grain boundary segregation of phosphorus and columnar grain growth during decarburization in
TRANSCRIPT
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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Magn. Mater. 254–255 (2003) 315–317.
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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