The aim of this project was to make a comparison between the changes in mechanical properties of mild steel quenched in various quenching mediums namely Vegetable oil, Brine solution, NaOH solution and Super-quenchant. Mild-Steel specimens for hardness test, tensile test and impact test were prepared and heated upto the austenizing range of temperature. After holding at that temperature for the necessary sintering time, they were immediately quenched in the four mediums. Upon carrying the various tests, it was observed that hardness of all the specimens increased at the expense of toughness. Further the rate of cooling influenced the hardness of the specimens. Specimens quenched in NaOH exhibited maximum increase in hardness and tensile strength of steel. Oil quenched steel showed rise in hardness and tensile strength with least decrease in toughness among the four mediums. Brine also improved the hardness and tensile strength but maximum reduction in toughness was encountered. Finally, superquenchant was found to be the best quenching medium with appreciable rise in the hardness and tensile strength at very less reduction in toughness.

1. Analysis of Mechanical Properties of Heat-treated
Mild Steel
- Saugata Chowdhury, Md. Nur Alom, Bhaskar J. Choudhury, Tanmoy Das
Mechanical Engineering Department, NERIST
Abstract
The aim of this article is to present a comparison between the
changes in mechanical properties of mild steel quenched in various
quenching mediums namely Vegetable oil, Brine solution, NaOH
solution and Super-quenchant. Mild-Steel specimens for hardness
test, tensile test and impact test were prepared and heated upto the
austenizing range of temperature. After holding at that
temperature for the necessary sintering time, they were
immediately quenched in the four mediums.
Upon carrying the various tests, it was observed that hardness of
all the specimens increased at the expense of toughness. Further the
rate of cooling influenced the hardness of the specimens. Specimens
quenched in NaOH exhibited maximum increase in hardness and
tensile strength of steel. Oil quenched steel showed rise in hardness
and tensile strength with least decrease in toughness among the
four mediums. Brine also improved the hardness and tensile
strength but maximum reduction in toughness was encountered.
Finally, superquenchant was found to be the best quenching
medium with appreciable rise in the hardness and tensile strength
at very less reduction in toughness.
Keywords: Mild Steel, Quenching medium, Mechanical properties,
Sintering time, Austenizing temperature
I. INTRODUCTION
In the present era, to meet the ever-increasing demands of the
people mechanical properties of the metals have to be altered.
This can be done either by changing the constituent elements or
by changing the microstructure through heat treatment. In our
work, we have dealt with heat treatment of mild steel with an
aim to obtain the best quenching medium which imparts the
optimum combination of strength, hardness and toughness. It is
a well established fact that upon rapid cooling or quenching a
well heated material, its hardness increases at the expense of
toughness. This happens due to the formation of martensite
having BCT (Body- centered Tetragonal) structure in which the
carbon particles get trapped in the iron matrix. [1]
Hardenability
of steel depends on the carbon and alloy content of steel but
maximum hardness depends only on the percentage of carbon
content.[2]
With an increase in carbon content, the hardness of
martensite increases. But heat treatment is generally carried on
steel with carbon content upto 0.6%.
Martensite is produced by controlling cooling time from
austenising temperature to martensite start temperature within 2
seconds. The rate of cooling depends on the quenching medium
which influences the hardness. Quenching takes place in three
distinct stages namely vapour blanket stage, boiling stage and
liquid cooling stage. Out of the three stages, boiling stage is
characterized by highest rate of heat transfer. The size and shape
of the vapour bubbles during this stage are important in
controlling the duration of this stage as well as its corresponding
rate. The difference in temperature between the boiling point
and actual temperature of the medium is the major factor
affecting the rate of heat transfer in liquid quenchants. [3]
Further, viscosity of the medium at this point also affects the
cooling rate since a less viscous medium will dissipate heat
faster than one of high viscosity. The final stage or the liquid
cooling stage of quenching is the most important in cooling and
reducing distortion and cracking.
In general, liquid quenching is performed in water, oil and more
recently in aqueous polymer solutions. Oil is valued for its
ability to offer rapid cooling over a wide range of temperatures.
[4]
They are classified in three distinct groups: conventional, fast
and martempering (hot quenching) with respect to their
quenching effect. When NaCl or CaCl2 is added in small amount
to water, it offers faster quench rates than plain water and is
used for steels with low hardenability. The salt in water raises its
specific heat and disrupts the vapour jacket that forms around
the quenched part. [5]
This fastens the rate of heat transfer and
thus the hardness increases. Unfortunately, it tends to accelerate

2. corrosion problems. Like salt, NaOH also increases the surface
tension of water which improves the uniformity and rate of
cooling. But due to violent reaction this is no more used. It has
been replaced by super-quenching medium which is actually
brine solution with surfactants added to it.[6]
The surfactant
encourage improved wetting of the quenchant on the steel due to
which further uniformity and improved rate of cooling is
obtained. This medium is suitable for low hardenability steels.
II. EXPERIMENTAL PROCEDURE
Material selection and Specimen preparation
The chemical composition of mild steel used (by%wt.) is given
as follows: C-0.6, Si-0.03, Mn-0.32, S-0.05, P-0.2, Ni-0.01,
Cu0.01, Cr-0.01 and Fe.
The test specimens for analysis of various mechanical properties
were prepared as per ASTM standard. For hardness test,
standard steel specimens of cylindrical shape were prepared
with dimensions 25mm diameter x 25mm height. Toughness test
specimens were prepared with following dimensions: length =
55mm, width = 10mm, thickness = 10mm, notch depth = 5mm.
Tensile test specimens with round cross section were prepared
with gauge diameter = 12mm and gauge length = 120mm.
Selection and Preparation of quenching medium
Four different quenching medium as discussed above were
selected after comparing their cooling rate with water and were
prepared as per the following composition [7]
:
Quenching mediums Cooling rate compared
to water
Sodium hydroxide (10%
solution)
2.06
Super quenching medium
(heavy brine + surfactant +
anti-bubbling agent)
> 1.96 (approx.)
Brine (7% Salt solution) 1.00-1.96 (approx.)
Oil 0.15-0.45 (approx.)
*While experimentation, volume of quenching medium used
was 10L.
Heat Treatment
The prepared specimen were heated to a temperature of 810C
(sufficiently above the upper critical temperature of mild steel)
and were held at that temp for 1 hr to let the specimen sinter
completely. After the heating was complete the specimens were
taken out and immediately dipped in the respective quenching
medium without much heat loss in between. The heating was
done in muffle furnace.
Mechanical Test
The tensile test was carried out in a UTM with 400kN load cell
under constant cross head speed. The Brinell hardness Test was
carried out with indentation ball diameter 10mm and load of
3000kgf. The charpy impact test was carried out with potential
energy of hammer being 300J and 135 being the angle of
inclination of hammer.
III. RESULTS AND DISCUSSION
In general heat treatment and quenching of mild steels resulted
in an increase in hardness, tensile strength and decrease in
toughness values. The test results of different mechanical
characteristics like tensile strength, toughness and hardness has
been tabulated below
S
l
.
N
o
.
Quenching
medium of
specimen
Cooling
rate as
compar
ed to
water
Ultimate
tensile
strength
(kN/mm2
)
Toughness
(Joules) BHN
1 Untreated
specimen
- 0.440 120 140
2 Brine Solution 1.0-1.96 0.774 40 198.
33
3 NaOH solution 2.06 0.840 71.33 226
4 Oil 0.15-
0.45
0.635 106.33 180.
33
5 Superquenchant >1.96 0.661 93.66 208.
33

3. 1. The hardness values varied in between 140 BHN – 226
BHN and it is highest for NaOH quenched mild steel
specimens and lowest for the untreated specimens. This
result shows that quenching greatly improved the
hardness of mild steel and the quenching media capable
of quicker cooling leads to increased hardness.
2. The tensile strength varied in between 0.440 kN/mm2
–
0.845 kN/mm2
and it is highest for NaOH quenched
mild steel specimens and lowest for the untreated
specimens. This results shows that quenching greatly
improved the tensile strength of mild steels.
3. The toughness values varied in between 71.33 Joules–
120 Joules and it is highest for untreated samples and
lowest for brine quenched specimens. The result shows
that quenching decreases the toughness of mild steels.
A comparative study of the changes in various mechanical
properties due to quenching in the four media has been
graphically represented below:
4. NaOH quenched mild steels has given the highest
hardness of 226 BHN and tensile strength of 0.845
kN/mm2
but it has become brittle with low toughness
of 71.33 J.
5. Brine quenched mild steels has given the high hardness
of 198.33 BHN and tensile strength of 0.778 kN/mm2
but it has become brittle with lowest toughness of 40 J.
6. Oil quenched mild steels has given the high toughness
of 106.33 J but has got low hardness and tensile
strength of 180.33 BHN and 0.633 kN/mm2
.
7. Finally, after comparioson of all the mechanical
properties it is noted that superquenchant has given the
best results because though it has become harder by a
great extent with high tensile strength but its toughness
has decreased by a lesser amount as compared to others
with that kind of high cooling rate.
8. In addition, studies have shown that the highest degrees
of corrosion resistance have been obtained through the
maximum rates of quenching.
IV. CONCLUSION
From the present study on “Analysis of Mechanical Properties
of Heat Treated Mild Steel” the following conclusions have
been drawn:-
140
198.33
226
180.33
208.33
0
50
100
150
200
250
BHN
Quenching Media
RESULTS OF HARDNESS TEST
120
40
71.33
106.33
93.66
0
20
40
60
80
100
120
140
Toughness(inJ)
Quenching Media
RESULTS OF TOUGHNESS TEST
0.44
0.774
0.84
0.635 0.661
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
UTS(inkN/mm2)
Quenching Media
RESULTS OF ULTIMATE TENSILE STRENGTH

4. 1. The mechanical properties of mild steel were
found to be strongly influenced by the process of
quenching.
2. Heat treatment followed by rapid cooling
(quenching) increases the hardness of steel by
great extent.
3. Process of rapid cooling increases the hardness but
decreases the toughness of the mild steel.
4. Oil is a very good quenching medium as it
increases hardness and tensile strength with the
least decrease in toughness.
5. Sodium hydroxide (NaOH) quenched steels gives
the highest hardness and tensile strength but it is
highly reactive.
6. NaOH can react with the container vessel if it is
metallic and the reaction is exothermic and safety
precautions should be taken while putting hot
specimen into NaOH bath.
7. Brine as a quenching medium also gives a high
hardness and tensile strength but it gives low
toughness because brine quenching sometimes
causes quench-cracking and it is also corrosive in
nature.
8. Superquenching medium is the best quenching
medium because it gives high hardness,
appreciable tensile strength with a low decrease in
toughness. It gives the best combination of
mechanical properties.
9. As a modification of brine quenchant,
superquenching medium is less corrosive in nature
and there is also very low possibility of cracking
due to uniform cooling rate.
V. REFERENCES
[1] Robert K. Nichols, PE. The metallurgical effects of weld
seam heat treating
[2] Steel founder’s society of America. Steel Castings
Handbook - Supplement 11
[3] Marco Fontecchio, Mohammed Maniruzzaman and
Richard D. Sisson, Jr. Quench Factor Analysis and Heat
Transfer Coefficient Calculations for 6061 Aluminum Alloy
Probes Quenched in Distilled Water
[4] Totten, G.E., Bates, C.E. & Clinton, N.A. (1993).
Handbook of quenchants and quenching technology.
Materials Park, OH: ASM International: 129.
[5] Houghton Fluid technology and service worldwide,
Madison and Van Bured Aves, Valley Forge. Houghton on
Quenching
[6] Robb Gunter, Central Virginia Blacksmith Guild, April-
1998
[7] http://www.tpub.com/steelworker1/12.htm