Introduction:

ROCKWELL HARDNESS TEST: Hardness is a measurement of material resistance to localised plastic deformation (e.g. small dent or scratch).

Rockwell harness test constitute the most common method used to measure hardness because there are so simple to perform and does not require special skill to operate. Its worldwide adoption is due to the many advantages provided by the test method. The test is fast, inexpensive, and relatively non-destructive, leaving only a small indentation in the material.

The Rockwell hardness test can provide significant information about steel materials, such as the tensile strength, wear resistance, and ductility. The test is generally useful for material selection, for process and quality control, and for acceptance testing of commercial products. Today, Rockwell hardness machines can be found in use in almost every testing environment.

Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specified period and removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from zero datum position as a result of the application of major load. The entire procedure requires only 5 to 10 seconds.

Use of a minor load greatly increases the accuracy of this type of test, because it eliminates the effects of repercussion in the measuring system and causes the indenter to break through slight surface roughness.

Experiment Procedure:

1. First is to examine the Rockwell hardness tester, insuring that the correct combination of weights and penetrators (1/16-inch ball) for the experiment are in place.

2. We make sure that the operating lever was in the position closest to the operator and that the correct major load was in position to be applied.

3. We inserted the proper indenter into the testing machine and 4. We place the test specimen on the anvil.5. Next, we slowly turn the elevating screw; this raises the anvil and specimen into contact

with the indenter. We continued to elevate the specimen until the initial load was fully applied, determined by the pointer being within 5 scale divisions of its upper vertical position. We then turned the bezel of the gage until “SET” line on the dial (the B 130 mark), set by a red arrow, was directly above the pointer.

6. After checking that the specimen was held firmly in place by the initial load, we released the operating lever with a smooth motion. Allowing the handle to move without interference until the major load was applied, we waited until the handle and pointer came to a rest.

7. We return the operating load to its original position with a smooth, gentle motion8. Finally, we recorded the result of the test off of the appropriate dial.9. We repeated this test for each specimen.

5

Results and Analysis

Tables and Graphs:

Variation of Hardness with % Rolling Reduction for Copper

% Rolling Reduction

Rockwell B

Hardness

  Test 1 Test 2 Test 3 HRB Average0% 57.9 54.4 52.2 54.8

10% 109.0 108.7 108.5 108.720% 109.8 109.5 109.2 109.530% 121.5 120.3 119.6 120.540% 119.5 119.0 118.8 119.150% 122.0 121.8 121.8 121.960% 126.8 126.4 126.0 126.470% 124.6 124.2 124.1 124.380% 124.9 124.4 124.5 124.6

Table 1

Variation of Hardness with % Rolling Reduction of Copper

5

Graph 1

Variation of Hardness with Annealing Temperature for Copper

Annealing Temperature

Rockwell B

Hardness

  Test 1Test 2 Test 3 HRB Average

100 121.9 121.5 120.8 121.4150 128.8 126.4 126.8 127.3200 127.0 126.1 125.8 126.3250 120.9 119.8 119.2 120.0400 86.9 86.4 86.2 86.5500 81.9 81.2 80.6 81.2

Table 2

Variation of Hardness with % Annealing Temperature (0C)

5

Graph 2

Most metals, the atoms are not sufficiently mobilised at ambient temperatures for new grains to form, and so the metal will remain hardened as it is clearly seen in graph 1, the high the % rolling reduction the high the hardness. For metals to form new stress-free grains (i.e. to recrystallize) it is necessary to heat the metal (i.e. to anneal) above a certain temperature, known as the recrystallization temperature.

The variation of hardness with % rolling reduction and annealing temperature for copper graph illustrates how the hardness changes according to the percentage reduction in thickness and in annealing temperature afterward.

The trend curve in the graph clearly indicates three different regions. The first section is the Recovery region, the second is the Recrystallization region, and the third is the Grain Growth region

5

In the first region, there is no much variation in the hardness which means there is no substantial change in the grain structure and orientation. This region may be considered as stress releasing region.

In the second region, there is a sudden drop in hardness; we can conclude that this is due to the formation of new stress – free grains with different grain orientations, strength reduced and increases in ductility.

In the third region (i.e. grain growth), as evidently seen on the graph, there is slow hardness drop this indicates that there was reduction in total grain boundary, large grains consume small grains by atom-by-atom across grain boundaries which implies that there is achievement of maximum recrystallized grains with different orientations, this result in decreased strength.

Discussion and Conclusion:In general Rockwell test procedure is the same regardless of the Rockwell scale or indenter being used. The indenter is brought into contact with the material to be tested, and a preliminary force (minor load) is applied to the indenter. The preliminary force is usually held constant for a set period of time, after which the depth of indentation is measured.

The hardness of the metal is measured by forcing an indenter into its surface. The indenter material which is usually a ball, pyramid, or cone, is made of a material much harder than the material being tested.

For most standard hardness tests a known load is applied slowly by pressing the indenter at 90 degrees into the metal surface being tested. After the indentation has been made, the indenter is withdrawn from the surface. An observed hardness number is then read off a dial (or digital display), which is based on the cross-sectional area of depth of the impression.

Testing precautions / Source of Error:There is tendency or possibility of errors to occur during or in the course of carrying out a measurement test in this regard Rockwell hardness test. However, the major factors that can contribute to measurement errors are; change of operator or environmental factors, and general cleanliness. Below is a summary: a. Inaccuracies also result if the test specimen is too thin, if an indentation is placed too

close to the specimen edge, the hardness number will decrease accordingly. b. To ensure an accurate test, the distance from the centre of the indentation to the edge of

the specimen must be at least two and one-half diameters.c. An indentation hardness test cold works the surrounding material. If another indentation

is placed within this cold worked area, the reading usually will be higher than the real value.

5

d. Generally, the softer the material, the more critical the spacing of indentations becomes. However, a distance three diameters from the centre of one indentation to another is sufficient for most materials.

e. It is a common practice to test most materials before they are accepted for processing, and before they are put into service to determine whether or not they meet the specifications required.

Conclusion:

It is a common practice to test most materials before they are accepted for processing, and before they are put into service to determine whether or not they meet the specifications required. One of these tests is for hardness.

This experiment shows that accuracy and precision are important aspects when performing hardness test. It was found, through various trials of the experiment, that the Rockwell is the best piece of equipment to use when carrying out hardness test. Moreover, a Rockwell machine doesn’t require any special skill to operate.

Reference:

William D. Callister Jr. Materials Science and Engineering and Introduction, 7th Edition, John Wiley and Sons Inc., 2007, pp 131- 132

Webpage

web.utk.edu/~kjohann1/mse201/lab2b.pdhttp://www.docstoc.com/docs/20502751/Lab-8---Hardness-Testingdocxhttp://www.tensiletest.com/products_services/dwnlds/SP960_5.pdfhttp://www.sut.ac.th/engineering/metal/pdf/MechMet/09_Hardness%20test.pdf

Dates accessed: December 20, 2010

The recrystallization temperature is 2000C

5