43

CHAPTER 4

EXPERIMENTAL INVESTIGATION

The various mechanical tests and procedures adopted in the present

study are explained in this chapter.

4.1 CHEMICAL COMPOSITION ANALYSIS

The material En 19 considered in study is obtained in the form of

20mm diameter rod. The chemical composition of En 19 steel is obtained by

using Optical Emission Spectroscope (OES) and it is reported in Table 4.1.

The obtained chemical composition of En 19 steel is within the ranges given

in the Industrial steel reference book (Bagchi and Kuldip Prakash, 1986).

Table 4.1 Chemical Composition of En 19 steel (wt %)

Sampledescription

C Si Mn P S Cr Mo

Raw material 0.45 0.35 0.75 0.017 0.019 1.19 0.21

Uncertainty +0.010 +0.013 +0.012 +0.003 +0.007 +0.007 +0.018

4.2 HARDNESS TEST

The hardness of steel is the resistance of steel to plastic

deformation. This can be determined usually by indentations. The Rockwell

44

Hardness test is carried out for CHT, SCT and DCT samples. The sample is

in the form of 10mm diameter and 30mm length.

4.3 SLIDING WEAR TEST

The wear tests of the En 19 steel are carried out in (DUCOM TR

20LE) pin on disk wear testing machine as per ASTM G99-95a (2005) by

volume loss method. The wear testing machine and monitor are shown in

Figure 4.1 and Figure 4.2.

Figure 4.1 Wear Testing Machine

Figure 4.2 Wear and Friction Monitor

45

The pin sample (10mm diameter, 30mm long) required for the wear

test is made of En 19 steel and a wear disc (160mm diameter, 8mm thickness)

is made of En 31 steel of hardness 64 HRC. The average surface roughness

value of the flat circular disc is 0.26 µm. The pin sample is fixed vertically

and the required load is applied against the rotating disk. Sliding occurs

between the stationary pin and a rotating disc. The amount of wear is found

out by measuring appropriate linear dimensions of pin sample prior to and

after wear test for all the samples. Alternately, the wear loss is also

determined by weighing the specimen before and after the tests using a

precision electronic weighing balance with an accuracy of 0.0001g. Since the

mass loss is measured, it is converted to volume loss using the density of the

specimen. In this study the experiments are categorized into two different

groups according to loads selected. The data acquired from the dry sliding

wear tests are presented as wear resistance of pin sample for different

treatments. The experimental parameters are tabulated in Table 4.2.

Table 4.2 Experimental Parameters for Dry Sliding Wear Test

Experimental

Parameters

Dry Sliding Wear Test at

Low Loads

Dry Sliding Wear Test

at High Loads

Conditions CHT, SCT and DCT CHT, SCT and DCT

Loads selected 10N, 20N and 30N 60N, 70N and 80N

Sliding Speed 1.57m/s2.8m/s, 3.2m/s and

3.6m/s

Number of

Samples Tested2 3

46

4.3.1 Wear Test Parameters at Lower Loads (10N, 20N, 30N)

The selected factors for the experiments are normal loads of 10N,

20N and 30N, sliding speed of 1.57m/s, disc rotational speed of 300 rpm and

the test duration of 900 s. The wear test is repeated two times under each

condition. The tangential frictional force and the linear wear are measured

with an accuracy of 0.1 + 2% of measured frictional force in Newton and 0.1

+ 1% of measured wear in micron. Linear measurements of wear are

converted to wear volume for all the samples.

4.3.2 Wear Test Parameters at Higher Loads (60N, 70N, 80N)

The wear tests are also performed for the three different higher

loads of 60N, 70N and 80N, three sliding velocities of 2.8 m/s, 3.2 m/s and

3.6 m/s and for three different treatment conditions namely CHT, SCT and

DCT as indicated in Bensely et al (2005). Every sample is tested for a period

of 720 s.

4.4 IMPACT TEST

The Charpy V notch impact test is a best method to determine

absorbed energy. The tough steels absorb more energy, whilst brittle materials

tend to absorb very less energy prior to fracture. The change in potential

energy of the impacting head (from and before impact to after fracture) is

determined with a calibrated dial that measures the total energy absorbed in

the breaking of the specimen. In this study, the Charpy V notch impact test is

used to measure the toughness values of CHT, SCT and DCT samples.

In the present research work, sixteen Charpy impact specimens are

machined from raw material (En 19 steel) according to ASTM standard

E23-02a. The specimen diagram is shown in Figure 4.3. The sixteen

47

machined samples are separated into four groups namely Group A, Group B,

Group C, and Group D. The conditions of each group of samples are

indicated in Table 4.3. Group A samples are reserved as such to find the raw

material (En 19) behaviour and the remaining Group B, Group C and Group

D samples are subjected to CHT, SCT and DCT, respectively.

All dimensions are in mm

Figure 4.3 Charpy Impact Test Specimen

Table 4.3 Impact Test Groups

Group Identification Conditions

Group A Raw Material

Group B CHT

Group C SCT

Group D DCT

45º

55 10

210

48

4.5 TENSION TEST

The results of tension tests are used in selecting steels for various

engineering applications. Tensile test values are specified in material

specifications to ensure quality.

The tensile strength of steel is the major concern. The strength is

measured in terms of either the stress essential to cause substantial plastic

deformation or the highest stress that the material can resist. These measures

of strength are used, with suitable caution (in terms of factor of safety), in

component design field. The other interest is the material’s ductility. This is a

measure of how much it can be deformed prior to fractures. Ductility is also a

siginificant parameter in material specifications to ensure toughness and

quality. It is incorporated rarely in component design. Low percentage

ductility in a tension test frequently is accompanied by low fracture resistance

under other forms of loading.

4.5.1 Tensile Strength

The significance of tensile strength and ductility is explained by

Charles Moosbrugger (2002). Tensile strength is the maximum load divided

by original cross-sectional area of the specimen. The tensile strength is the

value most often quoted from the results of a tensile test. However, it is a

value of little fundamental significance pertaining to the strength of steel. For

ductile steels, the tensile strength is observed as a measure of the maximum

load that steel can endure under the very restrictive conditions of uniaxial

loading. This value bears little relation to the useful strength of the steel

under the more complex situations of stress that usually are encountered. For

many years, it is normal to base the strength of structural members on the

tensile strength, suitably reduced by a factor of safety. The present trend is

the more rational approach on the static design of ductile metals on the yield

49

strength. However, because of the long practice of using the tensile strength

to describe the strength of the materials, it has become a familiar property,

and as such, it is a useful identification of a material in the same sense that the

chemical composition serves to identify a metal or alloy. Furthermore,

because the tensile strength is easy to determine and is a reproducible

property, it is useful for the purpose of specification and for quality control of

a product.

4.5.2 Percentage Elongation

The conventional measures of ductility, attained from the tension

test is the engineering strain at fracture (generally called elongation).

Elongation is represented in percentage. Ductile property is considered as

subjective, qualitative property of steel.

According to ASTM standard: E 8M (ASTM, 2004), tension tests

give information on strength and ductility of materials under uniaxial tensile

stresses. This kind of information may be useful in comparison of materials,

quality control, alloy development, and design under certain working

conditions. The results of tension test of specimens machined to standardized

dimensions from selected portions of a component or material may not totally

represent the strength and ductility properties of the whole end product or its

in-service behaviour in different working environments. These test methods

are considered satisfactory for acceptance testing of commercial shipments.

The test methods have been used extensively in the trade for this purpose.

The tension tests are performed for CHT, SCT and DCT conditions.

The tensile samples are separated into four groups, that is Group A, Group B,

Group C and Group D. Every group contain 4 samples. The group A samples

are not subjected to any treatment in order to find the behaviour of the raw

material (En 19 Steel) and the Group B, Group C and Group D samples are

50

subjected to CHT, SCT and DCT, respectively, as indicated in Table 4.4.

After the treatment, the samples are tested using tension testing machine. The

tensile tests are carried out according to ASTM standard “Standard test

methods for tension testing of metallic materials- designation: E8M” (ASTM

standard, 2004). The specimen diagram is shown in Figure 4.4.

Figure 4.4 Tensile Test Specimen

Table 4.4 Tensile Test Groups

Group Identification Conditions

Group A Raw Material

Group B CHT

Group C SCT

Group D DCT

4.5.3 Fractography

Fracture is the fragmentation or separation of a solid body into two

or more parts under the action of load. Fracture can be classified into two

general categories. They are ductile fracture and brittle fracture. Becker and

51

Lampman (2002) explain that the fractography is the science of revealing

loading conditions and environment that causes the fracture by a three

dimensional interpretation of the appearance of a broken component. If the

sample is well preserved and the metallurgist is knowledgeable, the fracture

appearance reveals details of the loading events that culminated in fracture.

An understanding of how cracks nucleate and grow microscopically to cause

bulk (macroscale) fracture is a vital part of fractography. Ductile fracture has

been defined rather ambiguously as fracture noticeable gross plastic

deformation. It occurs by a slow tearing of the metal with the considerable

energy. The indication of ductile failure is the micro-scale presence of

dimples on the fracture surface. The dimples appearance by a process

referred to as microvoid coalescence, where voids nucleate (initiate), grow,

and coalesce to develop the final fracture surface that is dimpled. For brittle

materials, increasing loads can be stable up to a point when a crack can grow

quickly with no prior gross deformation as pointed out by Becker and Dennis

Mcgarry (2002). It is characterized by a rapid rate of crack propagation and

very little micro deformation. In brittle fracture, the cracks run perpendicular

to the applied load. This perpendicular fracture leaves a relatively flat surface

at the break. The fracture features of broken tensile specimens are observed

by using Scanning Electron Microscope (SEM).

4.6 X-RAY DETERMINATION OF RETAINED AUSTENITE

The volume percent of retained austenite present in CHT, SCT and

DCT steel samples is obtained by X-ray diffraction intensity. The significance

and use of the X-ray determination of retained austenite is reported in ASTM

standard: E-975-00 (ASTM, 2004). Retained austenite is found in the

microstructure of heat-treated low-alloy and high strength steels have medium

(0.4 wt %) carbon contents. Retained austenite can be measured in a test

52

section or companion sample that is included in a heat-treated lot of steel as

part of a quality control practice.

The three circular cross section samples are machined for retained

austenite measurements by Xray Diffraction (XRD) analysis. Each sample has

dimensions of 10mm diameter and 10mm length, which are subjected to

CHT, SCT and DCT respectively. The measurement has been carried out on

sample after slight metallographic polishing to remove the oxidized layer.

XRD patterns have been collected using a Cu-K radiation, angle 2 , 38-

102°, step 0.1° and time 1s. The software called Material Analysis Using

Diffraction (MAUD) is used to calculate the percentage- retained austenite

from the XRD spectra, using the Rietveld method. This software is a common

diffraction analysis program primarily based on the Rietveld method. The

retained austenite is determined by comparing the intensities of diffraction

peaks arising from each the phase. The Rietveld is a fundamental method in

which the diffraction patterns from each phase is modelled and scaled to

provide a least squares fit to the diffraction pattern. The retained austenite

present in the samples of En 19 steel subjected to conventional heat treatment,

shallow cryogenic treatment and deep cryogenic treatment after tempering is

measured using X-ray diffraction profiles.

4.7 RESIDUAL STATE OF STRESS MEASUREMENT BY

X-RAY DIFFRACTION TECHNIQUE

X-ray diffraction techniques exploit the fact that when a metal is

under stress, applied or residual, the resulting elastic strains cause the atomic

planes in the metallic crystal structure to change their spacing. It measures

the absolute stress without the need for an unstressed sample for calibration.

When a beam of X-rays is incident on a poly crystalline material,

crystallographic planes diffract X-rays and Bragg’s law n =2dhklsin is

53

satisfied, which is put forward by Martinez et al (2003). Here n is an integer

indicating the order of diffraction, is the X-ray wave length, dhkl is lattice

spacing of the hkl planes, and is the diffraction angle on the hkl planes. In

this analysis, X-ray diffraction measurements are carried out on X-stress 3000

diffractometer (Stresstech Oy/Finland) to find out the residual micro- and

macro-stresses. A standard X-ray analyzer is shown in Figure 4.5. It

comprises gonimeter, main unit and software. The main unit of Xstress 3000

analyzer includes power supply, self contained liquid cooling system, controls

electronics and firmware, high voltage generator, all interlocks required for

complete safe use / operation. The universal power input of the unit is 90 -

240 Volts, 50 - 60 Hz.

Figure 4.5 X-ray Analyzer

This analysis has been conducted by using solid-state linear sensor

technique (MOS, Dual 512 pixels) with a gonimeter in modified psi geometry

(symmetry side inclination). X-Ray Diffraction tests are carried out at room

temperature in ambient air using CrK radiation for residual micro and macro

54

stress measurements. Cylindrical samples of size 15mm diameter and 10 mm

thick are used for the measurements. Microsoft Windows compatible software

X3000 is the user interface for the XSTRESS 3000 controls, data acquisition

and numerical analyses. This software is easy to use in applications with

powerful performance. Some of the functions are given below.

Powers up the XSTRESS 3000 system

Controls goniometer movements

Controls the safety functions

Calibrates the system automatically

Calculates stresses by d-sin2 method

Calculates stresses automatically

Calculates retained austenite contents using two, three or four

peak method

Creates graphical presentations of the results

The purpose of the investigation is to determine surface residual

stress after CHT, SCT and DCT conditions. The residual stresses can be

classified into three types based on the length scale. These stresses remain

homogenous over a large number of grains and equilibrium forces are

assumed over a large number of crystals. Here uniform strain occurs over

large distances. Macro stresses are strongly related with macroscopic

structures such as grain boundary area, grain shape, grain size, cracks,

porosity, and several other features.

De Oliveira et al (2008) point out that the microstresses are given

by two distinct types. The first one remains homogenous within one grain and

the forces are assumed to be in balance among adjoining grains. The second

one is homogenous over some inter atomic distances and the internal forces

55

remain in equilibrium around crystalline defects. Microstresses are formed by

non uniform strain that occurs over short distances. This strain is typically

within few grains or within a single grain of type 2 and type 3, respectively.

The present study measures the micro- and macro-stresses in both

longitudinal and transverse directions with respect to the bar axis by the X-ray

diffraction techniques as shown in the Figure 4.6. This technique measures

the changes in interplanar spacing caused by the residual stress. These

changes in the crystals (grains) correspond to the elastic constants and the

residual stress of the material.

Figure 4.6 Directions of the Measurements using X-ray diffraction (a)

Longitudinal Direction (b) Transverse Direction

Prevey (1996) mentions that macrostresses produce uniform

distortion of many crystals simultaneously, shifting the angular position of the

diffraction peak selected for residual stress measurement. Gale and Totemeier

(2004) also point out that when macrostresses are present in the lattice plane

spacing in the crystals (grains) change from their stress-free values to new

values corresponding with the residual stress and the elastic constants of the

material. This produces a shift in the position of corresponding diffraction, ie.

change in Bragg angle . Microstresses, however, give rise to the non-uniform

strain variations in interplanar spacing which broadens the diffraction rather

than causing a shift in their position. The data are obtained at beam angle in

the range comprising between -45° and 45° and the residual stresses are found

56

out by using the Chi-method ( -method) which is derived from the classical

sin 2 ) method. The test method for residual stress analysis by X-ray

Diffraction is described in SFS-EN 15305 (2008). The diffraction peak

position is found out by using cross correlation algorithm. Macroscopic

stresses are displayed in MPa. Microscopic stresses are presented as a relative

result, by expressing the peak broadening. With this kind of measurements,

the absolute micro stress value in MPa would not be obtained. The behavior

of micro stresses is more or less a scalar quantity, apart from vector form

macroscopic stresses. The properties and parameters chosen for the

experimentation are indicated in Table 4.5.

Table 4.5 Properties and Parameters for Residual Stress Measurements

Properties andParameters

Values

Poisson’s ratio 0.3

Young’s Modulus 211000 MPa

Diffraction angle 156.4º

Exposure time 5s

Due to restricted penetration of Cr K-alpha radiation in steel (4 m),

only the stresses in the outermost surface region could be determined. Hence,

stresses at 3 locations approximately 120 degree apart are determined on the

surface of the outer diameter of the samples. The residual state of stress is

measured for CHT, SCT and DCT samples. Three samples are tested at each

condition.

4.8 CORROSION TEST

Despite the improvement of the wear resistance, the investment in

an extra step in the steel treatment could not be justified if other properties

57

such as corrosion performance are dramatically decreased. In dual phase

steel, it is found that the increase of martensite phase leads to an increase of

mechanical properties and corrosion susceptibility in salt solution. This

suggests the need to study the effect of increasing martensite phase in the

corrosion resistance of En 19 steel upon cryogenic treatment.

The purpose of the corrosion study is to investigate the effect of the

shallow and deep cryogenic treatments on the general corrosion performance

of En 19 steel in alkali medium (Na2CO3). This research work is carried out

to determine whether the improvement of mechanical properties is

accompanied by an increase or decrease of the corrosion resistance. The

material considered in this study is obtained in the form of 10 mm diameter

and 15 mm height bars.

Potentiodynamic polarization technique is used to investigate the

corrosion performance of En 19 steel with three different treatments namely

conventional heat treatment (CHT), shallow cryogenic treatment (SCT), and

deep cryogenic treatment (DCT). Tests are conducted in a 500ml- Pyrex cell

using the specimens as the working electrode, saturated Ag/AgCl2 as

reference electrode and platinum mesh as counter electrode. The samples are

soldered to a copper wire current collector and then mounted in epoxy resin

allowing a fixed working electrode surface area of 0.785cm2. To study the

corrosion of the sidewall, the samples are double wrapped using adhesive

plastic foil allowing the same surface area to be exposed to the electrolyte.

Potentiodynamic polarization tests are carried out in 1M Na2CO3 solution at room

temperature between -1 and 0.2V with a potential sweep rate of 10mVs– 1. Prior to

immersion, the samples are polished using a series of grinding papers at 120,

180, 240, 360, 400, 600, 800, 1000, 1200, 2500, and 4000. The samples are

immersed in the test solutions during 25 minutes before polarization to allow

open circuit potential (OCP) stabilization. Fresh samples (metal and

58

electrolyte) are used for each measurement. All measurements are carried out

using Autolab© PGSTAT302N equipped with NOVA software for data

acquisition and analyses.

4.9 DAMPING TEST

In recent years more number of studies has been carried out to

determine the vibration characteristics of cantilever beams. Vibration is the

motion of a particle or a body or a system of connected bodies displaced from

a position of equilibrium. Normally vibrations are unwanted in machines and

structures as they produce wear and inducing fatigue which leads to the

failure of the system.

Frequency is defined as the number of repetition of a specific

forcing function or vibrating part over a definite unit of time. Amplitude

denotes the highest value of vibration or motion. It is represented in terms of

displacement, velocity or acceleration. Vibration which occurs under the

excitation of external forces is known as forced vibration. When the excitation

is oscillatory, the structure is forced to vibrate the excitation frequency. If the

frequency of excitation matches with one of the natural frequencies of the

structure, a condition of resonance is encountered, resulting in dangerous

large oscillations. The failure of most vital structures namely bridges,

buildings or airplane wings and machine components are an awesome

possibility under resonance. The assessment of the natural frequencies is very

important in preventing the amplitude of oscillation at response. The energy

given to the system dissipates during the motion. The damping is the

dissipation of the energy over time. The damping causes the vibration

amplitude to decay.

The aim of this study is to evaluate the frequency, inverse quality

factor (Q-1) and damping percentage of En 19 steel samples for the DCT, SCT

59

and CHT. The photograph of the Fast Fourier Transform (FFT) analyzer and

the schematic diagram of the damping test system are shown in Figure 4.7 and

Figure 4.8.

Figure 4.7 Photograph of Fast Fourier Transform Analyzer

Figure 4.8 Schematic Diagram of Damping Test System

Sample

PC Amplifier FFT Vibrator

Accelerometer Holder

60

A highly sensitive Fast Fourier Transform (FFT) analyzer is used

for measuring the vibration. The photon dynamic signal analyzer is is used to

measure the vibration of components accurately. This device has two to four

analog input channels and a waveform source tachometer channel. The

frequency responses are measured using LDS-Dactron’s RT Pro Dynamic

Signal Analysis Series software. RT pro software provides flexible and

comprehensive capabilities for dynamic signal analysis. The V400 series

vibrators are wide frequency band electro-dynamic transducers capable of

producing sine vector of force. The specification of V400 series vibrator is

sine force peak is 98.0 N, maximum working temperature is 30 ºC and the

useful frequency range is from 50 to 9000Hz. The Ling Dynamic system

PA500L amplifier is used for amplifying the signals. The accelerometer is

mounted on one end of the circular rods of diameter 10 mm and length 125

mm. V400 series vibrators are used as actuator and sensor to execute the

sample to measure the frequency response function (FRF). The inverse

quality factor Q-1 is calculated to determine the damping capacity. The

following equation 4.1 is used to calculate the inverse quality factor Q-1.

nF

FFQ 121

(4.1)

where F1 and F2 refer to half-power bandwidth frequencies and Fn is the

resonant frequency in the spectrum. RT pro software automatically calculates

the frequency, Q factor and damping percentage from the obtained frequency

spectrum.

4.10 MICROSTRUCTURAL ANALYSIS

The microstructures of CHT, SCT and DCT samples are studied

through scanning electron microscope (SEM). After Bakelite mounting, the

61

samples are polished on a series of emery papers of grit 80, 120, 200, 600,

800, 1000, and up to 1 m. Then the samples are polished by using diamond

paste and finally all the samples are etched with 2% nital.

4.11 OPTIMIZATION OF DEEP CRYOGENIC AND HEAT

TREATMENT

The engineering on surfaces of components to develop the life and

performance of parts is an active area of research. The selection of suitable

treatment conditions in the deep cryogenic treatment process is based on the

analysis relating to the various heat treatment parameters. Traditionally, the

selection of parameters is carried out by using the experience of heat

treatment engineers or by using the data provided by the designers or

manufacturers. In general, the parameters given by the manufacturers are only

related to the common steel grades. Therefore, the mechanical properties

produced after the heat treatment process is inappropriate. It is difficult to find

the optimal conditions using conventional experimental design methods. It is

also difficult to conduct the experiments if the number of process parameters

increases. To overcome this complexity, Taguchi Design of Experiment

(DOE), a well-known method is used to optimize the critical parameters of

any process. Mahapatra and Amar Patnaik (2007) point out that the Taguchi

method is a simple, effective and powerful experimental design used for

deriving optimal process parameters. Further, this approach requires

minimum experimental cost and reduces the effect of source of variation.

Paulo Davim (2003) mentions that the techniques of Taguchi consist of a plan

of experiments with the objective of acquiring data in a controlled way,

executing these experiments, in order to obtain information about the

behaviour of a given process. The experiments are conducted based on

Taguchi Design of Experiments in terms of orthogonal arrays to find the

effects and interactions of several process parameters. This study clearly

62

demonstrates the treatment conditions and the contribution of each factor.

The best combination of the parameters affecting the deep cryogenic

treatment process to reduce wear loss of En 19 steel is found out in the

present study.

4.11.1 Planning for Experimentation

Researchers found that the samples treated at -196ºC will enhance

wear resistance. In this study, the deep cryogenic temperature is selected as -

196ºC. Hence, this temperature is not considered as a factor for this

optimization study. The soaking temperature is fixed as -196ºC. In order to

find the influence of the factors in the deep cryogenic and heat treatment

process, four factors namely hardening temperature, soaking period,

tempering temperature and tempering time are considered in the current

study. These factors mainly affect the tribological behaviour of steels. In

conventional hardening process, steel is heated slowly in a furnace at the

temperature and then the heated steel is soaked at this temperature for the

specified period. This temperature is known as hardening temperature.

Hardening temperature is the very significant parameter which defines the

level of probable enhancement of wear resistance in deep cryogenic treatment

process. After hardening process, the steel is subjected to oil quenching at

room temperature. Due to the cooling, the FCC austenite is transformed into

BCT martensite. Martensite can form only if the diffusion controlled

transformation of austenite is suppressed. On a practical level, this is

accomplished by rapid cooling, for example in water or brine baths.

Therefore, medium carbon steels are alloyed with elements such as nickel,

chromium, molybdenum, which make it more difficult for the diffusion

controlled transformation to occur. However, such drastic cooling introduces

high surface tensile residual stresses and may cause quench-cracking. As a

result, martensite can be formed with less drastic cooling, such as oil

63

quenching. Therefore, the oil quenching is selected for treating En 19 steel as

explained in Krauss (1990). However, some amount of austenite is retained in

steel samples. This retained austenite is alleviated by way of deep cryogenic

treatment. By deep cryogenic treatment, the samples are cooled down from

room temperature to -196ºC and the samples are soaked for the predetermined

time at -196 ºC in the liquid chamber. This soaking time is called as soaking

period. Then the samples are slowly heated to room temperature. The final

process is the tempering process. It is the process of reheating the steel to

prescribed temperature, held at this temperature for an adequate span of time,

and then cooled to ambient temperature. The respective temperature for this

process is called tempering temperature and time duration is called the

tempering period.

Cylindrical steel samples of 10mm diameter and 30mm length are

used for the wear tests and hardness tests. The En 19 steel samples are first

hardened at the specified temperature for one hour. After hardening process,

the samples are quenched in oil at 30 ºC. The deep cryogenic treatment is

then carried out for the specified soaking period. Finally, the tempering

process is performed at the prescribed temperature and time.

4.11.2 Taguchi Method

Taguchi’s robust design is an easy, systematic and efficient method

to determine the best combination of the design parameters. Prasanta Sahoo

and Sujan Kumar Pal (2007) stated that Taguchi method achieves the

integration of design of experiments (DOE) with the parametric optimization

of the process yielding the desired results. The orthogonal array requires a set

of well-balanced (minimum experimental runs) experiments. In this method,

main parameters, which are assumed to have an influence on the process, are

located at different rows in a designed orthogonal array. With such an array,

64

completely randomized experiments can be conducted. Taguchi’s method

uses the statistical measure of performance, called signal-to-noise ratio (S/N),

which is logarithmic functions of desired output to serve as objective

functions for optimization. The ratio depends on the quality characteristics of

the product/process to be optimized. The three categories of S/N ratios are

used normally: lower-the- better, higher-the-better and nominal-the-best. The

parameter level combination that maximizes the appropriate S/N ratio is the

optimal setting. For the case of minimization of friction or wear, lower-the-

better characteristic needs to be used.

The full factorial experiment for optimizing the four factors and

three levels would require 81 experiments for the analysis. But, the Taguchi

design of method requires only 9 or 27 experiments. The purpose of

conducting an orthogonal experiment is to find the best combination of design

parameters and also to determine the significance of each parameters.

In the present study, Taguchi method-based robust design, a L27

orthogonal array is employed for the experimentation to analyze the influence

of factors, to achieve greater accuracy. Furthermore, a large orthogonal array

is selected for the design of experiments to find all the possible interaction

effects between the main factors. Ross (1998) points out that Taguchi has

created a transformation of the repetition data to another value, which is a

measure of the variation present. The transformation is the signal to noise

ratio. The S/N ratio is the ratio of signal to noise where the signal represents

the desirable and noise represents the undesirable value. The S/N ratio

consolidates several repetitions (at least two data points are required) into one

value that reflects the amount of variation presents. The lower the better

output response is the type where it is desired to minimize the result, with the

ideal target being zero. This is stated by Genich Taguchi et al (2004). The

65

equations for calculating S/N ratio for lower the better characteristics are

given in equation 4.2 for finding minimum wear of En 19 steel samples.

n

iiy

nNS

1

21log10/ (4.2)

where n is the number of repetitions of the experiment and y i is the measured

value of experimental data. Sung H. Park (1996) has introduced a method

called Pareto ANOVA in order to find the significance of all the factors and

the interaction between the factors. Pareto ANOVA is a quick and easy

method for analyzing results. This method enables to find the significance of

factors and interactions. The ANOVA analysis is also performed to find the

percentage contribution of the selected factors. The dry sliding wear tests are

carried out to find the linear wear for 27 steel samples. The pin is made of En

19 steel and the disk is made up of En 31 hardened steel. The experimental

parameters for the wear test are given in Table 4.6. The amount of wear loss

is determined by measuring appropriate linear dimensions of both samples

before and after the test. Linear dimensions of wear loss are frequently

measured since the mass loss is often too small to measure precisely.

Table 4.6 Wear Test Parameters for Optimization study

Experimental Parameters

Conditions DCT

Load selected 50 N

Sliding Speed 1.57m/s

Number of

Samples Tested2