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CHAPTER - 1
INTRODUCTION
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1. INTRODUCTION
1.1 SPRING
A spring is defined as an elastic body, whose function is to distort when loaded
and to recover to its original shape when the load is removed. The various important
applications of springs are as follows:
a) To cushion, absorb or control energy due to either shock or vibration as in car
springs, railway buffers, air-craft landing gears, shock absorbers and vibration
dampers.
b) To apply forces as in brakes, clutches and spring loaded valves.
c) To control motion by maintaining contact between two elements as in cams and
followers.
d) To measure forces as in spring balances and engine indicators.
e) To store energy as in watches.
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1.2 TYPES OF SPRINGS
There are various types of springs theses are: coil springs, leaf springs, torsion
bars and air springs.
1.2.1 Coil springs: is a mechanical device which is typically used to store energy and
subsequently release it to absorb shock, or to maintain a force between contacting
surfaces.
1.2.2 Leaf springs: are suspension springs made up of several thin, curved, hardened-
steel or composite-material plates attached at the ends to the vehicle under-body.
1.2.3 Torsion bars: are a long straight steel bar fastened to the chassis at one end and
to a suspension part at the other which when twisted provides the spring force.
1.2.4 Air springs: is a mechanical device using confined air to absorb the shock of
motion.
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CHAPTER – 2
LITERATURE SURVEY
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2.1 LITERATURE SURVEY
This chapter deals with the review of work done related to topic. Firstly the
review is on the tensile test which is a standardized mechanical test which is widely
used to reveal the mechanical behaviour of lot of materials. It is an important test to
ensure that the particular material can withstand the given load and given conditions.
Morestin developed a logic which computes the deformation of steel sheet in
press forming after spring back. Work hardening of steel involves modifications of
the elastic properties of the material, e.g. an increase in its yield stress. It can also be
the cause of an appreciable decrease in the Young modulus. However, this property
diminishes as the plastic strain increases. The purpose of the experiments with
microcomputer-controlled tensile test machine is to indicate that the diminution can
reach more than 10%, of the initial value after only 5% plastic strain. In spite of this
fact, a lot of elastic-plastic software does not take into account the decrease in the
Young modulus with plastification even though it may lead to obvious differences
among results. So, as an application they developed the software. The software takes
into account the decrease in the Young modulus and its results are very close to
experimental values. They noticed a recovery of the Young modulus of plastified
specimens after few days, but not for all steels tested. The Young modulus vs. plastic
strain allows a better numerical analysis of an elastic plastic phenomenon such as
spring back. The introduction of the behaviour in other software simulations of metal
forming seems equally as necessary as the correct determination of the general work-
hardening parameters of the material. Its influence on the shape of the specimen
remains to be shown. The recovery shows as well that, for the metals already excited,
the apparent Young modulus cannot be used as a permanent damage indicator for
plastic strains lower than 15%.
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CHAPTER – 3
SPECIAL PROCESS
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3.1 HEAT TREATMENT
This is the heating and cooling of a solid metal or alloy in such a way as to
obtain desired conditions or properties. The term heat treatment process is in and of
itself only a very generic term; it covers all specific methods. Thus emphasis will be
made on those forms of heat treatment that are most commonly used in the spring
industry these are: Annealing, Normalizing, Hardening and Tempering.
3.2 NORMALIZING / STRESS RELIEVING
Heating to a suitable temperature of 225°C for 15 minutes, holding long
enough to reduce residual stresses, and then cooling slowly enough to minimize the
development of new residual stresses. It relieves the stresses that occur as a result of
the spring forming operation. It also returns the material to the strength levels prior to
the forming operation and can actually increase the strength to levels greater than
originally supplied.
3.3 HARDENING (WATER AND OIL QUENCHING)
Quenching can be done by plunging the hot steel in water. The water adjacent
to the hot steel vaporizes, and there is no direct contact of the water with the steel.
This slows down cooling until the bubbles break and allow water contact with the hot
steel. Water quenching produces steel with a very high hardness but also results in
very brittle and fragile steel with a low tensile strength also. As the water contacts and
boils, a great amount of heat is removed from the steel. With good agitation, bubbles
can be prevented from sticking to the steel, and thereby pre-vent soft spots. Water is a
good rapid quenching medium, provided good agitation is done. However, water is
corrosive with steel, and the rapid cooling can sometimes cause distortion or cracking.
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3.4 TEMPERING
Tempering is usually done after quenching, it involves re-heating of the steel in
order to reduce the hardness of the quenched steel and improve the ductility,
toughness and strength of the spring. Tempering is usually done hand in hand with
quenching and is usually a tradeoff between hardness and toughness/strength of steel.
This research is aimed at evaluating the effect of normalizing, hardening and
tempering on the impact toughness, hard-ness and tensile strength of springs.
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CHAPTER – 4
FAILURE MODE & EFFECT ANALYSIS
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4.1 FAILURE MODES OF A SPRING
Mechanical springs are used in machine designs to exert force, provide
flexibility, and to store or absorb energy. Springs are manufactured for many different
applications such as compression, extension, torsion, power, and constant force.
Depending on the application, a spring may be in a static, cyclic or dynamic operating
mode. A spring is usually considered to be static if a change in deflection or load
occurs only a few times, such as less than 10,000 cycles during the expected life of
the spring. A static spring may remain loaded for very long periods of time. The
failure modes of interest for static springs include spring relaxation, set and creep.
Cyclic springs are flexed repeatedly and can be expected to exhibit a higher
failure rate due to fatigue. Cyclic springs may be operated in a unidirectional mode or
a reversed stress mode. In one case, the stress is always applied in the same direction,
while in the other, stress is applied first in one direction then in the opposite direction.
Figure 1 shows the difference in deflection and stress between these two operating
modes. For the same maximum stress and deflection between a unidirectional and
reversed stress spring, the stress range for the reversed stress spring will be twice that
of the unidirectional spring and therefore a shorter fatigue life would be expected.
Dynamic loading refers to those intermittent occurrences of a load surge such
as a shock absorber inducing higher than normal stresses on the spring. Dynamic
loading of a spring falls into three main categories: shock, resonance of the spring
itself, and resonance of the spring/mass system. Shock loading occurs when a load is
applied with sufficient speed such that the first coils of the spring take up more of the
load than would be calculated for a static or cyclic situation.
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This loading is due to the inertia of the spring coils. Spring resonance occurs
when the operating speed is the same as the natural frequency of the spring or a
harmonic of the natural frequency. Resonance can cause greatly elevated stresses and
possible coil clash resulting in premature failure. Resonance of the spring/mass
system occurs when the spring is required to carry a mass attached to its moving end
and the combined system is subject to resonance at a much lower cycle rate than the
spring alone. Failure modes for dynamic loading of a spring include fracture of the
spring material due to shock pulses and resonance surging.
Springs tend to be highly stressed because they are designed to fit into small
spaces with the least possible weight and lowest material cost. At the same time they
are required to deliver the required force over a long period of time. The reliability of
a spring is therefore related to its material strength, design characteristics, and the
operating environment.
Most springs are made of steel and material strength of the spring is usually
listed in terms of tensile strength in relation to the expected spring stress. Corrosion
protection of the spring steel has a significant impact on reliability and therefore
material properties, the processes used in the manufacturing of the spring, operating
temperature, and corrosive media must all be known before any estimate of spring
reliability can be made. Spring reliability is also directly related to the surface quality
and the distribution, type and size of sub-surface impurities in the spring material.
Common materials of construction for springs include spring steel, stainless steel,
nickel base alloy, and copper base alloy or bronze. Spring steel is any variety of steels
that are normally of the high-carbon or alloy type. High carbon spring steels are
probably the most commonly used material for springs except for those to be used in
high or low temperature environments or for shock or high impact loads.
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Fig 4.1 Unidirectional Stress
Fig 4.2 Reversed Stress
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4.2 DUCTILE FRACTURE
Ductile fracture is been defined as fracture occurring with appreciable gross
deformation. Ductile fracture in tension is usually preceded by a localized reduction
in diameter called necking. Very ductile metals may actually draw down to a line or a
point before separation. This kind of failure is usually called rupture.
Fig 4.3 Stages in the formation of a cup-and-cone fracture.
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The stages in the development of a ductile “cup-and-cone” fracture are
illustrated in fig 4.3 Necking begins at the point of plastic instability where the
increase in strength due to strain hardening fails to compensate for the decrease in
cross-sectional area. This occurs at the maximum load or at a true strain equal to the
strain-hardening coefficient. The formation of a neck introduces a triaxial state of
stress in the region. A hydrostatic component of tension acts along the axis of the
specimen at the center of the necked region. Many fine cavities form in this region,
and under continued straining these grow and coalesce into a central crack. This crack
grows in a direction perpendicular to the axis of the specimen until it approaches the
surface of the specimen. It then propagates along localized shear planes at roughly
450 to the axis to form the “cone” part of the fracture Universal Testing Machine is
used to conduct the tensile test. Two general types of machines are used in tension
testing.
(1) Load controlled machine and
(2) Displacement controlled machines.
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4.3 CYCLIC MODES OF SPRING OPERATION
4.3.1 FAILURE MODES
Springs of all types are expected to operate over very long periods of time
without significant changes in dimension, displacement or spring rate, often under
changing loads. Considering these requirements, potential failure modes include
yielding, fatigue, corrosion fatigue, fretting fatigue, creep, thermal relaxation,
buckling, and force-induced elastic deformation. The operating life of a mechanical
spring arrangement is dependent upon the susceptibility of the materials to corrosion
and stress levels (static, cyclic or dynamic). The most common failure modes for
springs are fracture due to fatigue and excessive loss of load due to stress relaxation.
By definition, objects that are loaded under purely oscillatory loads fail when their
stresses reach the material’s fatigue limit. Conversely, objects that are loaded under
purely static loads fail when their stresses reach the materials yield limit σyield
. For
springs that have a mixture of σmean
and σalt
stresses, the Soderberg Criterion provides
a way to calculate a failure limit. Mean stress is plotted on one axis and alternating
stress on the other.
4.3.2 FATIGUE STRESS
All springs have finite fatigue limits, the limit depending fatigue stress and the degree
of fluctuating loads. The four most common fatigue stress conditions include constant
deflection, constant load, unidirectional stress and reversed stress. A spring inside a
valve assembly is an example of a constant deflection where the spring is cycled
through a specified deflection range. An example of a constant load spring is the use
of vibration springs under a dead weight where the load applied to the spring does not
change during operation but the deflection will. A unidirectional stress is one where
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the stress is always applied in the same direction such as used in the return spring of
an actuator. A reversed stress is applied first in one direction then in the opposite
direction such as used in a regulator valve. The three stages to a fatigue failure
include crack initiation, crack propagation and finally fracture of the spring material.
Static springs can be used in constant deflection or constant load applications.
A constant deflection spring is cycled through a specified deflection range, the loads
on the spring causing some set or relaxation which in turn lowers the applied stress.
The spring may relax with time and reduce the applied load. Under constant load
conditions, the load applied to the spring does not change during operation. Constant
load springs may set or creep, but the applied stress is constant. The constant stress
may result in fatigue lives shorter than those found in constant deflection applications.
A corrosive environment may accelerate the time to fatigue failure, corrosion
reducing the load-carrying capability of a spring and its life. The precise quantitative
effect of a corrosive environment on spring performance is difficult to predict.
Springs are almost always in contact with other metal parts. If a spring is to be
subjected to a corrosive environment, the use of inert materials provides the best
defense against corrosion. Protective coatings can also be applied. In special
situations, shot peening can be used to prevent stress corrosion and cathodic
protection systems can be used to prevent general corrosion. The spring material is
normally more noble (chemically resistant to corrosion) than the structural
components in contact with it because the lesser noble alloy will be attacked by the
electrolyte.
Surging (resonant frequency response) can occur in high-speed cyclic
applications if axial operating frequencies approach the axial natural frequency of the
helical-coil spring. If the material and geometry of the axially reciprocating spring are
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such that its axial natural frequency is close to the operating frequency, a traveling
displacement wave front is propagated and reflected along the spring with about the
same frequency as the exciting force. This condition results in local compressions and
rarefactions producing high stresses and/or erratic forces locally, with consequent loss
of control of the spring-loaded object. Surging of a valve spring, for example may
allow the valve to open erratically when it should be closed or vice versa
4.3.3 SPRING RELAXATION
Springs of all types are expected to operate over long periods of time without
significant changes in dimension, displacement, or spring rates, often under
fluctuating loads. If a spring is deflected under full load and the stresses induced
exceed the yield strength of the material, the resulting permanent deformation may
prevent the spring from providing the required force or to deliver stored energy for
subsequent operations. Most springs are subject to some amount of relaxation during
their life span even under benign conditions. The amount of spring relaxation is a
function of the spring material and the amount of time the spring is exposed to the
higher stresses and/or temperatures.
Static springs can be used in constant deflection or constant load applications.
A constant deflection spring is cycled through a specified deflection range, the loads
on the spring causing some set or relaxation which in turn lowers the applied stress.
The spring may relax with time and reduce the applied load.
Elevated temperatures can cause thermal relaxation, excess changes in spring
dimension or reduced load supporting capability. Under constant load conditions, the
load applied to the spring does not change during operation. Constant load springs
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may set or creep, but the applied stress is constant. The constant stress may result in
fatigue lives shorter than those found in constant deflection applications.
In many applications, compression and extension springs are subjected to
elevated temperatures at high stresses which can result in relaxation or loss of load.
This condition is often referred to as "set". After the operating conditions are
determined, set can be predicted and allowances made in the spring design. When no
set is allowed in the application, the spring manufacturer may be able to preset the
spring at temperatures and stresses higher than those to be encountered in the
operating environment.
A highly stressed spring will set the first several times it is pressed. Relaxation
is a function of a fairly high stress (but usually lower then that required to cause set)
over a period of time. Creep in the spring may lead to unacceptable dimensional
changes even under static loading (set). A spring held at a certain stress will actually
relax more in a given time than a spring cycled between that stress and a lower stress
because it spends more time at the higher stress. The amount of spring relaxation over
a certain period of time is estimated by first determining the operating temperature,
the maximum amount of stress the spring sees and how long the spring will be
exposed to the maximum stress and the elevated temperature over its lifetime.
4.4 MISCELLANEOUS FAILURE MODES
Most extension spring failures occur in the area of the spring end. Extension
springs are designed to become longer under load and their maximum length must be
controlled for long life. Their turns are normally touching in the unloaded position
and they have a hook, eye or some other means of attachment. For maximum
reliability, the spring wire must be smooth with a gradual flow into the end without
tool marks, sharp corners or other stress risers. The spring ends should be made as an
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integral part of the coil winding operation and the bend radius should be at least one
and one-half times the wire diameter.
Other failure mechanisms and causes may be identified for a specific
application to assure that all considerations of reliability are included in the
prediction. For example, another failure mode to be considered is the hardness of the
spring material that can be sensitive to plating and baking operations. Table 1 is a list
of failure modes and causes for mechanical springs.
4.5 FAILURE MODE AND EFFECTS ANALYSIS (FMEA)
This section provides some guidelines for performing the FMEA for
mechanical springs. The FMEA is normally performed by first identifying the failure
modes and then estimating the probability of occurrence for each identified failure
mode. Failure modes have been discussed in the previous section. Their probability of
occurrence depends on the material strength with respect to operating stress which in
turn depends to a large extent on the following items:
Type of spring
Compression,
Extension,
Torsion.
Size of spring
Diameter,
Length
Spring material
Monel,
Music,
High-Carbon,
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Wire diameter
Loading
Static,
Cyclic,
Dynamic
Operating temperature
Spring design
Spring Index,
Bends
Manufacturing methods & quality control
Tool Marks,
Pits
Hook design
Operating environment
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CHAPTER – 5
QUENCHING FUNDAMENTALS
AND
PROPERTIES
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5.1 QUENCHING OIL FUNDAMENTALS
Quench oil serves two primary functions:
1. It facilitates hardening of steel during quenching.
2. It enhances wetting of steel during quenching to minimize the formation of
undesirable thermal and transformational gradients, which may lead to distortion or
cracking.
When hot metal is quenched, a vapor envelope is initially formed around the
hot metal as it is immersed in the oil. The stability of this vapor envelope and thus the
ability of the oil to harden steel is dependent on the metal surface irregularities, the
presence of oxides, surface wetting agents (which accelerate the wetting process and
destabilize the vapor envelope), and the presence of other oil degradation by-products.
Upon further cooling, the vapor envelope collapses, resulting in so-called
nucleate boiling, which is the fastest heat transfer.
Nucleate boiling is a type of boiling that can take place under certain
conditions. It is the process of forming steam bubbles within liquid in micro cavities
adjacent to the wall if the wall temperature at the heat transfer surface rises above the
saturation temperature while the bulk of the liquid is sub-cooled. The bubbles grow
until they reach some critical size at which point they separate from the wall and are
carried into the main fluid stream. There the bubbles collapse because the temperature
of bulk fluid is not as high as at the heat transfer surface where the bubbles were
created. Heat and mass transfer during nucleate boiling has a significant effect on the
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heat transfer rate. This heat transfer process helps to quickly and efficiently carry
away the energy created at the heat transfer surface.
When the temperature of the hot steel interface is less than the oil’s boiling
point, nucleate boiling will stop and convective cooling will begin.
Oil degradation is often accompanied by sludge and varnish formation. These
by-products do not adsorb uniformly on the steel surface as it is being quenched,
resulting in cooling rate variations and thermal gradients.
Another source of non-uniform heat transfer is water contamination of the
quench oil. Water causes thermal gradients and lower viscosity.
Effects of Contaminants.
5.1.1 VISCOSITY
Of all the variables that can affect the maximum cooling rate during nucleate
boiling, temperature has the most significant effect on the maximum cooling rate.
Increasing the temperature increases the maximum cooling rate due to the change in
viscosity. At room temperature, the oil is viscous and does not wet the surface of the
part well. As the viscosity decreases with increased temperature, the result is better
wetting of the part and consequently better heat transfer.
5.1.2 SOOT
Soot has the second largest impact on maximum cooling rate. The maximum
cooling rate increases as the amount of soot in the oil increases. This is due to the soot
particles functioning as nucleation sites for bubble formation during nucleate boiling.
Soot also causes the temperature of maximum cooling to increase.
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5.1.3 SALT
Salt crystals have an effect similar to soot particles since they do not dissolve in
oil and form nucleation sites for bubble formation during nucleate boiling.
5.1.4 WATER
Water increases the maximum cooling rate and substantially decreases the
temperature of maximum cooling. This increases the chances of distortion of the part
by increasing the thermal gradients within the part.
5.1.5 HYDRAULIC FLUID
Contamination with hydraulic fluid increases the maximum cooling rate and the
temperature at which maximum cooling occurs. Because hydraulic fluids are miscible
in quench oil, the properties of the quench oil change. The boiling point of the
mixture will likely increase, causing an increase in maximum cooling rate and the
temperature at which maximum cooling rate occurs.
5.1.6 OXIDATION
Oxidation causes the maximum cooling rate and the temperature of maximum
cooling to decrease, which is caused by increases in viscosity of the quench oil. This
in turn causes a decrease in wetting. Increase in viscosity also causes bubble
formation to become more difficult while the maximum cooling rate and the
temperature of maximum cooling is reduced.
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5.2 PRECAUTIONS
5.2.1 PERCENT WATER
This contaminant in amounts as low as 1,000 parts per million (ppm) can cause
foaming, fires, and explosions.
5.2.2 FLASH POINT
This value should be as high as possible. Changes usually indicate
contamination or degradation. Low flash points increase the chance of fires.
5.2.3 PERCENT SLUDGE
This is the result of oxidation and polymerization.
5.2.4 PERCENT ASH
Increased inorganic ash content indicates degradation.
5.2.5 KINEMATIC VISCOSITY
As oil degrades, viscosity usually increases. Some contaminants reduce
viscosity and flash point.
5.2.6 NEUTRALIZATION NUMBER
Increased oxidation causes the oil to become more acidic.
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5.2.7 QUENCHING SPEED
Either a GM Quenchometer test or a cooling rate curve should be used to
evaluate the cooling/quenching characteristics of the oil.
5.3 QUENCHING BATHS
When steel heated above the critical point is plunged into a cooling bath, the
rapidity with which the heat is absorbed by the bath affects the degree of hardness;
hence, baths of various kinds are used for different classes of work. Clear cold water
is commonly employed and brine is sometimes substituted to increase the degree of
hardness. Sperm [whale oil] and lard oil baths are used for hardening springs, and raw
linseed oil is excellent for cutters and other small tools. The effect of a bath upon steel
depends upon its composition, temperature, and volume. The bath should be amply
large to dissipate the heat rapidly, and the temperature should be kept about constant,
so that successive pieces will be cooled at the same rate. Greater hardness is obtained
from quenching in salt brine, and less in oil, than is obtained by the use of water. This
is due to the difference in the heat-dissipating qualities of these substances. When
water is used, it should be "soft," as unsatisfactory results will be obtained with "hard"
water. If thin pieces are plunged into brine, there is danger of cracking, owing to the
suddenness of the cooling.
The temperature of the hardening bath has a great deal to do with the hardness
obtained. In certain experiments a bar quenched at 41 degrees F. showed a
scleroscopic hardness of 101. A piece from the same bar quenched at 75 degrees F.
had a hardness of 96, while, when the temperature of the water was raised to 124
degrees F., the bar was decidedly soft, having a hardness of only 83.
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The higher the temperature of the quenching water, the more nearly does its
effect approach that of oil, and if boiling water is used for quenching, it will have an
effect even more gentle than that of oil; in fact, it would leave the steel nearly soft.
With oil baths, the temperature changes have little effect on the degree of hardness.
Parts of irregular shape are sometimes quenched in a water bath that has been
warmed somewhat to prevent sudden cooling and cracking. A water bath having one
or two inches of oil on top is sometimes employed to advantage for tools made of
high-carbon steel, as the oil through which the work first passes reduces the sudden
action of the water.
Irregularly shaped parts should be immersed so that the heaviest of thickest
section enters the bath first. After immersion, the part to be hardened should be
agitated in the bath; the agitation reduces the tendency of the formation of a vapor
coating on certain surfaces, and a more uniform rate of cooling is obtained Various
oils, such as cotton-seed, linseed, lard, whale oil, kerosene, etc., are also employed;
many prefer cotton-seed oil. Linseed has the objection of becoming gummy, and lard
oil has a tendency to become rancid. Whale oil or fish oil give satisfactory results, but
have offensive odors, although this can be overcome by the addition of about three
per cent of heavy "tempering" oil.
A quenching solution of a 3 per cent sulphuric acid and 97 per cent of water
will make hardened carbon steel tools come out of the quenching bath bright and
clean. This bath is sometimes used for drills and reamers which are not to be polished
in the flutes after hardening. Another method of cleaning drills and similar tools after
hardening is to pickle them in a solution of 1 part hydrochloric acid and 9 parts water.
This method is satisfactory for reamers and tools which are not to be polished in the
flutes after hardening.
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Fig 5.1 Oil Quenching
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5.3.1 OIL QUENCHING BATHS
Oil is used very extensively as a quenching medium as it gives the best
proportion between hardness, toughness and warpage for standard steels. Special
compounded oils of the soluble type are now used in many plants instead of such oils
as fish oil, linseed oil, cotton-seed oil, etc. The soluble properties enable the oil to
make an emulsion with water. A good quenching oil should possess a flash and fire
point sufficiently high to be safe under the conditions used and 350 degrees F. should
be about the minimum point. The specific heat of the oil regulates the hardness and
toughness of the quenched steel, and the greater the specific heat, the harder the steel
will be. Specific heats of quenching oils vary from 0.20 to 0.75, the specific heats of
fish, animal, and vegetable oils usually being from 0.2 to 0.4, and of soluble and
mineral oils, from 0.5 to 0.7. The oil should not contain water, gum when used, have a
disagreeable odor or become rancid. A great many concerns use paraffin and mineral
oils for quenching, while a few use crude fuel oils. The quantity of steel that can be
quenched per gallon of oil depends on the fluidity of the oil, or its draining qualities.
The so-called "refrigerating qualities" are really the capacity of the oil to remove the
heat from the steel at a fast rate and then radiate its own heat to the atmosphere.
5.3.2 TANKS FOR QUENCHING BATHS
The main point to be considered in a quenching bath is to keep it at a uniform
temperature, so that each successive piece quenched will be subjected to the same
heat. The next consideration is to keep the bath agitated, so that it will not be of
different temperatures in different places; if thoroughly agitated and kept in motion,
as is the case with the bath shown in Fig.3.2, it is not even necessary to keep the
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pieces in motion in the bath, as steam will not be likely to form around the pieces
quenched. Experience has proved that if a piece is held still in a thoroughly agitated
bath, it will come out much straighter than if it has been moved around in an
unagitated bath.
Fig 5.2 Tanks for Quenching Baths
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5.4 WATER QUENCHING
Fig 5.3 Water Quenching
In Fig 4.3 is shown a water or brine tank for quenching baths. Water is forced
by a pump or other means through the supply tube into the intermediate space
between the outer and inner tank. From the intermediate space it is forced into the
inner tank through holes as indicated. The water returns to the storage tank by
overflowing from the inner tank into the outer one and then through the overflow pipe
as indicated. The water or brine tank of a more common type. In this case the water or
brine is pumped from the storage tank and continuously returned to it. If the storage
tank contains a large volume of water, there is no need of a special means for cooling.
Otherwise, arrangements must be made for cooling the water after it has passed
through the tank. The bath is agitated by the force with which the water is pumped
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into it. The holes at A are drilled at an angle, so as to throw the water toward the
center of the tank.
The oil quenching tank in which water is circulated in an outer surrounding
tank for keeping the oil bath cool. Air is forced into the oil bath to keep it agitated.
Fig. 6 shows a water and oil tank combined. The oil is kept cool by a coil passing
through it in which water is circulated, which later passes into the water tank.
Fig 5.4 Ordinary Type of Quenching Tank
Fig 4.4 shows the ordinary type of quenching tank cooled by water forced through a
coil of pipe. This can be used for either oil, water or brine. It shows a similar type of
quenching tank, but with two coils of pipe. Water flows through one of these and
steam through the other. By this means it is possible to keep the bath at a constant
temperature.
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5.5 OBJECTIVES OF HEAT TREATMENTS
Fig 5.5 Heat Treatments
Heat Treatment is the controlled heating and cooling of metals to alter their
physical and mechanical properties without changing the product shape. Heat
treatment is sometimes done inadvertently due to manufacturing processes that either
heat or cool the metal such as welding or forming.
Heat Treatment is often associated with increasing the strength of material, but
it can also be used to alter certain manufacturability objectives such as improve
machining, improve formability, restore ductility after a cold working operation. Thus
it is a very enabling manufacturing process that can not only help other manufacturing
process, but can also improve product performance by increasing strength or other
desirable characteristics.
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Steels are particularly suitable for heat treatment, since they respond well to
heat treatment and the commercial use of steels exceeds that of any the material.
5.6 FURNACE
Furnace is a device used for heating. The name derives from Latin fornax,
oven. In American English and Canadian English usage, the term furnace on its own
refers to the household heating systems based on a central furnace (known either as a
boiler or a heater in British English), and sometimes as a synonym for kiln, a device
used in the production of ceramics. In British English, a furnace is an industrial
furnace used for many things, such as the extraction of metal from ore (smelting) or in
oil refineries and other chemical plants, for example as the heat source for fractional
distillation columns. The term furnace can also refer to a direct fired heater, used in
boiler applications in chemical industries or for providing heat to chemical reactions
for processes like cracking, and is part of the Standard English names for many
metallurgical furnaces worldwide. The heat energy to fuel a furnace may be supplied
directly by fuel combustion, by electricity such as the electric arc furnace, or through
induction heating in induction furnaces.
Fig 5.6 Furnace
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5.7 TENSILE TEST.
Tensile test is an important standard engineering procedure to
characterizeproperties related to mechanical behaviour of materials. To properly
describe the response of the material during the actual loading conditions, the
variation in geometry of the specimen has to be considered. Although the behaviour
of the material in elastic limit is of considerable importance but the knowledge
beyond elastic limit is also relevant since plastic effects with large deformation takes
place in number of manufacturing processes. The mechanical behaviour of Corten
Steel, used in manufacturing of railway coaches are important properties used in the
crash analysis of the component. Finite Element Method being a widely used tool for
analysis due to revolution in computer field is used for the analysis of the
components. The present work describes the behavior of Corten Steel sheet specimens
in plastic range. Finite element method was employed for the analysis of tensile test.
Fig 5.7 Layout of UTM
36
The engineering tensile test also known as tension test is widely used to
provide basic design information on the strength of material and as an acceptance test
for the specification of the materials. Tensile tests are simple, relatively inexpensive,
and fully standardized. By pulling on something, it can be very quickly determined
how the material will react to forces being applied in tension. As the material is being
pulled, its strength along with how much it will elongate can be find out. A lot about a
substance can be learned from tensile testing. As the machine continues to pull on the
material until it breaks, a good, complete tensile profile is obtained. A curve will
result showing how it reacted to the forces being applied. In the tensile test a
specimen is subjected to a continually increasing uniaxial tensile force while
simultaneous observations are made of the elongation of the specimen. Fig 3.8 shows
a typical stress-strain curve for a metal.
Fig 5.8 Stress - Strain Curve
For the very small strains involved in the early part of the test, the elongation of
a measured length is recorded by an extensometer. The load is increased gradually,
and at first the elongation and hence the strain, is proportional to the load and hence to
the stress.
37
This relation (Hooke’s Law) holds up to a value of the stress known as the limit
of proportionality (Point A). Hooke’s law ceases to be obeyed this point, although the
material may still be in the “elastic” state. The point B shows the elastic limit. If the
material is stressed beyond this point, some plastic deformation will occur. The next
important occurrence is the yield point C, at which the metal shows an appreciable
strain even without further increase in load. For materials showing no definite yield, a
proof stress is used to determine the onset of plastic strain After yielding has taken
place, further straining can only be achieved by increasing the load, the stress-strain
curve continuing to rise up to the point D. The strain in the region from C to D is 100
times the strain in the system from O to C, and is partly elastic (i.e. recoverable), but
mainly plastic (i.e. permanent strain). At this stage (D) the bar begins to form a local
“neck”, the load falling off from the maximum until fracture at E. The proportional
limit is the stress at which the stress-strain curve deviates from linearity. The slope of
the stress-strain curve is the modulus of elasticity. Plastic deformation begins when
the elastic limit is exceeded. As the plastic deformation of the specimen increases, the
metal becomes stronger so that the load required to extend the specimen increases
with further straining. Eventually the load reaches a maximum value. The maximum
load divided by the original area of the specimen is the ultimate tensile strength. For a
ductile metal the diameter of the specimen begins to decrease rapidly beyond
maximum load, so that the load required to continue deformation drops off until the
specimen fractures. Since the average stress is based on original area of the specimen,
it also decreases from maximum load to fracture.
38
Fig 5.9 The Engineering Stress-Strain Curve
An engineering stress-strain curve is constructed from the load-elongation
measurements (fig 5.9). The stress used in this stress-strain curve is the average
longitudinal stress in the tensile specimen. It is obtained by dividing the load by the
original area of the cross-section of the specimen.
S = P A0
39
The strain used for the engineering stress-strain curve is the average linear
strain, which is obtained by dividing the elongation of the gauge length of the
specimen, δ, by its original length.
The shape and magnitude of the stress-strain curve of a metal will depend upon
its composition, heat treatment, prior history of plastic deformation, and the strain
rate, temperature, and state of stress imposed during the testing. The parameters
which are used to describe the stress-strain curve of a metal are the tensile strength,
yield strength or yield point, percentage elongation and reduction of area. The first
two are strength parameters; the last two indicate ductility. In the elastic range, strain
is measured by an “extensometer” attached to the gauge length. In the elastic region
stress is linear proportional to strain. When the load exceeds a value corresponding to
the yield strength, the specimen undergoes plastic deformation. It is permanently
deformed if the load is released to zero. The stress to produce continued plastic
deformation increases with increasing plastic strain i.e. the metal strain-hardens. The
volume of the specimen remains constant during plastic deformation, and as specimen
elongates, it decreases uniformly along the gauge length in cross-sectional area.
Initially the strain hardening more than compensates for this decrease in area and the
engineering stress continues to rise with increasing strain.
Eventually, a point is reached where the decrease in specimen cross-sectional
area is greater than the increase in deformation load arising from strain hardening.
40
This condition will be reached first at some point in the specimen that is slightly
weaker than the rest. All further plastic deformation is concentrated in this region, and
the specimen begins to neck or thin down locally .Because the cross-sectional area
now is decreasing far more rapidly than the deformation load is increased by strain
hardening, the actual load required to deform the specimen falls off and the
engineering stress likewise continues to decrease until fracture occurs. Many varieties
of fractures can occur during the processing of metals and their use in different types
of service.
41
CHAPTER- 6
FORMULA USED
42
6.1 FORMULA
6.1.1 STRESS AND STRAIN RELATIONSHIP
When a specimen is subjected to an external tensile loading, the metal will
undergo elastic and plastic deformation. Initially, the metal will elastically deform
giving a linear relationship of load and extension. These two parameters are then used
for the calculation of the engineering stress and engineering strain to give a
relationship as illustrated in using equations as follows
where is the engineering stress
is the engineering strain
P is the external axial tensile load
Ao is the original cross-sectional area of the specimen
Lo is the original length of the specimen
Lf is the final length of the specimen
43
The unit of the engineering stress is Pascal (Pa) or N/m2 according to the
SI Metric Unit whereas the unit of psi (pound per square inch) can also be used.
6.1.2 YOUNG'S MODULUS (E)
During elastic deformation, the engineering stress-strain relationship follows
the Hook's Lawand the slope of the curve indicates the Young's modulus (E).
Young's modulus is of importance where deflection of materials is critical for
the required engineering applications. This is for examples: deflection in structural
beams is considered to be crucial for the design in engineering components or
structures such as bridges, building, ships, etc. The applications of tennis racket and
golf club also require specific values of spring constants or Young's modulus values.
6.1.3 YIELD STRENGTH (Y)
By considering the stress-strain curve beyond the elastic portion, if the tensile
loading continues, yielding occurs at the beginning of plastic deformation. The yield
stress, σ y, can be obtained by dividing the load at yielding (Py) by the original cross-
sectional area of the specimen (Ao) as shown in equation.
44
The yield point can be observed directly from the load-extension curve of the
BCC metals such as iron and steel or in polycrystalline titanium and molybdenum,
and especially low carbon steels. a). The yield point elongation phenomenon shows
the upper yield point followed by a sudden reduction in the stress or load till reaching
the lower yield point. At the yield point elongation, the specimen continues to extend
without a significant change in the stress level. Load increment is then followed with
increasing strain. This yield point phenomenon is associated with a small amount of
interstitial or substitutional atoms. This is for example in the case of low-carbon
steels, which have small atoms of carbon and nitrogen present as impurities. This
dislocation pinning is related to the upper yield point as indicated in figure 4 a). If the
dislocation line is free from the solute atoms, the stress required to move the
dislocations then suddenly drops, which is associated with the lower yield point.
Furthermore, it was found that the degree of the yield point effect is affected by the
amounts of the solute atoms and is also influenced by the interaction energy between
the solute atoms and the dislocations.
6.1.4 ULTIMATE TENSILE STRENGTH TS
Beyond yielding, continuous loading leads to an increase in the stress required
to permanently deform the specimen as shown in the engineering stress-strain curve.
At this stage, the specimen is strain hardened or work hardened. The degree of strain
hardening depends on the nature of the deformed materials, crystal structure and
chemical composition, which affects the dislocation motion. FCC structure materials
45
having a high number of operating slip systems can easily slip and create a high
density of dislocations. Tangling of these dislocations requires higher stress to
uniformly and plastically deform the specimen, therefore resulting in strain hardening.
If the load is continuously applied, the stress-strain curve will reach the maximum
point, which is the ultimate tensile strength (UTS, σ TS). At this point, the specimen
can withstand the highest stress before necking takes place. This can be observed by a
local reduction in the cross sectional area of the specimen generally observed in the
centre of the gauge length.
6.1.5 ELONGATION
Tensile ductility of the specimen can be represented as % elongation or %
reduction in area as expressed in the equations given below in equation 8 and 9.
46
Where,
Af is the cross-sectional area of specimen at fracture.
The fracture strain of the specimen can be obtained by drawing a straight line
starting at the fracture point of the stress-strain curve parallel to the slope in the linear
relation. The interception of the parallel line at the x axis indicates the fracture strain
of the specimen being tested.
47
CHAPTER - 7
METHODOLOGY
48
7.1 METHODOLOGY
Springs represent the most important groupof engineering materials as they
have widest diversity of applications of any of the engineering materials. The majority
of the specifications are based on the chemical composition of the steels because it
indicates the required heat treatment data, i.e. phase transformation temperatures and
critical cooling rate of selective material. Therefore, any raw material that will be
treated must be firstly analyzed to know chemical composition.
The first step in process is heat treatment cycle of material. The total heating
time should be just enough to attain uniform temperature through the section
of the part to enable not only the completion of phase transformation, but also to
obtain homogeneous stage.
First stage of tempering : Up to 300°C
Second stage of tempering : 350°C
Third stage of tempering : 400°C
Fourth stage of tempering : 450°C
Fifth stage of tempering : 450°C
Sixth stage of tempering : 450°C
49
This UTM tests were carried in a material testing centre which was named
THIRUMALA MATERIAL TESTING CENTRE, which is at Hosur.
7.2 ABOUT THE TESTING CENTRE
The THIRUMALA MATERIAL TESTING CENTRE at Hosur was established
in the year 2007. It has accreditation from the NABL – NATIONAL ACCREDITION
BOARD FOR LABORATORIES. It is run by the CEO Mr. Sridhar Reddy, who did
his M.Tech in Metallurgy. Our UTM tests were carried under his supervision. Also
his ideas and thoughts helped us a lot in completing the project successfully.
The above specimens were heat treated and they were tested in UTM machine.
All the mechanical properties of the specimens like Yield Strength, Tensile Strength
and Elongation. The results of the tests were tabulated and they are shown in the table
below.
50
TABLE 7.1 - Tabulated values of the Tensile Test.
YIELD LOADYIELD
STRENGTH
TENSILE
LOAD
TENSILE
STRENGTHELONGATION YIELD LOAD
YIELD
STRENGTH
TENSILE
LOAD
TENSILE
STRENGTHELONGATION
N N/mm2 N N/mm2 % N N/mm2 N N/mm2 %
1 NORMAL MATERIAL 46000 920 63000 1260 8 74000 1162 85000 1335 8
2STRESS RELEIVING PROCESS
(225°, 15 MINS)65000 1300 70000 1400 6 41500 652 83500 1312 6
3 300° , 1 HOUR, WATER QUENCH 57000 1140 67500 1350 10 69500 1092 81500 1280 6
4 350° , 1 HOUR, WATER QUENCH 51500 1030 62000 1240 10 62000 974 77500 1217.78 12
5 400° , 1 HOUR, WATER QUENCH 53000 1060 65000 1300 10 70000 1099.93 82500 1296.35 12
6 450° , 1 HOUR, WATER QUENCH 73000 1460 74000 1480 5 73000 1141 88000 1375 5
7 500° , 1 HOUR, WATER QUENCH 71000 1420 72500 1450 6 69500 1086 83000 1297 6
8 550° , 1 HOUR, WATER QUENCH 69000 1380 70000 1400 7 65000 1016 73000 1141 7
9 300° , 1 HOUR, OIL QUENCH 47500 950 67000 1340 8 81000 1272.78 88500 1390.63 18
10 350° , 1 HOUR, OIL QUENCH 45000 900 62000 1240 14 70000 1099.93 81000 1272.78 14
11 400° , 1 HOUR, OIL QUENCH 52500 1050 63500 1270 10 64000 1005.65 79000 1241.35 10
12 450° , 1 HOUR, OIL QUENCH 68000 1360 70000 1400 5 74500 1164 91000 1422 5
13 500° , 1 HOUR, OIL QUENCH 65000 1300 66500 1330 6 71000 1111 84000 1313 6
14 550° , 1 HOUR, OIL QUENCH 625000 1250 63700 1274 7 68000 1063 77000 1204 7
THIRUMALESA MATERIAL TESTING CENTER
TEST CERTIFICATE - UTM VALUES
SL.NO PARTICULARS
Ø8 mm Ø9 mm
51
Fig 7.1 UTM Machine
Fig 7.2 Specification of the UTM Machine
52
7.3 GRAPHS
Various graphs were plotted against the temperature. The change in behavior of
the specimens for various temperatures can be clearly seen in the graphs plotted.
The various graphs are
Yield strength vs. Temperature (Ø8 mm & Ø9 mm)
Tensile strength vs. Temperature (Ø8 mm & Ø9 mm)
Elongation vs. Temperature (Ø8 mm & Ø9 mm)
53
7.3.1 GRAPHS FOR DIAMETER 8 MM
Fig 7.3 YIELD STRENGTH vs. TEMPERATURE, DIA 8
Fig 7.4 TENSILE STRENGTH vs. TEMPERATURE, DIA 8
Fig 7.5 ELONGATION vs. TEMPERATURE, DIA 8
54
7.3.2 GRAPHS FOR DIAMETER 9 MM
Fig 7.6 YIELD STRENGTH vs. TEMPERATURE, DIA 9
Fig 7.7 TENSILE STRENGTH vs. TEMPERATURE, DIA 9
Fig 7.8 ELONGATION vs. TEMPERATURE, DIA 9
55
7.4 SOFTWARE ANALYSIS
The values obtained from the experiment were tested in software and both
practical and analytical results were compared. The software “SOLIDWORKS” was
used to get the analytical report. Obtained analytical results are shown below for
diameter of 8 mm and 9 mm.
56
7.4.1 ANALYTICAL REPORT FOR DIA 8 MM
Fig 7.9 Analytical Diagram for Stress Analysis – dia 8mm
300°C, OIL QUENCH 350°C, OIL QUENCH
500°C, OIL QUENCH
450°C, OIL QUENCH
550°C, OIL QUENCH
400°C, OIL QUENCH
57
7.4.2 ANALYTICAL REPORT FOR DIA 9 MM
Fig 7.10 Analytical Diagram for Stress Analysis – dia 9 mm
300°C, OIL QUENCH 350°C, OIL QUENCH
400°C, OIL QUENCH
450°C, OIL QUENCH
500°C, OIL QUENCH
550°C, OIL QUENCH
58
CHAPTER - 8
COST ACCOUNTING
59
8. COST ACCOUNTING
Material Cost for Dia 8 mm - 800
Material Cost for Dia 9 mm - 1200
Straightening Cost - 300
Cutting Charges - 200
UTM Tests - 4800
Other Costs - 500
Total Cost - 7800
60
CHAPTER - 9
CONCLUSION
61
9. CONCLUSION
With respect to the experiment that was carried out, the readings were observed
and it was found that the heat treated specimen with a temperature of 450°C can
withstand maximum load when compared to all other specimens. The commonly used
stress relieving method of 225°C with 15 minutes can withstand a maximum load of
70000 N. On the other hand the specimen heat treated for 450°C can withstand a
maximum load of 74000 N. There by the 6% increase in load can be seen clearly.
So, we suggest the Spring Material of Grade III with heat treatment of 450°C can be
used to for many industrial and automobile applications.
62
CHAPTER - 10
REFERENCES
10. REFERENCES
63
1. Min Shan HTUN1, Si Thu KYAW2 and Kay Thi LWIN3, (July 30, 2012)
Journal on “Effect of Heat Treatment on Microstructures and Mechanical
Properties of Spring Steel”
2. HOU Weiguo, ZHANG Weifang, LIU Xiao, WANG Zongren, and DING
Meili, (21 February 2011) Journal on “Failure Analysis of Aviation Torsional
Springs”
3. ChokriCherif, André Seidel, AyhamYounes, Jan Hausding (No4, December
2010) Journal on “Evaluation of a Tensile Test for the Determination of the
Material Behaviour of Filament Yarns under High Strain Rate”
4. GurpreetKaur - Thapar Institute Of Engineering And Technology. Journal on
“Experimental And Numerical Analysis Of Tensile Test”
5. Morestin, F., and Boivin, M.,” On the Necessity of Taking into Account the
Variation in the Young Modulus with Plastic Strain in Elastic-Plastic
Software”,Nuclear Engineering and Design ,Vol.162.pp.107- 116,1996.
6. “The Design Data” Book by PSG Tech.
7. R.S. Khurmi - Book on “Theory on Machines”
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