CHAPTER 4 HEAT TREATMENT

Transcription

1 27 CHAPTER 4 HEAT TREATMENT 4.1 INTRODUCTION Pistons are subjected to different type of heat treatments in order to promote better bonding characterisation. The bonding nature is observed through Scanning Electron Microscope (SEM) and the phases present at the bonding region are observed through X-ray Diffraction (XRD) study. The shear strength test comparison is done on the heat treated and as cast pistons. 4.2 HEAT TREATMENT Heat treatment plays vital role to increase the bond strength and hardness of the material. It is often associated with increasing the strength of material, but it can also be used to change certain manufacturability objectives. It also enhances the desirable properties of the component without changing the shape. It leads to increase the performance of the component Specimen Preparation Typical macrograph of a piston with the cast iron insert is presented in Figure 4.1. The as cast piston is sectioned into different segments to study its characteristics. Typical specimens are presented in Figure 4.2. Preliminary trials are conducted on these specimens to arrive at heat treatment schedules for the piston.

2 28 CI Al Al Figure 4.1 Piston with cast iron insert CI Al Figure 4.2 Typical specimen of piston

3 Heat Treatment for Pistons The pistons are subjected to heat treatments at different temperatures with tempering and without tempering. Based on the preliminary studies, it is subjected to the following heat treatments: As cast piston without any heat treatment. Piston heat treated at 503 K for 7 hours and quenched in air. Piston heat treated at 773 K for 9 hours and quenched in water then tempered at 473 K for 2 hours METALLURGICAL EXAMINATIONS Scanning Electron Microscope The bonded region of aluminum and cast iron is observed through SEM. To have better understanding, treated and untreated pistons are subjected to the micro structural studies. Metallurgical characteristics of as cast and heat treated pistons are studied through scanning electron microscope. The changes of phases present at aluminum cast iron bonding region (Al-CI) are analyzed through XRD test. Specimens are cut from the heat treated and un treated pistons. Specimens are polished and etched with Nital (10%) as an etching agent. Typical microstructures of the untreated (as cast) pistons are presented in Figures 4.3 and 4.4. At lower magnification, no remarkable discontinuity is observed, however, at higher magnification, de bonding zones are observed at the interface. Cracks are also observed at the nearby zone of bonding. This lack of integrity naturally results in reduced strength and associated poor qualities.

4 30 Microstructures of the air quenched piston are presented in the Figures 4.5 and 4.6. In this case, clear bond interface is observed even at higher magnification (x1500). Better integrity of aluminum and cast iron is observed. Presence of cracks at the bonding interface is minimized. From the Microstructure, it can be clearly seen that aluminum and cast iron are perfectly bonded. No remarkable voids and cracks are observed at the interface. Microstructure at higher magnification (x1500) reveals the same. White layer, observed at the bonding zone reveals the diffusion of aluminum through cast iron part. This kind of perfect registry improved the strength. SEM micrographs of water quenched piston are presented in Figures 4.7 and 4.8. From the figures, it can be understood that the bonding interface is continuous which is comparable with the air quenched specimen. No debonding zones are observed at the interface. Cracks are observed at either side of the bonding zone. More number of cracks are observed at the aluminum side.

5 31 Figure 4.3 Typical micrograph of as cast piston Figure 4.4 Typical micrograph of as cast piston (higher magnification) Figure 4.5 Typical micrograph of as air quenched piston

6 32 Figure 4.6 Typical micrograph of air quenched piston (higher magnification) Figure 4.7 Typical micrograph of water quenched piston

7 33 Figure 4.8 Typical micrograph of water quenched piston (higher magnification) X Ray Diffraction Normally X Ray Diffraction study is used to analyze the phase present with information on lattice type orientation spacing (Venkatesh V.C et al 1982). The phases present in Al-Si and Fe system can be understood from the ternary phase diagram (Ragavan.V 2002). Fe Al 2, Fe 2 Al 5 and Fe Al 3 are the possible intermediate phases in Fe-Al system as explained in Table 4.1. There are nine forms of ternary compounds in Al-Si and Fe system.

8 34 Table 4.1 Al-Fe-Si crystal structure [54] Phase % Composition Al Fe Si Al 2 Fe 3 Si Al 2 FeSi Al 2.7 FeSi Al 15 Fe 6 Si Al 4.5 FeSi Al 63.5 Fe 20.5 Si Al 6 Fe 4 Si Al 2 Fe 3 Si Al 4 Fe 1.7 Si Specimens of treated and untreated are subjected to XRD studies in order to understand the typical changes in phases. These changes affect the performance of pistons. Figure 4.9 gives the XRD profile of as cast Al- CI piston. Peaks of iron, iron carbide, aluminum and its intermetallic compounds are observed. However, in this case, presence of very hard phases is not noticed. XRD profile of air quenched specimen is presented in Figure No remarkable changes are observed in the air quenched specimen when compared with the intensity of the peaks of untreated specimen. Figure 4.11 shows the XRD profile of the water quenched specimen. Compared with previous specimens (as cast and air quenched) there are remarkable changes in the profile. Presence of hard phases (martensite) with increased intensity is observed. Presence of such hard phase increases the hardness of the zone.

9 Aluminum; Al Fe2 Al3 Si3 Aluminum; Al Hagg Carbide Hagg Carbide Hagg Carbide Aluminum; Al Hagg Carbide Aluminum; Al

10 Hagg carbide, Fe5 C2 Fe2 Al3 Hagg carbide, Fe5 C2 Aluminum Iron Carcide; Al Fe3C Hagg carbide, Fe5 C2 Hagg carbide, Fe5 C2 Hagg carbide, Fe5 C2 Aluminum Iron Carcide; Al Fe3C Hagg carbide, Fe5 C2

11 Iron, Fe Hagg carbide, Fes c2, Iron carbide, Fe4 C0.63 Alumnium Iron, Fe 2 Al3 Hagg Carbide Fe5C2 ; Iron Iron Carbide Marternsite; C0.12 Fe1.88; Iron Carbide ; Fe4 Aluminum Iron ; Al 0.4 Fe 0.6; Iron Carbide; Fe4 Iron Carbide; Marternsite; C0.12 Fe1.88; Iron Carbide ; Fe4

12 HARDNESS SURVEY The polished specimens of Al-CI (treated and untreated) are studied through hardness survey. Hardness data was taken using Brinell hardness tester at five different locations on the specimen at either side. The average data was calculated. Equivalent hardness in Vickers scale is reported. The hardness variation for as cast, air quenched and water quenched piston of aluminum and cast iron sides are presented with the equivalent Vickers hardness (HV) value in Figures 4.12 and 4.13 respectively. Hardness chart As cast Air quenched Water quenched Specimen Al Figure 4.12 Hardness chart of Aluminum Hardness chart CI 130 As cast Air quenched Water quenched Specimen Figure 4.13 Hardness chart of Cast iron

13 39 Compared to as cast piston, the hardness of aluminum and cast iron are observed to be increasing due to the heat treatments such as air and water quenching. Precipitation strengthening and possible phase changes have occurred in the treated pistons. This might be the reason for the enhancement of hardness. Compared to air quenched pistons, water quenched ones exhibit higher hardness on either side. 4.4 STRENGTH TEST The bimetallic piston has to be machined to get its nearest net shape. There is a possibility of separation of cast iron insert from aluminum body during machining. This is due to the shear stress, which occurs during machining of such bimetallic pistons. Hence, interfacial bond between aluminum and cast iron should not break/disintegrate during machining. The shear strength estimation gives an idea of machining capacity of the pistons. Shear strength testing or push out test is normally done to determine the strength between the reinforcement and base alloy. Typical study was also carried out by Durrant et al (1996) for squeeze cast aluminum with mild steel insert. Shear/push out test is done to estimate the bonding strength. Fuel Instruments and Controls Ltd, make Universal Testing Machine (UTM) is used to determine the shear strength. A round pin is used to apply load at the Al-CI interface. The load and displacement data are recorded from UTM. Typical positions of round pin prior and after the application of load are presented in Figure 4.14 and Figure 4.15 respectively.

14 40 Figure 4.14 Typical load sketch before shearing Figure 4.15 Typical load sketch after shearing Complete piston is supported and the load is applied to evaluate the strength related aspect. Typical load test on piston is done, and is presented in Figure 4.16.The crack portion due to shear is also shown in Figure 4.17.

15 41 Figure 4.16 Typical application of load on piston Figure 4.17 Cracked piston after strength test

16 42 Typical behavior of air quenched piston, when subjected to shear strength is presented in Figure Figure 4.18 Typical strength test result of air quenched piston The typical data of breaking load, ultimate load and elongation are presented in Table 4.2. Table 4.2 Typical strength test result Parameter Peak load Displacement at Max force Breaking Load Maximum Displacement Result 45.5 KN 2.9 mm KN 3 mm Similarly, strength test also conducted for heat treated and untreated pistons. Typical shear strength (load at which bond breaks) data of the treated and untreated pistons are presented in Table 4.3.

17 43 Table 4.3 Strength result of different pistons Specimen As cast Air quenched Water quenched Strength (KN) From the Table 4.3, it can be observed that the as cast piston (without any heat treatment) exhibits the least strength among the tested specimens. It is due to the poor bonding of aluminum and cast iron insert. Air quenched specimen exhibits better strength when compared to untreated specimens. However, water quenched specimen exhibit the highest strength among all. 4.5 SUMMARY This chapter emphasizes the characteristics of various heat treated and as cast pistons. Bonding nature is analyzed through SEM study. XRD study reveals the phases at the interface. Hardness and shear strength data revealed the mechanical characteristics of the pistons. Performance of treated and as cast pistons are compared and shown in Table 4.4. Table 4.4 Comparison of different treated pistons Properties Untreated Treated Treated (Air quenched) (Water quenched) XRD No hard phases Hard phases found Hard phases found SEM Poor bonding Good bonding Cracks at either side Hardness Least High Very high Shear Strength 145 Mpa 177 Mpa 220 Mpa