during Decarburization in Plain Carbon Steels
N. H. Heo1 and J. K. Lee2
1KEPCO Research Institute, Munji-dong, Yusung-ku, Daejeon 305-380,
Korea 2PILETA Co, Munji-dong, Yusung-ku, Daejeon 305-380,
Korea
During decarburization of plain carbon steels, the grain boundary
segregation concentration of phosphorus increased with increasing
bulk phosphorus content and with decreasing decarburization
temperature. The grain growth kinetics decreased with increasing
bulk phosphorus content which is due to the grain boundary pinning
effect of highly segregated phosphorus. After decarburization at
973K for 24 h, the columnar grain growth following the abnormal
grain growth was observed in the steel containing a low bulk
phosphorus content, while the steel containing a high bulk
phosphorus content showed only the abnormal grain growth behavior.
Such grain growth behaviors can be understood in the light of the
abnormal grain growth driven by the grain boundary carbides and the
solute drag effect of highly segregated phosphorus on moving grain
boundaries. During decarburization at 1173K, only the normal grain
growth was observed due to the absence of grain boundary carbides,
regardless of the bulk phosphorus content. The decarburization
reaction in the present study can be expressed by the parabolic
relationship x ¼ kðDtÞ1=2 where x is the decarburization depth, k
the reaction coefficient, D the diffusivity of carbon and t the
decarburization time. [doi:10.2320/matertrans.M2010283]
(Received August 26, 2010; Accepted November 15, 2010; Published
January 25, 2011)
Keywords: fracture, grain growth, ferrous alloy, steel
1. Introduction
It is well-known that low alloy steels tempered in the range
623–873K or slowly cooled through this temperature range often
exhibit an increase in the ductile-brittle transition temperature
and a change in the low temperature fracture mode from
transgranular to intergranular.1–4) This is due to the grain
boundary segregation of impurities such as P, As, Sb and Sn. The
grain boundary segregation of P can be enhanced when the dissolved
carbon content is decreased.5)
A similar situation is observed during decarburization of low alloy
steels containing phosphorus.6) The addition of chro- mium to
carbon steels can also result in such a change in fracture mode.7)
This is because the formation of chromium carbides during holding
or using at an intermediate temper- ature decreases the dissolved
carbon content and as a result increases the grain boundary
segregation concentration of phosphorus.
In this paper, grain boundary segregation of phosphorus, abnormal
and columnar grain growth during decarburization are investigated
in plain carbon steels.
2. Experimental
Plain carbon steel plates of 3mm thickness in which the bulk
phosphorus content is mainly different were prepared through vacuum
induction melting and hot-rolling processes. The chemical
compositions of the prepared steels are shown in Table 1. Tensile
specimens with a dimension of 25mm (gauge length) 4mm (width) 1.5mm
(thickness), which were machined from the plates in the hot-rolling
direction, were decarburized for 2, 6, 12 and 24 h under a wet
hydrogen atmosphere. The decarburization temperature was fixed at
973 and 1173K. The wet hydrogen atmosphere was simply produced by
flowing hydrogen of 5 liter/min and nitrogen of 2 liter/min into
water at 323K. The decarburiza-
tion was performed in a quartz tube of an outer diameter of 31mm
which was heated with a three zone tube furnace. After the
decarburization, the quartz tube was pulled out from the tube
furnace and then air-cooled. Tensile tests were performed after
holding in liquid nitrogen for 10min in order to investigate the
effect of grain boundary segregation concentration of impurities on
fracture strength. The cross- head speed was 1mm/min. The grain
boundary segregation behavior of impurities was investigated with
Auger electron spectroscope (AES, Perkin Elmer 700). AES specimens,
which were machined from the decarburized tensile speci- mens, were
fractured after chilling with liquid nitrogen for about 30min under
a vacuum of about 1 107 Pa or better to minimize the post-fracture
contamination. Changes in microstructure and fracture mode with
decarburization time, temperature and bulk phosphorus content were
investigated, using an optical microscope (OM, LEICA DMI 5000M) and
a scanning electron microscope (SEM, JSM 6360). Etchant for
microstructure analyses was 3% nitric acid solution.
3. Results and Discussion
Figure 1 shows changes in grain boundary segregation concentration
of impurities with bulk phosphorus content and decarburization
temperature after decarburization for 24 h. The specimens of the
1.5mm thickness were mostly decarburized after 24 h, regardless of
the decarburization temperature. The grain boundary segregation
concentration of phosphorus increased with increasing bulk
phosphorus content, and the overall segregation concentration was
much higher at 973 than at 1173K. Peaks of carbon, nitrogen and
oxygen were additionally observed at 1173K. Changes in fracture
mode of the AES specimens with decarburization temperature and bulk
phosphorus content are shown in Fig. 2. The fracture mode was
changed from intergranu- lar + transgranular to intergranular, as
the decarburization
Materials Transactions, Vol. 52, No. 2 (2011) pp. 219 to 223 #2011
The Japan Institute of Metals EXPRESS REGULAR ARTICLE
temperature decreases and the bulk phosphorus content increases.
Figure 3 shows tensile test results in liquid nitrogen after
decarburization at 1173K for 24 h. As expected in Fig. 1, the
fracture strength decreased with increasing bulk phosphorus content
and consequently with increasing grain boundary segregation
concentration of phosphorus. The fracture mode was also changed
from mostly transgranular to intergranular with increasing grain
boundary segregation concentration of phosphorus.
Figure 4 shows changes in optical microstructure of Heats 1 and 3
with decarburization time at 973K. First of all, the abnormal grain
growth rate was much faster in Heat 1, although the decarburization
kinetics was similar in Heats 1 and 3. The decarburization zone
depth was about 240 mm at the initial stage and increased with
increasing time. In Heat 1, a big grain started to appear within
the interior of the decarburization zone after decarburization for
2 h. The number of the big grain increased abruptly after
decarburi-
dN (E
dN (E
dN (E
P PC N O P
(a) (c)(b)
0 250 500 750 1000 0 250 500 750 1000
0 250 500 750 10000 250 500 750 10000 250 500 750 1000
Fig. 1 Changes in grain boundary segregation concentration of
impurities with bulk phosphorus content and decarburization
temperature
after decarburization for 24 h: (a) Heat 1, (b) Heat 2 and (c) Heat
3.
Table 1 The chemical compositions of the prepared steels
(mass%).
Heats P C N Cr S Mn Ni Cu Si Fe
Heat 1 0.005 0.196 <0:001 0.021 0.020 0.596 0.020 0.010 0.298
Balance
Heat 2 0.019 0.203 <0:001 0.020 0.026 0.610 0.021 0.010 0.302
Balance
Heat 3 0.092 0.203 <0:001 0.021 0.017 0.602 0.021 0.010 0.301
Balance
40 µµm 973 K 40 µm 973 K 40 µm 973 K
40 µm 1173 K 40 µm 1173 K 40 µm 1173 K
(a) (b) (c)
Fig. 2 Changes in fracture mode of the AES specimens with
decarburization temperature and bulk phosphorus content: (a) Heat
1,
(b) Heat 2 and (c) Heat 3.
220 N. H. Heo and J. K. Lee
zation for 6 h. With further decarburization, the decarburiza- tion
zone of Fig. 4(a3) is divided into three regions: a surface region
I consisting of the big grains impinging together, other region II
only with very small grains and another interior
region III mixed with small and bigger grains. After
decarburization for 24 h, the big grains grew finally into a
typical columnar shape within the interior of the decarburi- zation
zone. As shown in Fig. 4(b), the abnormal grain growth was observed
also in Heat 3 during decarburization, but the columnar grain
structure was not formed. From Fig. 5 obtained from Fig. 4, because
the gradient in the log (decarburization depth) versus log
(decarburization time) plot was evaluated as about 0.5 (exactly,
0.49), the decarburization reaction in the present study can be ex-
pressed by a parabolic relationship between the decarburiza-
(a) (b) (c)
100
200
300
400
500
600
700
(a) (b) (c)
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Fig. 3 Changes in fracture strength and fracture mode with bulk
phosphorus content in liquid nitrogen after decarburization at
1173K for
24 h: (a) Heat 1, (b) Heat 2 and (c) Heat 3.
300 µµm
300 µµm
300 µµm
300 µµm
300 µµm
300 µµm
300 µµm
Fig. 4 Cross-section views after decarburization at 973K for
various
hours: (a) Heat 1 and (b) Heat 3. Here, the symbol l means
the
decarburization zone depth.
-1.6
-1.5
-1.4
-1.3
-1.2
-1.1
-1.0
(a)
(b)
Fig. 5 Plots obtained from Fig. 4: (a) a decarburization depth
versus
decarburization time plot and (b) a log (decarburization depth)
versus log
(decarburization time) plot obtained from (a).
Grain Boundary Segregation of Phosphorus and Columnar Grain Growth
during Decarburization in Plain Carbon Steels 221
tion depth (x) and the time (t). As a result, from the
decarburization zone depth expressed by x ¼ kðDtÞ1=2 and the carbon
diffusion coefficient in ferrite D ¼ D0 expðQ=RTÞ where D0 is 6:2
103 cm2s1 and Q is 19.2 kcalmole1,8)
the reaction coefficient k at 973K was equal to 0.60. Based on Fig.
5, about 50 ks (14 h) is enough for the complete decarburization of
the 1.5mm thick plate. Such a parabolic relationship between x and
t was also observed at 1173K. Using the carbon diffusion
coefficient in austenite D ¼ D0 expðQ=RTÞ where D0 is 0.1 cm2s1 and
Q 32.4 kcal mole1,8) the reaction coefficient k at 1173K was
approx- imately equal to that at 973K, as expected from the same
decarburization reaction.
The effect of decarburization temperature on grain growth behavior
was investigated for the comparison with that at 973K of Fig. 4. As
shown in Fig. 6(a), the columnar grain structure and the abnormal
grain growth behavior was not observed after decarburization at
1173K, irrespective of P contents. Additionally, the grain growth
near the free surface to a depth of about 70 mm was severely
inhibited. This is due to the film shape of grain boundary oxides
which are composed of Fe, Mn, Si and O.9) As shown in Fig. 6(b),
fine carbide particles (white particles) were observed at grain
boundaries and interiors in front of the abnormally growing grains
during decarburization at 973K.
Generally, the grain boundary segregation concentration increases
with increasing bulk content and with decreasing temperature.2,3)
The segregation behavior of phosphorus, which are shown in Fig. 1,
can be therefore understood in this direction.2,3) Also the
equilibrium segregation concentration is inversely proportional to
the solubility,10,11) explaining the segregation behavior of carbon
and nitrogen in Fig. 1. The reason why the grain boundary
segregation behavior of sulfur is not observed in the present study
can be attributed to two factors: the reaction H2 þ S½segregated or
dissolved ! H2S under the decarburization atmosphere containing wet
hydrogen and no solubility of sulfur in ferrite region below 1173K
which results in the reaction Mnþ S ! MnS.12,13) Comparing Figs. 4
and 6, severely oxidized grain boundaries and interiors near the
free surface are formed only at 1173K. Therefore, the oxygen peak
observed only at the grain boundary facets of 1173K in Fig. 1 is
probably due to an
oxidation atmosphere at 1173K which enables oxygen to diffuse into
inner grain boundaries along surface grain boundaries.
In previous research,14) during annealing in argon atmo- sphere at
1473K the strong grain boundary pinning effect of the segregated
sulfur was observed in 100 mm thick 3% silicon steels containing a
low bulk sulfur content of 30 ppm. The much slower growth rate in
the sample with 0.092%P (Heat 3) of Figs. 4(b) may therefore be
attributed to the strong grain boundary pinning effect of the
segregated phosphorus in Fig. 1(c).
On the other hand, abnormal grain growth can occur when normal
grain growth is strongly inhibited. The main factors which lead to
the abnormal grain growth are surface effects,14–16) second-phase
particles17–19) and texture.20,21)
Some researches22,23) in which the starting material was a rolled
lamination steel22) and a rolled 3% silicon steel23) have been
performed on the abnormal grain growth behaviors occurring during
decarburization. In the laminated steel, the abnormal grain growth
has been attributed to the interaction between dislocations arising
from the cold-rolling and carbide particles. In the silicon steel,
the abnormal grain growth was responsible for the composition of
oxide- separator. Based on the researches,17–19) many grain bounda-
ry carbide particles in Fig. 6(b) suggest a possibility for the
abnormal grain growth not at 1173K but at 973K. At the initial
decarburization stage at 973K, specific grains larger than average
grain size in the decarburization zone grow abnormally at the
expense of other smaller grains, in order to decrease the grain
boundary energy. As the decarburization time increases, the bigger
grains in the regions I and III grow at the expense of the smaller
grains in the region II. In order to further decrease the grain
boundary energy after the lateral impingement of the growing
grains, the bigger grains in the region III continue to consume the
smaller grains in front of the growing grains, resulting in a
typical columnar grain structure of Fig. 4(a4).
During decarburization at 1173K, in order to minimize the total
grain boundary energy, the smaller grains do not wait for only the
growth of the specific grains but grow actively due to their high
grain boundary mobility at the temperature, resulting in the normal
grain growth.
(a) 150 µµm 50 µm(b)
Abnormally growing grains
Fig. 6 Cross-section views of Heat 1 after decarburization: (a)
1173K for 24 h and (b) 973K for 6 h. Figs. (a) and (b) were
obtained from
OM and SEM, respectively.
4. Summary
Effects of decarburization on grain boundary segregation behaviors
of solutes and grain growth behaviors have been investigated in
three Fe-0.2C-P plain carbon steels with the phosphorus
concentrations of 0.005, 0.019 and 0.092%. The decarburization
reaction in the present plain carbon steels is expressed by the
parabolic relationship x ¼ kðDtÞ1=2 where x is the decarburization
depth, k the reaction coefficient, D the diffusivity of carbon and
t the decarburization time. After decarburization, the grain
boundary segregation concentra- tion of phosphorus increased with
increasing bulk phospho- rus content and was higher at 973 than at
1173K. During decarburization, the growth kinetics was faster in
steels with lower phosphorus contents. This can be attributed to
the solute drag effect of highly segregated phosphorus on moving
grain boundaries. The fracture strength in liquid nitrogen after
decarburization for 24 h decreased with increasing bulk phosphorus
content. After decarburization at 973K for 24 h, a typical columnar
grain structure was observed in the sample with 0.005%P. This is
due to the abnormal grain growth driven by grain boundary carbide
particles. After decarbu- rization at 1173K, such a columnar
structure was not formed due to the absence of grain boundary
carbide particles.
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