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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

Al2.7FeSi2.3 45.0 16.7 38.3Al15Fe6Si5 57.7 23.1 19.2Al4.5FeSi 69.2 15.4 15.4

Al63.5Fe20.5Si16 93.5 20.5 16.0Al6Fe4Si6 37.5 25.0 37.5Al2Fe3Si4 22.2 33.3 44.4Al4Fe1.7Si 59.7 25.4 14.9

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

4.10. 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.

Aluminum; Al

Fe2 Al3 Si3Aluminum; Al Hagg Carbide

Hagg Carbide

Hagg Carbide

Aluminum; Al

Hagg Carbide

Aluminum; Al

Hagg carbide, Fe5C2

Fe2Al3 Hagg carbide, Fe5 C2

Aluminum Iron Carcide; Al Fe3C

Hagg carbide, Fe5 C2

Hagg carbide, Fe5C2

Hagg carbide, Fe5C2

Aluminum Iron Carcide; Al Fe3C

Hagg carbide, Fe5C2

Hagg carbide, Fes c2, Iron carbide, Fe4 C0.63

Alumnium Iron, Fe2Al3

Iron, Fe

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

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4.3 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.

Figure 4.12 Hardness chart of Aluminum

Figure 4.13 Hardness chart of Cast iron

Hardness chart

75

90

105

120

135

150

1 2 3

Specimen

Al

As cast Air quenched Water quenched

Hardness chart

130

140

150

160

1 2 3

Specimen

CI

As cast Air quenched Water quenched

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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.

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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.

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Figure 4.16 Typical application of load on piston

Figure 4.17 Cracked piston after strength test

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Typical behavior of air quenched piston, when subjected to shear

strength is presented in Figure 4.18.

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 Result

Peak load 45.5 KN

Displacement at Max force 2.9 mm

Breaking Load 39.050 KN

Maximum Displacement 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.

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Table 4.3 Strength result of different pistons

Specimen As cast Air quenched Water quenched

Strength (KN) 32 39 44

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 UntreatedTreated

(Air quenched)

Treated

(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