[PDF] 1 Engr 270 AA -- Materials Science Experiment &# 4 -- Hardness

127009_7EXP4_Rockwellhardness.pdf 1Engr 270 AA -- Materials Science

Experiment # 4 -- Hardness Testing of Materials

Purpose:

This experiment aims to introduce the students to the Rockwell hardness testing and its relations

to other mechanical properties such as yield strength and ultimate strength. It also introduces the

students to the variability of materials properties, and the use of measures of central tendency and dispersion to quantify such variability.

Background:

The general concept of hardness as a property of materials having to do with solidity and firmness of

outline is easily comprehended, but not a single measure of hardness that is universally applicable to all

materials 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 now

in use. Some of these definitions are

1. Resistance to permanent indentation under static or dynamic loads --indentation hardness

2. Energy absorption under impact loads -- rebound hardness

3. Resistance to scratching -- scratch hardness

4. Resistance to abrasion -- wear hardness

5. Resistance to cutting or drilling -- machinability

Such definitions generally develop with the necessity for some way to qualitatively express performance

requirements under differing conditions of service. In spite of their apparent divergence in meaning, the

method 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 bring

these fundamental forces into play in the same way or the same extent, thus no method of measuring

hardness 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 to

resilience, 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 a

given 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 important

respect: The standards must be scrupulously observed measurements must be exact. This contrasts, for

example, with a compression test on concrete: although major differences in specimen size has some

minor 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, a

concrete test were designed to check what total force breaks a standard cylinder, all cylinders would have

to be of exactly the same diameter. 2Rockwell 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 the

depth to which the same penetrator has been driven by a definite light (initial or minor) load --see Figure

1. 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 load

for this penetrator is 150 kg. For unhardened steels, phosphor-bronze, aluminum, brass, cast iron, and

many metals and alloys that are not extremely hard, a specially hardened steel balI, 1/16" in diameter, is

used as a penetrator. When used in combination with a standard major load of 100 kg, the respective scale

is 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, each

combination establishing a different hardness scale. The material being tested and the thickness of this

material determine the type of penetrator and major load to be used (see reference Table I). A Rockwell

Superficial 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, especially

since 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 surfaces give low readings. Dirt, scale, oil, grease, burrs or ridges on the lower surface will squeeze down during the test and give low readings.

2. Assure that tilting of the specimen does not occur during major load application. A direct

vertical line of load between base and penetrator is essential. The slightest tilting of the specimen 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 are

worthless. 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 calibration

hardness 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 rough

or improper handling. Its results are reproducible when properly made but meaningless under any condition that precludes accurate measurement. 3

There are other hardness testing machines in addition to the Rockwell and Brinell systems. In table Ill and

IV, hardness conversion data are provided for some of the more common hardness scales. The data of

Table IV include empirical correlation between hardness and strength for hardenable carbon and allow

steels. As a rule-of-thumb, the tensile strength of such steel can be approximated to be one-half of the

Brinell 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 been

calibrated 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 the

flat anvils are for flat pieces. If long pieces are to be tested, an accessory item, the jack rest, should be

used to properly support these workpieces.

Workpieces should be free of scale, dirt, grease, etc. The smoother the surface to be tested, the more

accurate 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 by

turning the wing nut located on the underside of the body so that the scale pointer is centered to the correct load zone. Turning clockwise will lower the scale pointer, while counter-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, coming to rest at the twelve o'clock position. Move dial bezel so that pointer reads absolute zero.

The minor load has now been applied.

Caution: If pointer travels past the twelve o'clock position and cannot be compensated by turning the 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 not

snap!! Dial pointer will move counter-clockwise. Wait until dial pointer stops (approximately 4-6 seconds). Lower the cam handle with a smooth steady motion to its horizontal position. Pointer will move clockwise. The number indicated by the pointer is the Rockwell number. See the scale chart to determine which color numbers to read for the particular scale being used. Note that superficial testers have only one set of numbers. 4

Laboratory Activity:

Obtain samples to be tested from the instructor. Measure and record the Rockwell hardness of each

sample using the "C" scale first. If the readings obtained are too low, measure the hardness again using

the "B" scale. Make sure that the surfaces have been properly prepared; prepare surfaces by removing

oxides if necessary.

Prepare a table for the test data. Always take three or more readings for each workpiece to insure that

representative readings are being obtained. Calculate the average and standard deviations of the readings

for 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 or

comments.

3. Summarize the results of observations in a table.

S pecimen Rockwell Hardness Metal Type Brinell Hardness Yield Strength Tensile Strength 1 2 3 4 5

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

Scale Symbol

Penetrator Major Load, kg Dial Figures

B 1/16" ball 100 Black

C Diamond 150 Black

A Diamond 60 Black

D Diamond 100 Black

E 1/8" ball 100 Red

F 1/16" ball 60 Red

G 1/16" ball 150 Red

H 1/8" ball 60 Red

K 1/8" ball 150 Red

L 1/4" ball 60 Red

M 1/4" ball 100 Red

P 1/4" ball 150 Red

R 1/2" ball 60 Red

S 1/2" ball 100 Red

V 1/2" ball 150 Red

Table II: HARDNESS PROPERTIES OF SOME METALS

METAL ROCKWELL HARDNESS

Aluminum

a. Annealed B27 b. Cold rolled B38 c. Precipitation hardened B75

Brass

a. Soft B42 b. Hard B77

Cast Iron

a. Gray B83 to B99 b. Chilled or White C42 to C60 c. Malleable B31

Copper

a. Soft B37 b. Hard B60

Iron (Wrought) B48

Magnesium B37

Nickel B54

Steel

a. 1020 B74 b. 1040 B76 c. 1050 B86 d. 1090 B95 e. 2340 C48 f. 4140 C25 g, 5150 C4 5