effects of continuous cooling on hardness and microstructural

Original Scientific Paper
Aweda, E. O., Dagwa, I.M., Dauda, M., Dauda, E.T.
EFFECTS OF CONTINUOUS COOLING ON HARDNESS AND MICROSTRUCTURAL
PROPERTIES OF LOW CARBON STEEL WELDED PLATE
Received: 1 August 2013 / Accepted: 23 September 2013
Abstract: In this study, the effects of continuous cooling on the hardness and microstructural properties of low
carbon steel (LCS) plates using different coolants were investigated. The hardness was measured as well as the
microstructural examination at distances; 10mm to 45mm in-steps of 5mm from the fusion zone. When hardness
value of the control (unwelded) sample was compared with the welded samples that were cooled in air, water, and
salty water, it was discovered that the hardness values increased by 3.18%, 4.40% and 10.5% respectively. The
improvement in hardness values in air cooled samples was attributed to the transformation of austenite into pearlite
in a ferrite matrix. While, for the water and salty water cooled samples, more martensite regions were observed at
the grain boundaries of ferrite matrix with salty water having much more martensites.
Key words: Continuous cooling, hardness, micro structural studies, low carbon steel.
Efekti kontiunalnog hlađenja na tvrdoću i mikrostrukturne osobine niskougljenične čelične zavarene ploče. U
ovom radu ispitivani su efekti kontinuiranog hlađenja na čvrstoću i mikrostrukturna svojstava niskougljenične
čelične ploče gde su korišćena različita sredstava za hlađenje. Merena je tvrdoća kao i ispitivanje mikrostrukture na
rastojanjima: 10mm do 45mm u - koracima od 5mm od zone zavarivanja. Kontrolom vrednosti tvrdoće uzorka koji
je poređen sa zavarenim uzorcima koji se hlade u vazduhu, vodi i slanoj vodi, otkriveno je da su vrednosti tvrdoće
porasle za 3,18% , 4,40% i 10,5% respektivno. Poboljšanje vrednosti tvrdoće vazdušnim hlađenjem uzoraka je
pripisano transformaciji austenita u perlit. Dok je za hlađenje vodom i slanom vodom uzoraka, ima više martenzitnih
regiona na granicama zrna ferita odnosno korišćenjem slane vode dobija se martenzitna struktura.
Ključne reči: Kontinualno hlađenje, tvrdoća, mikrostrukturna svojstva, niskougljenični čelik.
thickness with air weld recording a more consistent
increase. From the results, it was observed that the
hardness for underwater welding is almost twice that of
the welding in air around weld metal and heat affected
zone (HAZ), and remarkable differences were not
observed between both welding processes in the base
metal away from the weld zone. Currently [6], there is
a strong interest in studying the effect of cooling rate
on the mechanical properties and microstructure of
industrial processed steels. Calik [6] has studied the
effect of cooling rate on hardness and microstructure of
AISI 1020, AISI 1040, and AISI 1060 steels and shown
that the microstructure of these steels can be changed
and significantly improved by varying the cooling
rates. Adedayo and Oyatokun [7] studied the effect of
saline water cooling on service quality of a welded
AISI 1013 Carbon steel plate by varying the coolant
flow rates.
Therefore, in this study the effect of simultaneous
cooling (using different cooling media such as air,
water and salty water (3.5percent salty in water)) and
welding at varying distances from the weld fusion line
on the hardness property and the microstructures of low
carbon steel plates was investigated.
1. INTRODUCTION
Low carbon (LC) steel materials are susceptible to
hardness property change when they are welded
depending on factors such as: cooling medium, rate of
cooling, electrode type and composition, welding
parameters, etc. The LC steel contain up to about 0.25
percent carbon and are used for general engineering
construction work involving severe cold working
especially in sheet form such as car bodies, furniture,
tubing and magnetic materials[1]. The commonest
method used in joining of LCS parts is the Shielded
metal arc welding, (SMAW). Furthermore; it is used in
the oil and gas sector. In the offshore environments it is
used in the construction, building, repair and
maintenance of ships, the surrounding water which is
salty serves as a cooling medium. Studies [2] have
shown that depending on the environment, welding can
alter the carefully designed microstructure of steels as a
result of heat affected zone thermal cycles that exceed
the transformation temperature [3]. Depending upon
the heating and cooling cycles involved, different types
of microstructures are obtained in weld bead and the
heat affected zone (HAZ).
This leads to varying mechanical properties such as
hardness of different zones of a weldment [4]. Fukuoka
et al [5] welded steel plates of sizes 2mm, 8mm,
14mm, 16mm and 19mm using gas shielded arc
welding for both air and underwater welding. Cooling
rate was observed to increase with increase in plate
2. EXPERIMENTAL PROCEDURE
The elemental composition (see Table 1) of the low
carbon steel was determined using X-Ray Fluorescence
(XRF) at the Universal Steels, Ikeja, Lagos. The
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weld joints. The grinding process was carried out using
silicon carbide papers of various grits starting with the
coarsest to the smoothest in the following order: 120,
240, 320, 400 and 600 grits sizes respectively. Coolant
was applied intermittently in order to avoid the heating
up of the sample which could alter its hardness
property by annealing process as well as to wash away
the particles that were removed from the surface. The
polished surfaces were shinny and free of abrasive
particles.
cooling media used were: air, water and salty water
(3.5 percent of sodium chloride in water)
2.1 Specimen Preparation
Fifty four samples were prepared for both hardness
and microstructural tests from a 5mm thick plate. Eight
(8) samples each were cooled with water and salty
water. Hardness test and metallographic examination
were carried out on the same samples. For each cooling
position T1 to T8 (in steps of 5mm from 10mm to
45mm from the fusion zone), three samples were
produced in order to obtain the average value. T1 was
located at the furthest distance away from the weld
joint, while, T8 was the closest to the weld joint. Nine
(9) of the samples were used as control test samples.
The effect of continuous cooling on hardness was
observed at eight (8) different positions. Before the
samples were butt welded, they were chamfered at
angles 30o creating a vee-groove and leaving a root face
and a root gap of 2mm.
2.5 Etching and Microscopy
The polished surfaces of the samples were etched
with Nital and examined under a metallurgical
microscope. The microscope was used to study the
microstructure of the polished surface under different
conditions of welding and cooling. A magnification of
100 was used.
3. RESULTS AND DISCUSSION
Element
Average
Element
Average
content (%)
content (%)
C
0.1460
Nb
0.0001
Si
0.297
Al
0.0001
Mn
0.285
B
0.0001
S
0.027
W
0.0001
P
0.021
Mo
0.0001
Cr
0.040
V
0.0001
Ni
0.018
Ti
0.007
Cu
0.028
Fe
99.131
Table 1. Average Content of Elements in theMaterial
3.1 Cooling Curves
The cooling curves for the various cooling media
air, water and salty water are presented as follows:
3.1.1 Air Cooled Samples
A rapid fall in temperature was observed from peak
temperature (1200oC) to 640oC within 16s as shown in
Fig. 1 which, is followed by a gradual fall to ambient
temperature.
2.2 Hardness Test
ASTM A370 standard was used to prepare hardness
test samples. The hardness values of the fifty-four
samples were obtained using Identec (Diamond
Rockwell) Universal Hardness Testing Machine (Type
8187.5 LKV Model B) [Minor load= 10kgf, Total
load= 60kgf, Scale= A, Indenter=diamond cone
(1200)].
2.3 Welding Process
The shielded manual arc welding (SMAW) machine
and E6013 electrode were used during the welding
process. The welding parameter setting for the SMAW
machine was set at: voltage; 18V, current; 250A, and
average speed; 3.1mm/s; heat transfer efficiency factor
f1 (for SMAW) was 0.65. Hence, the heat input was
computed to be 1 453.06 J/mm. A constant mass flow
rate of the coolant was set at 718.2 g/s throughout the
work. The temperature at selected distances from the
fusion zone was measured using a digital thermometer
(TECPEL, DTM 307). The cooling process was aided
by coolant flow guide.
Fig.1. Cooling Curve for Air Cooled Sample
3.1.2 Water Cooled Samples
Figure 2 shows the cooling curve for the water
cooled samples. It was observed that the closer the
cooling position is to the weld zone, the faster the
increased temperature drop because of the thermal
gradient. The curve T1 is the farthest from the
weldment while T8 is the closest to it.
The temperature dropped from 1200oC to 600oC
within 8s. However, It took 52s for the sample’s
temperature to drop to the ambient temperature.
2.4 Metallurgical Examination
Fifty-four (54) samples were ground and
subsequently
polished
towards
metallographic
examination. An abrasive wheel was used to grind the
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may be due to the faster cooling rate of salty water,
which resulted in higher martensite formation. Also,
the increased presence of fine dispersion of small
particles in the pro-eutectoid ferrite and pearlitic ferrite,
which will prevent the dislocation movement, may
have also contributed to the higher Rockwell hardness
number of the salty water cooled sample.
Fig.2. Cooling Curves of Water Cooled Samples
3.1.3 Cooling Curves for Salty Water Cooled
Samples
The cooling curves for salty water cooled samples
are shown in figure 3. It was observed that it took only
4s for the temperature to drop from 1200oC to 400oC.
The cooling effect of salty water was observed to
increase with decrease in cooling distance away from
weld zone.
This cooling curve for salty-water cooling shows a
more rapid cooling rate than air and water cooling. This
rapid cooling experienced resulted in an increase in
hardness of the samples that were cooled with salty
water. Calik[6] has shown that the micro hardness of
steels increases with the cooling rate and also carbon
content.
Cooling
medium
Minor
Load (kgf)
Total
Load (kgf)
Weldment
(kgf)
HAZ
(kgf)
Weldment
(N)
HAZ
(N)
Fig.4. Weld Hardness (Water Cooled (WC) and SaltyWater Cooled (SWC) Samples)
Air
10.00
60.00
48.50
47.20
475.79
463.03
Table 2. Hardness Values of Air Cooled Sample
3.3 Microstructures
The microstructures of the samples cooled in air
(control), water and salty water are presented in plates
1 to 6. Each of the plates has a magnification of 100.
Fig.3. Cooling Curves of Salt Water Cooled Test
Samples
Plate 1: Control sample
3.2 Mechanical property:
( a) Air Weld
3.2.1 Hardness
Rockwell Universal Hardness testing machine was
used to carry out hardness tests on the welded samples.
Figure 4 presents the results of water cooled (WC) and
salty water cooled (SWC) samples. The control sample
had a hardness of 45.7kgf.
The hardness values of water cooled samples have
lower Rockwell hardness values compared to salty
water cooled samples as presented in figure 4. This
22
From the micrographs presented in plate 3, at 45mm
from the weld, ferrite structure is seen to be more and
generally dispersed in the microstructure. The ferrite
structures are the light-coloured regions of the
structure. The amount of pearlite was observed to
increase as the cooling distance from the weld reduces
(that is, as the material is cooled at a distance nearer to
the weld zone).
(b) Air HAZ
3.3.2 Heat Affected Zone (salty water)
Plate 2: Weld zone and Heat Affected Zone (cooled
under room temperature)
Plate 4 shows the heat affected zone (HAZ) of the
salty water cooled samples. The dark areas are the
martensite regions and they appear to be more
especially for 15mm plates. The other plates, such as
45mm have lighter appearance. These are pearlite
structures (a mixture of iron ferrite and cementite).
The control sample in plate 1 shows ferrite and
pearlite layers because of the low carbon content in the
material and the grain structure is coarse. The ferrites
are the white areas while pearlites are the dark areas.
Ferrite, which has a body centred cubic (bcc) structure
transforms into austenite (a high temperature phase
with face centred cubic (fcc) crystal structure). During
the cooling cycle, the austenite transforms back into
ferrite or other metastable phases as seen in plate 2.For
low cooling rate as in the air cooled sample, the
austenite was observed to transform into ferrite.
In plate 2a, the weld zone has fine grains whereas
its heat affected zone as shown in plate 2b has coarse
grains. This is attributed to the faster cooling rate at the
weld zone. It was observed that there was no sufficient
time for grain growth and nucleation in the weld zone.
15mm
45mm
3.3.1 Heat Affected Zone (Water)
Ibarra, et al, [8], observed in his work that due to
the rapid cooling that occurs in wet weld, the heat
affected zone of most welded mild steel are coarsegrained and martensitic. This was also observed in the
heat affected zone micrographs shown in plates 3 and
4. The salty water cooled samples have more
martensitic structures than water and air cooled
samples.
Plate 4: Heat Affected Zone (Salty Water Cooled)
The micrographs presented in this section show the
samples cooled in 3.5percent salty in water. The
cooling rate of the salty water cooled samples was
faster than the water cooled samples. The grains were
also coarse but it was observed that there was an
increase, though minimal, in the amount of pearlite
layers. At 10mm and 15mm from the weld zone,
martensitic structures were observed.
15mm
3.3.3
Weld (water cooled)
Weld zone microstructures for water cooled are
presented in plate 5. The grain structures were fine and
finger-like. This is because of the rapid cooling rate at
the weld zone. The micrographs farther away from the
weld show more ferrite structures and some few
pearlite layers.
45mm
3.3.4
Weld (salty cooled)
The micrographs above (plate 6), are also fine
grained. The microstructure of the 10mm and 15mm
samples show some martensitic layers with some
pearlite regions. It was observed that the martensite in
the water cooled weld micrographs were more evenly
dispersed than that of the salty water cooled weld zone
Plate 3: Heat Affected Zone(Water Cooled) at some
distances from the fusion zone
23
micrographs. The microstructures of samples cooled far
from the weld zone were also more ferritic.
The increase in pearlite and martensite in the salty iii.
water cooled samples resulted in an increase of their
hardness and strength.
iv.
15mm
v.
45mm
5.
REFERENCES
[1] Ibhadode, A.O.A., Introduction to Manufacturing
Technology, Ambik Press, Benin-City, Nigeria,
2001
[2] Pang,W., Ahmed,N. and Dunne, D. Hardness and
microstructural Gradient in the heat Affected Zone
of Welded Low- Carbon quenched and Tempered
Steels, Austral Asian Welding Journal,Volume 56,
Second Quarter, pp36-48, 2011
[3] Shome , M., Gupta, O. P and Mohanty, O. N. A
Modified Analytical Approach for Modelling Grain
Growth in the Coarse Grain HAZ of HSLA Steels.
Scripta Materialla 50: Pp 1007- 1010, 2004
[4] (Parmar, R.S., Welding
Engineering and
Technology, Khanna Publishers, India, 2007.
[5] Fukuoka T. and Fukui S., Analysis for Cooling
Process of Underwater Welding -Comparison with
Welding in Air, Bulletin of the M.E.S.J., Vo1. 22,
No.2. PP 88-92,1994.
[6] Calik, A., Effect of cooling rates on the hardness
and microstructure of AISI 1020, AISI 1040, and
AISI 1060 steels, International Journal of Physical
Sciences Vol.4(9),pp 514-518, 2009. Available
online at hhtp://www.academicjournals.org/IJPS
[7] Adedayo, S.M. and Oyatokun, V.O., Effect of
saline water cooling on service quality of a welded
AISI 1013 Carbon steel plate. Annals of faculty of
engineering Hunedoara-International Journal of
Engineering, Tome XI, Fascicule 2 pp 25-28, 2013.
[8] Ibarra S., Liu S. and Olson D.L., "Underwater Wet
Welding of Steels", Welding Research Council
Bulletin, no. 401, PP 1-39,1995.
Plate 5: Weld Zone Microstructures (Water Cooled)
15mm
45mm
Plate 6: Weld Zone Microstructures (Salty Water
Cooled)
4.
during welding and the low carbon content of the
material.
The closer the cooling position, the more the
cooling effect and the more the hardness and
metallurgical properties were influenced.
As the cooling rate in the weld zone and heat
affected zone varied with cooling distance from the
fusion zone of the weld, the phases formed from
the transformation was observed to also vary with
cooling distance.
From the hardness test results, the coarse grained
heat affected zone (CGHAZ) had high hardness
values compared with the rest of the heat affected
zone (HAZ) and unaffected areas of the steel
because of the large grain size and high cooling
rate in the region.
Authors: Aweda, E. O.1, Dr. Dagwa, I.M.2, Dr.
Dauda, M3., and Dr. Dauda, E.T.4
1,3
Department of Mechanical Engineering 4Department
of Materials and Metallurgy Engineering, Faculty of
Engineering, ABU Zaria. 2Department of Mechanical
Engineering University of Abuja, Abuja, Nigeria
Email address:
oluwarotimi.aweda@yahoo.com,
dagwaim@gmail.com, muhammaddauda@yahoo.com,
emmayustoi@yahoo.com
CONCLUSION
The following conclusions were drawn:
The samples cooled with salty water had higher
hardness values possibly due to the formation of
martensitic structure after quenching, while, air
cooled samples were mostly ferrite.
ii.
It was observed that the variations in the
microstructures were minimal; this was possibly
due to the rapid heat input, very short soaking time
i.
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