EXP4 Rockwell Hardness

8/12/2019 EXP4 Rockwell Hardness

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Engr 270 AA -- Materials ScienceExperiment # 4 -- Hardness Testing of Materials

Purpose:

This experiment aims to introduce the students to the Rockwell hardness testing and its relationsto other mechanical properties such as yield strength and ultimate strength. It also introduces thestudents to the variability of materials properties, and the use of measures of central tendencyand dispersion to quantify such variability.

Background:

The general concept of hardness as a property of materials having to do with solidity and firmness ofoutline is easily comprehended, but not a single measure of hardness that is universally applicable to allmaterials has yet been devised. The fundamental physics of hardness is not yet clearly understood.

A number of different arbitrary definitions of hardness form the basis for the various hardness tests nowin use. Some of these definitions are

1. Resistance to permanent indentation under static or dynamic loads -- indentation hardness2. Energy absorption under impact loads -- rebound hardness3. Resistance to scratching -- scratch hardness4. Resistance to abrasion -- wear hardness5. Resistance to cutting or drilling -- machinability

Such definitions generally develop with the necessity for some way to qualitatively express performancerequirements under differing conditions of service. In spite of their apparent divergence in meaning, themethod of test implied by each definition has a certain useful field of application.

All the hardness measures are functions of interatomic forces, but the various hardness tests do not bringthese fundamental forces into play in the same way or the same extent, thus no method of measuringhardness uniquely indicates any other single mechanical property. Although some hardness tests seem to

be more closely associated than others with tensile strength, some appear to be more closely related toresilience, to ductility, and so forth. In view of this situation, it is obvious that a given type of test is of

practical use only for comparing the relative hardness of similar materials on a stated basis. The result of ball-indentation tests on steel, for example, have no meaning when compared with results of such tests performed on rubber but serve nicely to evaluate the effectiveness of a series of heat-treatments on agiven steel or even to classify steels of various compositions.

The fact that hardness is arbitrarily defined sets hardness tests apart from most others in an importantrespect: The standards must be scrupulously observed measurements must be exact. This contrasts, forexample, with a compression test on concrete: although major differences in specimen size has someminor effect, it makes no difference, for example, whether the specimen diameter is 6 inches (152.4 mm)or 150 mm (provided, of course, that the correct value is used to compute the area). If, however, aconcrete test were designed to check what total force breaks a standard cylinder, all cylinders would haveto be of exactly the same diameter.

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Rockwell Hardness Testing Method

In this experiment, we will use the Rockwell hardness testing method. The Rockwell hardness number is based on the additional depth to which a test point or ball is driven by a heavy (major) load beyond thedepth to which the same penetrator has been driven by a definite light (initial or minor) load --see Figure1. The penetrator type and the load establish the respective Rockwell hardness scale. The penetrator C,for the scale used to test exceptionally hard bodies, is a spheroconical diamond. The standard major loadfor this penetrator is 150 kg. For unhardened steels, phosphor-bronze, aluminum, brass, cast iron, andmany metals and alloys that are not extremely hard, a specially hardened steel balI, 1/16 in diameter, isused as a penetrator. When used in combination with a standard major load of 100 kg, the respective scaleis the Rockwell B scale (denoted by the symbol R(B)).

For making special comparative tests, the Rockwell hardness testers can be furnished with ball penetrators of 1/8, 1/4, and 1/2 and these may be used with 150 kg, 100 kg, or 60 kg major loads, eachcombination establishing a different hardness scale. The material being tested and the thickness of thismaterial determine the type of penetrator and major load to be used (see reference Table I). A RockwellSuperficial hardness tester uses smaller minor and major loads and is intended for very shallow

penetration of especially thin specimens.

Many precautions are necessary to obtain true hardness results with the Rockwell instrument, especiallysince the scale reading is a depth of penetration measurement. Precautions include the following:

1. Both the upper and lower surfaces should be flat, smooth, and clean. Curved surfacesgive low readings. Dirt, scale, oil, grease, burrs or ridges on the lower surface willsqueeze down during the test and give low readings.

2. Assure that tilting of the specimen does not occur during major load application. A directvertical line of load between base and penetrator is essential. The slightest tilting of thespecimen will change the reading, while leverage (holding by hand one end of a long specimen)may fracture the diamond penetrator.

3. Curved surfaces underestimate true hardness and with small diameters the readings areworthless. Small round pieces should be flattened by filing and grinding.

4. Thin, hard specimens are best tested using the appropriate scale.

5. The accuracy of the hardness tester should be checked against appropriate calibrationhardness blocks before tests are made on unknown specimens.

6. Successive hardness indentations should be spaced at least one and one-half diameters apart;otherwise, successive hardness values will overestimate true specimen hardness.

The Rockwell Hardness Tester is a fine and delicate testing machine and can be easily damaged by roughor improper handling. Its results are reproducible when properly made but meaningless under anycondition that precludes accurate measurement.

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There are other hardness testing machines in addition to the Rockwell and Brinell systems. In table Ill andIV, hardness conversion data are provided for some of the more common hardness scales. The data ofTable IV include empirical correlation between hardness and strength for hardenable carbon and allowsteels. As a rule-of-thumb, the tensile strength of such steel can be approximated to be one-half of theBrinell hardness value in KSI (1 KSI = 1000 psi).

Testing of Workpieces

The instructor will first calibrate the Rockwell Tester with the correct scale. After the unit has beencalibrated to the proper scale, testing can be done on the sample.

Select the anvil that will properly support the workpiece. The V anvils are for round test pieces and theflat anvils are for flat pieces. If long pieces are to be tested, an accessory item, the jack rest, should beused to properly support these workpieces.

Workpieces should be free of scale, dirt, grease, etc. The smoother the surface to be tested, the moreaccurate the readings. Follow the procedure summarized below to make the actual tests:8

1. The scale pointer should be located in the proper scale zone. This is accomplished byturning the wing nut located on the underside of the body so that the scale pointer iscentered to the correct load zone. Turning clockwise will lower the scale pointer, whilecounter-clockwise will raise it.

2. Insert penetrator.3. Select test block that corresponds to the scale being used. Select the proper anvil to

properly support the sample.4. Rest test block on anvil, turn handknob clockwise to raise the anvil assemble. When

contact is made with the penetrator, the dial pointer will move in a clockwise direction.Continue turning handknob until dial pointer makes two revolutions of the dial, comingto rest at the twelve oclock position. Move dial bezel so that pointer reads absolute zero.The minor load has now been applied.

Caution: If pointer travels past the twelve oclock position and cannot be compensated by turningthe dial bezel, repeat step #4 above. Do not back up the pointer by turning handknob counter-clockwise,

5. Raise cam handle with a smooth, steady motion to its vertical position (2-3 seconds). Do notsnap!! Dial pointer will move counter-clockwise. Wait until dial pointer stops (approximately 4-6seconds). Lower the cam handle with a smooth steady motion to its horizontal position. Pointerwill move clockwise. The number indicated by the pointer is the Rockwell number. See the scalechart to determine which color numbers to read for the particular scale being used. Note thatsuperficial testers have only one set of numbers.

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Laboratory Activity:

Obtain samples to be tested from the instructor. Measure and record the Rockwell hardness of eachsample using the C scale first. If the readings obtained are too low, measure the hardness again usingthe B scale. Make sure that the surfaces have been properly prepared; prepare surfaces by removingoxides if necessary.

Prepare a table for the test data. Always take three or more readings for each workpiece to insure thatrepresentative readings are being obtained. Calculate the average and standard deviations of the readingsfor each sample tested. Identify the sample tested (type of metal) based on the observed physical

properties (color, texture) and the observed hardness using Table II on page 5 of this lab handout.

Report:

1. Please follow the recommended format for laboratory reports. 2. Describe in detail the Rockwell test procedures. Include any relevant observations orcomments. 3. Summarize the results of observations in a table.

S ecimen Rockwell Hardness Metal T e Brinell Hardness Yield Stren th Tensile Stren th12345

4. Construct the following:a. A graph correlating the Rockwell hardness readings versus yield strength for both the B scale and the C

scale for all the materials. b. A graph correlating the Brinell hardness versus yield strength for all the materials.c. A graph correlating the Brinell hardness versus tensile strength.5. Describe and discuss the significance of the results shown in the graphs generated in 4.

Figure 1. Diagram of the Principle of the Rockwell Hardness Test.

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Table I: DESIGNATIONS FOR THE VARIOUS SCALES OF ROCKWELL HARDNESS TESTER

Scale Symbol Penetrator Major Load, kg Dial FiguresB 1/16 ball 100 Black

C Diamond 150 BlackA Diamond 60 BlackD Diamond 100 BlackE 1/8 ball 100 RedF 1/16 ball 60 RedG 1/16 ball 150 RedH 1/8 ball 60 RedK 1/8 ball 150 Red L 1/4 ball 60 RedM 1/4 ball 100 RedP 1/4 ball 150 RedR 1/2 ball 60 RedS 1/2 ball 100 RedV 1/2 ball 150 Red

Table II: HARDNESS PROPERTIES OF SOME METALS

METAL ROCKWELL HARDNESS Aluminum

a. Annealed B27b. Cold rolled B38c. Precipitation hardened B75

Brassa. Soft B42b. Hard B77

Cast Irona. Gray B83 to B99b. Chilled or White C42 to C60c. Malleable B31

Coppera. Soft B37b. Hard B60

Iron (Wrought) B48Magnesium B37Nickel B54

Steela. 1020 B74b. 1040 B76c. 1050 B86d. 1090 B95e. 2340 C48f. 4140 C25g, 5150 C4 5

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EXP4 Rockwell Hardness