440 410 420 393 400 375 380 357 Rockwell C hardness No., HRC 70 69 68 68 67 66 66 65 64 63 62 62 61 60 59 58 57 56 55 54 53 52 51 50 48 47 45 44 42 40 38 35 Note: Brinell hardness numbers in parentheses are beyond the normal range and are presented for information only (a) 10 mm (0.4 in.) diam tungsten carbide ball; 3000 kgf load Source: ASTM E 140 (Ref 6) Fig Hardness conversions for malleable iron (a) Conversion from Brinell to Rockwell G scales for malleable iron (b) Conversion from Brinell to Rockwell B, C, and G scales for pearlitic malleable iron Fig Relationship between observed and converted hardness values, as influenced by carbon equivalent, for gray iron containing type graphite Nonferrous Alloys To a great extent, the same general guidelines apply to both nonferrous and ferrous materials Indentation spacing, proximity to edges, thickness of testing material, and the selection of indenter and load combinations are all factors that influence hardness readings With very few exceptions, nonferrous metals are generally softer than steels and cast irons Brinell testing and Rockwell testing with ball indenters under a variety of test loads are most often used Many of the higher strength or higher hardness nonferrous metals can be accurately tested with the Brinell test method when the workpiece is of sufficient thickness and size The Brinell test is the preferred test for wrought aluminum alloys and large nonferrous castings, which are usually tested with the 500 kgf load Some high-strength alloys such as titanium-base alloys that are phase transformation or age hardened can be tested with the 3000 kgf load Diamond indenters are sometimes used— notably, the Rockwell A scale Some multiphased cast nonferrous alloys that are too soft for Brinell testing will require the Rockwell or Vickers test methods Typical Rockwell scales used for a wide variety of nonferrous metals and other materials are listed in Table Very small nonferrous metal parts made of extremely thin sheet, strip, or foil are tested by microindentation methods Aluminum and aluminum alloys are tested frequently for hardness to distinguish between annealed, coldworked, and heat treated grades The Rockwell B scale (100 kgf load with a 1.58 mm, or in., steel ball indenter) generally is suitable in testing grades that have been precipitation hardened to relatively high strength levels For softer grades and commercially pure aluminum, hardness testing usually is done with the Rockwell F, E, and H scales For hardness testing of thin gages of aluminum, the 15T and 30T scales of the Rockwell superficial tester are recommended Approximate hardness conversions for wrought aluminum are listed in Table 11 Table 11 Approximate equivalent hardness numbers for wrought aluminum products Brinell hardness No., 500 kgf, mm ball, HBS Vickers hardness No., 15 kgf, HV Rockwell hardness No B scale, E scale, 100 kgf, 100 kgf, ⅛in in ball, ball, HRE HRB H scale, 60 kgf, ⅛in ball, HRH Rockwell superficial hardness No 15T scale, 30T scale, 15W scale, 15 30 15 kgf, ⅛in kgf, in kgf, in ball, ball, ball, HR15W HR15T HR30T 160 155 150 145 140 135 130 125 120 115 110 105 100 95 90 85 80 189 183 177 171 165 159 153 147 141 135 129 123 117 111 105 98 92 91 90 89 87 86 84 81 79 76 72 69 65 60 56 51 46 40 … … … … … … … … … … … … … … 108 107 106 89 89 89 88 88 87 87 86 86 86 85 84 83 82 81 80 78 … … … … … … … … 101 100 99 98 … 96 94 91 88 77 76 75 74 73 71 70 68 67 65 63 61 59 57 54 52 50 95 95 94 94 94 93 93 92 92 91 91 91 90 90 89 89 88 75 86 34 84 104 76 47 87 70 80 28 80 102 74 44 86 65 74 … 75 100 72 … 85 60 68 … 70 97 70 … 83 55 62 … 65 94 67 … 82 50 56 … 59 91 64 … 80 45 50 … 53 87 62 … 79 40 44 … 46 83 59 … 77 Source: ASTM E 140 (Ref 6) Copper and Copper Alloys Because copper alloys vary so widely in hardness, a wide range of indenters and loads may apply to this family of alloys Beginning at the top of the range, the precipitation-hardenable alloys (such s C17000, C17200, and C17300) may be regarded as essentially the same as steel in their hardened condition because they are generally within the range of 36 to 45 HRC Therefore, these alloys can be tested satisfactorily with the Rockwell C scale For thinner gages, the 15H or 30H scale is used The Brinell test, using 1500 to 3000 kgf loads, is also appropriate for testing the harder copper alloys When these alloys are in the annealed or cold-worked condition, the Rockwell B scale is recommended, or the 15T or 30T scale for very thin sections When the indenter is penetrating the test material too deeply with the B scale, a lighter load or larger ball, such as that used for Rockwell E or F scale, must be used Approximate hardness conversions for wrought copper alloys and cartridge brass are listed in Tables 12 and 13, respectively ... 5,723 5,660 5,598 5,537 5 ,47 7 5 ,41 8 5,360 0.019 5,137 5 ,083 5,030 4, 978 4, 927 4, 877 4, 827 0.020 4, 636 4, 590 4, 545 4, 500 4, 456 4, 413 4, 370 0.021 4, 205 4, 165 4, 126 4 ,087 4, 049 4, 012 3,975 0.022 3,831... 501.6 48 5.5 47 0.2 45 5.6 44 1.6 42 8.3 41 5.6 40 3 .4 391.8 380.6 369.9 359.7 359.7 349 .9 340 .5 331 .4 322.7 3 14. 4 306 .4 298.6 291.2 2 84. 0 277.1 270.5 2 64. 1 257.9 251.9 246 .1 240 .6 235.2 230.0 2 24. 9 220.0... 510.0 49 3.5 47 7.8 46 2.8 44 8.5 43 4.9 42 1.9 40 9 .4 397.5 386.1 375.2 3 64. 8 3 64. 8 3 54. 7 345 .1 335.9 327.0 318.5 310.3 302.5 2 94. 9 287.6 280.6 273.8 267.2 260.9 2 54. 9 249 .0 243 .3 237.8 232.5 227 .4 222.5