Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 832-835
Published Online August 2012 (http://www.SciRP.org/journal/jmmce)
Effect of Normalizing and Hardening on Mechanical
Properties of Spring
O. R. Adetunji*, P. O. Aiyedun, S. O. Ismaila, M. J. Alao
Mechanical Engineering Department, College of Engineering, Federal University of Agriculture, Abeokuta, Nigeria
Email: *adetunjiolayide@yahoo.co.uk
Received May 27, 2012; revised July 7, 2012; accepted July 30, 2012
ABSTRACT
This study was carried out to investigate the effect of heat treatment (Normalizing and Hardening) on the mechanical
properties of springs. The springs were made from mild steel rod having a diameter of 6 mm, a total of 15 springs were
made. The springs were then subjected to various heat treatment processes which included; normalizing, hardening and
tempering. The heat treated springs were then subjected to various test in other to determine their mechanical properties,
these included; impact toughness test, hardness test and tension test. The normalized spring had more strength, was
harder and was much tougher than both the annealed and as received springs. The water quenched springs were the
hardest of all the heat treated springs, were very brittle and had the lowest percentage elongation. Their strength was
also lower than that of the normalized and as received springs. The water quenched and tempered springs had better
mechanical properties required for spring making, they had the optimum combination of hardness, strength and tough-
ness when compared w i t h the other heat treated sprin g s .
Keywords: Normalizing; Hardening; Tempering; Impact Toughness; Tension; Spring
1. Introduction
A spring is defined as an elastic body, whose function is
to distort when load ed and to recover to its origin al shape
when the load is removed. The various important appli-
cations 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 damp-
ers.
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.
There are various types of springs theses are: coil
springs, leaf springs, torsion bars and air springs [1].
1) Coil springs: is a mechanical device which is typi-
cally used to store energy and subsequently release it to
absorb shock, or to maintain a force between contacting
surfaces.
2) Leaf springs: are suspension springs made up of
several thin, curved, hardened-steel or composite-mate-
rial plates attached at the ends to the vehicle under-
body.
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.
4) Air springs: is a mechanical device using confined
air to absorb the shock of motion?
1.1. Heat Treatment
This is the heating and cooling of a solid metal or alloy
in such a way as to obtain desired conditions or proper-
ties. The term heat treatment process is in and of itself
only a very generic term; it covers all specific methods
[2]. Thus emphasis will be made on those forms of heat
treatment that are most commonly used in the spring in-
dustry these are: Annealing, Normalizing, Hardening and
Tempering.
1.2. Normalizing/Stress Relieving
Heating to a suitable temperature, 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
*Corresponding author.
Copyright © 2012 SciRes. JMMCE
O. R. ADETUNJI ET AL. 833
actually increase the strength to leve ls greater than origi-
nally supplied [3].
The normalizing of steel is carried out by heating ap-
proximately 100˚F (38˚C) above the upper critical tem-
perature (Ac3 or Acm) followed by cooling in air to
room temperature Normalizing is often considered from
both a thermal and a micro structural standpoint. In the
thermal sense, normalizing is austenitizin g followed by a
relatively slow cooling.
1.3. Hardening (Water 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 cor-
rosive with steel, and the rapid cooling can sometimes
cause disto rtion or cracking [ 3].
1.4. 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.
2. Experimental Methodology
The springs were made using mild steel rods having a
diameter of 6 mm. The following were the steps taken
during the making of the springs. The mandrel and one
end of the steel rod were clamped together with the use
of a g clamp. The steel rod was clamped to the beginning
of the mandrel [4-6].
a) The steel rod was then wound round the mandrel at
the desired pitch.
b) When the desired number of turns was reached i.e.
8 turns. The steel rod wa s then cut off from the unwound
steel rod with the help of a saw.
c) The mandrel with the spring wound round it was
both removed from the g-clamp after which the spring
was remove d from the mandrel.
2.1. Normalizing/Stress Relieving
In normalizing the springs were heated to 550˚C, 700˚C
and 900˚C respectively, after the desired temperature had
been obtained they were removed from the furnace and
allowed to cool in air till room temperature was obtained.
They will have a different property if the springs have
cooled down to room temperature in the furnace.
2.2. Hardening (Water Quenching)
In water quenching t he spri ng s were heat ed to about 850˚C
to ensure conver sion to austenite had been achieved. Th e
springs were then taken out of the furnace and placed in a
bath of water to ensure that rapid cooling of the spring
occurred.
2.3. Tempering
In tempering the three water quenched steel were heated
to 300˚C, 500˚C and 600˚C respectively and were then
allowed to cool down gradually. Th is was done to relieve
some of the stress that occurred during water quenching
and to also reduce the hardness of the steel rod, thus
making it more tougher.
The remaining three springs were not subj ected to any
heat treatment and shall thus be used for comparison with
the other heat treated springs.
Three new set of springs are normalized, this is done
by heating them to the following respective temperature
550˚C, 700˚C and 850˚C after which they are removed
from the furnace and allowed to cool in air to atmos-
pheric temperature.
1) Three new set of springs are then heated to 850˚C
after which they are removed from the furnace and
placed in a bath of water, a process known as water
quenching.
2) The three set of springs which have been water
quenched from 850˚C are then tempered by heating them
to the following temperature 300˚C, 500˚C and 600˚C
respectively after which they are allowed to cool at a
slow rate.
2.4. Impact Toughness
The impact test is done with an impact tester. This is
used to determine toughness of the steel wire used for the
spring the impacter was allowed to fall from a certain
height in other to crush the steel rod. The height from
which the impacter is released can be used to measure
the degree of hardness of the steel rod by the amount of
energy absorbed by the rod before fracture.
2.5. Tensile Test
The tensioning machine was used to determine the
strength of the spring. A force is applied axially via
Copyright © 2012 SciRes. JMMCE
O. R. ADETUNJI ET AL.
Copyright © 2012 SciRes. JMMCE
834
3. Result and Discussion
weights on the tensioning machine. The amount of force
required to produce a certain amount of deflection was
then recorded for all the spring samples. The results obtained for tension test of normalized springs
are contained in Table 1, tension test for water quenched,
tempered and as received springs in Table 2, impact
toughness and hardness test for normalized springs in
Table 3 and impact toughness and hardness test for har-
dened and tempered springs in Table 4.
2.6. Hardness Test
The surface hardness test was measured by Matsuzawa
DXT3 Rockwell test device according to the ISO stan-
dards, this was done to determine the hardness of the
various heat treated springs. The water quenched spring
was subjected to an incremental load of 4 kg and an
extension of 0.15 cm was measured until an extra load
of 4 kg was added to the 16 kg mass and an extension
of 0.3 cm was measured, which indicated that 16 kg
was the yield load of the water quenched springs, simi-
larly the yield load of the normalized and as received
springs can be obtained from Section 3 (Result and Dis-
cussion).
Table 1. Tension test for normalized springs.
Load
(kg)
Extension (cm)
Normalized at
550˚C
Extension (cm)
Normalized at
700˚C
Extension (cm)
Normalized at
850˚C
4 0.5 0.6 0.5
8 1.0 1.2 1.0
121.5 1.8 1.5
162.0 2.4 2.0
202.7 3.3 2.5
Table 2. Tension test for water quenched, tempered and as received springs.
Load (kg) Extension (cm) Water
Quenched at 850˚C Extension (cm) Tempering
at 300˚C Extension (cm) Temperi ng
at 500˚C Extension (cm) Tempering
at 600˚C As Received
spring
4 0.15 0.45 0.55 0.6 0.4
8 0.3 0.9 1.1 1.2 0.8
12 0.45 1.3 1.6 1.8 1.2
16 0.6 1.7 2.1 2.4 1.6
20 0.9 2.0 2.8 3.0 2.0
24 2.5 3.8 2.6
Table 3. Impact toughness and hardness test for nor m alized spr i ngs.
S/No Spring Description YL
(kg) Yield Stress
(kg/mm) Extension
(cm) % ElongationToughness
(Joules) Hardness
(RC)
1 Normalized at 550 16 0.555 2.4 25 45 50
2 Normalized at 700 16 0.555 3.0 30 49 47
3 Normalized at 850 24 0.848 3.0 37.5 58 38
Table 4. Impact toughness and hardness test for hardened and tempered springs.
S/No Spring description YL
(kg) Yield Stress
(kg/mm) Extension
(cm) % ElongationToughness
(Joules) Hardness
(Rc)
1 Water quenched at 850 16 0.565 0.6 7. 5 20 49
2 Tempered at 300 20 0.565 1.7 21.25 47 45
3 Tempered at 500 16 0.707 2.6 32.25 55 38
4 Tempered at 600 20 0.565 2.4 31 45 35
O. R. ADETUNJI ET AL. 835
3.1. Normalized Springs
As the steel is heated above the critical temperature,
about 1335˚F (724˚C), it undergoes a phase change, re-
crystallizing as austenite. Continued heating to the hard-
ening temperature, 1450˚F - 1500˚F (788˚C - 816˚C) en-
sures complete conversion to austenite. At this point the
steel is no longer magnetic, and its color is cherry-red,
normalizing has a slower rate of cool than annealing and
this accounts for the springs having a greater strength
than the annealed spring, and also accounts for it being
harder and less ductile than the ann ealed springs [7,8].
3.2. Hardening and Tempering
As the steel is heated above the critical temperature,
about 1335˚F (724˚C), it undergoes a phase change, re-
crystallizing as austenite (i.e. it changes from body cen-
tered cubic to face centered cubic) Continued heating to
the hardening temperature, 1450˚F - 1500˚F (788˚C -
816˚C) ensures complete conversion to austenite, the
springs are then cooled suddenly by quenching in a bath
of water, a new crystal structure, martensite, is formed as
seen in the micro structural analysis above. Martensite is
characterized by an angular needle-like structure and
very high hardness, as seen from the hardness test carried
out. While martensitic steel is extremely hard, it is also
extremely brittle and will break, chip, and crumble with
the slightest shock. Furthermore, internal stresses remain
in the spring from the sudden quenching; these will also
facilitate breakage of the spring. Tempering relieves
these stresses and causes partial decomposition of the
martensite into ferrite and cementite. The amount of this
partial phase change is controlled by the tempering tem-
perature. The tempered steel is not as hard as pure mart-
ensite, but is much tougher. This can be observed from
the result tabulated above where by the water quenched
sprigs have the highest hardness and are the least ductile
from all the springs, but after the water quenched steel
were tempered it was observed that the hardness of the
spring reduced, the toughness and ductility increased
when compared to the water quenched springs.
4. Conclusions
The normalized spring had more strength, was harder and
was much tougher than as received springs.
The water quenched springs were the hardest of all the
heat treated springs, were very brittle and had the lowest
percentage elongation. Their strength was also lower than
that of the normalized and as received springs.
The tempered water quenched springs had better me-
chanical properties required for spring making, they had
the optimum combination of hardness, strength and
toughness when compared with the other heat treated
springs.
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