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254 Chiang Mai J. Sci. 2012; 39(2)
Chiang Mai J. Sci. 2012; 39(2) :
254-262http://it.science.cmu.ac.th/ejournal/Contributed Paper
Effect of Carburizing via Current Heating Techniqueon the
Near-Surface Microstructure of AISI 1020 SteelChatdanai
Boonruang*[a], Wareelak Kumpangkeaw [a], Kittichai Sopunna
[b],Natthaphol Chomsaeng [c] and Suparut Narksitipan [d][a]
Department of Physics and Materials Science, Faculty of Science,
Chiang Mai University,
Chiang Mai 50200, Thailand.[b] Department of Physics, Faculty of
Science and Technology, Sakon Nakhon Rajabhat University,
Sakon Nakhon 47000, Thailand.[c] Faculty of Gems, Burapha
University, Chanthaburi Campus, Chanthaburi 22170, Thailand.[d]
Division of Materials Science, Faculty of Science, Maejo
University, Chiang Mai 50290, Thailand.*Author for correspondence;
e-mail: [email protected]
Received: 26 October 2011Accepted: 16 March 2012
ABSTRACTAISI 1020 steels with 1.8 cm diameter and 1.8 cm height
were heat treated by
heating at 950°C, quenching and then tempering at 650°C for 60
min. After that, theywere ground with 1000 grid SiC paper and then
polished with 0.3 mm alumina paste.The steels were carburized using
the current heating technique with the applied electricalpowers of
40-240 W for 20 min. The near-surface phases and microstructure of
the steelswere characterized using the optical microscope, scanning
electron microscope, energydispersive X-ray spectroscopy and X-ray
diffractometer. The surface hardnesses of thesteels were measured
using microhardness tester. The fractions of pearlite at the
surfaceof the carburized steels are high and they are decreased as
the distance from the surfaceincreased reflected the effect of
carbon diffusion on the microstructure of the steel.However, the
fraction of Fe3C was not high enough to be detected by the XRD.
Fe2O3and Fe3O4 could be formed on the steel during the carburizing
process. The hardness ofthe steel increased with increasing applied
electrical power. The hardness of the steelcarburized at 240 W is
the highest which is 44.87 % increased compared to theuncarburized
steel.
Keywords: carburizing, current heating technique, AISI 1020
steel, near-surface microstructure,hardness
1. INTRODUCTIONFor hardening process in low carbon
steels, work hardening is more favourablethan carburizing
because it could be simplydone during the processing of steels. In
thework hardened steels, the strength would
be increased but the ductility would bedecreased. In the
carburized steels, themechanical properties of the surface andthe
interior would be different. The strengthof the surface would be
increased due to
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Chiang Mai J. Sci. 2012; 39(2) 255
the precipitation of Fe3C in pearlite [1] andthe ductility of
the interior would be increaseddue to the effect of annealing.
Moreover,the carburized steels have better surfaceproperties such
as wear resistance andcorrosion resistance.
Current heating technique is the newdiffusion coating technique
in carburizingapplied from the nanofiber fabricationtechnique [2]
and conventional packcarburizing [3]. This technique is verysimple,
low cost, low energy consumptionand causes very low contamination
tothe environment. The DC power supplyis applied to the charcoal or
graphite causingthe change in electrical energy into thermalenergy
leading to the increase in temperatureof the matters and diffusion
coefficientof carbon. The nascent carbon from thecarbonaceous
medium is absorbed intothe metal by diffusion mechanism. Inour
previous works, this technique wassuccessfully applied to the
carburizing ofTi and TiAl alloy [4-5]. In this study, it isthe
first time that current heating techniqueis applied to low carbon
steel since theother techniques have been applied formany years. We
expect the different resultfrom the previous works because
thesolubility of carbon in low carbon steelsis higher than that of
Ti and TiAl alloysand their microstructures are different.Current
heating technique could be developedand applied to carburizing of
any metal.
The aim of this work is to study thenear-surface microstructure
of the AISI 1020steel carburized via current heating technique.The
effect of carbon diffusion on themicrostructure will be studied
using XRD,SEM, EDS and microhardness test.
2. MATERIALS AND METHODSAISI 1020 steel samples with 1.8 cm
diameter and 1.8 cm height were prepared.
The samples were heat treated by heatingat 950°C, quenching and
then tempering at650°C for 60 min in order to obtainpearlite
structure and relief the effect ofwork hardening. After that, they
wereground with 1000 grid SiC paper andthen polished with 0.3 mm
alumina paste.The samples were placed amidst 8.5 g of20 μm graphite
powders pressed againstthem with a pressure of 10.3 kPa in a2.5 cm
inner diameter, 0.2 cm thicks and6.0 cm height glass tube. The
glass tubewas placed between two Cu electrodes in thechamber as
shown in Figure 1. Evacuationwas done in order to reduce the
pressurein the chamber down to 66.0 kPa. Ar gaswas fed into the
chamber in order to attainthe atmospheric pressure and it
keptflowing with the rate of 2 l/min. In thecarburizing, the direct
current was appliedacross the samples with the fixed
electricalpowers of 40, 80, 120, 160, 200 and 240 Wfor 20 min. The
temperatures were measuredby using the type K thermocouple. At
theend of the process, the samples wereunpacked and cooled down to
roomtemperature in air.
The near-surface phases and micro-structure of the samples were
characterizedusing the JSM-6335F JEOL scanningelectron microscope
equipped with EDSinstrument operated at 15 kV and MiniflexIIRigaku
X-ray diffractometer with Cu targetoperated at 30 kV and 15 mA with
step widthof 0.015° and scanning speed of 10°/min.The surface
hardness of the samples weremeasured using Buehler
microhardnesstester operated at 200 gf load with 15 sload time in
Vickers mode. The hardnessvalues in Pa were evaluated using
theequation
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256 Chiang Mai J. Sci. 2012; 39(2)
Figure 1. Schematic of the chamber for carburizing.
where F = load (N) d = size of indentation (m).
After that, cross-sectional-near-surfacemicrostructures of the
samples were observedusing optical microscope. The samples
wereprepared for the observation by coldmounting, grinding with
1000 grid SiC paper,polishing with 0.3 mm alumina paste and
thenetching with 3% Nital solution.
3. RESULTS AND DISCUSSIONThe carburizing temperature
measured
using the thermocouple is not the truetemperature of the sample
because it isnecessary to disconnect the tip of thethermocouple
from the sample in order toavoid the disturbance of the applied
directcurrent on the thermoelectric current in thethermocouple.
However, the temperaturemeasured using the thermocouple and thetrue
temperature of the sample could beapproximately equivalent because
the tip ofthe thermocouple and the sample was onlyseparated by the
2 mm thick glass-tube walland the graphite which is a good
thermalconductor.
The plots of temperature and timeduring the carburizing are
shown in Figure 2.
The trend shows that the temperatureincreased rapidly in the
beginning periodof the process and continuously increasedbut slower
until the end of the process.
The temperature increased as theapplied electrical power
increased as well.The applied electrical power has an influenceon
the temperature and heat generated tographite and sample during the
carburizing.According to Arrhenius equation forinterstitial
diffusion [6], the mobility anddiffusion coefficient of carbon in
a-Fewere increased as temperature increasedleading to the
increasing in the depth ofdiffusion and carbon concentration in
thenear-surface region of the sample [7].
The cross-sectional optical micrographsfor uncarburized sample
and the samplescarburized at the powers of 80, 160 and240 W are
shown in Figure 3. It is suggestedthat the fine grains shown in
Figure 3(a)caused by the recrystallization occurredduring the
tempering. Figure 3(b)-(d) revealthe effect of applied electrical
power onthe carbon diffusion and the near-surfacemicrostructure of
AISI 1020 steel. It showsthat the pearlite, a fine lamellar
structurecomposed of a-Fe and Fe3C [8], is increased
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Chiang Mai J. Sci. 2012; 39(2) 257
as carbon concentration increased. Themicrostructure of
uncarburized sampleconsists of ferrite in majority and the
fine colonies of pearlite dispersed regularyin ferrite. The
fractions of pearlite incarburized samples are higher than that
of
Figure 2. Carburizing temperature as a function of the applied
electrical power.
Figure 3. Near-surface-cross-sectional image for (a)
uncarburized AISI 1020 steel andthe steels carburized at the powers
of (b) 80, (c) 160 and (d) 240 W.
the uncarburized one and pearlite increasedas applied electrical
power increased.
For carburized samples, the fractionsof pearlite at the surface
are high and theyare decreased as the distance from thesurface
increased reflected the effect ofcarbon diffusion on the
microstructure ofAISI 1020 steel. According to Fe-C phasediagram
[9], the transformation of a-Fe to
a-Fe + Fe3C occurred when the solubilitylimit of C in a-Fe was
exceeded so thatthe increase in pearlite in AISI 1020steel
corresponds to the increase in carbonconcentration. Therefore, the
carbonconcentration is the principal factorcontrolling the
microstructure of the AISI1020 steel.
XRD patterns for uncarburized sample
Time (min)
Tem
pera
ture
(�C
)
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258 Chiang Mai J. Sci. 2012; 39(2)
and the samples carburized at the electricalpowers of 80, 160
and 240 W are shownin Figure 4. The pattern for uncarburizedsample
shows the detection of -Fe but nodetection of Fe3C forming in the
pearlitestructure as shown in Figure 3. Accordingto Fe-C phase
diagram, using the lever rule,
Figure 4. XRD patterns for uncarburized AISI 1020 steel and the
steels carburized at theelectrical powers of 80, 160 and 240 W.
the calculated fraction of -Fe and Fe3Cfor the composition of
0.2wt%C atroom temperature are 0.971 and 0.029,respectively. This
shows that the fractionof Fe3C is 0.030 times of that of α-Feand
this small amount of Fe3C could notbe detected by the XRD. For the
carburized
samples, there was detection of -Fe andalso no detection of Fe3C
even the pearlitewas increased. It is suggested that thefraction of
Fe3C and the depth ofcarburizing were not high enough to bedetected
by the XRD. C was detected in thecarburized samples. It is
reasonable thatthe peaks of C in the XRD patterns camefrom the
excess carbon deposited onthe surface of the samples as a
solidcarbonaceous layer [10], not from thematrix of the samples,
because C is lessthermodynamically stable than Fe3C in a-Fe.Fe2O3
and Fe3O4 were also detected in thecarburized samples. There are 2
suggestionsfor the formation of the oxides. The firstone is the
oxides came from the reactionbetween the a-Fe and the oxygen in the
airtrapped among the graphite powders duringthe carburizing
process. The second one is
the oxides came from the reaction betweenthe -Fe and the oxygen
in the air exposedto the surface of the sample after the
removalfrom the chamber. The intensity of -Fepeaks are decreased
with the increasingapplied electrical powers due to the formationof
Fe3C, C, Fe2O3 and Fe3O4 on the surfaceof the samples reduced the
fraction of
-Fe in the near surface region.SEM micrographs for the
samples
carburized at the electrical powers of80 and 160 W are shown in
Figure 5. Themicrographs reveal the morphology ofthe sample
surfaces affected by theformation of oxides and the deposition
ofcarbon. According to the XRD patternsin Figure 4, it is suggested
that the oxidesnucleated and grew as the applied electricalpower
increased. The results correspond tothe EDS spectra for the samples
shown in
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Chiang Mai J. Sci. 2012; 39(2) 259
Figure 6. The intensities of C and O peaksfor the sample
carburized at the electricalpower of 160 W are higher than those
of80 W reflecting the increase in the dissolutionof C and O with
the increasing appliedelectrical power. The micrographs in Figure
5revealed the mechanism in the formationand growth of the oxides.
The oxygen
Figure 5. SEM micrographs for the AISI 1020 steels carburized at
the electrical powersof (a) 80 and (b) 160 W.
reacted with iron to form the nuclei ofthe oxides later then
became the equiaxialoxide grains as shown in Figure 5 (a). Sincethe
oxygen concentration increased, thereaction between Fe and O was
proceeded.Atoms diffused across boundary of nearestneighbor grains,
as a result, small grainsshrank and disappeared and other
grains
Figure 6. EDS spectra for the AISI 1020 steels carburized at the
electrical powers of (a)80 and (b) 160 W.
became larger. Growth and combinationof the grains are shown in
Figure 5 (b).Carbon could deposit on the oxides butthere’s no
significant formation of carbon-oxide compound detected by XRD.
Thedissolved carbon could react with Fe andform pearlite.
Pearlite is a lamellar structurecomposes of soft-ductile a-Fe
and hard-brittle Fe3C. In general, hardness andstrength of steels
could be improved,without the effect of work hardening, by
increasing in the hard-carbon-rich content[11] or the hard phase
such as Fe3C. Fe3Chas an orthorhombic structure and itrequires high
energy for motion ofdislocation and plastic deformation
whencompared to that of BCC -Fe leading tohigh resistance to
plastic deformation andpenetration. Dislocation requires highenergy
to move across the lamellarstructure of pearlite when compared
tothat of BCC -Fe as well. The increase inpearlite and Fe3C with
the increasing
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260 Chiang Mai J. Sci. 2012; 39(2)
Figure 7. The plot of hardness of AISI 1020 steel as a function
of applied electrical powers.
applied electrical power causes the increasein hardness of AISI
1020 steel as shown inFigure 7.
Hardness of uncarburized sample is thelowest at 1.56±0.097 GPa.
The hardnessesof carburized samples are increased asthe applied
electrical power increased.In general, carbon diffuses in BCC
structureof a-Fe via interstitial mechanism. At theapplied
electrical powers of 40-160 W, smallnumber of carbon diffused into
-Fe andsmall number of interstitial sites wereoccupied by those
carbon. Therefore, freecarbon could diffuse through the
unoccupiedinterstitial sites easily leading to the increasein
pearlite and hardness in high rate. Theresult is quite different
for the samplescarburized at the applied electrical powersof 200
and 240 W. Large number of theinterstitial sites were occupied and
free carbonwere obstructed by those carbon occupiedinterstitial
sites leading to the increase inpearlite and hardness in low rate.
The hardnessesof the samples carburized at 160, 200 and240 W are
nearly the same which about2.20 - 2.26 GPa corresponding to their
carboncontent and pearlite which are nearly the
same as well. Large quantities of carbondiffused across the
surfaces of the samplesin the long distance causing the largenumber
of pearlite formed in the near-surface region of those samples
leadingto the high hardness values compared tothose of low applied
electrical powers.The hardness of the sample carburizedat 240 W is
the highest which is 44.87 %increased compared to
uncarburizedsample. However, the deviations of thehardness of the
samples carburized at highapplied electrical powers were higher
thanthose of low applied electrical powers.It is suggested that the
surface roughnessof those samples were high due tothe formation of
oxides, carbide anddeposition of carbon on the surfacedisturbed the
hardness indentation.The hardness depth profiles for the AISI1020
steel are shown in Figure 8. Thetrend of the depth profiles is
decreasedas the distance from the surface increases.The profiles
reveal the effect of carbondiffusion and the formation of
pearlite,resulting from the effect of appliedelectrical power, on
the hardness of the steel
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Chiang Mai J. Sci. 2012; 39(2) 261
without the effect of the oxide formed onthe surface of the
steel.
The effect of carburizing via currentheating technique on AISI
1020 steel wassuccessfully studied. The study for mediumand high
carbon steel will be takinginto account in our future work.
4. CONCLUSIONSThe fraction of pearlite in AISI 1020
steel increased as applied electrical powerincreased. The
fractions of pearlite at thesurface of the carburized steels are
high andthey are decreased as the distance fromthe surface
increased reflected the effectof carbon diffusion on the
microstructureof AISI 1020 steel. There were detection of
-Fe, C Fe2O3 and Fe3O4 in the carburizedsteel, but no detection
of Fe3C due to itsfraction was not high enough to be detectedby the
XRD. The increase in pearlite and Fe3Cwith the increasing applied
electrical powercauses the increase in hardness of the steel.The
hardness of the sample carburized at240 W is the highest which is
44.87 %increased compared to uncarburized sample.
Figure 8. Hardness depth profile for uncarburized AISI 1020
steel and the steelscarburized at 80, 160 and 240 W.
Distance from surface (mm)
Har
dnes
s (G
Pa)
The deviation in hardness of the steelcarburized at high applied
electrical powerwas high due to the formation of oxides,carbide and
deposition of carbon on thesurface disturbed the hardness
indentation.
ACKNOWLEDGEMENTThis research is supported by Faculty
of Science, Chiang Mai University.
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