Mechanism of Abnormal Surface Carbon Content Reduction in Nb-bearing Case Hardening Steel Gas Carburized after Machining

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In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing steel decreases substantially with increasing the machining speed before carburizing. In the present study, a systematic study was made to clarify the effects of Cr,Nb addition and machining speed on the Cs. Uniform Cr oxide film was observed on the surface of specimens exhibited a large decrease in Cs. The carbon concentration profile was calculated assuming that carbon absorption was ceased after the complete coverage of specimen surface by Cr oxide film. It is confirmed that the present calculation fits well with the measured carbon concentration profile of low Cs. Moreover, it is shown that the value of Cs becomes smaller as the time span between the start of carburization and the time of Cr oxide film formation becomes shorter. The role of Nb is considered to be the retardation of recovery, recrystallization and grain growth through the pinning effect of Nb(CN) during carburizing. The high density of lattice defects introduced by machining is maintained by Nb(CN) and promotes faster Cr diffusion. In the present study, it has been clarified that in machined specimens of Nb-bearing steel, entire coverage of specimen surface by Cr oxide film during carburizing occurs in steels containing even a small amount of Cr like 1%.

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ISIJ International, Vol. 55 (2015), No. 1, pp. 227–234

# Mechanism of Abnormal Surface Carbon Content Reduction in Nb- bearing Case Hardening Steel Gas Carburized after Machining

Yutaka ETO, 1)* Minoru UMEMOTO 2) and Masakatsu YOSHIDA 2)

1) Heat Treatment Engineering Department, AISIN AW Co., Ltd., 10 Takane Fujii-cyo Anjo-shi Aichi, 444-1192 Japan.
2) Department of Graduate School of Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho,
Toyohashi, Aichi, 441-8580 Japan.

( _Received on July 17, 2014; accepted on October 26, 2014; originally published in Tetsu-to-Hagané,_
_Vol. 99, 2013, No. 11, pp. 642–650_ )

In our previous paper, we reported that the surface carbon concentration (Cs) in carburized Nb-bearing
steel decreases substantially with increasing the machining speed before carburizing. In the present
study, a systematic study was made to clarify the effects of Cr,Nb addition and machining speed on the
Cs. Uniform Cr oxide film was observed on the surface of specimens exhibited a large decrease in Cs.
The carbon concentration profile was calculated assuming that carbon absorption was ceased after the
complete coverage of specimen surface by Cr oxide film. It is confirmed that the present calculation fits
well with the measured carbon concentration profile of low Cs. Moreover, it is shown that the value of Cs
becomes smaller as the time span between the start of carburization and the time of Cr oxide film formation becomes shorter. The role of Nb is considered to be the retardation of recovery, recrystallization and
grain growth through the pinning effect of Nb(CN) during carburizing. The high density of lattice defects
introduced by machining is maintained by Nb(CN) and promotes faster Cr diffusion. In the present study,
it has been clarified that in machined specimens of Nb-bearing steel, entire coverage of specimen surface
by Cr oxide film during carburizing occurs in steels containing even a small amount of Cr like 1%.

KEY WORDS: Nb-bearing steel; machining; gas carburizing; surface carbon concentration; Cr oxide layer.

1. Introduction

Many of the power transmission parts of vehicle, such as
gear parts and axle components, are subjected to a carburizing treatment after being formed via machining to enable
additional functionality, such as fatigue strength and wear
resistance. Gas carburizing is widely used within the automotive industry as the method of heat treatment for these
kinds of parts because of its excellent suitability for mass
production and its ease of operation. 1) During normal operations, the surface carbon concentration (Cs) is adjusted to
approximately 0.7% to 0.8% in many cases to ensure that the
parts have the required fatigue strength and wear resistance.

The gas carburizing process involves an equilibrium reaction between the carbon potential (CP) of a carburizing
atmosphere (an atmospheric gas containing both carbon
monoxide and carbon dioxide gases) and the surface of the
steel. This reaction allows the carbon to penetrate into the
steel. In our previous study, it was found that even when the
gas carburizing was carried out in a carburizing atmosphere
with a CP of approximately 0.8%, there were cases where
the Cs of the steel largely decreased down to approximately
0.4%. 2) In our preliminary experiment, the effect of Nb addition and machining conditions on the decrease in Cs was
studied. The results showed that in the case of Nb free

SCM420 steel the machining conditions before carburizing
had only a small effect on the Cs. On the other hand, in the
case of SCM420 steel with a small amount of Nb, a large
abnormal decrease in the Cs was observed 2) when subjected
to gas carburizing after high-speed machining. The cause of
this abnormal decrease in Cs after machining the Nbbearing steel has not been clarified. Therefore, in this
research, alloys containing different amounts of Cr and Nb
were subjected to gas carburizing after machining to clarify
the effects of these elements and machining on the abnormal
decrease in Cs after gas carburizing.

2. Experimental Procedure

Table 1 shows the chemical compositions of the steel and
alloys used in the present study. The case hardened steel of
SCM420 (JIS G4053), which is commonly used for vehicle
parts, was set as a base material, 12 alloys with different Cr
and Nb contents were prepared.

gas carburizing was carried out in a carburizing atmosphere The alloys were cast into 50 kg ingots via vacuum meltwith a CP of approximately 0.8%, there were cases where ing, forged to φ65 mm at 900°C, water cooled after being
the Cs of the steel largely decreased down to approximately subjected to a solution treatment at 1 300°C for 10.8 ks,
0.4%. 2) In our preliminary experiment, the effect of Nb addi- which is followed by the normalizing treatment at 900°C for
tion and machining conditions on the decrease in Cs was 3.6 ks. Specimens with shape and size shown in Fig. 1 were
studied. The results showed that in the case of Nb free machined from these bars. After machined with various cut
- Corresponding author: E-mail: i31171_etou@aisin-aw.co.jp ting depths, cutting speeds, and feed rates, specimens were
DOI: http://dx.doi.org/10.2355/isijinternational.55.227 subjected to the gas carburizing using a continuous gas car

ISIJ International, Vol. 55 (2015), No. 1

Table 1. Chemical composition of materials (mass%).

C Si Mn P S Cr Mo Nb

Steel A (0Cr0Nb) 0.20 0.25 0.80 0.023 0.020 <0.01 0.15 <0.005

Steel B (0Cr0.05Nb) 0.20 0.25 0.80 0.023 0.022 <0.01 0.15 0.050

Steel C (0Cr0.1Nb) 0.20 0.24 0.80 0.020 0.020 <0.01 0.15 0.100

Steel D (0.5Cr0Nb) 0.20 0.25 0.79 0.020 0.019 0.50 0.14 <0.005

Steel E (0.5Cr0.05Nb) 0.20 0.26 0.79 0.020 0.019 0.50 0.16 0.050

Steel F (0.5Cr0.1Nb) 0.20 0.26 0.79 0.021 0.019 0.50 0.14 0.100

Steel G (1.0Cr0Nb) 0.20 0.25 0.79 0.020 0.019 1.00 0.15 <0.005

Steel H (1.0Cr0.05Nb) 0.20 0.26 0.79 0.019 0.018 1.00 0.14 0.050

Steel I (1.0Cr0.1Nb) 0.20 0.25 0.79 0.020 0.020 1.00 0.14 0.100

Steel J (2.0Cr0Nb) 0.20 0.26 0.80 0.019 0.020 2.00 0.14 <0.005

Steel K (2.0Cr0.05Nb) 0.20 0.26 0.80 0.020 0.020 2.00 0.15 0.050

Steel L (2.0Cr0.1Nb) 0.20 0.26 0.79 0.020 0.020 2.00 0.15 0.100

Fig. 1. Schematic illustration of specimen shape.

burizing furnace. The φ50 mm portions of the specimens
shown in Fig. 1 were clamped and the φ60 mm outer diameters portions were subjected to turning operations during
the machining process. Machining was performed using
NL-2500 (manufactured by MORI SEIKI Co., Ltd.) with
cemented carbide tips (Sandvik CNMG120408-MM 2025)
in wetting condition. Two machining conditions with high
and low-speed were selected. The high-speed machining
condition was with cutting depth of 2.0 mm, cutting speed
of 5.8 m/s, and feeding rate of 0.5 mm/rev. The low-speed
machining condition was with cutting depth of 0.1 mm, a
cutting speed of 0.8 m/s, and feeding rate of 0.1 mm/rev. In
addition, electro-polishing was used to obtain specimen surface with an ideal un-machined state.

Figure 2 shows gas carburizing condition applied. The CP
in the figure denotes carbon potential. The gas carburizing
was performed using the roller-hearth type continuous gas
carburizing furnace (manufactured by DOWA Thermotech
Co., Ltd.) for a total of 13.2 ks at 950°C. After carburizing,
the specimens were quenched into oil kept at 150°C. Propane gas was used as the carburizing gas.

After the specimens were subjected to the gas carburizing
process, the carbon concentration distribution was measured
using an electron prove micro-analyzer (EPMA) and the
microstructure was observed with a field emission scanning
electron microscope (FE-SEM). An elemental analysis and
composition analysis were performed on the surfaces of the

Fig. 2. Gas-carburizing process.

specimens using X-ray photoelectron spectroscopy (XPS)
and energy dispersive X-ray spectroscopy (EDS). For the
observation of microstructure 5% nital was used as an etching solution.

3. Experimental Results and Discussion

3.1. Effects of Cr and Nb Addition on the Abnormal
Decrease in Cs
The 12 alloys containing different amounts of Cr and Nb
were subjected to gas carburizing after being machined
using one of three machining conditions: electropolishing,
low-speed machining, or high-speed machining. After gas
carburizing, the Cs was measured using the EPMA. Figure
3 shows the change in the Cs as a function of Cr content.
Figure 3(a) shows the change in the Cs for the specimens
gas carburized after electropolishing (shorten ECS: electropolished and carburized specimens). In the specimens
without Nb, the Cs stayed at an almost constant value,
regardless of the amount of Cr. On the other hand, in the
specimens with 0.05% and 0.1% Nb, the Cs decreased with
increasing the amount of Cr. The amount of decrease in Cs
was very slight and even when the Cr content was 2.0%, the
decrease in Cs was only about 0.1%C. Figure 3(b) shows the
change in the Cs as a function of Cr content for the specimens gas carburized after low-speed machining (shorten
LMCS: low-speed machined and carburized specimens). In
the LMCS, the Cs remained almost constant at 0.9%C in the
Nb-free steel regardless of the Cr content. However, in the
0.05 Nb specimen and the 0.1 Nb specimen, the Cs
decreased with the increase in Cr content. When the Cr content was 2.0%, the Cs of the 0.05 Nb specimen was 0.6%C
and the Cs of the 0.1 Nb specimen was 0.55%C. In the
LMCS, the Cs of the 0.1 Nb specimens was about 0.05%C
lower than the Cs of the 0.05 Nb specimens at all Cr contents. Figure 3(c) shows the change in the Cs as a function
of Cr content for the specimens gas carburized after highspeed machining (shorten HMCS: high-speed machined and
carburized specimens). In these HMCS, the Cs of the Nbfree specimens remained constant, regardless of the amount
of added Cr, the same as the LMCS. However, in the
0.05 Nb and 0.1 Nb specimens, there was a large decrease
in Cs with the increase in the amount of Cr. When the
amount of Cr was 1.0%, the Cs of the 0.05 Nb specimen was
0.58% and the Cs of the 0.1 Nb specimen was 0.5%. Even
when the amount of Cr was increased to 2.0% the Cs
remained at approximately the same levels obtained when

ISIJ International, Vol. 55 (2015), No. 1

Fig. 3. Surface carbon concentration as a function of Cr and Nb content. (a) electropolished, (b) low speed machined and
(c) high speed machined.

Fig. 4. Contour of surface carbon concentration as a function of Cr and Nb content in carburized specimens after (a) low
speed machining and (b) high speed machining.

the amount of Cr was 1.0%. In both the HMCS and LMCS,
the specimens with the same Cr content showed a similar
trend of a decrease in Cs with the increase in Nb content.
The results described above indicated that the abnormal
decrease in Cs became pronounced with the addition of the
Cr and Nb. In addition, it was also found that the higher the
speed in machining, the abnormal decrease in Cs appeared
more clearly.

Figure 4 shows the relationship between the contents of
Cr and Nb and the Cs when the specimens were subjected
to gas carburizing after being machined under low-speed
and high-speed machining conditions. Figures 4(a) and 4(b)
show the results from the low-speed and high-speed
machining, respectively. The numbers near the data points
indicate the Cs values. The curves in the figures indicate the
boundaries where Cs = 0.9%, 0.8%, 0.7%, 0.6%, and 0.5%.
These results in Fig. 4 indicate that the Cs tends to decrease
with the increase in the Cr and Nb contents for both lowspeed and high-speed machining. It was found that the higher the speed in the machining conditions, the larger the
decrease in Cs, even when the amounts of added Cr and Nb
were small. However, when Cr or Nb was added separately,
a large decrease in Cs did not occur, regardless of the
machining conditions. Therefore, it was found that the
simultaneous addition of both Cr and Nb is essential for this
large decrease in Cs to occur. In summary, the above results

clearly reveal that specimens containing both Cr and Nb,
which are then subjected to machining, exhibit a decrease in
Cs. Furthermore, this decrease becomes larger with the
increase in the amounts of Cr and Nb and the speed of the
machining.

3.2. Microstructure Observations of Surfaces of Specimens with Abnormal Decreases in Cs after Gas
Carburizing
Figure 5 shows top surface (a) and cross section (b) SEM
micrographs of a high-speed machined 2.0 Cr 0.05 Nb specimen which showed a large decrease in Cs. Figure 5(a) indicates that the surface of the specimen is covered with a
dense product fine particles formed during carburizing. Figure 5(b) indicates that the surface is covered with a film
thickness of approximately 600 nm.

Underneath the film, a pearlite structure that was produced as a result of slack quenching is observed. It is considered that below the film is the metal substrate. Figure 6
shows the results of EPMA analysis taken from a cross section of the specimen to study the distributions of Cr and
oxygen (O). The EPMA analysis reveals that Cr and O are
concentrated on the surface of the specimen. There is a
strong possibility that the film that formed on the surface of
the specimen after gas carburizing is Cr oxide. The chemical
state of Cr on the surface of the specimen was then analyzed

ISIJ International, Vol. 55 (2015), No. 1

using XPS. Figure 7 shows the analysis results of the binding energy peak of XPS associated with Cr after sputtered
the surface to a depth of approximately 20 nm. In the case
of the metal Cr, the spectrum peaks for Cr2p3/2 and Cr2p1/2
are at approximately 574.4 eV and 584.2 eV, respectively.
In the case of Cr oxide, the peaks for Cr2p3/2 and Cr2p1/2 are
at approximately 576.5 to 579.5 eV and 586.3 to 589.3 eV
respectively. 3) The peaks of the 2.0 Cr 0.05 Nb specimen,
which exhibited a large decrease in Cs, were observed near

Fig. 5. SEM images of carburized 2.0%Cr0.05%Nb specimen after
high speed machining. (a) surface and (b) cross section.

Fig. 6. Cross sectional EPMA analysis of 2.0%Cr0.05%Nb specimen carburized after high speed machining.

to 577 eV and 586.5 eV. This clearly confirmed that the film
that formed on the surface of the specimen was Cr oxide. In
our present and previous study, Cr oxide has not been
observed on specimens after machining and before carburizing. 4) Therefore, it can be concluded that the Cr oxide
observed in the present study was produced during the gas
carburizing process.

3.3. Effects of Elements Addition and Machining on the
Formation of Cr Oxide Film
The effects of Cr and Nb addition, as well as the machining
conditions on the surface structure were observed. The effect
of Cr content in 0.05% Nb-bearing specimens subjected to
gas carburizing after high-speed machining was studied
first. Figure 8 shows SEM micrographs of the top surface
of specimens with various Cr content. Cr free specimen
(Figs. 8(a) and 8(e)) showed disk-shaped product as large as
micrometer size and small holes mostly along grain boundaries. The small holes appeared as black were considered to
be the traces of small oxides that peeled off during preparing
the specimens. Figure 9 shows the results of a composition
analysis performed on the surface of a 0 Cr 0.05 Nb specimen using EDS. A strong correlation of the locations with
high concentrations of silicon (Si), manganese (Mn), and

Fig. 7. XPS spectrum around Cr after 1 min sputtering
(2.0%Cr0.05%Nb specimen carburized after high speed
machining).

Fig. 8. Surface SEM images of specimens carburized after high speed machining. (a)(e) 0%Cr, (b) 0.5%Cr, (c) 1.0%Cr
and (d) 2.0%Cr in 0.05% Nb bearing steel.

ISIJ International, Vol. 55 (2015), No. 1

Fig. 9. Surface EDS images of 0%Cr0.05%Nb specimen carburized after high speed machining.

oxygen indicating the formation of Si and Mn oxides on the
surface during carburizing.

The SEM photographs of Figs. 8(a), 8(e) and 9 indicate
that the outside of oxides are pearlite that was produced as
a result of lowered hardenability. Outside of oxides iron (Fe)
was also detected in the composition analysis using EDS, so
it was concluded that this was the metal substrate. In contrast to the Cr free specimen, 0.5 Cr specimen showed no
trace of disk-shaped Si and Mn oxides (Fig. 8(b)). Instead,
the formation of new particles with a diameter of approximately 0.3 μm was observed. Rest of the surface was considered to be the metal substrate from the pearlite structure.
The number and the area fraction of new particles increase
with the increase in Cr content. Particles cover the entire
surface of both the 1.0 Cr specimen (Fig. 8(c)) and the 2.0 Cr
specimen (Fig. 8(d)). XPS was used to analyze the chemical
state of the Cr in 0.5 Cr and 1.0 Cr specimens and the results
are shown in Fig. 10 . From the positions of the XPS spectrum peaks, it was concluded that the product material on
the surfaces of both the 0.5 Cr specimen and 1.0 Cr specimen is Cr oxide same with the case of 2.0 Cr as shown in
Fig. 7. It was noticed that the grain size of Cr oxide is smaller in 2.0 Cr specimen than that of 1.0 Cr specimen. In summary, the present study revealed that in machined 0.05%
Nb-bearing specimens the amount of Cr oxide during gas
carburizing increases with increasing Cr content and Cr
oxide covers almost entire surface when the content exceeds
more than 1.0%.

The effect of Nb addition on the surface structure was
investigated. 1.0% Cr-bearing specimens with different
amounts of Nb were subjected to gas carburizing after highspeed machining. Figure 11 shows SEM micrographs of top
surface of specimens with various amount of Nb. It is seen
that the surface is partially covered with Cr oxide in Nb-free
specimen and covered completely in 0.05 and 0.10 Nb bearing specimens. Compared 0.05 and 0.10 Nb bearing specimens, the grain size of Cr oxide is almost same.

The effect of machining on the surface microstructure
was investigated. Specimens of 1.0% Cr and 0.05% Nb were
subjected to gas carburizing either after electropolishing
(ECS) or high-speed machining (HMCS). SEM micrographs
shown in Fig. 12 compares surfaces of these two types of
conditions. It is seen that the formation of Cr oxide is less
pronounced in ECS than HMCS. In the ECS, the formation

Fig. 10. XPS spectra around Cr after 1 min sputtering (0.5%Cr and
1.0%Cr specimens carburized after high speed machining
in 0.05%Nb bearing steel).

of Cr oxide is observed only along the grain boundaries. On
the other hand, in the HMCS, a Cr oxide film that covers
almost entire surface was observed. These observations
indicate that even in specimens with the same chemical
composition, the formation of Cr oxide is promoted by
machining. It is thought that the pronounced formation of Cr
oxide is due to the enhanced diffusion of Cr to specimen
surface by the lattice defects that are introduced during
machining.

SEM micrographs shown in Fig. 13 compares the cross
sectional area of 0.05% Nb-bearing specimens with various
Cr content machined in high-speed before subjected carburizing. It is seen that Cr oxide on the surface increases with
increase in Cr content. Cr oxide completely covers the surface in specimens containing more than 1.0%Cr and the
thickness of Cr oxide layer increases with increasing Cr
content. From the picture the thickness of the Cr oxide layer
was measure as approximately 350 nm in 1.0%Cr and
600 nm in 2.0%Cr. It was noted that the Cr oxide layer produced in the 2.0 Cr specimen have more uniform thickness
compared to the one in the 1.0 Cr specimen.

Using the same specimens subjected to SEM observation,
XPS analysis was performed to determine how deep Cr and

ISIJ International, Vol. 55 (2015), No. 1

Fig. 11. Surface SEM images of specimens carburized after high speed machining. (a) 0%Nb, (b) 0.05%Nb and (c)
0.1%Nb in 1.0% Cr bearing steel.

Fig. 13. Cross sectional SEM images of specimens carburized after high speed machining. (a) 0%Cr, (b) 0.5%Cr, (c)
1.0%Cr and (d) 2.0%Cr in 0.05%Nb bearing steel.

Fig. 12. Surface SEM images of carburized 1.0%Cr0.05%Nb
specimens. (a) electropolished and (b) high speed
machined. Fig. 14. XPS depth profiles of 0.05%Nb specimens carburized
after high speed machining.

Fe could be detected from the surface. In this analysis, the
sputter rate in the depth direction from the surface was
20 nm/min.
Figure 14 shows the results of XPS analysis. It is seen
that Cr concentration peaks becomes deeper as the Cr content increased. In the 0.5 Cr specimen and 1.0 Cr specimen
the Cr concentration peak was found at depths of approximately 150 nm and 300 nm, respectively. In the 2.0 Cr specimen the Cr maintained a high atomic concentration of
about 30% even down to 600 nm in depth. Fe distribution
is shown in the right hand side of Fig. 14. Although Fe was
detected from the surface of the 0 Cr specimen it detected
at deeper depth as the Cr content increases. In the 2.0 Cr
specimen, almost no Fe is detected, even at a depth of
600 nm. As described above, a homogeneous Cr oxide layer
was produced on the surfaces of both the 1.0 Cr and 2.0 Cr
specimens. The Cr peak concentration depth and the depth
at which Fe concentration starts to increase in these specimens are practically the same as the thickness of the Cr
oxide film shown in Fig. 13. Since the Fe was not detected
in the surface layer, it is concluded that the Cr oxide film
observed on the surface of the specimen is Cr2O3 not containing Fe. In the 0.5 Cr specimen, the estimated thickness
of the Cr oxide layer is about 100 nm by XPS measurement

and about 250 nm measured by SEM observation. This difference is considered to occur from the incomplete coverage
by Cr oxide layer in 0.5 Cr specimen.

Summarizing the above mentioned results obtained using
SEM, EDS, and XPS the following points were clarified
about the formation of the Cr oxide film during gas carburizing. 1) The Cr oxide layer that is produced on the surfaces
of the specimens increased in conjunction with the increase
in the Cr and Nb content. In specimens containing Cr, a
dense Cr oxide layer forms only when Nb is added at the
same time as Cr. 2) The complete coverage of specimen
surface by Cr oxide layer was observed in the specimens
containing more than 1.0% Cr and more than 0.05% Nb subjected to high-speed machining. 3) In the specimens not subjected to machining, Cr oxide layer forms only partially
even the Cr and Nb are added at the same time. Thus addition of both Cr and Nb and machining is necessary to obtain
the complete coverage of specimen surface by Cr oxide layer.

The present study confirmed for the first time that the formation of the Cr2O3 layer which is responsible for the large
decrease in Cs is produced by the addition of Nb and performing machining even when the amount of Cr addition is
as low as 1%.

ISIJ International, Vol. 55 (2015), No. 1

3.4. Calculation of Carbon Concentration Distribution
when Oxide Layer Forms during Gas Carburizing
The present study confirmed a strong correlation between
the Cs and the Cr oxide layer. Since the Cr oxide layer was
not detected before carburizing, it is considered that the
oxide layer grew during the carburizing process. It is expected that the Cs is related to the formation time of the Cr oxide
layer during carburizing process. Since the solubility of carbon in Cr oxide is undetectable small amount and the carbon
cannot diffuse within the Cr oxide, 5) the carbon absorption
during carburizing occurs until the surface of the steel specimen covered by the Cr oxide film. After the oxide layer has
coated the steel surface, the carbon that had already penetrated into the steel prior to the formation of the oxide layer
can diffuse within the specimen. The diffusion of carbon
after the oxide layer coating might be the cause of the
decrease in Cs.

To verify this idea, the relationship between the time that
carbon ceases penetration from surface and Cs was calculated. The distribution of the carbon concentration was calculated by solving Fick’s second law, which is shown in Eq.
(1) below, via the finite difference method.

............................. (1)

∂

⎛

⎜
⎝

∂ _C_ ⎛ ∂

= _D_ ⎜
∂ _t_ ⎝ ∂

_C_ ⎛ ∂ 2 _C_ ⎞

_t_ = _D_ ⎜⎝ ∂ _x_ 2 ⎟⎠

_C_ ⎛ ∂ 2 _C_

_t_ = _D_ ⎜⎝ ∂ _x_ 2

⎞

∂ _x_

where, _C_ is the carbon concentration (mass%), _D_ is the
diffusion coefficient, and _x_ is the distance from the surface.
For diffusion coefficient, the experimental value obtained
by Zhang _et al._ 6) that are shown in Eq. (2) below were used.

- 7
_D_ = 7 7. × 10 exp (− 107 400 / _RT_ )

............. (2)

Fig. 15. Surface carbon concentration as a function of time carburizing is ceased.

Fig. 16. Comparison of measured and calculated carbon concentration profiles of 2.0%Cr0.1%Nb specimens carburized
after high speed machining. Carburizing was ceased after
2.5 ks.

took for the Cr oxide film to form on these specimens was
estimated from the Cs and shown in Fig. 15 by the arrows.
It is seen that as the Cr and Nb content increases, the time
it takes for the Cr oxide film to form becomes shorter.
Figure 16 shows the calculated carbon concentration distribution assumed that the carbon penetration into the specimen stopped 2.5 ks after the start of carburization. In the
figure measured carbon concentration distribution in the 2.0
Cr 0.1 Nb specimen was shown for comparison. A good correlation between the calculated and measured values is seen.
The simulation described above showed that the origin of
the substantial decrease in Cs is the formation of a Cr oxide
layer on the surface of the specimen during the carburizing
process, which inhibited the penetration of the carbon into
the specimen. Furthermore, after this layer forms, the carbon
that already penetrated into the specimen prior to the formation of oxide layer diffuse in the specimen. The Cr oxide
layer is probably thin when it becomes effective to stop the
penetration of the carbon, but it continues to grow during
‘carburizing process’ and becomes several hundred nm in
thickness, such as those shown in Fig. 13, by the time that
the gas carburizing is finished.

where, _D_ is the carbon diffusion coefficient (m 2 ·sec –1 ), _R_
is the gas constant (J·K –1 ·mol –1 ), and _T_ is the absolute temperature (K). In the simulation Eqs. (1) and (2) were used
under the following two assumptions. 1) The three levels of
CP during gas carburizing shown in Fig. 2 were applied:
CP = 1.10%, 1.05%, and 0.90%. Since Cs depends on the
alloying elements in specimens, Cs was calculated over the
range of Cr 0 to 2.0% and Nb 0 to 0.1% using the reported
equations. 2) To compare the calculated Cs among various
composition of alloys, the average values of Cs in each of
the 3 carburizing stages was set as Cs = 1.19%, 1.13%, and
0.97%. 2) It is assumed that during gas carburization the
coverage of surface by Cr oxide layer occurs in a short time.
Once surface is completely covered by Cr oxide layer, the
carbon penetration (or decarburization) no longer occurs
and only the carbon diffusion within the steel takes place.
The relationship between the time it takes for the carbon to
stop penetrating into the specimens from the surface ( _i.e._,
the time Cr oxide film forms) and the Cs was calculated
based on the two assumptions described above. Figure 15
shows the calculated Cs as a function of the formation time
of Cr oxide layer. It is seen that the shorter the time it takes
for the carbon to stop penetrating into the specimen from the
surface, the lower the Cs. Cs value of 0.5%C can occur
when Cr oxide layer forms after 2.5 ks from the start of the
gas carburizing. This value is almost the same as the lowest
Cs value that was actually measured (0.49%) in the 2.0 Cr
0.1 Nb specimen. In Fig. 15, the data of the specimens containing Cr 1.0% and 2.0% and Nb 0.05% and 0.10% subjected to high-speed machining were plotted. The time it

ISIJ International, Vol. 55 (2015), No. 1

4. Conclusions

In the present study, the effects of Cr and Nb contents and
machining condition on the decrease in surface carbon content (Cs) after gas carburizing were investigated using 12
alloys with composition close to a conventional case hardening steel. The origin of the decrease in Cs was discussed.
The main results obtained from this research are as follows.

(1) A large decrease in Cs up to 0.5%C was observed
in specimens containing both Cr and Nb and subjected to
machining prior to the gas carburizing.

(2) The decrease in Cs becomes larger as the Cr and Nb
content are increased and the speed of the machining is
increased.

Fig. 17. Schematic illustration of Cr oxide layer formation during
carburizing in machined specimen.

3.5. Mechanism Causing Decrease in Cs
Based on the above mentioned results, the mechanism of
the Cr oxide layer formation in the specimens of Nb-bearing
steel subjected to gas carburizing after machining which
results in the large decrease in Cs was considered. Figure
17 shows a schematic illustration of the Cr oxide film formation. After machining, surface layer contains many lattice
defects. Due to the heating during gas carburizing, the density of lattice defects in the deformed layer decrease due to
the recovery and recrystallization. In Nb free specimen, the
density of lattice defects would rapidly decrease in conjunction with the rise in temperature and the defects would have
no effect on the Cs. In contrast, in the specimens containing
Nb, the Nb (CN) acts as pinning particles that retard or
inhibit the recovery, recrystallization, and grain growth of
the machined structure during gas carburizing. 7–9) Fast diffusion of Cr occurs along lattice defects and form Cr oxide
on the surface instead of inside specimens. It is thought that
increasing the speed of machining also increases the density
of lattice defects acting as diffusion paths. This then leads
to more rapid Cr diffusion to the surface of the specimen,
which causes the formation of Cr oxide in shorter time. Consequently, the abnormal decrease in Cs that was observed in
this research is considered to be due to the high-density lattice defects that were introduced by the machining and those
are maintained by the Nb (CN) during the gas carburizing.
The Cr passes through these lattice defects, diffuses to the
surface of the specimen, and then a dense Cr oxide film
forms on the surface of the steel. Furthermore, the addition
of cerium (Ce) to Fe- (10-20) Cr alloy promotes the production of Cr oxide. 10) Therefore, it was considered that the Nb
(CN) in this research may also become a nucleation site
when the Cr oxide is produced. The possibility of this is supported by the fact that the addition of Nb to the ECS caused
only a minor decrease in Cs, as shown in Fig. 3(a). Detailed
study on the effect of Nb on the abnormal decrease in Cs is
expected.

(3) In the electro-polished sample, the decrease in Cs is
less than 0.1%C. This decrease is much smaller than that of
machined sample.

(4) In the Cr bearing sample, Cr oxide was produced on
the surface of the steel after gas carburizing. A uniform and
dense Cr oxide layer formed as the Cr and Nb content
increased and the speed of the machining increased. It was
concluded that the Cr oxide, which does not dissolve carbon
into a solid solution, was responsible to prevent the penetration of the carbon into the sample.

(5) The effect of machining on the formation of Cr
oxide layer is considered to be the fast diffusion of Cr along
grain boundaries and dislocations. Nb (CN) is expected to
act as pinning particles that retard the annihilation of the lattice defects at the carburizing temperature.

(6) To estimate the decrease of Cs by Cr oxide layer, a
carbon diffusion simulation during carburizing process was
carried out. The results indicated that Cs decreases with time
after the formation of Cr oxide and the earlier the formation
of Cr oxide layer, the larger the decrease in Cs. The calculated value of Cs was in the range measured in the present
experiment and supporting that the origin of the decrease in
Cs is the formation of Cr oxide which prevents the penetration of carbon into specimen.

REFERENCES

1) T. Naito: Practical Carburizing Technology, Nikkan Kogyo Shimbun,
LTD., Tokyo, (1979), 3.
2) Y. Eto, M. Umemoto, K. Sasaki and M. Yoshida: _Tetsu-to-Hagané_,
99 (2013), 706.
3) J. Chastain and R. C. King, Jr.: Handbook of X-ray Photoelectron
Spectroscopy, ULVAC-PHI, Chigasaki, (1995).
4) Y. Eto, M. Umemoto and M. Yoshida: _Tetsu-to-Hagané_, 99 (2013),
564.
5) I. Wolf and H. J. Grabke: _Solid State_ _Comm._, 54 (1985), 5.
6) X. Zhang, R. Takahashi, T. Akiyama and J. Yagi: _Tetsu-to-Hagané_,
83 (1997), 299.
7) T. Murakami, H. Hatano and H. Yaguchi: _Tetsu-to-Hagané_, 92
(2006), 448.
8) Y. Maehara, T. Kunitake and N. Fujino: _Tetsu-to-Hagané_, 67 (1981),
362.
9) M. Kubota and T. Ochi: _Nippon Steel Tech. Rep._, 378 (2003), 72.
10) T. N. Rhys-Jones, H. J. Grabke and H. Kudielka: _Werkstoffe_
_Korrosion_, 38 (1987), 65.

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