Project Topics
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Experimental procedure
The procedure of how the experiments were carried out are explained here. For this work a quantitative study was carried out. The reason to why a quantitative study was chosen is because experimental work was needed in order to achieve results for the question in the project.
Chemical composition/casting
For this thesis work the school got piston rings with different materials that are to be used in this thesis work. For this work, two different piston rings were used. Each of the two rings had different material, the materials were unalloyed compacted graphite iron (CGI) and FM produced compacted graphite iron (CGI). Two piston rings, one fully ferritic and the other fully pearlitic, were cast as 800 mm in diameter, 60 mm in height, they were to be processed and examined for this thesis work.
Both alloys consist of more than ten materials, the content of each piston ring is presented in Table 1.
The piston rings were both primary made out of iron but the only thing that differed from them was the chemical composition or the microstructure. One of the material was called unalloyed compacted graphite iron (CGI) and the other material was called FM produced CGI. Throughout this whole project the unalloyed material was labeled as “A” and the FM produced was labeled as “B”, these labels were used to ease the work.
Sample preparation
To begin with, the as-cast piston rings, as shown in Figure 2, were cut in four equally sized pieces. This means they were cut into a quarter of a circle. The reason that they were cut was because the piston rings were too big for the second machine that was going to cut them into small cubes. For cutting the piston rings, a handheld circular saw was used. It was cut by the help of my supervisor and another student that also works on his thesis work.
Once each ring was cut into four pieces (a total of eight different rings, four pieces for each material. The small pieces were then cut into cubed shapes by an automatic saw machine. The machine was already programmed by a teacher in the workshop for cutting the quarter of the ring. The quarter of the ring was still heavy and it was hard to hold it in place at first so a special tool shaped like the letter « C » was produced by the workshop personnel. This tool helped when the pieces were cut by holding it so it wouldn’t drop from the other side when it was cut off the small piece. The tool that was produced was made of wood and was a good aid that made cutting simple and more important, it increased the level of safety when using the automatic saw machine.
Each quarter of ring was cut into small cube-shaped pieces, from each quarter of ring approximately 15 small cubes could be made. In total from each piston ring we got approximately 60 small pieces that could be used in this work. For each cube it took about three minutes to cut. A cube from the work can be found in Figure 3.
When all the pieces were cut they need to be machined by a CNC machine in the workshop. The workshop personnel made a software for the machine so that the cubed sample could be machined into a cylindrical sample. The pieces were machined by milling process. For each sample it took about seven minutes to machine with about 2400 steps in the software. The CNC that was used for this work can be found in Figure 4.
A special tool was used to hold the samples in place when they were being milled. This was important because the sample should be held in place tightly and if it were to move it would have changed the cylindrical shape that was needed for this work. Each 7-minute step cut only half of the sample because the other half was held in place so it wouldn’t move. When all the halves were milled it was time for the other half. Now we needed to use another tool to keep the sample in place because it was circle shaped now compared to the original squared shape. The same software was used for milling here as well, the only thing that was changed was the tool to hold the sample in place. In Figure 3 the sample can be seen in the two stages of the milling process. Figure 4 shows the sample during the milling process.
Heat treatment practices
The next step in the process after all the samples from both the piston rings were machined was the heat treatment step. The JU-cast workshop was booked by the supervisor were there were three furnaces that would be used for this step in the process. The reason to why three furnaces were used was because the first furnace was used for pre-heating the samples, the second furnace was used for the heat treatment and the third furnace was used for the salt bath. In Figure 5., the schedule for heat treating was used. By following this schedule, the work could be done easily and it was easy to keep track of what was to be done.
The furnace for preheating was important to use because if the samples were in room temperature and the samples would be put in the second furnace then the temperature would be affected greatly. This happens because the samples are too cold if compared to the temperature of the furnace. So, a furnace just for preheating was necessary in this case, it was used to decrease the amount of error sources and therefore give a more accurate result. In Figure 6 the furnaces that were used are shown, each furnace had a different temperature.
The furnaces took some hours to get to the right temperature and therefore the temperature for each furnace was set hours prior to the time of the treatment by the supervisor. Out of all the three furnaces, the furnace used as salt bath took the most time to reach the desired temperature.
The salt bath furnace hade in total nine openings that could be used to put samples in, see Figure 7. To use the time wisely a certain pattern that can be seen in the picture was made. The work efficiency was highly increased by using this pattern. In each column there were four samples and with same time from the austempering process. A schedule was used during the heat treatment as can be seen in Figure 8.
Polishing
The other half of the sample that was cut earlier, was polished in four different stages by a polishing machine that had a preset software that was used just to polish these samples. By polishing the samples, the scratches could easily be removed. Without any scratches, a clear and good picture could be taken by the microscope. The pictures that were taken for this other half were used for the micro-structure analysis.
Hardness measurement
After the austempering and austenitising process the samples were cut in half. The first half was used to measure the hardness of the material and it was done by hardness testing machine. The process involves using an indenter. In this project an indenter of the material tungsten carbide was used. A weight of 750 kg was used to make a dent on the sample surface. After this, the diameter of the dents on the surface were measured by microscope.
Microstructural analysis
After the pictures were taken by the microscope in four different magnifications, it was time for the samples to be etched. The etching process involved using an acid on the surface to show the content of the surface clearer. After this process, more pictures were taken in five different magnifications. Figure 9 shows the microstructure after the etching process, the etchant that was used is called Nital 2 %, it is a solution consisting of nitric acid and ethanol.
Phase analysis
This time an extra ten pictures were taken in the maximum magnification (100x) for use later on in the project.
The ten pictures that were taken with 100x magnification were used for phase analysis. By using a software, different parts of the picture could be colored to show the content in percentage. Figure 10 shows the microstructure before the phase analysis and Figure 11 shows the microstructure after the phase analysis.
Validity and reliability
The validity and the reliability of this project has been taken into account in order to achieve accurate and legitimate results. By performing a quantitative study for this work, results could be achieved for this work. The experimental work was necessary in order to answer the questions.
Validity
To achieve valid results for this project, certain measures were taken throughout the whole experimental process. There are many examples of measures that were taken to make sure accurate results were attained.
During the heat treatment process three different furnaces were in use. Each furnace was set to a specific temperature. Even though the furnaces couldn’t keep the exact temperature that was set, because of many factors, the temperature was monitored constantly. The frequent monitoring of the temperature was to a big help as it showed if the furnace temperature needed to be increased or decreased. With a frequent monitoring, the temperature could be kept in the desired range and therefore a more accurate and valid result could be achieved.
Another example of validity can be found during the hardness test process. In order to measure the hardness of each sample, five dents were made on the surface. However, before starting to use the machine for the samples that are to be used in this project, a test sample was used. The test sample helped in making sure the hardness test machine was in good condition and that it was ready to be used on the samples for this project.
Reliability
Another thing to take into consideration when writing a report is the reliability of the results. In order to make sure that reliable results could be achieved from the experiments, different measures were taken.
One of many measures that was performed was to take several pictures with the microscope, this helped for the phase analysis. Ten pictures were taken across the surface of the sample. If only one picture was taken then it could have affected the results because the picture could maybe show an area that doesn’t represent the sample as a whole. By taking ten pictures for the phase analysis, an even more accurate result could be achieved. Not only were ten pictures taken but the standard deviation was taken into account as well when measuring the content in percentage. The standard deviation adds to the reliability and shows for the reader that the result is spread in a certain deviation that is stated.
Another way for reliable results was to clean the equipment and samples. For example, if a sample was not clean before the pictures were taken then the picture would disturb the content. Even scratches affected the pictures as they were visible in the microscope and affected the picture. It was of huge importance to be careful and observant during the polishing process, but also to cover the polished sample surface with a plastic piece so as it would not get scratched.
Results
The results are presented under three different headings. Starting with the microstructure then the phase analysis and finally the hardness.
Microstructure
As-cast microstructure
Figure 12 (a) and (b) show the as cast microstructure of the fully ferritic and high-alloyed fully pearlitic CGI materials respectively, used in this study. The compacted graphite particles are visible in the microstructure.
Unalloyed sample, austenitised at 850 minutes and austempered at 325 oC for 30, 60, 90
As we see clearly in all heat-treated samples, presented in Figure 13, under various austenitising and austempering temperatures and times, the ausferrite matrix were evolved however, for 30 minutes austempering time there exists some areas which the austenite did not converted or transformed to the acicular ferrite and carbon-rich austenite. The amount of acicular ferrite will increase by austempering holding time. More closely the ausferrite formed matrix shows a coarse ausferrite needles would be resulted (austempering time of 120 minutes) which would be the consequence of the diffusion of the carbon and acicular growth phenomenon due to longer austempering time as observed in Figure 23.
Unalloyed sample, austenitised at 850 minutes and austempered at 375 oC for 30, 60, 90 oC for 60, 90 and 120 and 120 minutes
As can be seen in Figure 14, with changed time and temperature for austempering and austenitising, the amount of acicular ferrite and austenite doesn’t seem to change to significantly. However, by austenitising and austempering the sample for 120 min respectively, the acicular ferrite increased. This can also be seen in Figure 24.
Table of contents
1 Introduction
1.1 CLASSIFICATION OF CAST IRON
1.2 BACKGROUND
1.3 PROBLEM DESCRIPTION
1.4 PURPOSE AND QUESTIONS
1.5 DISPOSITION
2 Theoretical framework
2.1 LITERATURE SURVEY
2.2 POTENTIAL APPLICATIONS OF AUSTEMPERED DUCTILE AND COMPACTED GRAPHITE IRONS
3 Experimental procedure
3.1 CHEMICAL COMPOSITION/CASTING
3.2 SAMPLE PREPARATION
3.3 HEAT TREATMENT PRACTICES
3.4 POLISHING
3.5 HARDNESS MEASUREMENT
3.6 MICROSTRUCTURAL ANALYSIS
3.7 VALIDITY AND RELIABILITY
4 Results
4.1 MICROSTRUCTURE
4.2 PHASE ANALYSIS
4.3 HARDNESS
5 Discussion and conclusion
5.1 HOW DOES THE AUSTEMPERING PROCESS, AUSTEMPERING AND AUSTENITISING TIME AND TEMPERATURE, AFFECT THE MICROSTRUCTURE OF ORDINARY FULLY FERRITIC AND HIGH-ALLOYED CGI MATERIALS?
5.2 HOW DOES THE AUSTEMPERING PROCESS, AUSTEMPERING AND AUSTENITISING TIME AND TEMPERATURE, AFFECT THE MECHANICAL PROPERTIES, IN THIS STUDY HARDNESS, OF AUSTEMPERED CGI MATERIALS?
5.3 ERROR SOURCES
5.4 CONCLUSION
5.5 FURTHER WORK
Bibliography
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Heat treatment of Compacted Graphite Iron
