TYPES OF CAST IRON

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Method and implementation

Sample preparation 

In collaboration with the Federal Mogul, the piston ring was provided to perform heat treatment and make the samples to obtain mechanical properties. With the help saw machine, the samples were cut into pieces and then with the help of CNC machine tensile bars were prepared. The tensile bars were prepared according to the ASTM – E8/E8M – 16a standards with a gauge length of 30mm, gauge diameter 6mm, fillet radius of 6mm, diameter of the grip section was 10mm and the total length of the bar with grip section was 60mm.

Microstructure evaluation 

Microscopy analysis was used to identify the matrix of the microstructure and perform analysis. Olympus Light Optical Microscope (OLOM) was used for the analysis. A polished sample is used under an optical microscope to get a good result. The microscope is accompanied by Olympus steam image analysis software, which enhance us to get live image, measurements, and the analysis of the microstructure. The austempered samples are cut into half and the polished samples were placed in the microscope. Polishing is a method of preparing a perfect finish metallographic specimen for microscopic examination. The polished samples were washed with the ethanol, followed by the 30 sec ultrasonic agitation, washed with acetone and dried. The samples were etched with 2% Nital solution. Etching reveals the microstructure of the metal through selective etching solution. Pictures of both etched and unetched samples were taken at different magnifications.

Hardness 

Hardness test is used to determine the resistance a material towards permanent deformation by penetration of an indenter of another hard material. In this case, the hardness test was done on the austempered samples. The hardness tests used here were Brinell hardness test and the Vickers hardness test for the analysis. In Brinell hardness measurement, 750kgf load was applied onto the samples with a 5mm tungsten carbide ball. Whereas, 500gf of load was applied in the case of Vickers hardness measurement. Later the pictures were taken under the OLOM for the measuring the impact of indentations.

Heat treatment 

Heat treatment is the process in which the material changes its properties to achieve a much-improved mechanical and tribological properties. The process of heat treatment starts by placing the samples under pre-heating furnace. The pre-heating is a crucial process to pre-heat the samples and prepare themselves to be placed under the main furnace. Suppose the samples were placed under the main furnace with pre-heating, then there would be a great drop of temperature in the furnace and might get an incorrect result. The time plays a crucial role under austenitization temperature. The temperature in the pre-heating furnace was kept to 400oC and the samples were pre-heated for 30 minutes. After pre-heating, the samples were quickly placed under the main furnace. The temperature under the main furnace was set to 850oC. As the austenitization temperature was fixed, the samples were set to fixed austenitization time 60 minutes for the first batch and 90 mins for the second batch. After the austenitization process, the samples were ready to be submerge under the Salt bath heat-treatment furnace for the austempering process. For some set of samples, the temperature was set to 275oC and for the other set of samples it was set to 325, and 375oC. Same as the austenitizing temperature, the time plays a crucial role under austempering temperature. After 30 minutes, 1st set of samples were removed, and let them cool in the open air. Similarly, the 2nd, 3rd and 4th set of samples were removed after 60, 90, and 120 minutes.

Tensile test 

Tensile test is used to measure an object resistance to deform and finally fractures when maximum stress is induced. After the heat treatment process, the samples were further taken in account for tensile test, where the ultimate tensile strength, yield strength, and Young’s modulus is obtained from the tensile curve. The tensile machine used on the samples was Zwick Z100, and the testing was carried out according to ASTM-E8 at room temperature. A constant engineering strain rate (4×10-4 s-1) was used for tensile testing. The results were recorded and later they were converted from engineering stress strain curve to true stress strain curve with the help of MATLAB script.

Findings and analysis

Microstructure 

The samples of CGI were heat treated to obtain an optimum result in mechanical properties without changing microstructure of the graphite particles [22]. After the samples were heat treated, they were etched thus it influence on the surface profile at grain boundaries, which in results in microscopic inspection. From the figure 3 (a), it was observed that the polished samples show the mixed combination of flakes and spheroidal nodules. After etching, the microstructure of the austempered CGI was revealed as shown in figure 3(d), which signifies that the austempering was successful onto the samples, when compared to the etched conventional CGI as shown in figure 3(b).
From the figure (4, 5, 6, 7, 8, & 9), we observe that after material was heat treated, it forms a dark-needle shaped ferrite, known as acicular ferrite. The above figures represent the matrix of the austempered CGI, where the austenitising temperature is kept at 850oC with different times (60 and 90mins) and austempered at different temperatures (275, 325 and 375oC), and different time interval (30,60,90, & 120 mins). The acicular ferrite formation after the samples were etched shows the success of the austempering on the conventional CGI samples. In some pictures as shown in figure 8(a) and 9(a) there is visibility of lower bainites, whereas pictures shown in figure 4(a) and 5 (a) is fond of upper bainites. The visible white area is the untransformed austenite or retained austenite obtained after the heat treatment process. The untransformed austenite may form to martensite. The grey area etched region is identified as the carbon-enriched austenite [22].

nfluence of the austenitizing temperature and time on the mechanical properties.

From the figure 10,11, 12, and 13, shows the austempering temperature and time on Brinell and Vickers hardness of the austempered CGI. It shows clearly that with an increase in austempering temperature, the microstructure become coarser. The coursing behavior with retained austenite results in reduction of hardness and strength value but increases the ductility and impact energy of the material [23] . After the heat treatment of the conventional CGI, the material shows a significant growth in the hardness values. Initially with conventional CGI showed a result of 138HBW for Brinell and 181HV for the Vickers hardness, but after the heat treatment, there have been a gradual increase in hardness value. It was noticed that the low austempering temperature shows higher mechanical strength compared to the high austempering temperature, this is due to the formation of fine acicular ferrite and low amount of austenite. The maximum hardness is both Brinell and the Vickers hardness was observed in the low austempering temperature (275oC) and minimum for the high austempering temperature (375oC). There wasn’t change in the trend for all the three austempering temperatures (275,325 and 375oC) with the increase in the austempering time from 30 to 120 mins. The short austempering time has low fracture toughness but exhibits good hardness. From the results, the short time period (30 mins) was enough to produce the ausferrite structure in all the austempering temperatures. In the case of austenitising temperature 850oC and austenitizing time 90 mins, there was slightly increase in the hardness which might be the formation of the retained austenite to ferrite and carbides.
From the figure (14,15,16,17,18 and 19) as shown above signifies the tensile trend which was evaluated using the MATLAB script to determine the ultimate tensile strength, yield strength, and Young’s modulus. The bar graph shows the mechanical properties at different austempering temperatures at different time interval. A similar kind of results has been observed as that of the hardness test that is with the decrease in the austempering temperature there is increase in the tensile properties. From the graph it is observed that the low austempering temperature (275oC) has the higher tensile and hardness value is due to the lower ausferritic microstructure which results in improvement in hardness, strength and fatigue of the material [23]. But from the young’s modulus graph, it shows that there is not much of a difference under different austempering treatment conditions.
During the analysis of the stress strain curve under the tensile test, it was observed that the higher austempered temperature has significantly more elongation than that of the lower austempering temperature. With the increase in austempering temperature results in retained austenite and carbon enrichment, which results in elongation and ductility of the material. Whereas the lower austempering temperature results in low ductility but has comparatively higher hardness and tensile strength.
As it is a confidential document, it is not possible to show all the pictures and the graphs, but few of the tensile curves has been uploaded in the appendix.

Fracture surface analysis

The fracture surface of the tensile test of the samples of all the austempering treatment condition were analyzed by SEM studies. The figures shown above are the fractographs of the tensile test samples. The pictures reveal the ductile fracture for the higher austempering temperature (375oC) with some dimple and cleavage fracture. It is observed that the fractures formed under all the austempering treatment conditions were tend to brittle fracture. The pictures reveal that the lower austempering temperature has the mixture of quasi-cleavage and dimple fracture which results in brittle fracture. However, the high austempering temperature reveal more dimple which indicates the ductility of the material.

Discussion and conclusions

In this part the discussion and the conclusions of the methods as well as findings are made.

Discussion of method

The method of obtaining the Austempered CGI with the help of heat treatment was to obtain improved mechanical properties with compared with the conventional CGI. The acicular ferrite obtained after the heat treatment varies from the conventional CGI. To obtain the results, factors like time and temperature were taken in account. The samples were austenitised at 850oC for 60 and 90 mins, later they were austempered at 275, 325, and 375oC for 30, 60, 90 and 120 mins. The austenitization was performed under the furnace and the austempering was performed under the salt bath furnace.
To validate the results, analysis was performed to the austempered CGI to obtain the mechanical behavior. Microstructure analysis was performed for each austempered CGI sample on LOM. To obtain the mechanical properties, tensile and the hardness test was performed. The tensile test was performed on a constant engineering strain rate. While the hardness test was performed on two different tests. The hardness test used here were Brinell and Vickers Hardness and later pictures were taken under LOM for the analysis of the results. The tensile test performed onto the samples were then taken on for SEM for the fractography analysis.

Discussion of findings

The heat treatment performed onto the conventional CGI has obtained a significant improvement to the mechanical properties and shows the effect in the microstructure as well. It shows that with the increase amount of acicular ferrite, shows increase in mechanical properties. The conventional CGI was austenitised at 850oC for 60 and 90 mins, which was eventually it was austempered at 275, 325, & 375oC for 30,60,90 and 120 mins. The austempered samples when noticed under the LOM, showed a significant change in matrix structure, forming acicular ferrite. From the figure 4,5,6,7,8, and 9 shown above under the microstructure analysis has all obtained the acicular ferrite which signifies that the heat treatment performed onto the samples were successful.
It showed that lower austempering temperature (275oC) was enough to obtain the acicular ferrite. The process window for the formation of the acicular ferrite under lower and higher austempering temperature is like that of the ADI as mentioned above the theoretical background.
The mechanical tests performed onto the austempered samples have shown significantly increase in the mechanical properties when compared to that of the conventional CGI. The trend shown in figure 10,11,12, and 13 as shown above under the hardness test, signifies that samples under the lower austempering temperature, the samples have higher hardness values, whereas the higher austempering temperature has significantly lower hardness values. Similar kind of trend is observed under the tensile test results. The samples with high carbon saturated austenite has significantly decreased in the hardness but improved ductility of the samples. Overall, the austempered CGI has improved mechanical properties when compared to the conventional CGI.

Conclusions

The austempering process performed on the conventional CGI has significantly improved the mechanical properties. The samples of low austempering temperature (275oC) resulted in more hardness and tensile strength when compared to the samples of high austempering temperature but with the increase in austempering temperature results in increase in ductility of the material due to the high carbon stabilized austenite.
The heat treatment of the CGI from 275 to 375oC forms the acicular ferrite along with retained austenite. With an increase in austempering temperature, the microstructure becomes coarser. The austempering at lower temperature (275oC) results in greater volume of acicular ferrite and low amount of austenite which results in higher hardness and tensile strength, whereas the austempering at higher temperature results in coursing behavior with retained austenite which results in increase in ductility and impact energy. With the increase in austempering temperature from 275 to 325oC, the fracture mechanism of the samples changed from brittle to ductile fracture.

Contents
1 Introduction 
1.1BACKGROUND
1.2PURPOSE AND RESEARCH QUESTION
1.3DELIMITATION
1.4OUTLINE
2 Theoretical background 
2.1CAST IRON
2.2TYPES OF CAST IRON
2.3MATRIX STRUCTURE AND THEIR CHARACTERISTICS
2.4HEAT TREATMENT
3 Method and implementation 
4 Findings and analysis 
4.1MICROSTRUCTURE
4.2INFLUENCE OF THE AUSTENITIZING TEMPERATURE AND TIME ON THE MECHANICAL PROPERTIES.
4.3FRACTURE SURFACE ANALYSIS
5 Discussion and conclusions 
5.1DISCUSSION OF METHOD
5.2DISCUSSION OF FINDINGS
5.3CONCLUSIONS
6 References
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Effects of Austempering Process on Mechanical Behavior Properties of Compacted Graphite Iron