T55-Lab #1 – Hardness Testing
Object: To determine the hardness of two material specimens, one of which is unknown, and identify the unknown material.
Apparatus: Rockwell Hardness Machine
Materials: Aluminum Sample
References: Lab manual
MatWeb Property Data (http://www.matweb.com/search/GetProperty.asp)
Background: Hardness is a material’s resistance against plastic deformation. It is not a quantity defined by any of the fundamental units of measurement, such as pounds, feet and seconds. Therefore, hardness tests are relative to the apparatus on which they are conducted and the scale which was used. Although there are a number of hardness tests, the more common hardness test is the Rockwell Hardness Test. This test can be conducted using different scales, such as Rockwell Hardness Scales B and C. Different scales are used because not all materials register on one particular scale. Scale B (abbreviated HRB) uses a 1/16 in. metal ball, a 10 kg preload, and a 100 kg major load to indent the surface of the sample. Scale C (abbreviated HRC) uses a diamond cone indenter, a 10 kg preload, and a 150 kg major load on the samples.
Before the lab is conducted, the experimenter must determine the accuracy of the machine’s reading. The TA does this using a reference sample for which the hardness is known. Three readings are taken on the test sample for the scale in use; the readings are then averaged and subtracted from the known hardness for the material. The difference between them is the calibration. It is added to each hardness value arrived at during the experiment to give a more accurate idea of the hardness. If more than one scale is used, this procedure must be redone for each scale. If there is a variation, which may be related to the defects on the samples, you need to change the sample or the position of the sample.
In order to compare experimental results to a standard one, the percent error concept is useful. It is determined using the following equation:
Percent Error = ((Standard Value – Experimental Value)/Standard Value)
1. Begin by finding the calibration numbers for Rockwell Test B. Using the test sample, take three readings at each scale, and record them for later calibration.
2. Set the apparatus for the aluminum test. Use Rockwell test B: insert a 1/16 in. metal ball indenter and setup a 100 kg major load. Make sure the platform is clean (without any oil and dust).
3. Place the aluminum sample on the platform so that the indenter can hit a non-indented spot on its surface. Rotate the sample upward until it is preloaded, as is shown by the gauge on the screen.
4. Press start to begin the major load testing. After the test is finished, record the hardness value for the sample in Table I, below.
5. Repeat this procedure one more time for the same sample.
6. Now remove the aluminum sample and set the unknown sample on the platform. Rotate it into place until preloaded. Note that preloading is important, and ask the TA for assistance.
7. Repeat steps 4 and 5.
Table I: This table can be used to record the data found in the lab.
|Hardness # (HRB)|
|1st Reading||2nd Reading||3rd Reading|
Lab #1 Report Guidelines – Hardness Testing
1. Concise explanation what hardness is?
2. Why is hardness important?
3. Include the purpose of the experiment and its objectives.
1. Define Hardness
2. Explain how a Rockwell Hardness Machine produces a hardness number.
3. Explain the difference between Micro and Macro hardness
4. What is the difference between Rockwell and Brinell hardness
5. Talk about the Rockwell scales
6. What are some of the precautions to get good hardness numbers
7. List equations used (percent error).
1. Produce a table of the empirical data of known sample hardness with the average hardness and percent error.
2. Produce a table of the empirical data of unknown sample hardness and average.
1. Discuss what was learned from the known sample hardness data; i.e. what were the hardness average, standard value, and percent error? Was the error high than expected number? If so, why?
2. Discuss what was learned from the unknown sample hardness data; i.e. what was the hardness average value? Select ten candidates for the unknown sample from www.matweb.com. Arrange them in a table with their hardness numbers. Explain why they could be the unknown material. State which candidate you believe the sample to be, and justify your reasoning. By the way, only the justification will be graded.
1. Follow the format guidelines given in the manual.