THEORY

Steels go through a number of phases as they are heated and cooled. The material composition also changes in this process. By cooling the steels in different ways, different microstructures can be captured. These different ways of cooling also create different hardnesses the steels. For the case of the Jominy test bars, the bars were quenched from one end of the cylindrical test specimen. This creates different cooling rates throughout the bar. The hardness can then be tested using a hardness tester, and the hardness can be measured as a function of the distance from the quenched end. Finally, for the case of the 8 different buttons, different steel microstructures induced by different cooling, can be seen using a microscope.

TEST PROCEDURE

The Jominy test is an end-quench procedure for determining the hardenability of steel, outlined by the ASTM designation A255-85. For this procedure, cylindrical test specimen of both 4140 and 1045 steel were used. The apparatus used for the procedure consisted first of all of a small oven capable of heating the steel to its austenitizing temperature. Secondly, a fixture was used, which held the cylindrical test specimen in a vertical position as required by the ASTM Standard. The cylindrical test specimen was inserted into the fixture only after it had been heated to its austenitizing temperature for 30 minutes. The quench water was then directed to spray up at the test specimen contacting only the bottom end of the bar. The correct flow rate and water temperature was used as according to the ASTM Standard.

Upon completion of quenching, the test specimen had to have two flats ground on them along the entire length of the bars, approximately .015" deep. This was done with a carbide cutting tool in a mill. Special care was taken to disallow the test specimen from becoming too hot in this operation, because that could effect the heat treating results. After the flats were milled, one of the flats was stained with bluing, and marked off at every sixteenth of an inch, using a scale and a scribe.

Finally, the hardness of the cylindrical test specimen was tested on the hardness tester, using the Rockwell C scale. The hardness was first tested at the end of the specimen which was quenched, with more readings being taken at this end, as requested by the ASTM Standard.

The second part of the laboratory consisted of eight austenintized steel buttons. Four buttons were 1045 steel, marked #1-4, and four buttons were 4140 steel, marked #5-8. One button of each steel type was air cooled after being heated to there ausenitizing temperatures, one button of each was water quenched, and one button of each was annealed. The remaining two buttons were not austenitized.

Once cool, the buttons were polished using water sanders. Four different grits were used to achieve a finer and finer finish. When the buttons were nearly polished, they were finished off using the diamond grit, and then the alumina wheel.

In order to be able to more clearly see the microstructures, a drop of nitric acid was placed on the button, and then washed off. A specimen microscope was then used to view the different microstructures in the specimens.

RESULTS

A) Jominy Test Bars

The graph shown below is a plot of the hardness vs. distance from the quenched end for both the 1045 and 4140 steel test specimens.

From Figure 1, it can be seen that hardness of the Jominy bars indeed decreases as the distance from the end of the bar increases. Because the end of the bar experienced the greatest cooling rate, more of the carbon precipitated out in carbon nodules, and the material became harder. It could also be noted that martinsite forms in steels when they are quenched rapidly. Therefore the percent martinsite in the bar, very likely is greatest at the end of the bar, and decreases as the distance from the end of the bar increases.

Both 1045 and 4140 steels would be considered medium carbon steels (.25-.60wt% Carbon). However, from Figure 1 it can be seen that the 4140 steel bar was overall harder at each distance from the end of the bar. This is due to the fact that additions of other elements in the iron such as chromium, nickel, and molybdenum improves the capacity of these alloys to be heat treated. 1040 steel has a much lower alloy content than 4140, and therefore the data appears accurate.

B) Heat Treated Buttons

Upon viewing the heat treated buttons in this lab, different microstructures were found. In order to verify these observations, I used the phase diagram for iron-iron carbide, found on page 267 of the Material Science and Engineering book, written by William D. Callister, Jr. Furthermore, I also used the ASM Volume 7 Microstructure book.

From the phase diagram for iron-iron carbide, I found that a variety of microstructures are possible. For the situation when the steels were water quenched the most likely microstructure is austentite or martinsite. For the air cooled situation, the cooling curve, which can be approximated on the phase diagram passes through the regions which include pearlite and ferrite when the carbon content is low. Lastly, for the full annealed case, pearlite is extremely likely, with possibly ferrite or cementite.

Upon viewing the air cooled buttons, #1 and #5, pearlite was the dominant phase found. The #1 and #5 buttons very closely match picture 238 and picture 297 on page 32 of the ASM manual mentioned above. The button consists of pearlite (darker areas) and ferrite (light areas). Button #5 can be best described as both fine and course pearlite, along with lighter areas of ferrite.

Buttons #2 and #6 were both water quenched. Under the microscope, these two buttons appeared to have rather distinct areas, similar to dry up cracked mud. Looking at the phase diagram for these low carbon alloys, the gamma-phase, or austentite will be formed when the materials are quickly quenched after being brought to there austentizing temperatures. Figure 9.21 (b) in Material Science and Engineering on page 268 shows a photomicrograph of austentite. Similar to the buttons, the picture shows very distinct areas with sharp edges.

Upon viewing buttons #3 and #7, both distinct areas or colonies were found. Surrounding these areas was a plain lightly colored background. Using the ASM manual, picture 230 on page 31, and picture 296 on page 39 both show very similar structures for these alloys. The darker areas are pearlite, and the white areas are ferrite.