Brinell Hardness Testing Methods and Their Applicability

2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-012 (10 pages)

Devin R. Hess, Ph.D., General Motors Engine Materials Engineer, Pontiac, Michigan

Herbert W. Doty, Ph.D. General Motors Materials Technology, Pontiac, Michigan

ABSTRACT test, better known as the Product A, was introduced in 1975.4 The Leeb test is a form of the scleroscope test. Hardness is one of the most commonly specified requirements on automotive propulsion system One definition of hardness is a materials resistance to engineering drawings. It is used as a process control indentation. This is typically determined by measuring the check and as a proxy for strength, wear resistance, and permanent depth or diameter of the indentation left by the machinability. It is commonly assumed harder means testing method. The resulting hardness value depends on stronger, and while this is generally true, what are we the Product And the test parameters being used. Hardness really controlling when we measure hardness? Depending therefore is not a fundamental physical property of a on the material and the manufacturing process, hardness material. Thus, it is critical that the Product A parameters can be used to verify that the material has been exposed to be clearly defined when specifying or reporting a the appropriate thermal conditions and that the proper hardness value. One concern that often arises is how to microstructure is present. compare hardness values obtained from different test methods. This paper explores some of the issues that can arise when trying to compare between different scales within a given HARDNESS TEST METHODS REVIEWED IN THIS Product And when trying to convert from one test method PAPER to another and offers some rationale for these limitations. The focus of this paper is limited to cast iron and BRINELL HARDNESS aluminum engine components and the Brinell, Rockwell, The Brinell hardness test is an indentation type of test. It and Equotip (noted from here on as “Product A”) test is commonly specified to test materials that have a methods. structure that is too coarse or that have a surface that is too rough to be tested using another test method, e.g. Keywords: Hardness, Brinell, Rockwell, cast iron, castings and forgings. The Brinell method is performed aluminum by applying a predetermined test load (F) to a carbide ball of fixed diameter (D) into the workpiece. The load is held INTRODUCTION for a predetermined time period and then removed. The diameter of the resulting indentation is then measured Different hardness test methods have been developed and using a specially designed Brinell scope or an optical adopted to address a variety of problems. There are at system across at least two diameters, usually at right least 12 different methods that can be used for measuring angles to each other. These diameters are averaged (d) hardness. One of the earliest hardness testing methods and used to determine the surface area of the indentation. was the scratch test which was attributed to Reaumur in The hardness number is generated by dividing the test 1722.1 File hardness is one type of scratch test which has force by the surface area of the indentation, Equation 1. been used to ensure that proper surface hardening has The Brinell test was originally developed for determining been achieved in applications such as the induction heat the hardness of steels but has since been applied to almost treating of valve seats in cast iron cylinder heads. In 1900, all metals. Johan August Brinell’s ball hardness test was showcased at the Paris Exhibition.2 The Brinell test has become one 2 = Eqn. 1 of the most commonly used hardness tests. Another ( ) commonly used hardness test is the Rockwell hardness 𝐹𝐹 Where:𝐻𝐻𝐻𝐻𝐻𝐻 2 2 test which was introduced in 1919 by Stanley P. HBW = the Brinell𝜋𝜋𝜋𝜋 𝐷𝐷 hardness− �𝐷𝐷 −number 𝑑𝑑 Rockwell.1 The Vickers hardness test was introduced in 3 F = the test force in kgf 1922 by R. L. Smith and G. E. Sandland. The Vickers D = the diameter of the indenter ball in mm test was a variation on the Brinell test. The Leeb rebound d = the measured mean diameter of the indentation

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Standards for Brinell Hardness Testing indicates recommended force-diameter ratios for certain The two most commonly utilized standards for measuring materials and hardness ranges. That table is re-created in Brinell hardness are ASTM E10 and ISO 6506. ASTM Table 1.6 It is important to note that the standards use E10 is the standard followed in North America while ISO force-diameter ratio, but it is actually the force in kgf 6506 is the standard followed throughout much of the rest divided by the diameter squared. of the world. While these standards are essentially equivalent there are subtle differences which have been Table 1. Ratio 0.102xF/D2 for Different Metallic shown to result in different hardness readings. In Materials6 comparing these standards, it is important not only to Material Brinell Force-diameter point out the differences which exist but also to point out hardness ratio 0.102xF/D2 common requirements between the standards. (HBW) (N/mm2)

Steel, nickel 30 Common Requirements Between Standards ASTM E10 and ISO 6506 both describe equipment alloys, titanium alloys requirements, test piece requirements, and the test a procedure for performing Brinell hardness measurements. Cast iron < 140 10 Both standards require the use of a hard metal or tungsten ≥ 140 30 carbide ball as the indenter.5,6 Both standards show how Copper and < 35 5 to designate Brinell hardness by calling out the hardness copper alloys 35 to 200 10 number followed by the indenter type (e.g. HBW for hard > 200 30 metal ball), followed by the diameter of the indenter used Light metals and < 35 2.5 and the applied load in kgf.5,6 For example, 109 HBW their alloys 35 to 80 5, 10, 15 10/500 would be a hardness value of 109 using a tungsten > 80 10, 15 indenter with a diameter of 10 mm (0.39 in.) and an Lead, tin 1 applied load of 500 kgf (4903 N). Both standards also Sintered metal According to ISO 4498-1 recommended that the test be carried out at a temperature a - For the testing of cast iron, the nominal between 50 F(10 C) and 95 F(35 C).5,6 diameter of the ball shall be 2.5 mm, 5 mm, or 10 mm. A rather interesting commonality between the two standards is a comment made with respect to converting While ASTM E10 does not provide any recommendations to other hardness scales or to tensile strength. Both for what load should be used it does state that different standards make a statement cautioning against converting Brinell hardness numbers may be obtained if different to other hardness scales or into tensile strength, stating loads are used on the same diameter ball on the same that these conversions are approximations at best and material.5 ASTM E10 also states that direct comparisons should be avoided unless a reliable basis has been of results using different scales (indenter and load established through comparative tests.5,6 This cautionary combinations) can only be made using the same force- statement will be explored in further detail later in this diameter ratios.5 paper. In North America it has been common practice to use a An important, and often overlooked, requirement that is 500 kgf (4903 N) load with a 10 mm (0.39 in.) diameter common between both standards is the minimum and indenter when testing aluminum.7 This results in a force- maximum diameter of the indentation. Both standards diameter ratio of 5. According to ISO 6506, as shown in indicate that the test force be selected so that the resulting Table 1, a force-diameter ratio of 5 should only be used indentation is between 24% and 60% of the ball for light metals that have a hardness between 35 and 80. diameter.5,6 This common requirement also leads to one of Also as shown in Table 1, ISO 6506 recommends a force- the most significant differences between the two diameter ratio of 10 or 15 be used for light metals and standards which will be discussed in the next section. alloys with a hardness number greater than 80.

Differences Between Standards Most automotive cast aluminum components have a One difference between the ASTM E10 and ISO 6506 hardness that exceeds 80 HBW 10/500 with some standards is the required thickness of the test sample. components approaching 120 HBW 10/500. Figure 1 ASTM E10 suggests that the thickness of the material shows an overlay of this typical hardness range with both should be at least 10 times the depth of the indentation.5 the HBW 10/500 scale and the HBW 10/1000 scale. Whereas ISO 6506 only requires the thickness to be at least 8 times the depth of the indentation.6 The graph in Figure 1 has been created to show the hardness as a function of the indentation diameter. The Probably the most important difference between ASTM indentation diameter has been limited to fall within the E10 and ISO 6506 resides in the recommendation for 24% to 60% range specified by ASTM E10 and ISO what test force to use. ISO 6506 provides a table which 6506. As is illustrated in Figure 1, the typical range of

Page 2 of 10 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-012 (10 pages) hardness values for automotive cast aluminum alloys is VICKERS HARDNESS fully covered using the HBW 10/1000 scale whereas the The Vickers hardness test method, also referred to as a maximum hardness value that falls within the 24% to microhardness test method, is mostly used for small parts, 60% range is only 109 HBW 10/500. It is important to thin sections, or case depth work. Vickers is based on an note that the HBW 10/500 scale is a force-diameter ratio optical measurement system. ASTM E384 specifies a of 5 while the HBW 10/1000 scale is a force-diameter range of light loads using a diamond indenter to make an ratio of 10. Figure 1 supports the ISO 6506 indentation which is measured and converted to a recommendation that a scale with a force-diameter ratio hardness value.8 Test samples must be highly polished to of 10 should be used if the hardness exceeds 80. enable accurate measurement of the size of the impressions. A square base pyramid shaped diamond indenter is used for testing in the Vickers scale. Typically loads are very light, ranging from 10 gf (0.098 N) to 1 kgf (9.8 N); although “macro” Vickers loads can range up to 30 kgf (294.2 N) or more.

LEEB HARDNESS The Leeb hardness test is commonly referred to as the Product A. In this test method, a device fires a hard indenter sphere towards the surface of the material being tested. The velocity of the impact body is recorded in three main test phases: 1. The pre-impact phase, where the impact body is accelerated by spring force towards the surface of the Figure 1. Graph showing hardness versus indentation test piece diameter between 24% and 60% for a 10 mm diameter 2. The impact phase, where the impact body and the test indenter with typical hardness ranges for aluminum piece are in contact. The hard indenter tip causes the alloys overlaid. test material to elastically and plastically deform and is itself elastically deformed. After the impact body is fully stopped, elastic recovery of the test material and ROCKWELL HARDNESS the impact body takes place and causes the rebound The Rockwell hardness method measures the permanent of the impact body. depth of indentation produced by a load applied to a 3. The rebound phase, where the impact body leaves the material via an indenter. A variety of indenters may be test piece with residual energy that was not consumed used: conical diamond with a round tip for harder metals during the impact phase. to ball indenters with a diameter ranging from 1/16 in. The L-value, also known as the Leeb-number or Leeb (1.5875 mm) to ½ in. (12.7 mm) for softer materials. hardness (HL), is the ratio of the rebound velocity to the impact velocity of the impact body multiplied by 1000. A preload is first applied to a sample using a diamond or ball indenter. Preloads range from 3 kgf (29.4 N) used in COMPARING BETWEEN BRINELL HARDNESS the “Superficial” Rockwell scale to 10 kgf (98.1 N) used SCALES in the “Regular” Rockwell scales. After holding the preload force for a specified dwell time, the baseline Within the Brinell test, each combination of indenter and depth of indentation is measured. After the preload an load defines a different hardness scale. Similar to how additional load, called the major load, is added to reach Rockwell A, B, and C are different scales within the the total required test load. Total test load ranges from 15 Rockwell hardness test procedure; HBW 10/500, HBW kgf (147.1N) to 150kgf (1471N) with the superficial and 10/1000, HBW 10/3000, HBW 2.5/62.5, HBW 5/250, etc. regular Rockwell tests while Rockwell microhardness test are all different scales within the Brinell hardness test loads range from 500 kgf (4903 N) to 3000 kgf (29420 procedure. Both ASTM E10 and ISO 6506 caution N). This force is held for a predetermined amount of time, against comparing between scales but suggest that if the known as the dwell time, to allow for elastic recovery. force-diameter ratio is maintained then the resulting The major load is then released, returning to the minor hardness value would be approximately the same.5, 6 load. After holding the minor load for a specified dwell time, the final depth of indentation is measured. Some part print requirements callout a Brinell hardness number without a scale leaving the part manufacturer to The Rockwell hardness value is derived from the use the ‘common industry practice’ to perform the difference in the baseline and final depth measurements. measurement. As has been implied by the comparison in This difference is converted to a hardness number on a the previous section, the ‘common industry practice’ 100-point scale. could be different depending on what region of the world the manufacturer resides. Some part print requirements

Page 3 of 10 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-012 (10 pages) require Brinell hardness be measured in areas where a and the HBW 2.5/62.5 scale. Both of these scales have a smaller indenter and lighter load is required. Studies were force-diameter ratio of 10 so according to ISO 6506 and carried out to understand the effect of using different ASTM E10 the resulting hardness values should be Brinell hardness scales to measure aluminum castings and approximately the same.5, 6 Figure 3 shows a graph with to quantify any differences. the HBW 2.5/62.5 scale on the y-axis and the HBW 10/1000 scale on the x-axis. For the samples tested, the EXPERIMENTAL PROCEDURE HBW 2.5/62.5 scale read on average 7 points higher than Samples of castings were obtained such that the the HBW 10/1000 scale. Individual readings were found microstructure in a localized region would be similar and to be as much as 17 points different. with sufficient material to enable measurement with different Brinell hardness scales. Comparisons were made looking at the HBW 10/500 versus the HBW 10/1000 scales, the HBW 10/1000 versus the HBW 2.5/62.5 scales, and the HBW 10/500 versus the HBW 2.5/62.5 scales. These are the most common scales used and represent a comparison between different force-diameter ratios, 5 versus 10, and different scales but with a common force-diameter ratio of 10.

Six different aluminum alloy and heat treatment combinations were sampled. The results from all of the combinations were plotted on the same graph to see if alloy and heat treatment had an influence on the results, Figure 2. The graph was formatted to create an isoplot with a range of 80 to 130 on both the x and y axis. The expectation is that if the readings for both scales were Figure 3. Graph showing a comparison between the equivalent, then all of the data would lie on the 45-degree HBW 2.5/62.5 scale and the HBW 10/1000 scale. line formed by drawing a line from the origin (80 HBW The difference between the HBW 10/500 and HBW 10/500 – 80 HBW 10/1000) to the upper right corner (130 2.5/62.5 scales were also examined, Figure 4. It was HBW 10/500 – 130 HBW 10/1000). The results were found that the average differences were additive going very close to equivalent, but the hardness readings from HBW 10/500 to HBW 10/1000 to HBW 2.5/62.5. obtained using the HBW 10/1000 scale were higher than On average the difference between the HBW 10/500 scale those obtained using the HBW 10/500 scale. On average, and the HBW 2.5/62.5 scale was 10 points with individual the HBW 10/1000 readings were approximately 3 points measurements being as high as 21 points different. higher than the HBW 10/500 readings. Individual readings were found as high as 9 points different between the two scales.

Figure 4. Graph showing a comparison between the HBW 2.5/62.5 scale and the HBW 10/500 scale.

Figure 2. Graph showing a comparison between the Based on these findings, a universal offset probably HBW 10/1000, force-diameter ratio = 10, and HBW cannot be applied to adjust readings from one scale to 10/500, force-diameter ratio = 5, for six different aluminum alloy–heat treatment combinations. another. Also published equations or charts that convert between one scale and another most likely are not The same six alloy and heat treat combination samples accurate for applying to cast aluminum materials. There were then evaluated to compare the HBW 10/1000 scale was a lot of scatter in the Brinell hardness readings as

Page 4 of 10 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-012 (10 pages) well which compounds the issues of trying to convert to ensure that the presence of the mount material did not between scales. impact the hardness readings.

MEASUREMENT APPARATUS Figure 6 is a micrograph showing the indentation made in There are a couple of types of devices that can be used for a fine microstructure 319-F sample using the HBW measuring the diameter of the indentations made.5, 6 The 10/500 scale. This is the standard scale used in foundries Type A device is a microscope that can make in North America. When the indentation was read using continuously variable measurements such as image the metallograph a hardness value of 80.2 HBW 10/500 analysis equipment.5 The Type B device is a hand-held was calculated. When the indentation was read using a scope with fixed graduated lines.5 A couple of different stereoscope a hardness value of 84.6 HBW 10/500 was Type A measurement devices were examined. A calculated. comparison was made between measuring the diameter using a stereoscope connected to a computer with image analysis software and a metallograph connected to a computer with the same image analysis software. Figure 5 shows the average results of the study.

Figure 5. Graph showing that the average hardness reported when measuring the indentation with a Figure 6. Micrograph showing the microstructure stereoscope is higher than the average hardness sampled using the HBW 10/500 scale. reported when measuring the indentation with a metallograph. Figure 7 is a micrograph showing the indentation made in The study was performed on six 319-F samples. Two the same sample as shown in Figure 6 using the HBW samples were a fine microstructure, two samples were a 10/1000 scale. This would be the recommended scale coarse microstructure, and two samples were selected in based on requirements in ISO 6506. When the indentation between the extremes. On average, the stereoscope was read using the metallograph a hardness value of 81.1 resulted in a slightly higher reported hardness value than HBW 10/1000 was calculated. When the indentation was when the reading was taken using a metallograph. This is read using a stereoscope a hardness value of 88.2 HBW likely due to the greater ability to resolve the edge of the 10/1000 was calculated. indentation in the metallograph than in a stereoscope. An evaluation of the Type B device was not performed Figure 8 is a micrograph showing the indentation made in however it is anticipated that since the resolution is lower the same sample as shown in Figures 6 and 7 using the with a Type B device, the reported hardness would be HBW 2.5/62.5 scale. This is an equivalent force-diameter higher. ratio as the HBW 10/1000 scale. When the indentation was read using the metallograph a hardness value of 85.0 EFFECTS FROM MICROSTRUCTURE HBW 2.5/62.5 was calculated. When the indentation was There were concerns that the microstructure may affect read using a stereoscope a hardness value of 86.7 HBW the hardness readings. Perhaps Silicon particles were 2.5/62.5 was calculated. influencing the hardness or even porosity. Micrographs were prepared and then indentations were performed on the polished micrographs so that we could see the microstructure that is covered by the indentations. The readings taken on the micrographs were compared to hardness readings that were taken on unmounted samples

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Figure 7. Micrograph showing the microstructure Figure 9. Micrograph showing the microstructure sampled using the HBW 10/1000 scale. sampled using the HBW 2.5/62.5 scale in a coarse microstructure.

While an exact influence of the microstructure was not able to be determined, the micrographs shown in Figure 6 through Figure 9 do show that the indentations in all scales used were sufficiently large enough to average any minor influences from eutectic volume and porosity.

CONVERTING BETWEEN HARDNESS TEST MEASUREMENT METHODS

The previous section demonstrated some of the risks in converting between scales within the Brinell hardness test method. Sometimes because of test equipment limitations, production feasibility, or sample limitations a different hardness test method may be used to perform the test and the results are converted back to the hardness test method that is specified for the component. A couple of examples of this would be the use of the Product A hardness test unit and the use of the Rockwell test method for cast iron Figure 8. Micrograph showing the microstructure components and then converting the results back to sampled using the HBW 2.5/62.5 scale. Brinell hardness values.

Figure 9 is a micrograph showing the indentation made in PRODUCT A HARDNESS TESTING a coarse microstructure 319-F sample. The coarse The Product A hardness test is a quick and portable test microstructure shows more porosity than the fine which makes it a desirable alternative to the benchtop microstructure sample. When the indentation was read Brinell hardness test for many foundries. In North using the metallograph a hardness value of 85.2 HBW America, the hardness value obtained from the Product A 2.5/62.5 was calculated. When the indentation was read hardness test is usually reported as an HBW 10/500 value. using a stereoscope a hardness value of 88.9 HBW A correlation study between the Product A unit and a 2.5/62.5 was calculated. Porosity can be seen in the benchtop Brinell hardness unit was performed. A bottom of the indentation. Even though porosity was tolerance parallelogram was used to understand how sampled in this measurement, the hardness value did not process control limits are influenced by using a Product A read softer than the fine microstructure sample which had hardness test unit versus a stationary bench Brinell no porosity. hardness test unit, Figure 10.

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Figure 10. Tolerance parallelogram showing a comparison between a stationary bench Brinell test unit on the y-axis and a Product A handheld test unit on the x-axis.

Figure 10 indicates that the process control range between Figure 11. Hardness impressions on cylinder bore wall the Product A test unit and the benchtop Brinell hardness sections using various hardness scales. unit is not equivalent. In this particular study, the process control range for the Product A unit is slightly lower than for the benchtop Brinell unit.

Figure 10 is only an example showing that a difference exists between the Product A hardness test unit and the benchtop Brinell hardness unit. Extensive studies of this difference have not been undertaken. It is recommended that if a portable device is going to be used a correlation study between the readings obtained with the portable device and a benchtop Brinell hardness unit be performed.

ROCKWELL TO BRINELL CONVERSION In a large gray cast iron diesel block application, the cylinder bores are induction hardened to improve wear resistance. In order to obtain consistent response to Figure 12. Average hardness of heat-treated gray cast induction hardening, the as-cast cylinder bore iron measured using 4 different hardness scales and microstructure must be controlled to very low levels of on 2 levels of microstructural fineness. ferrite. For process control, the Brinell hardness of the In Figure 12 notice that the spread in the data are roughly bore was specified as the control method. Figure 11 the same at each hardness level / heat treat condition; visually shows what can happen when the scale normally however, the ranking of the hardness scales changes with specified for gray cast iron is used ignoring the ASTM both test scale and microstructural fineness. Wall sections E10 requirements on indentation diameter size and are denoted with dashed lines and open markers whereas location with respect to test piece dimensions. the bearing journal locations exhibit solid lines and solid

markers. Figure 12 indicates a lack of a systematic bias Several hardness scales were compared in order to and thereby renders any attempt to provide direct determine the appropriate specification for these as-cast conversion from one scale to another problematic. bores. Samples of both cylinder bores and main bearing saddles were subjected to various heat treatments in order to obtain test pieces with a range of hardness values. The HARDNESS AND PROPERTIES heat treatments used were as-cast, annealed at 1652F The measurement of Brinell hardness is sometimes used (900C) for 1 hour and furnaced cooled, and Normalized at as a proxy for mechanical properties. One popular 1787F (975C), 1751F (955C), 1571F (855C), and 1391F relationship is shown in Equation 2 below.9 (755C) each for 1 hour followed by air cooling. Each material combination was hardness tested using Rockwell = 500 Eqn. 2 B and three Brinell scales; HBW 10/3000, HBW 5/750, and HBW 2.5/187.5. The hardness results are compared T𝑈𝑈h𝑈𝑈𝑈𝑈ere have been∗ 𝐵𝐵𝐵𝐵 several𝐵𝐵 papers written trying to correlate in Figure 12. Each point represents the average of 6 hardness to strength and even to microstructure. Basaj, et. hardness readings. al. showed that Brinell hardness could be correlated with

the pearlite content and the tensile and yield strength in

Page 7 of 10 2020 AFS Proceedings of the 124th Metalcasting Congress Paper 2020-012 (10 pages) as-cast ductile iron.10 Tash, et. al. developed a model for compression test resulting in non-correlated modulus predicting the hardness value in 319 aluminum.11 These values. correlations have limited use, however. TEMPERATURE DEPENDENCE Cáceres investigated the relationship between Vickers Hardness testing is used for evaluating the temperature hardness, flow stress, and yield strength in cast Mg-Al that some components in an engine are exposed to. alloys.12 In this paper Cáceres showed a strong correlation Specifically, temperature check valves and valve seats are between the Vickers hardness and the strain hardening used to understand the temperature that those components exponent. Cáceres also showed a strong correlation reach during engine operation. Temperature check valves between the proof stress and the strain hardening and valve seats are specially made using a steel that has a exponent. This work showed that as the strain hardening known and documented hardness response to temperature. exponent increases both the proof stress and the Vickers Cylinder heads are built using these temperature check hardness decreased. valves and valve seats and then run for a short duration on a dynamometer. The valves and valve seats are then From a physics perspective, the hardness value has to evaluated for hardness profile and the resulting hardness relate to some physical property of the material that is values are used to predict what temperature the material being tested. For indentation tests, as the indenter is being saw. pressed into the material the local stress is decreasing as the surface area over which the load is being applied is Philosophically, this same technique should be applicable increasing. An attempt was made to experimentally in age hardenable aluminum alloys. When an age determine this by using a Mechanical Testing System hardenable aluminum alloy is exposed to temperature, the (MTS) and wrought aluminum plates. A fixture was strengthening precipitates coarsen through diffusion of the manufactured that would allow a Brinell hardness elements from smaller precipitates to larger precipitates. indenter to be mounted to the MTS test frame. The Brinell Depending on the starting condition of the material, the hardness indenter was a 5 mm (0.197 in.) diameter ball so precipitate growth could result in either an increase or a a 250 kgf (2452 N) load was used. The load and decrease in strength. Since the rate of diffusion is time- displacement were recorded as the load was applied. The based, if the hardness value is known and the exposure crosshead displacement was used to calculate the surface time can be estimated then the temperature that the area of the spherical cap formed by the indenter being material was exposed to can also be estimated. pressed into the material. This surface area was used to calculate the applied stress during loading and the Figure 14 shows the effect that various temperature and decrease in the localized stress. A compression test on the time exposures have on a heat treated A356 sample. This material was also performed and overlain on the stress- particular A356 material was supposed to be heat treated displacement graph, Figure 13. to a T7 condition by aging at 374 F (190 C) for 3 hours with a resulting initial hardness of 101 HBW 2.5/62.5. However, as can be seen in Figure 17, additional aging of the material at 302 F (150 C) increased the hardness thus it may be concluded that the initial heat treat condition of the material did not actually result in a T7, overaged, condition. Exposure to temperatures of 392 F (200 C) and above did result in a rapid initial decrease in hardness becoming asymptotic around 40 HBW 2.5/62.5. The fact that the hardness values at 482 F (250 C) and 572 F (300 C) converged towards 40 HBW 2.5/62.5 might suggest that this is the intrinsic hardness limit for this particular composition.

Figure 13. Graph showing an overlay of compression test, hardness indentation, and local instantaneous stress for a 6061 aluminum sample.

It was anticipated that the compression test curve and the hardness curve would be able to align and that the point at which the local instantaneous stress and the compression curve intersected would be between the proof stress and compressive yield strength of the material. This experimental attempt did not prove to be successful however as the system stiffness in the hardness tests were apparently higher than the system stiffness in the

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Figure 14. Graph showing the effect of temperature Figure 16. Change in hardness for 319-F aluminum and time exposure on the hardness of an A356-T7 showing a hardness increase at 302 F (150 C) and a aluminum. hardness decrease at higher temperatures.

Figure 15 shows the change in hardness as a function of time and temperature. A change in hardness that is positive might indicate that the sample is not truly in an overaged condition but rather slightly underaged while a change in hardness that is negative should indicate overaging and softening of the material.

The change in hardness graph for as-cast (F temper) aluminum 319 clearly shows this aging effect, Figure 16. There is a marked increase in hardness when 319-F is exposed to 302 F (150 C) and a smaller, more immediate increase in hardness when exposed to 392 F (200 C) with a subsequent drop in hardness indicating that the material has now become overaged. This conclusion is supported by the mechanical property data shown in Figure 17. Figure 17. Tensile strength for 319 aluminum as a function of temperature.

While it does appear to be possible to use hardness to determine the temperature exposure of age hardenable aluminum alloys, it is clear that each alloy decreases hardness to a different level. Al-Si-Mg-Cu alloys (e.g. 319) tend to maintain their hardness better at elevated temperatures than do Al-Si-Mg alloys (e.g. 356). Sufficient experimentation was performed in this study to indicate trends, but additional experimentation would need to be performed before a predictive model could be developed.

SUMMARY Figure 15. Graph of the change in hardness for A356- T7 aluminum exposed to different temperature and The measurement of hardness can be used for a variety of times. purposes. While hardness itself is not an inherent physical property of materials it is a physics-based measurement and can be used as a proxy for material properties. However, there are some limitations to this statement. Some of the limitations identified during these studies are: • There is no universal equation that can relate mechanical properties to hardness for every material.

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• Converting between scales within a hardness test 8. ASTM International, “Standard Test Method for method or between hardness test methods should be Microindentation Hardness of Materials,” ASTM avoided. E384, (01 June 2017). 9. Askeland, Donald R., The Science and Engineering This study identified some specific recommendations: of Materials, Third Edition, p 210–211 (1993). • For automotive cast aluminum alloys Brinell 10. Basaj, L. J., Dorn, T. A., Headington, F. C., hardness should be read using a force-diameter ratio Rothwell, M. D., Johnson, B. D., Heine, R. W., that is equal to 10. This enables measurements that “Tensile Properties Continuum with Brinell Hardness are valid per ASTM E10 and ISO 6506 to be made of As–Cast Ductile Iron,” AFS Transactions, p 671– over a greater range of hardness values. 677, paper 99-123, (1999). • If the hardness test method specified cannot be 11. Tash, M., Samuel, F. H., Mucciardi, F., Doty, H. W., performed due to sample dimensions, then an Valtierra, S., “Experimental Correlation between alternative hardness test method should be agreed Metallurgical Parameters and Hardness in Heat– upon for use. Conversion between hardness test Treated 319 Alloys: A Quantitative Study Using methods or scales should be avoided. Factorial Analysis,” AFS Transactions, (2006). • If an alternative test method is going to be used for 12. Cáceres, C. H., “Hardness and Yield Strength in Cast process control, then a correlation study between the Mg–Al Alloys,” AFS Transactions, paper 02–001, specified hardness Product And the alternative (2002). method should be performed to establish the appropriate process control ranges. • Hardness can be used as a proxy to estimate the temperature exposure in aluminum however the estimates must be experimentally established.

ACKNOWLDEGEMENTS

The authors would like to acknowledge the GM Pontiac Materials Lab for their support in sample preparation and evaluation.

REFERENCES

1. Chandler, H., “Hardness Testing Second Edition,” ASM International, Materials Park, OH (2004) 2. Wahlberg, A., “On Brinell’s method of determining hardness and other properties of iron and steel,” The Journal of the Iron and Steel Institute, vol. 59, issue 1, pp. 243–298 (1901). 3. Smith, R. L., Sandland, G. E., “An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness,” Proceedings of the Institution of Mechanical Engineers, vol. 102, issue 1, pp. 623–641 (June 1, 1922). 4. https://www.leebhardnesstesters.com/portable_hardn ess_testers/leeb_hardness_testers (17 July 2019) 5. ASTM International, “Standard Test Method for Brinell Hardness of Metallic Materials,” ASTM E10, (01 July 2018). 6. ISO, “Metallic materials–Brinell hardness test–Part 1: Test method,” ISO 6506-1, Third edition, (2014– 10–01). 7. ASM International, “Hardness Testing,” ASM Handbook Volume 8: Mechanical Testing and Evaluation, (2000).

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