Me272 - Lab 4 Results

Results and Discussion Each sample was investigated under a microscope and had its hardness tested after polishing in order to determine the microstructure. Below are images from microscopic investigation of Sample #1:

Image 1: Microscopic Investigation of Sample 1

Image 2: Microscopic Investigation of Sample 1

Image 3: Microscopic Investigation of Sample 1

The images reveal this sample to be pearlite. When the carbon-iron alloy is heated above the eutectoid temperature for a period of time, it becomes the stable form austenite. Gamma-phase iron austenite is able to hold much more carbon in solution. However, when it is cooled, the carbon falls out of solution and produces different microstructures. Existing in one phase, austenite is like a blank slate from which different iron-carbon microstructures can be produced based on the cooling. Pearlite is formed when the alloy is slowly cooled below the eutectoid temperature. Pearlite is a lamellar structure; it consists of alternating layers or plates of ferrite and cementite (Fe3C). As seen in Image 2 and Image 3, there are islands of pearlite in a sea of austenite in the sample. The yellow is the primary phase, while there are patches of pearlite structures within it. Image 1 shows a section which is mostly pearlite, which highlights the alternating plate structure of pearlite.

Below is an image from Sample #2:

Image 4: Microscopic Investigation of Sample 2

When pearlite is heated treated at just below the eutectoid temperature (i.e. 700 degrees Celsius) for a long period of time, sphereoidite is formed from pearlite. The alternating plates of cementite are changed into spheres in order to lower their Gibbs Free Energy by increasing their surface area. This causes grain growth. Image 4 shows sphere-like patches of cementite surrounded by the alpha-iron primary phase.

Below are images from Sample #3:

Image 5: Microscopic Investigation of Sample 3

Image 6: Microscopic Investigation of Sample 3

Image 7: Microscopic Investigation of Sample 3

When austenite is cooled quickly to a low temperature, martensite is formed. This rapid temperature change does not allow enough time for carbon to diffuse out of the gamma phase. Martensite has a diffusionless phase transformation. The quick cooling allows for little grain growth, resulting in grains that are very fine. As seen in Image 4, 5, and 6, martensite grains take on a fine, needle-like appearance.

Hardness Testing

The microstructure of the iron-carbon alloy affects its material properties. The amount of grain growth affects the hardness of the material, as given by the Hall-Petch relationship:

σ y = σ0 + k y d −1 / 2

where σy is the yield strength, d is the grain diameter, and σ0 and ky are both material constants. According to this relationship, smaller grain size causes a material to be stronger because there is a greater amount of grain boundary to impede slipping motion.

The microstructure of each iron-carboy alloy can be determined from its hardness. Since martensite is cooled very quickly from austenite, there is very little grain growth. This results in a very strong and hard material. Pearlite, while not as strong as martensite, is still a relatively strong material. The alternating plate structure add to its strength since there the plate structures stick to each other. This prevents deformation in the structure. Fine pearlite, for example, is stronger than coarse pearlite because fine pearlite has more alternating plate layers.

Spheroidite is the weakest of the three. While the sphere structures help prevent deformation, it is not as effective as pearlite’s plate structure or martensite’s fine grain structure. Spheroidite’s are allowed to grow larger than the other two, therefore it has less grain boundary to reinforce the material. Spheroidite should have the lowest results on the hardness tester since it is more ductile than the other two.

Below are the results from the Rockwell B Hardness Tester:

Sample #1

Sample #2

Sample #3

76.8

68.8

87.3

77.7

72.4

92.6

80.7

69.5

89.8

78.8

69.5

90.2

77.2

69.8

86.3

Table 1: Load for Rockwell Testing of Three Samples

For Sample #2, the first value of 39.0 was discarded because it was most likely the oxide layer breaking.

Sample #1

Sample #2

Sample#3

Average

78.2

70.0

89.2

Standard Deviation

1.57

1.39

2.50

Table 2: Average Load and Standard Deviation for Rockwell Testing of Three Samples

Figure 1: Bar Chart of Average Values of Hardness Load from Rockwell Hardness Tester

The results from the hardness tester corroborate with the images. Sample 3 is the hardest, therefore it should be martensite. Martensite is the hardest because its small grains impede deformation. The second hardest is Sample 1, which was shown to be pearlite. Pearlite’s plates impede slippage in the structure. Sample 2 has the lowest load in the hardness test, which shows it to be spheroidite. Spheroidite is the most ductile of the three due to its low grain boundary area.