Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.2, pp 79-92, 2009
jmmce.org Printed in the USA. All rights reserved
A Study of Processing Parameters on the Carburization of C2R Steels
P.O. Atanda1*, O.E. Olorunniwo1, L.E. Umoru1, and A.D. Adeyeye2
1Department of Materials Science and Engineering
Obafemi Awolowo University, Ile-Ife. Nigeria.
2Department of Industrial Engineering, University of Ibadan. Nigeria
*Correspondence Author email: [e-mail] Phone: [phone]
ABSTRACT
This study reports an investigation of the effect of carburizing variables – temperature, time and
percentage of energizer –on the case properties of C2R steel obtained from HMT Ltd. India. A
carburizer consisting of hardwood charcoal and coke respectively in the ratio of 2:1 was used
for the research with sodium carbonate as the energizer. The carburizing box was filled with 20
mm thick carburizer compound prior to fixing the steel samples in place. The specimens were
carburized using different percentages of energizers (10, 20 and 30%) at different temperatures
(820, 860, 900 and 940oC) for different times (from one to five hours). All the specimens were
quenched from carburizing temperature, ground to polished surfaces and then etched for ten
minutes in 25% Nital. The case depth was measured using the calibrated ocular of an inverted
metallurgical microscope fixed at 100 x magnification. The hardness values of the C2R steel
cases were measured with a micro hardness tester that uses diamond pyramid indenter. The
results of the study showed that the average hardness of the C2R steel cases increased with
temperature for any given carburizing time and temperature. For a given percentage energizer
and temperature, the case depths and hardness increased with time. Increase in the percentage
energizer however decreased the activation energy required for diff usion to occur exp o ne nti ally.
Keywords : Carburizer, Energizer, Heat treatment, Case depth, Activation Energy, Diffusion.
1. INTRODUCTION
The service conditions of many steel components such as gears, cams, valves etc, make it
necessary for them to possess both hard and wear resistant surfaces but tough and shock resistant
79

80 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
cores [1]. A low carbon steel of approximately 0.1% carbon will be tough while a high carbon
steel of about 0.9% or more will possess adequate hardness (and inherently low toughness) when
suitably heat-treated. The combination of hard, wear resistant surface and tough core required in
the aforementioned components involves the treatment of a shock- resistant steel so as to alter
the nature of the surface in order to increase the hardness while the core remains more or less
unaltered.
There are two major processes through which such an alteration of the surface layers of steel
components may be carried out, namely, (i) processes which impart surface hardness by
changing the microstructure of the surface skin without changing the chemical composition of
the surface. Such steel must not have a carbon content of less than 0.4% for them to be amenable
to hardening (by any of flame, induction, laser, and electron beam hardening) (ii) processes
which impact surface hardness by changing the surface chemistry of the steel by diffusing
carbon, nitrogen or both carbon and nitrogen into its surface. Steels for this process may have a
carbon content of about 0.1%. Examples of this latter process include carburizing, nitriding,
cyaniding, diffusion coating, and hard surfacing.
The method of carburizing was selected for this investigation because it has the advantages of
ease of operation, adaptability and portability of the equipment required, ability to heat-treat
component after surface- finishing (since there is little oxidation, decarburization or distortion),
and the ease of producing deeper zones of case depth.
It is known that the inward diffusion of the carbon takes place at a rate which depends on the
chemical composition of the steel, the carburizing temperature and the chemical composition of
the carburizing mixture [2, 3]. Established is a concentration gradient in respect of carbon from
the skin to the core of the steel. With increase in the carburizing activity of the carburizing
compound, the concentration is made steeper; the depth of penetration is not however increased
to the same extent.
In general, the case depth is controlled by the adjustment of the carburizing time and
temperature. There is a limit to which temperature can be increased in case hardening. At high
temperatures, the structure of the core deteriorates. It badly affects the diffusion too. At elevated
temperature, the rate at which the diffusing element is deposited on the surface of the specimen
is greater than the rate at which it diffuses towards the core of the steel. This leads to uneven
distribution of the concentration of the element and a high concentration which may lead to the
formation of networks of chemical compounds such as carbides and nitrides, which impacts high
brittleness to the surface layer. The yield strength of the core of a carburized component may be
exceeded, particularly as the core is in a state of tensile stress [4].

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 81
Although the stress is within the fatigue limit, the stress in the case is due both to the increased
strength of the surface layer and to the development of favourable compressive residual stresses
close to the surface.
For a successful carburization, a control of all these parameters; the carburizing temperature,
carburizing time, and chemical composition of the carburizing compound must be effected
[5,6,9].
In this work, the effect of carburizing variables on C2R steel used in machine tool operation for
the manufacture of different machine tool components has been studied with the aim of
improving both the production processes and the manufactured components used in machine
tools operation [7, 8]. The components studied include gears of different shapes and contours,
like the spiral, bevel, worm, taper, helical, etc.
2. MATERIALS AND METHODS
The composition of the C2R steel used in this work is given in the Table 1 below. The
carburizer was made in the ratio 2:1, hardwood charcoal and coke, with sodium carbonate
(Na2CO3) as energizer. The composition of carburizer used was shown in Table 2 below. The
steel, C2R, is normally used for the manufacture of different kind of gears, shafts, worm-wheel,
rack and pinion etc, in machine tool building. A 22.0mm diameter cylindrical specimen was used
in order to avoid “edge” effect during carburizing. Clay was used to lute the lid of the
carburizing container.
Table 1. Chemical composition of C2R steel (Diameter of specimen=22mm, Hardness= 240(HB)
C(%) Si(%) Mn(%) P(%) S(%) Cu(%) Cr(%) Ni(%) Sn(%)
0.15 0.23 0.50 0.040 0.040 0.025 0.10 0.011 0.05
The box was first filled with the carburizer compound about 20mm thick which was then
rammed and the components were placed about 25mm away from the sides of the carburizing
box. A digital chemical balance was used to weigh the generator (63% charcoal and 37% of
coke) and the energizer (Na2CO3) in the ratios 9:1, 8:2, and 7:3. A cylindrical steel container
(diameter 60mm) was used to contain the carburizer and the specimen. Carburizing was done in
a heat-treatment furnace (electrically energized).
Grinding machine and rotary polishing machine were used to prepare the specimen for case
depth measurement. A metallurgical microscope with calibrated ocular was used to measure the
case depth and microhardness tester (diamond pyramid) to measure the hardness of the case.

82 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
The generator (63% charcoal and 37% coke) and energizer (Na2CO3) were weighed respectively
according to the formulation in Table 2. The specimens were packed with the carburizer in the
carburizing container making sure that the specimen was covered with about 20mm of
carburizer. The lid was c o at e d w i t h clay and placed in the furnace.
The specimens were carburized at different temperatures (820, 860, 900 and 940oC), for different
lengths of time (from one to five hours) and different percentages of energizers (10,20 and 30).
All the specimens were quenched from the carburizing temperature, sectioned, ground and
polished. The specimens were etched with 25% Nital (25% HNO3 and 75% ethyl alcohol) for
about ten minutes. Case depth was measured with micrometer screw gauge and hand lens. The
hardness of the case was measured with microhardness tester using diamond pyramid indenter.
Table 2. Composition of carburizer.
Generator (wt%) Energizer (wt%)
90
80
70
10
20
30
3. RESULTS AND DISCUSSION
In order to analyze the results of this investigation, particularly as regards the effect of the
carburizing variables on the depth and hardness of C2R steel cases, graphs of Figs.1 to 13 have
been used to summarize pictorially the effects of the investigated variables. Also, plates 1a, 1b,
2a and 2b are included to reveal the effects of carburizing time, temperature and proportion of
carburizing energizer, Na2CO3 on case depth microstructures.
It is clear from Figs. 1-3 that the case depth increases with increase in carburizing time and
temperature. Significantly, the three figures show that 820oC should not be considered as a
carburizing temperature for C2R steel because of its case depth that was relatively low even up
to higher carburizing times. Temperatures 860oC and 900oC for carburizing exhibited similar
features in terms of their slopes, an indication that they have comparable activation energies [3,
10]. For all the three energizer compositions investigated, the graphs show that 940oC stands out
as an effective carburizing temperature.
Figures 4, 5 and 6 show that carburizing time almost exhibited linear relationship with case
depth. Generally, the depth of the cases also increased with carburiz ing time and tempera ture.
The effect of energizer’s proportion on case depth is contained in Figs. 7 and 8. It can be seen
that for any given carburizing time and temperature case depth increases as the percentage
energizer increases. This is probably so because increase in energizer composition increases the
nascent carbon on the surface of the steel. In other words, there would have been an increase in

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 83
the carbon potential of the carburizer and consequently a higher diffusion rate of carbon into
steel leading to an increased case depth as the percentage of the energizer (Na2CO3) increases.
O
C 940
O C 900
860 O
C
C820 O
234 51
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Case Depth (mm)
Time (hours)
Fig. 1. The Relationship between Case depth (mm) and
temperature for differen t c arb urizing time using 10%
N
a2CO3 (as quenched).
It can also be found in the figures earlier referred to that for any given carburizing temperature
and time the hardness of the case increases with the percentage of energizer used in the
carburizer. This is so because higher proportion of energizer implies an increase in carbon
potential and hence more carbon would have diffused into the steel than when lower percentage
of energizer was used.
An increase in carbon content in the case lead to an increase in its hardenability on quenching
and since there is more carbon in the case with high percentage of energizer, the case would be
harder.
The effects of carburizing temperature and time on the hardness of case are shown in Figs 9, 10
and 11. The average hardness of the case increases with temperature for any given carburizing
time and percentage energizer. The explanation for this is that temperature increase diffusion
rate, leading to a case of higher carbon content than a case developed at a lower temperature.
This leads to the formation of more martensite on quenching and hence increased hardness. The
increase of solubility of carbon in austenite wi t h t e mperature is another factor.

84 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
1.4
960 O C
900O C
860 O C
C820 O
23
Time(hours) 4 51
1.2
1
Case depth (mm)
0.8
0.6
0.4
0.2
0
Fig 2 The relationship between case depth(mm) and temperature for different
carburizing time using 20% Na2CO3 (s quenched)a
1.6
940 O C
900 O C
860 O C
C820 O
1234
1.4
1.2
1
Case depth (mm)
0.8
0.6
0.4
0.2
0
5
Time (hours)
Fig 3The relationship between case depthm) nd temperature for different(m
carburizing time using 30% Na a
CO
3 (as quenched) 2

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 85
C940 O
C 900 O
C860 O
C820 O
4 12 3
Time ( hours)
750
700
650
600
550
500
450
400
Average case hardness (HB)
5
Fig 4 The relationship between average case hardness(HB) and
carburizing time at various temperatures using 10% Na2CO3
(as quenched).
850
940 O C
900 O C
860 O C
820 O C
2 1
800
750
700
Case hardness (HB)
650
600
550
500
450
400
345
Time (hours)
Fig 5 The relationship between average cas and carburizing
time at various temperatures using 20% Na2CO3 (as quenched)

86 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
900
940 O
900 O
C860 O
O C820
2
1
C
850 C
800
Case Hardness (HB)
750
700
650
600
550
500
345
Time (hours)
Fig 6 The relationship between case hardness(HB) and carburizing time at
various temperature using 30% Na2CO3.( as quenched)
1.2
1.4
30% Na2CO3
20% Na2CO3
10% Na2CO3
1
Case depth, mm
0.8
0.6
0.4
0.2
0
0123456
Carburizing time, hous
Fig. 7 The relationship between case depth and proportion of energizer(Na2CO3) at carburizing
temperature of 900oC

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 87
Fig 8 The relationship between case depth and proportion of energizer(Na2CO3) at carburizing
temperature of 940oC
0
Carburizing time, hours
3012
1.6
1.4
1.2
Case depth, mm
1
0.8
10% Na2CO3
20% Na2CO3
30% Na2CO3
0.6
0.4
0.2
456
5 hours
4 hours
3 hours
2 hours
1 hour
920 9
C
o
00 880860840820
750
700
650
600
550
500
450
400
800
Average Case hardness(HB)
940 960
Temperature
Fig.9 The relationship between average case hardness and temperature
(as quenched)for various carburizing time using 10% Na CO3 2

88 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
5 hours
4 hours
3 hours
2 hours
1 hour
920 860
Temperature oC
900880840820
800
850
800
750
550
600
650
700
Average case hardness(HB)
500
450
400 940 960
Fig.10 The relationship between average case hardness and temperature
for various carburizing time for 20% Na2CO3 (as quenched)
3
5 hours
4 hours
3 hours
2 hours
1 hour
860 840 820
900
850
800
750
700
650
600
550
500
450
400
800
Average Case hardness(HB)
880 90
o 0
C 920 940 960 Temperature
Fig.11 The relationship between case hardness and temperature for different
carburizing time using 30% Na2CO3 (as quenched))

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 89
Fig 12 : The variation of linK with I/T for various carburizing time using 10% Na2CO3
(Energizer) as quenched.
Fig 13. The variation of linK with I/T for various carburizing time using 20% Na2CO3
(Energizer) as quenched.

90 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
Plate 1(a)
Macrostructure of a specimen,
carburized at 9400C for 3 hours
The carburizer cont ai ned 20%
Na2CO3 Etched in 25% Nital.
Magnification : x 2
Plate 1(b)
Macrostructure of a specimen,
carburized at 9400C for 4 hours
The carburizer cont ai ned 20%
Na2CO3 Etched in 25% Nital. x2
Magnification : x2
Plates 1(a)and 1(b) show the effect of carburizing time on the case depth .The ring shape layer in the case The
inner ring is the transition zone where the carbon content of the case decreases gradually into the carbon content of
the case .
Plate 2 (a)
Macrostructure of a specimen,
carburized at 9000C for 3 hours
The carburizer cont ai ned 20%
Na2CO3 Etched in 25% Nital.
Magnification: x2
Plate 2 (b)
Macrostructure of a specimen,
carburized at 9000C for 3 hours
The carburizer cont ai ned 20%
Na2CO3 Etched in 25% Nital. x2
Magnification : x2
Plates 2(a)and 2(b) show the effect of temperature on case depth. The ring shape layer is the case. The inner
ring is the transition zo ne where the carbon content if the case decreases gradually into the carbon content of the
core.

Vol.8, No.2 A Study of Processing Parameters on the Carburization of C2R Steels 91
The effect of the carburizing variables on the activation energy of the process is contained in
Figs. 12 and 13. The two figures show that for any given carburizing time, the activation energy
decreases as percentage energizer increases. The shape of the curves suggests that the rate of
change of activation energy with percentage energizer is instantaneous. Since increase in
energizer concentration increases the carbon potential (i.e. increases the CO to CO2 ratio) [2, 6],
it implies that the number of carbon atoms at the surface of the steel will also be increased
leading to a decrease in the activation energy. In addition the number of atoms that possess
enough energy to diffuse also increases. Also obvious from these figures is the fact that
activation energy decreases as time increases. The relation is an exponential one and the rate of
change of activation energy with time is instantaneous. Decrease in activation energy is higher at
shorter period than longer period.
The decrease in activation energy with increase in time may be due to increase in carbon
concentration in the surface layer. However, decrease in the rate of a change of activation energy
with time may be as a result of the decreasing carbon potential of the carburiz er wi th t ime.
4. CONCLUSIONS
The depth of resulting case and the hardness obtained depends on the factors summarized below
(a) The case depth obtained increase with temperature, time and percentage energizer.
(b) The case hardness increases with the percentage ene rgizer and carburizing time for
a given temperature.
(c) For a given temperature and percentage energizer, case depth and hardness
increases with time.
(d) For a given percentage energizer and time increase, in temperature increase the
case depth and hardness.
(e) For a given time and temperature, increase in percentage energizer increases the
case depth and its hardness.
(f) Increase in the percentage of energizer decreases the activation energy for
diffusion to occur exponentially.
(g) Increase in time decreases the activation energy for diffusion to occur
exponentially.
REFERENCES
[1] America Society of Metals(1964); Metals Hand book, Heat-treating, cleaning and finishing.
8th ed.
[2] M.A. Majid ad H.C. Child; Effect of carburizing on unidirectional bending fatigue strength of
low alloy steel. (1983). Journal of Metals Technology, Vol. 10 page 173.
[3] Lakhtin,Yu.M(1977); Engineering Physical Metallurgy a n d H ea t T reatment. Mir Publishers,

92 P.O. Atanda, O.E. Olorunniwo, L.E. Umoru, and A.D. Adeyeye Vol.8, No.2
Moscow.
[4] Smith, W.F(1970); Principles of Materials Science a nd E ng in ee ri n g. 2nd ed. Mc GrawHill Int.
ed.
[5] Simon, E.N(1974); Dictionary of Metal Heat Treatment. Frederic Muller Ltd, London.
[6] Prabhudev K.H (1997) Handbook of Heat treatment of Steels. 4th ed. Tata Mc GrawHill
Publishing Company Ltd. New Delhi.
[7] Rajput R.K(2002) Materials Science and Engineering. 2nd ed. S.K.Katara & Sons, New Delhi.
[8] Degarmo E. Paul, Black J.T & Kohser A. Ronald(1997) Materials and Processes in
Manufacturing. 8th ed. Prentice-Hall of India Private Ltd., New Delhi.
[9] George S.Brady and Henry R. Clauser(1989) Mater ials Handbook.12th ed. Mc Graw-Hill
Publishing Company, New York.
[10] Higgins R.A (1983) Engineering Metallurgy Part 1.