OBJECTIVES
• Utilize the procedures for grinding, polishing and etching steel metallurgical samples to reveal the microstructure.
• Observe the grain structure of low carbon steel at a magnification in excess of 4000X displayed on a video monitor.
• Experimentally determine the densities of steel samples from measurements of true mass and measurements of apparent mass when samples are submerged in water.
• Analytically determine the theoretical density of body-centered cubic (BCC) iron based on a crystal structure model.
• Perform a statistical test of hypothesis comparing sample densities from measurements with the theoretical density of iron.
PROCEDURES
Metallography
• Obtain a steel sample from the lab instructor.
• Grind one end of the sample using the grinding platforms, starting with the coarse grit paper and ending with the fine grit paper.
• Polish the same end of the sample, first with the wheel containing 0.30-micron alumina polishing compound and then with the wheel containing 0.05-micron alumina polishing compound. Make sure the sample is cleaned with clear water before each polishing step
to avoid contamination of the polishing wheels.
• Etch the polished end of the sample for about 15 - 20 seconds in NITAL (3% nitric acid in methanol). Rinse the sample immediately in clear water and dry it with a blow dryer.
• Place your sample under the microscope and focus on a clear area that shows the grain structure of the metal on the video monitor. Try to identify several different types of grains by coloration and try to identify grain boundaries. Sketch several grains of the
microstructure.
• Use a balance to measure the true mass of your sample, M1, in grams.
• Use a length of fine thread and a small piece of tape to suspend the sample from the pan of the balance into a beaker of water. Make sure that the sample is completely submerged and not touching the sides of the beaker.
• Record the mass of the submerged sample, Ms, in grams.
• Return your sample to the instructor.
• Obtain the values of MT and Ms from all other groups in your lab section such that a statistical sample for the density can be generated.
DATA ANALYSES
• Using the mass data for each sample, calculate the true densities of the steel samples as follows:
P ( MT ( MT MS )) PH20
where: pluo = density of water = 1.0 g/cm3
• Using one unit cell as a basis, calculate the theoretical density of body-centered cubic iron, pFe, at room temperature.
• Use your sample data to test the hypothesis that the density of the steel used in this lab equals the theoretical density of iron, calculated above, versus the alternative that the density of this steel is not equal to that of iron. Use small sample methods (student's "t" distribution) and a 5% level of significance. Note that, as stated above, this is a two-sided test of hypothesis. Make sure that you clearly state your hypothesis and the alternative hypothesis, and state your conclusion in a clear and concise sentence.