Fundamentals of Rockwell Hardness Testing

Transcription

1 Funamentals of Rockwell Harness Testing Funamentals of Rockwell Harness Testing

2 Funamentals of Rockwell Harness Testing Like the Brinell, Vickers, Knoop, Scleroscope an Leeb tests - all of which fall in the general category of inentation harness tests - the Rockwell test is a measure of the resistance of material, specifically metals, to permanent inentation. Inentation harness is not a funamental property of a material. However, reliable relationships have been establishe between the various tests an important properties of materials, such as tensile strength an machinability. Furthermore, inentation harness has become one of the most reliable controls of the heat treatment an quality of manufacture parts. Rockwell testing is covere by ASTM test metho E 18. While all inentation harness tests generally serve the same purpose, each one has efinite avantages that make the test more applicable to certain types of materials an part geometries. Brinell is use primarily on forgings an cast iron. The reboun test is use on large rolls. Vickers an Knoop tests are use on very small or thin parts an for case epth eterminations on parts such as gear tooth profiles. The Rockwell test is the most popular inentation harness test an is use in a wie variety of applications. Avantages of the Rockwell Test There are several reasons for the popularity of the Rockwell test. The test itself is very rapi. On a manually operate unit, a Rockwell test takes only five to ten secons, epening upon the size an harness of the specimen, as well as pre-loa an well time. Also, the inentation is extremely small an usually oes not nee to be remove by machining, making this a non-estructive test. A Rockwell C scale test on harene steel, for example, penetrates to a epth of approximately inch, with the iameter of the inentation only inch, which is barely visible. The Rockwell test is applicable to a wie range of part sizes. Sheet metal as thin as inch can be teste on the Rockwell superficial tester, an as long as the surface area is large enough, there is no actual limitation to the size of your specimen. The Rockwell test is base on measurement of the epth of penetration with the harness number rea irectly from the ial gauge or igital isplay that is part of every tester. In comparison, tests such as the Brinell an Knoop require optical measurement of the iameter an length respectively. Direct inication of the Rockwell harness number is possible only because of the unique feature of the application of the minor loa (preliminary test force) which seats the penetrator in the work an establishes a reference or SET position from which the epth of penetration uner the heavier or major loa (total test force) can be measure. This SET point establishes the same starting point with every specimen. Principle of Test The Rockwell test consists of measuring the aitional epth to which a carbie ball or Brale iamon penetrator is force by a heavy (major) loa beyon the epth of a previously applie light (minor) loa (SET point). The minor loa is applie first an a SET position is establishe on the ial gauge or isplacement sensor of the Rockwell tester. Then the major loa is applie. Without moving the piece being teste, the major loa is remove an, with the minor loa still applie, the Rockwell harness number is automatically inicate on the ial gauge or igital isplay. The Brale iamon penetrator is use for testing materials such as harene steels an cemente carbies. The carbie ball penetrators, available with 1 /16 inch, 1 /8 inch, 1 /4 inch, an 1 /2 inch iameter, are use when testing materials such as steel-copper alloys, aluminum an plastics to name a few. Rockwell testing falls into two categories: Regular Rockwell testing (e.g., C an B scales) an Rockwell superficial testing (e.g., 30 N an 30 T scales). High Rockwell harness numbers represent har materials an low numbers soft materials. Brale is Wilson 's traemark for a iamon penetrator with a conical shape, an inclue angle of 120, an a spherical tip with a raius of mm, all in accorance with ASTM E

3 Regular Rockwell Testing In regular Rockwell testing the minor loa is always 10 kgf (kilograms of force). The major loa can be any of the following loas: 60 kgf, 100 kgf or 150 kgf. No Rockwell harness value is specifie by a number alone. It must always be prefixe by a letter signifying the value of the major loa an type of penetrator (e.g. HRC 35). A letter has been assigne for every possible combination of loa an penetrator, as given in Table 1. Each test yiels a Rockwell harness value on your tester. Testers with ial gauges have two sets of figures: re an black. When the Brale iamon penetrator is use, the reaings are taken from the black ivisions. When testing with any of the ball penetrators, the reaings are taken from the re ivisions. Testers with igital isplays have a scale selection switch, allowing an automatic isplay of the Rockwell harness number on its screen. The regular Rockwell scales are establishe such that an infinitely har material will rea 100 on the iamon penetrator scales an 130 on the ball penetrator scales. Scale Symbol Penetrator A* Brale * 60 B 1 /16 in ball 100 C Brale 150 D Brale 100 E 1 /8 in ball 100 F 1 /16 in ball 60 G 1 /16 in ball 150 H 1 /8 in ball 60 K 1 /8 in ball 150 L 1 /4 in ball 60 M 1 /4 in ball 100 P 1 /4 in ball 150 R 1 /2 in ball 60 S 1 /2 in ball 100 V 1 /2 in ball 150 Table 1: Regular Rockwell scales. * Two scales- cabie an steel. Loa in Kilograms- Force One regular Rockwell number represents a penetration of mm ( inch). Therefore, a reaing of C60 inicates penetration from minor to major loa of (100 to 60 Rockwell points) x mm = mm or inch. A reaing of B80 inicates a penetration of (130 to 80 Rockwell points) x = mm or inch. Superficial Rockwell Testing In superficial Rockwell testing the minor loa is always 3 kgf. The major loa can be one of the following loas: 15 kgf, 30kgf or 45 kgf. As with the regular scales, a scale esignation has been assigne for every possible combination of loa an penetrator as given in Table 2. Infinitely har material will rea 100 on both the iamon an ball penetrator superficial scales. One superficial Rockwell number represents a penetration of mm ( inch). Scale Symbol Penetrator 15 N N Brale N N Brale N N Brale T 1 /16 in ball T 1 /16 in ball T 1 /16 in ball W 1 /8 in ball W 1 /8 in ball W 1 /8 in ball X 1 /4 in ball X 1 /4 in ball X 1 /4 in ball Y 1 /2 in ball Y 1 /2 in ball Y 1 /2 in ball 45 Table 2: Superficial Rockwell scales. Loa in Kilograms- Force 3

4 Selecting the Proper Scale In many instances Rockwell harness tolerances are inicate on rawings. At times, however, the Rockwell scale must be selecte to suit a given set of circumstances. Many Rockwell applications are covere by the B an C scales, which are use for testing steel an copper as well as their alloys. The ever-increasing use of materials other than steel an brass, as well as thin materials, is making it even more important to have some basic knowlege of the factors that must be consiere in choosing the scale that will assure an accurate Rockwell test. The choice is not only between the regular harness tester an superficial harness tester, with three ifferent major loas for each, but also between the iamon penetrator an the steel an carbie ball penetrators - a combination of 30 ifferent scales. A valuable source of information pertaining to Rockwell scales in use by inustry on a wie variety of materials is the American Society for Testing an Materials ( Many specifications will be foun uner both ferrous an non-ferrous metals in which the applicable Rockwell scale is given. In some cases tolerances are also state. In the event no specification exists or there is oubt about the suitability of the specifie scale, an analysis shoul be mae of four controlling factors important in the selection of the proper scale. These factors are foun in the following categories: h Type of material h h h Thickness of specimen With of area to be teste Scale limitations Types of material inclue in ASTM Designation E 18 is a listing of all regular Rockwell scales an typical materials for which these scales are applicable. This table provies an excellent starting point for choosing the correct scale, loa an penetrator to be use for your test (Table 3). While Table 3 inclues only the regular Rockwell scales, this information can be a helpful guie even when one of the superficial scales may be require. For example, note that the C, A an D scales are use on har materials such as steel an tungsten carbie. Any material in this harness category woul be teste with the iamon penetrator. The choice to be mae is whether the C, A, D, 45 N, 30 N or the 15 N scale is applicable. In any event, the possible scales have been reuce to six. The next step is to fin the scale, whether it be regular or superficial, which will guarantee accuracy, sensitivity an repeatability. This will normally be etermine by a sample size, thickness an harness. Testing uner the correct conitions is the objective of any measuring instrument, particularly the Rockwell tester. Scale Symbol B C A D E F G H K L M P R S V Typical Applications of Scales Copper alloys, soft steels, aluminum alloys, malleable iron, etc. Steel, har cast irons, pearlitic malleable iron, titanium, eep case harene steel an other materials harer than B 100 Cemente carbies, thin steel an shallow case harene steel Thin steel an meium case harene steel an pearlitic malleable iron Cast iron, aluminum an magnesium alloys, bearing metals Anneale copper alloys, thin soft sheet metals Phospor bronze, beryllium copper, malleable irons, Upper limit G 92 to avoi possible flattening of ball Aluminum, zinc lea Bearing metals an other very soft or thin materials, incluing plastics (see ASTM D 785). Use smallest ball an heaviest loa that o not give anvil effect Table 3: Typical scale applications. 4

5 Thickness of Specimen The material immeiately surrouning a Rockwell inentation is consiere col-worke. The extent of the col-worke area epens on the type of material an previous work harening of the test specimen. The epth of material affecte has been foun by extensive experimentation to be on the orer of 10 times the epth of the inentation. Therefore, unless the thickness of the material being teste is at least 10 times the epth of the inentation, an accurate Rockwell test cannot be expecte. This minimum thickness ratio of 10:1 shoul be regare only as an approximation. Computation of the epth of penetration for any Rockwell test requires only simple arithmetic, however charts exist that isplay minimum thickness values alreay. These minimum thickness values (Table 5, page 13) generally o follow the 10:1 ratio, but they are actually base on experimental ata accumulate on varying thicknesses of low carbon steels, harene an tempere strip steel. A typical example of the use of the minimum thickness an conversion tables shoul be helpful. Consier a requirement to check the harness of a strip of steel inch thick of approximate harness C63. Accoring to Table 5, material in the C63 range must be approximately inch for an accurate Rockwell C scale test. Therefore, this specimen shoul not be teste on the C scale. It is necessary, at this point, to etemine the approximate converte harness on the other Rockwell scales equivalent to C63. These values, taken from the conversion chart (Table 6, page 14), are: D73, A83, 45 N 70 N, 30 N, 80 N, 15 N an 91.5 N. Referring once again to Table 5, for harene inch material there are only three Rockwell scales to choose from: 45 N, 30 N an 15 N. The 45 N scale is not suitable as the material shoul be at least 45 N, 74 N. On the 30 N scale, inch material must be at least 30 N, 80 N. On the 15 N scale the material must be at least 15 N, 76 N. Therefore, either the 30 N or 15 N scale may be use. After all limiting factors have been eliminate an a choice exists between two or more scales, the scale applying the heavier loa shoul be use. The heavier loa will prouce a larger inentation covering a greater portion of the material, an a Rockwell harness number more representative of the material. Therefore the conversion chart will also show that a one point ifference on the HRC scale is 0.5 on the HR30 N scale. Smaller ifferences in harness can be etermine using the 30 N scale. The above approach woul also apply in etermining the scale to use which woul accurately measure the correct harness when approximate case epth an harness are known. Minimum thickness charts an the 10:1 ratio serve only as guies. After etermining the Rockwell scale base on minimum thickness values, an actual test shoul be mae an the surface irectly beneath the inentation examine to etermine if the material was isturbe or if a bulge exists. If so, the material was not sufficiently thick for the applie loa, resulting in a conition known as anvil effect, an the Rockwell scale applying the next lighter loa shoul be use. On softer materials the high stress concentration ue to insufficient thickness will result in flow of the material. When either anvil effect or flow exists the Rockwell harness number obtaine may not be a true value. It is not allowe to use several specimens, one on top of the other. The slippage between the contact surfaces of the several specimens makes a true value impossible to obtain. The one an only exception is in the testing of plastics: use of several thicknesses when anvil effect is present is recommene in ASTM Designation D 785. Specifications o exist, an in fact are in common use, permitting minimum thickness values below those establishe by the above approach. However, in most instances these specifications are wiely use an serve the purpose of proviing comparison information. An example is ASTM Designation B 36 for brass sheet an strip, where the Rockwell B scale is referre to for thicknesses of inch. an the Rockwell 30 T scale for thicknesses of inch for inspection purposes. For this application, the Rockwell test is a substitution for a tensile test for which correlation has been establishe on thicknesses to inch an inch. 5

6 When testing specimens where anvil effect exists, the conition of the supporting surface of the anvil must be carefully watche. After a number of tests this surface will become marre, or a small inentation will be prouce. Either conition will affect the Rockwell test, since uner the major loa the test material will sink into the inentation in the anvil an a lower reaing will result. If a specimen has been foun after testing to be too thin, the anvil surface shoul be inspecte an if amage, relappe or replace. The anvil shoul not be use if it is marre. When testing with a ball penetrator on the superficial tester on a specimen where anvil effect or material flow is present, a iamon spot anvil can be use in place of the stanar steel anvil. If testing is restricte to the ball penetrators an the superficial scale loas, there is no anger of amaging the har iamon surface when testing thin materials. Furthermore, with materials that flow uner loa the har polishe iamon provies a somewhat stanarize frictional conition with the unersie of the specimen. Conversion Conversion charts are of value only as a guie. Rockwell harness numbers shoul be reporte on the same scale on which the actual test was mae (this is a irect reaing as oppose to a converte reaing). In the event it is absolutely necessary to report converte values, the fact that the numbers are converte shoul be inicate. Comparison between ifferent harness scales on a conversion chart oes not apply if the actual test was improperly mae. For example, if sheet metal is so thin that tests mae on the Rockwell B scale show bulges on the unersie of the sheet, then values of Rockwell 30 T cannot be obtaine by merely carrying those incorrect B scale values to the chart but can only be foun by actually making a test on the 30 T scale. If an accurate 30 T scale reaing is etermine by test, the conversion chart may be referre to for the equivalent B scale harness numbers although the material is too thin for an accurate B scale test. Proper use of the chart requires a vali an proper reaing. With of Area In aition to the limitation of inentation epth into a specimen for a given thickness an harness, there is also a limiting factor on the minimum with of material. If the inentation is place too close to the ege of a specimen (or too close to another inent), the material will yiel an the Rockwell harness number will ecrease accoringly. Experience has shown that to assure an accurate test, the istance from the center of the inentation to the ege of the specimen must be at least 2 1 /2 iameters. Therefore, when testing in a narrow area, the with of this area must be at least five iameters when the inentation is place in the center. The appropriate scale must be selecte for this minimum with. While the iameter of the inentation can be calculate, for practical purposes the minimum istance can be etermine by an inentation harness test col works the surrouning material. If another inentation is place within this col worke area, the Rockwell harness test will be affecte. Usually the reaings will be higher than obtaine on the virgin material. Experience tells us the istance from center to center of inentations must be at least three iameters. Usually the softer the material, the more critical the spacing, but three iameters will be sufficient for most materials. Wilson Rockwell 2000 series tester performing harness test on sheet using iamon spot anvil. 6

7 Scale Limitations Use of the iamon penetrator when reaings fall below HRC20 is not recommene since there is loss of sensitivity. Brale iamon penetrators are not calibrate below HRC25. If use on softer materials there may not be agreement in results when replacing the penetrators. Another scale shoul be use, for example, the B scale. There is no limitation to the harest material that can be teste with the iamon penetrator. However, the C scale shoul not be use on tungsten carbie. The material will fracture, or the iamon life will be consierably reuce. The A scale is the accepte scale in use toay by the carbie inustry. The carbie A scale inenters are calibrate to the harness levels maintaine by the CCPA (Cemente Carbie Proucers Association) an therefore give ifferent reaings than normal HRA Steel Brales. Although scales using the ball penetrator (for example, the B scale) range to 130, reaings above approximately 100 shoul be one with caution. In this region, ue to the blunt shape of the penetrator, the sensitivity of most scales is poor. Also, with the smaller iameter penetrator there is anger of flattening the ball uner the high pressure evelope on the small area of contact. If values above 100 are obtaine, the next heavier loa or next smaller penetrator shoul be use. If reaings below zero are obtaine, the next lighter loa or larger penetrator shoul be use. Carbie balls are now use for Rockwell testing. The harer carbie balls are less likely to be amage by reaings over 100. Reaings below zero are not recommene on any Rockwell scale, primarily ue to the confusion an possible misinterpretation that can result from the use of negative values. Specimen Support It is important for the accuracy of the test that the specimen be hel securely uring the application of the major loa. Consier that one regular Rockwell harness number represents a vertical movement of the penetrator of approximately 80 millionths of an inch ( inch) an one superficial Rockwell number approximately 40 millionths of an inch ( inch). A vertical shift of the part being teste of only inch will lower the Rockwell reaing by more than 10 numbers on the regular an 20 numbers on the superficial scale. Sheet metal, small pieces or other pieces that o not have flat supporting surfaces are teste on the peestal spot anvil, which has a small elevate flat surface. An anvil with a large flat surface shoul be use for supporting large parts. Anvils with a supporting surface greater than approximately three inches in iameter shoul be attache to the elevating screw by a threae section rather than inserte in the anvil hole in the elevating screw. Roun work shoul be supporte in a harene Vee anvil or in a cylinron anvil, which consists of two harene parallel cyliners. When testing small rouns, it is essential that the center of the Vee be aligne with the center of the penetrator. The piece must be straight. Irregularly shape pieces must be properly supporte on specially esigne fixtures if an accurate test is to be mae. Tubes an hollow pieces must be supporte by a manrel to ensure rigiity uner the test loas. This will prevent the tube from eforming uner the major loa, thus causing a lower harness reaing. On non-homogeneous materials a scale shoul be selecte which will give relatively consistent reaings. If a ball penetrator too small in iameter or too light is use the resultant inentation will not cover an area sufficiently representative of the material to yiel consistent harness reaings. Gooseneck extension for ease of internal testing. 7

8 Cylinrical Corrections The Rockwell test is a measure of the resistance of the material being teste to permanent inentation. There is less lateral support to the penetrating force on a convex surface than on a flat surface. Consequently, the penetrator will sink further into the material with the result that the Rockwell harness number will be lower on the convex surface than on a flat piece of the same material. For a concave surface (or ID) the opposite is true. Above iameters of one inch the ifference is negligible but for iameters of one inch an smaller the effect of the curvature must be taken into consieration. Approximate Cylinrical Correction Values will be foun in Table 7. The cylinrical corrections are ae to the ial gauge or igital isplay reaing when testing on the OD (convex surface) an subtracte when testing on the ID (concave surface). On iameters own to 1 /4 inch the regular Rockwell scales can be use; the superficial scales can by use to 1 /8 inch. Below 1 /8 inch cylinrical corrections are not available nor is Rockwell testing normally recommene. The Knoop harness test is preferre on iameters uner 1 /8 inch. To avoi the problem of curvature, a flat can be groun in the area to be teste. But in oing so, extreme care must be exercise not to affect the harness of the part, an the groun area must provie the minimum 2 1 /2 inch iameter istance from the center of the inentation to the ege of the flat. Only results obtaine on flat surfaces or values correcte for curvature shoul be use when referring to conversion charts. When testing small iameter work, alignment of the piece with the penetrator is vital. The smaller the iameter, the more critical this alignment becomes. The Importance of Stanarize Testing When two or more parties must agree on the harness of a material or part, it becomes vitally important that the parties have a mutual unerstaning of how the test will be performe. It is not unusual for a supplier, for example, to promise his customer that all parts shippe will fall within a certain harness range. Prior to shipment, a wise supplier performs a harness test on the parts, either on a sample basis or as a 100% check. The customer normally inspects the same parts as they are receive. Unless an agreement has been mae on such things as the apparatus to be use, preparation of the test specimen, calibration, test proceure an metho of reporting, the possibility of isagreement is high. Most companies look to recognize stanars organizations to provie this information. These stanars organizations generally form committees to pool the knowlege an experience of users from various inustries an formulate etaile written stanars. In the U.S., the American Society for Testing an Materials has prepare ASTM Designation E 18 entitle 'Stanar Methos for Rockwell Harness an Rockwell Superficial Harness of Metallic Materials. This has become the recognize stanar for Rockwell testing in the Unite States an many countries aroun the worl. The comparable ISO Stanar is In some cases, testers may be use, which for reasons of spee or portability, o not perform a stanar Rockwell test. Caution shoul be exercise when reporting results from these testers, as the reaings may not be ientical to those mae on a stanar Rockwell harness tester for all scales, materials, or part geometries. Stanar V. Shallow V. Cylinron. Plane. Spot. Diamon spot anvil. Gooseneck. 8

9 Stanarize Test Blocks If a tester is in constant use, verifying your tester with test blocks shoul be a aily proceure. This check inicates if the tester is out of calibration or if the penetrator is amage. If a tester is use throughout a given scale, the recommene practice is to check it at the high, low an mile range of this scale. For example, to check the complete C scale, the tester shoul be checke at C63, C45 an C25. If, on the other han, only one or two ranges are use, test blocks shoul be chosen which fall within ±5 harness numbers on any scale using the iamon penetrator an ±10 numbers on any scale using the ball penetrator. A minimum of five tests shoul be mae on the stanarize surface of the block an the average must fall within the tolerances inicate on the block for the tester to be consiere in calibration. In aition, the sprea of the five reaings (i.e., the ifference between the highest an lowest reaing) must not be greater than the value specifie in Table 4 for the harness range of the test block. Only the stanarize surface shoul be use, as this is the only surface for which the values marke on the sie of the block apply. If the average of the five reaings falls outsie the test block limits, the ifference between this average an the test block average can be note as the error of the machine, an this error can be compensate for when evaluating test results. Recommene practice, however, is either to clean an examine the tester accoring to the instruction manual or to call in a factory traine service representative. Range of Stanarize Harness Test Blocks Rockwell C Scale: 60 an Greater Below 60 Rockwell B Scale: 45 an Greater Below 45 to 1.5, inclusive Rockwell A Scale: 80 an Greater Below 80 to 60.5, inclusive Rockwell 30 N Scale: 77.5 an Greater Below 77.5 to 41.5, inclusive Rockwell 30 T C Scale: 46.2 an Greater Below 46.2 to 15.0, inclusive Note: See ASTM E 18 for other scales. Table 4: Range of test block reaings. The Repeatability of the Test Block Reaings Shall be not Greater Than: Before assuming that the tester is the prime cause of reaings outsie of test block limits, replace the ball in the ball penetrator or examine the iamon point of the iamon penetrator. Replace the penetrator if it is amage. Spacing of inentations on the test block is important. The same rule applies as when testing any material: the istance from center to center of inentations must be at least three iameters. The reaings, usually higher, from any inentations space too closely together shoul be isregare. Rockwell calibration set. 9

10 Apparatus Stanley P. Rockwell, who was a metallurgist in a New Englan ball bearing manufacturing plant, esigne the Rockwell harness tester in At that time there was no entirely satisfactory metho for controlling the harness of ball bearing races an many other harene steel parts. As a result of his stuy an experiments for a means of accurately measuring their harness, he invente the tester that has become known as the Rockwell harness tester. The wor Rockwell as applie to the tester an also as applie to test blocks has long been registere as a traemark in the Unite States an many other countries. Wilson Instruments is the original manufacturer of the Rockwell harness tester an Rockwell test blocks. The early moel Rockwell tester consiste of a stury, hollow cast frame, together with a plunger that hel the testing point at one en; the other en was abutte against a elicate measuring evice. A series of levers with knife-eges connecte that plunger with a weight. By shifting the position of this weight, more or less weight was applie to the testing point at will, to suit testing conitions. This weight, originally calle the final weight, was applie an release by a han lever. An elevating screw with chuck or anvil hel the work. An initial pressure was applie by compressing a spring in the hea of the machine. The harness was rea irectly as the increment of epth cause by the increment in loa as inicate by a measuring evice. Since 1919 the tester has been refine an improve. However, the test principle remains the same. Moern Rockwell scale harness testers are available in the following general categories: h Bench moels - igital an ial h h h Portable an mobile moels Automatic/prouction moels Moels for special applications Bench Moels The bench moel Rockwell testers are precision instruments with loa application, measurement, an other features strictly in accorance with ASTM Stanar E -18. The original system was one of weights an levers, operating on a knife-ege fulcrum, with a free-floating friction less plunger system. The plunger ro carries the penetrator an moves up an own within the hea of the machine. Only the penetrator contacts the surface being teste, an the point of test is visible at all times. The most recent system is a close-loop system, which uses a loa cell an motorize loa application. The greatest egree of harness in a metal that can be teste is limite only by the ability of a iamon penetrator to withstan the stress. Depening upon the age of the tester, the rate of loa application is controlle either by a ashpot mechanism or through a motorize mechanism, which is micro-processor controlle. Bench moel testers are available for testing on the regular or superficial scales. A combination tester, known as the Rockwell Twintester, can perform tests on both regular an superficial scales. Bench moel Rockwell testers can be equippe with a ial gauge or igital isplay of harness. Most manufacture tester moels are motorize for automatic application an removal of the major loa. Bench moel testers are available in a wie range of vertical capacities: the smallest is approximately 6 inches an the largest as great as 14 inches. The throat epth for most moels is approximately 5 1 /4 inches. In choosing the vertical capacity, not only must the size of the part be consiere, but also the size of the anvil or supporting fixture. Since 1993 Wilson Instruments has been owne by Instron Corporation. As a result, a new series of Rockwell testers have been esigne using Instron s close loop technology. These new close loop instruments have avance Rockwell testing to a new level of accuracy an repeatability. Harness family. 10

11 Automatic/Prouction Moels With the ever-increasing cost of labor an the great emphasis on quality proucts, there is a growing nee for automatic harness testers. Fully-automatic systems can be custom engineere to suit the testing requirements for a particular part. These systems can be interface with an existing prouction line to provie automatic an rapi Rockwell testing. The basic component of a fully automatic test system is a test hea such as the Wilson Instruments highspee Rockwell test hea. The Wilson Instruments high-spee Rockwell test hea performs an inentation type harness test, using the same principle as the stanar Rockwell test. The minor an major loas for the high-spee tester are evelope by feeing two known air pressures into a bellows assembly in proper sequence. The application of a known air pressure on a fixe air bellows will prouce a known force. The epth of penetration is measure by a unique electronic system. These limits are simply set on the control console by screw ajustments. Colore lights inicate if the part is soft, within limits, or too har. The stanar Rockwell test requires approximately three to six secons to complete the loaing, testing an unloaing cycle. The custom high-spee harness tester can be esigne to require less than one secon to go through the same cycle. There is consierably less time for plastic flow of the sample uring the test. This ifference in time can prouce small but measurable ifferences in epth of penetration between the stanar Rockwell test an the high-spee test. Therefore, the manufacturer cannot claim that the high-spee tester performs a true Rockwell test, but it oes prouce a test that gives similar results. For ease of application, the high-spee tester is calibrate using stanar Rockwell test blocks. This system automatically remembers the minor loa position an the major loa position an then subtracts the two to obtain the esire result. The output has a DC voltage that is equal to 0.1 volts per point or 10 volts full scale. High an low harness limit controls are inclue to allow automatic harness sorting. Rockwell series 2000 tester integrate with Instron universal test system esigne for automatic loaing an testing with a robot. 11

12 Portable Moels Many ifferent types of portable testers are available. The ieal choice is the tester that hanles the job for which it is intene while proviing the egree of accuracy require. Because of the limitations impose on the esign to keep weight to a minimum, portable harness testers are not as a rule as accurate as bench moel testers, therefore, they shoul only be consiere when the part cannot be easily brought to the tester. In fact, some esigns o not operate on the Rockwell principle, but they are usually grauate in 'equivalent Rockwell C scale harness number' an as such are of the orer of accuracy of a conversion chart. Conclusion The Rockwell tester, while a rugge piece of equipment, is also a precision instrument with many features suitable for prouction testing. To utilize this instrument to its full potential, consieration must be given to the factors governing selection of the appropriate scale as well as the curvature of the specimen an the importance of aequate support uner test. With all limiting factors taken into account an with properly esigne equipment in goo calibration, the Rockwell test can be a versatile proceure capable of measuring small ifferences in harness. Several portable testers that follow the Rockwell principle are available. In one such unit, the only eviation from the proceure followe on a bench moel tester is the clamping of the part in the C clamp of the unit. The loas are applie by calibrate springs. Many portable testers are available for unusual applications (for example, internal testing), an the manufacturer shoul be contacte for aitional information. Moels for Special Applications Rockwell harness testers have been esigne for a variety of special applications. The harness of plastics can be measure on stanar igital Rockwell testers. On certain igital Rockwell testers, well an recovery times are programmable. The user can select the uration of time uring which the major loa is applie (between 1 an 99 secons) an the amount of time after removal of the major loa before the epth is measure (also between 1 an 99 secons). High temperature testing can be performe with a special Rockwell tester equippe with a furnace an temperature recoring system. Special penetrators permit testing up to F. Custom esigne frames can be built to test parts too large for bench testers. Special testers are also available for testing o-shape parts or internal or har-to-reach surfaces. Wilson Instruments mobile moel M51. Wilson Instruments portable moel M3. Wilson Instruments moel M-250 Portable LEEB type harness tester with printer. 12

13 About Conversion Charts (Table 6) Although conversion tables ealing with harness can only be approximate, it is of consierable value to be able to compare ifferent harness scales. This table is base on the assumption that the metal teste is homogeneous to a epth several times as great as the epth of the inentation. The inentation harness values measure on the various scales epen on the work harening behavior of the material uring the test, an this in turn epens on the egree of previous col working of the material. The B-scale relationships in the table are base largely on anneale metals for the low values an col worke metals for the higher values. Therefore, anneale metals of high scale B-scale harness, such as austenitic stainless steels, nickels an high nickel alloys, o not conform closely to these general tables. Neither o col-worke metals of low B-scale harness, such as aluminum an softer alloys. Special correlations are neee for more exact relationships in these cases. Where applicable, the values are consistent with ASTM E 140 Tables 1 an 2, an ASTM A 370 Tables 3A an 3B. All other conversions shown, incluing the microficial number values, were evelope in the Wilson Instruments stanars library. Table 5: Minimum thickness chart. X: No minimum harness These values are approximate only an this chart is intene primarily as a guie. Materials thinner than shown in this chart may be teste on the Tukon TM microharness tester. The thickness of the specimen shoul be at least 1.5 times the iagonal on the inentation when using the 136 iamon pyrami inenter, an at least 1 /2 times the long iagonal when using the Knoop inenter. 13

14 Table 6: Conversion charts Note 1: A 10 mm steel ball is use for 450 BHN an below. A 10 mm carbie ball is use above 450 BHN. Note 2: The tensile strength relation to harness is inexact, even for steel, unless it is etermine for a specific material. The values shown are approximate only an these charts are intene primarily as a reference guie. 14

15 Cylinrical work for correction to be ae to observe Rockwell number for scales inicate. These corrections are approximate only an represent the averages to the nearest 1 /2 Rockwell number. Table 7: Cylinrical corrrection chart

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