PURPOSE:

The purpose of this lab is to determine what effect heat treating and then cooling has on the hardness and grain structure of two different types of steel. The types of steel that will be utilized during the course of this experiment are 1045 which is a plain carbon steel containing 0.45 wt% carbon, and 4140 which is an alloy steel containing 0.40 wt% carbon, and approximate alloy compositions of 1.0 wt% chromium and 0.20 wt% molybdenum.

TEST SPECIMEN PREPARATION:

Before the start of the lab, the test specimens of steel were specially prepared depending on the type of testing that was to be done. In the first part of the lab which will determine the hardness of the test specimen as a function of position, the specimens were prepared as dictated by the Jominy End-Quench hardness test. Three test specimens were prepared. The specimens were cylindrical in shape, each having a diameter of 1 inch and a length of 4 inches. The specimens were heated in a high temperature oven until they were at an austenitizing temperature of 1700 degrees Fahrenheit. This part of the procedure took approximately 3 hours. Two of the specimens were then were removed from the oven one-at-a-time and placed vertically in a fixture which was mounted in such a way so that the lower end was directly over a water jet that was used to quench the specimen. The lower end of the specimen was positioned 0.5 inches above the orifice. A valve was then opened allowing a jet of water that was approximately 62 degrees Fahrenheit to come in contact with the bottom face of the specimen. This position was maintained for ten minutes per specimen. The third specimen was allowed to cool on its own in air. After all three specimens had cooled overnight, they were prepared for the hardness testing that was to follow. Each of the three specimens had approximately 0.015 milled off from opposite sides of the bar on the end that was in contact with the water during the quenching process. This was done to make flat areas for the hardness testing. The milling that was done had to be done carefully so that the specimens were not subjected to overheating which would change the properties of the material. After the milling procedure was done, one of the flats on each of the specimens was stained using machinist's dye. Then by using a scribe, points were marked off on the stained side of the specimen every of an inch to determine the location of the points of interest for the Rockwell hardness testing.

In the second part of the lab, the same two types of steel will be used, but the preparation is different. For this part of the lab, four buttons of each type of steel will be used. The buttons each had a diameter of 1 inch, were approximately 1 inch thick, and had an identification number stamped into the face of each. There were four buttons of each type of steel. All of the buttons except for numbers 4 and 8 were also heated in the oven until they were at an austenitizing temperature of 1700 degrees Fahrenheit. Then buttons number 1 and 5 were removed and allowed to air cool, buttons number 2 and 6 were quenched in water, and buttons number 3 and 7 were left in the oven for an extended period of time for the purpose of annealing. Buttons number 4 and 8 were not subjected to any form of heat treating or cooling.

PROCEDURE:

The lab was done as two separate procedures. In the first procedure, the hardness as a function of position along the test specimen will be determined using a Rockwell C hardness tester. The data that is gathered from this test will then be used to plot hardenability curves for each of the specimens. In the second procedure, the microstructure of the test specimens will be examined using a microscope to determine the phase that is present after each of the specimens is subjected to different types of cooling procedures. The specially prepared test specimens were then utilized as follows.

The bars that were prepared for the hardness testing were put into a Rockwell hardness testing machine. The bars were positioned such that one of the flat surfaces rested on the lower surface of the tester, and the impacting point came down onto the other. The free end of the bar was also suspended so that the data that was gathered could be as accurate as possible. The impacting point was brought down onto the bar at each of the scribed points and allowed to remain there until the test apparatus signaled that it was through recording the data. The display of the hardness on the Rockwell C scale, and the position of the test was recorded for later use.

The buttons had to be polished on one of the ends until the surface had a mirror like appearance. This was accomplished using a progressively finer grit wet sandpaper with each phase of the polishing procedure. The water sanders had 4 different grit sand papers beginning with a course one that was rated as 260 grit and finishing up with a fine which was rated as 600 grit. After the surface was as polished as the water sanders would allow, the specimen was finished on a twin wheel water polisher. The twin wheel water polisher used a diamond wheel first, then to complete the process an alumina wheel. After completion of the entire process, each of the eight buttons had a mirror-like finish on one of their ends.

The buttons were then dried, and a drop of nitric acid was applied to each of the polished faces. The acid was allowed to stand on the surface of the button until the surface started to darken. The acid was then washed off, and the microscope was used to observe the grain structure of the material.

DESCRIPTION OF APPARATUS:

The machining of the test specimens was done on a Bridgeport vertical mill. The polishing was done with a LECO water sanding system and a LECO two wheel water polishing machine. The hardness testing was done using a LECO R-600 Rockwell hardness tester set on the C scale. The grain structure was viewed using an Olympus SO41 microscope.

PRESENTATION OF DATA:

From the data that was gathered in the lab, the following hardenability curves were generated for 1040 steel, 4140 steel, and an unmarked steel specimen:

The microstructure that was observed for each of the four buttons of each of the two types of steel that were examined and their make-up are as shown.

DISCUSSION OF RESULTS:

From the hardenability curves that were generated, it is readily apparent that the hardness of the material drops off sharply as the cooling time increases. The end of the sample that was in contact with the water jet cooled more rapidly than the rest of the specimen. Since the hardenability curves do show that this is most definitely the hardest part of the specimen, it can then be said that the cooling rate does in fact determine the hardness of the material.

After examining the microstructure in each of the buttons of steel that were subjected to the same type of cooling processes, it is also apparent that even though the two types of steel were different in make-up, that the cooling process generated similar microstructures. By looking at the appropriate phase diagram and then knowing what type of material you started with, the temperature that it was raised to, the type of cooling process and cooling rate that it was subjected to, you should be able to predict with reasonable accuracy just what type of microstructure and therefore material properties that the finished product will posses.