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. 4.2.1 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.
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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.
4.2.1 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.
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Figure 4.1 Piston with cast iron insert
Figure 4.2 Typical specimen of piston
Al Al
CI
CI
Al
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4.2.2 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.
4.2.3 METALLURGICAL EXAMINATIONS
4.2.3.1 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.
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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.
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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
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Figure 4.6 Typical micrograph of air quenched piston (higher
magnification)
Figure 4.7 Typical micrograph of water quenched piston
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Figure 4.8 Typical micrograph of water quenched piston (higher
magnification)
4.2.3.2 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 Al2,
Fe2 Al5 and Fe Al3 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.
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Table 4.1 Al-Fe-Si crystal structure [54]
Phase% Composition
Al Fe SiAl2Fe3Si3 25.0 37.5 37.5Al2FeSi 50.0 25.0 25.0