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Effect of rare earths on the carburization of steel
Materials Science and Engineering A267 (1999) 162 – 166
Letter
Effect of rare earths on the carburization of steel Z.-X. Yuan a, Z.-S. Yu b, P. Tan a, S.-H. Song c,* a
Department of Materials, Wuhan Yejin Uni6ersity of Science and Technology, Wuhan, Hubei 430081, People’s Republic of China b Department of Materials Physics, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Rebublic of China c Institute of Polymer Technology and Materials Engineering, Loughborough Uni6ersity, Loughborough, Leicestershire LE11 3TU, UK Received 9 July 1998; received in revised form 25 January 1999
Abstract The effects of rare earths (RE) on the carburization of steel are examined with four 0.2%C steels doped and undoped with RE. Clearly, both RE in carburizer and RE in steel may accelerate the carburizing process. RE in carburizer is more effective at enhancing carburization than RE in steel. The mechanism for this enhancement of carburization is that the RE oxide enhances the medium-sample interface reaction during carburizing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Rare earths; Carburization; Segregation; Grain boundaries
1. Introduction
2. Experimental procedure
As is well known, carburization is one of the effective approaches to enhancing the surface-layer strength and wear resistance of steel. It is to increase the carbon concentration in the surface-layer of steel components so that the components have adequate strength and strongly increased wear resistance in the surface-layer after quenching and tempering. In the 1980s, it was found [1,2] that rare earths (RE) added to gas carburizer could accelerate the carburizing process considerably by an activation reaction with the surface of components. Soon after, it was also found [3 –5] that RE could accelerate the carbonitriding process by 15–20% as compared to carbonitriding without the involvement of RE, simultaneously bringing about an increase in the fatigue strength of low-carbon steels. It is anticipated on the basis of the above information that RE doped in steel may be able to accelerate the carburizing process. The present work was aimed at investigating the effect of RE both in steel and in carburizer on the carburization of steel.
Four 0.2%C steels, with and without RE addition, in the form of mischmetal were prepared by vacuum induction melting. All ingots 20 kg each were forged into sheets 13× 13× 1000 mm in size and then cut into samples 13×13× 6 mm. All the samples were normalised at 900oC prior to carburization. Chemical composition of the steels is shown in Table 1. The samples, well polished by mechanical polishing, were carburized in a pit-type gas carburizing furnace with a power of 35 kW at 850 and 910oC for 1, 2, 3, and 4 h, respectively. Details on the carburizing furnace may be seen in [6]. The carburizer was kerosene, with or without Ce2O3 oxides. It was made by the following method. A solution of 15 ml methanol and 15 ml acetone with 8 g Ce2O3 is mixed with every 3 kg kerosene. In the course of carburization, the kerosene dropped into the furnace at a rate of 50–70 d/min (drops per minute) in the boost stage and 30–40 d/min in the diffusion stage. The carburizing atmosphere pressure was kept positive in the range of 14–20 mbar during carburizing. Approximate composition of the gas is listed in Table 2. After carburizing, some samples were directly water-quenched and the others directly air-cooled.
* Corresponding author. Tel.: +44-1509-263171; fax: +44-1509223949. E-mail address: [email protected] (S.-H. Song)
0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 0 6 1 - 1
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166 Table 1 Chemical composition of the experimental steels (wt.%)
163
There is no apparent difference in microstructure between RE-free and RE-containing samples.
Steel
C
Si
Mn
P
S
RE
1 2 3 4
0.20 0.19 0.18 0.19
0.28 0.26 0.27 0.28
0.48 0.49 0.49 0.48
0.009 0.013 0.005 0.014
0.011 0.010 0.004 0.002
– 0.024 0.032 0.130
3.2. Carburizing kinetics
Table 2 Approximate composition of the carburizing gas (vol.%) CnH2n+2
CnH2n
CO
CO2
H2
O2
N2
10–15
0.6
10–20
50.4
60–70
50.4
5
The distance from the sample surface to the 50% pearlite position was taken as the case depth. The case depth was measured by optical microscopy for the as-air-cooled samples mechanically polished and etched by an ethanol solution of 4% nitric acid. Fig. 1 gives an example for its determination. In the measurement of case depths, five samples were employed for each condition and the arithmetic mean of data points acquired was taken as the measured result. The microhardness distribution from the sample surface to the centre was measured by an HX-500 microhardness testing machine under constant load (100 g) and loading time (15 s) with five water-quenched samples for each condition. In the measurement of case depth, the arithmetic mean of data points obtained was taken as the measured result.
3. Results
3.1. Microstructure For the sample air-cooled directly after carburization, microstructure in the carburized layer, as shown in Fig. 1, is pearlite plus ferrite in the inner part and pearlite plus cementite in the outer part. This is because, after carburization, the carbon concentration increases on going from the matrix to the surface.
Fig. 2 represents the carburizing kinetics of the four experimental steels, with and without RE oxide in carburizer, at 850 and 910oC, respectively. Clearly, the kinetic curves are parabolic. They may be divided into two stages. The first stage is from the beginning to 1 h and the second one is from 1 to 4 h. The curve slope in the first stage is larger than that in the second stage. RE in steel and RE in carburizer both have an enhancing effect on carburization. The case depth increases with increasing RE content in steel until 0.032 wt.%. When the RE content is greater than 0.032 wt.%, the case depth no longer increases. The enhancing effect of RE in carburizer is stronger than that of RE in steel. The combined enhancing effect of RE in steel and RE in carburizer is stronger than their individual effect.
3.3. Potency of enhancement The potencies of carburization enhancement caused by RE in steel and RE in carburizer individually are represented in Table 3 and Table 4, respectively, and their combined potency in Table 5. Here the potency of enhancement, P, is given by P= xi − xo
(1)
where xo is the case depth of the RE-free sample without RE in carburizer for evaluation of the enhancement potency of RE in steel or the case depth of the RE-free and RE-doped samples without RE in carburizer for evaluation of the enhancement potency of RE in carburizer; and xi is the case depth of the RE-free and RE-doped samples, either without RE in carburizer for evaluation of the enhancement potency of RE in steel or with RE in carburizer for evaluation of the enhancement potency of RE in carburizer. Table 3, Table 4 and Table 5 may be summarised as follows. When the RE content is about 0.032 wt.% (Steel 3) in steel, the enhancing effect is the strongest. In general, RE in carburizer is more effective than RE in steel in enhancing the carburization and the combined effect of RE in steel and RE in carburizer at 850oC is nearly the same as that at 910oC except for the 1-h carburized sample.
3.4. Microhardness
Fig. 1. Typical optical micrograph showing how to determine the case depth.
Microhardness of the as-quenched 4-h-carburized samples for Steels 1 and 3 are illustrated in Fig. 3 as a function of distance from the sample surface. Clearly, the microhardness of RE-doped Steel 3 is somewhat higher than that of RE-free steel at the same distance to
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Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
Fig. 2. The case depth as a function of carburizing time for (a) Steel 1, (b) Steel 2, (c) Steel 3, and (d) Steel 4 air-cooled directly after carburizing at 850 and 910oC (C1 and C3: 850 and 910oC without RE in carburizer, respectively; C2 and C4: 850 and 910oC with RE in carburizer, respectively).
the surface. The difference in microhardness between Steels 1 and 3 in the scenario without RE in carburizer is smaller for the 850oC-carburized samples than for the 950oC-carburized ones (see Fig. 3a). This is because the difference in the potency of carburization enhancement is smaller for the 850oC-carburized samples than for the 950oC-carburized ones (see Table 3 and Table 4). Also, the microhardness is more enhanced by RE in carburizer than by RE in steel because of the same reason as in enhancing the case depth, i.e. RE in carburizer is more effective than RE in steel in enhancing the carburization, which will be discussed in Section 4. It may be seen from Fig. 2 and Fig. 3 that, at the same distance from the surface, the microhardness increases generally with increasing case depth. This is reasonable because, at the same distance from the surface, the carbon concentration in as-quenched martensite within the carburized layer would increase with increasing case depth.
4. Discussion It is well known [7] that thermochemical treatment of
the steel may be divided into four steps. They are reaction in medium, external diffusion, interface reaction, and internal diffusion. Of these four steps, the slowest step determines the rate of thermochemical treatment. In general, the reaction in medium and the external diffusion are quite fast. As a consequence, the rate of thermochemical treatment is controlled by either interface reaction or internal diffusion. It may be seen from Fig. 2 that the kinetic curves are parabolic, which follow the diffusion distance-time relationship, x= K(Dt) 1/2, where x is the case depth, D is the diffusion coefficient of carbon, t is the carburizing time, and K is a material constant. The microhardness distribution presented in Fig. 3 corresponds to the carbon distribution from the surface to the matrix. It may be concluded that RE in steel and RE in carburizer both facilitate the carbon supply on the surface so as to have more carbon diffusing into the samples, i.e. RE in steel and RE in carburizer may both be able to speed up the interface reaction on the sample surface, but RE in carburizer is more effective than RE in steel due to the larger quantity adsorbed on the sample surface.
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
In general, the RE oxide is very stable and difficult to dissociate at carburizing temperatures and in carburizing environments. As a result, the RE oxide itself cannot penetrate into the sample. RE involved in RE oxides can however activate the sample surface and enhance transport of carbon atoms from the carburizer into the sample [8], i.e. RE in carburizer can play a catalytic role in carburization. As is well known [9,10], the RE oxide has been employed as a catalyst in the petrochemical and automotive industries for quite a long time. It can accelerate transformation of CO to CO2. In our case, kerosene is a hydrocarbon and can decompose into CO and CnH2n + 2 during carburizing. CO and CnH2n + 2 have the following reactions on the medium-sample interface: 2CO[C]+CO2
(2)
CnH2n + 2 n[C]+(n + 1)H2
(3)
Since RE elements are very high electropositive and reactive, RE in steel may easily combine with oxygen adsorbed on the sample surface to form RE oxides so as to activate the surface. This, as described above, is
165
beneficial to carburization. In addition, RE and their oxides may be able to enhance transformation of CO2 (adsorbed on the surface) to CO and decomposition of CO or CnH2n + 2 (adsorbed on the surface) to active carbon atoms ([C]). The above two processes are both beneficial to carburization. RE elements are very active to free surfaces and grain boundaries in steels and segregate strongly at these surfaces and boundaries [11–16]. At carburizing temperatures, RE atoms segregated at grain boundaries diffuse to the surface through fast grain-boundary diffusion and then proceed to diffuse to all over the sample surface through even faster surface diffusion. RE on the surface can enhance the interface reaction as RE in carburizer. However, if the RE content is too high, RE and Fe may form brittle RE-Fe intermetallic compounds which do not contribute to RE segregation at grain boundaries. As a consequence, excessive RE in steel cannot contribute any more to the enhancement of carburization. Although there is no experimental evidence about RE distribution from the sample surface to the matrix, the preceding discussion should be reasonable from the standpoint of diffusion theories.
Table 3 Potency of enhancement caused by rare earths (RE) for carburization at 850 oC, P (mm) Time (h)
1 2 3 4
P for RE in steel
P for RE in carburizer
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0 0 0 0
0.03 0.05 0.08 0.09
0.06 0.08 0.11 0.14
0.05 0.09 0.09 0.10
0.08 0.13 0.20 0.27
0.21 0.25 0.27 0.27
0.29 0.37 0.38 0.41
0.28 0.34 0.34 0.38
Table 4 Potency of enhancement caused by rare earths (RE) for carburization at 910oC, P (mm) Time (h)
1 2 3 4
P for RE in steel
P for RE in carburizer
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0 0 0 0
0.04 0.02 0.03 0.06
0.13 0.11 0.13 0.17
0.11 0.05 0.09 0.22
0.14 0.11 0.14 0.13
0.30 0.31 0.33 0.32
0.35 0.39 0.39 0.37
0.34 0.37 0.35 0.24
Table 5 Combined potency of enhancement caused by rare earths (RE) both in steel and in carburizer for carburization at 850 and 910oC, P (mm) Time (h)
1 2 3 4
P at 850oC
P at 910oC
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0.08 0.13 0.20 0.27
0.24 0.30 0.35 0.35
0.35 0.45 0.49 0.55
0.33 0.43 0.43 0.48
0.14 0.11 0.14 0.13
0.34 0.33 0.36 0.38
0.48 0.50 0.52 0.54
0.45 0.42 0.44 0.46
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Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
5. Summary Both RE in carburizer and RE in steel accelerate the gas carburizing process at 850 and 910oC. The catalytic mechanism is that the RE oxide enhances the mediumsample interface reaction during carburizing. The most appropriate RE content in 0.2%C steel may be about 0.032 wt.%. The catalytic effect of RE in steel is weaker than that of RE in carburizer.
References
Fig. 3. Microhardness distribution from the surface to the matrix in the as-water-quenched carburized samples for Steels 1 and 3: (a) carburizing temperature = 850oC and (b) carburizing temperature = 910 oC (C1 and C2: Steels 1 and 3 without RE in carburizer, respectively; C3 and C4: Steels 1 and 3 with RE in carburizer, respectively; carburizing time = 4 h).
In terms of the above discussion, that the difference in microhardness between Steels 1 and 3 in the case without RE in carburizer is smaller for the 850oC-4-hcarburized samples than for the 950oC-4-h-carburized ones (see Fig. 3a) may be attributed to a lower diffusion rate of RE atoms at 850oC leading the potency of carburization enhancement to be smaller for the 850oCcarburized samples than for the 950oC-carburized ones (see Table 3 and Table 4).
.
[1] Y.-D. Wei, Z.-R. Liu, X.-H. Cheng, Effect of the activity of rare earth elements on carburizing, in: Proceedings of 4th International Congress on Heat Treatment of Materials, Berlin, vol. 2, 1985, pp. 858 – 866. [2] Y.-D. Wei, Z.-R. Liu, X.-H. Cheng, H.-F. Jiang, Acta Metall. Sinica B 19 (5) (1983) 197. [3] Z.-R. Liu, Y.-D. Wei, X.-H. Cheng, C.-Y. Wang, H.-F. Jiang, B.S. Gong, Effect of rare-earth elements on carbonitriding process and properties on steels 20CrMnTi and 20CrMnMo, in: Proceedings of 5th International Congress on Heat Treatment of Materials, Budapest, vol. 2, 1986, pp. 1168 – 1175. [4] Z.-R. Liu, X.-H. Cheng, Met. Sci. Technol. (China) 5 (3) (1986) 13. [5] X.-H. Cheng, Mater. Mech. Eng. (China) 10 (6) (1986) 10. [6] Metals Handbook, Ninth Edition, Vol. 4, American Society of Metals, Metals Park, OH, 1981, p. 135. [7] Z.-S. Kong, Principles of Thermochemical Treatment, Aerospace Press, Beijing, 1992, p. 6. [8] S.-Q. Wang, L.-D. Wang, A.-H. Yang, Met. Heat-treatment (China) 3 (1988) 52. [9] Rare Earths, Metallurgical Industry Press, Beijing, 1978. [10] W.-X. Zhu, Rare Earths (China) 3 (1984) 21. [11] Z.-X. Yuan, J.H. Li, S.-Y. Feng, C.J. Wu, Behaviour of cerium in boundary segregation and temper embrittlement of steels, in: Heat Treatment Shanghai 83, Institute of Metals, London, 1984, pp. 422 – 427. [12] C.-J. Wu, P.-G. Li, D.-B. Zhang, X.-L. Zhao, The influence of cerium on the equilibrium grain boundary segregation of antimony and the grain boundary migration in pure iron, in: New Frontiers in Rare Earth Science and Applications, vol. 2, Science Press, Beijing, 1985, pp. 1253 – 1257. [13] D. Luo, G.-H. Xing, H.-L. Zhou, S.-S. Yan, Z.-Z. Yu, X.-H. Luo, J. The Less-Common Metals 93 (1983) 278. [14] L.-Q. Shi, J.Z. Chen, D.-O. Northwood, J. Mater. Eng. Perform. 1 (1992) 21. [15] L. Wei, L. He, T.-Y. Hsu, Z.-Y. Xu, Scripta Metall. Mater. 29 (1993) 273. [16] Q. Lin, T.-J. Yao, A.-S. Liu, N. Chen, W. Ye, Z.-S. Yu, X.-L. Lu, J. Rare Earths 14 (3) (1996) 189.
Letter
Effect of rare earths on the carburization of steel Z.-X. Yuan a, Z.-S. Yu b, P. Tan a, S.-H. Song c,* a
Department of Materials, Wuhan Yejin Uni6ersity of Science and Technology, Wuhan, Hubei 430081, People’s Republic of China b Department of Materials Physics, Uni6ersity of Science and Technology Beijing, Beijing 100083, People’s Rebublic of China c Institute of Polymer Technology and Materials Engineering, Loughborough Uni6ersity, Loughborough, Leicestershire LE11 3TU, UK Received 9 July 1998; received in revised form 25 January 1999
Abstract The effects of rare earths (RE) on the carburization of steel are examined with four 0.2%C steels doped and undoped with RE. Clearly, both RE in carburizer and RE in steel may accelerate the carburizing process. RE in carburizer is more effective at enhancing carburization than RE in steel. The mechanism for this enhancement of carburization is that the RE oxide enhances the medium-sample interface reaction during carburizing. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Rare earths; Carburization; Segregation; Grain boundaries
1. Introduction
2. Experimental procedure
As is well known, carburization is one of the effective approaches to enhancing the surface-layer strength and wear resistance of steel. It is to increase the carbon concentration in the surface-layer of steel components so that the components have adequate strength and strongly increased wear resistance in the surface-layer after quenching and tempering. In the 1980s, it was found [1,2] that rare earths (RE) added to gas carburizer could accelerate the carburizing process considerably by an activation reaction with the surface of components. Soon after, it was also found [3 –5] that RE could accelerate the carbonitriding process by 15–20% as compared to carbonitriding without the involvement of RE, simultaneously bringing about an increase in the fatigue strength of low-carbon steels. It is anticipated on the basis of the above information that RE doped in steel may be able to accelerate the carburizing process. The present work was aimed at investigating the effect of RE both in steel and in carburizer on the carburization of steel.
Four 0.2%C steels, with and without RE addition, in the form of mischmetal were prepared by vacuum induction melting. All ingots 20 kg each were forged into sheets 13× 13× 1000 mm in size and then cut into samples 13×13× 6 mm. All the samples were normalised at 900oC prior to carburization. Chemical composition of the steels is shown in Table 1. The samples, well polished by mechanical polishing, were carburized in a pit-type gas carburizing furnace with a power of 35 kW at 850 and 910oC for 1, 2, 3, and 4 h, respectively. Details on the carburizing furnace may be seen in [6]. The carburizer was kerosene, with or without Ce2O3 oxides. It was made by the following method. A solution of 15 ml methanol and 15 ml acetone with 8 g Ce2O3 is mixed with every 3 kg kerosene. In the course of carburization, the kerosene dropped into the furnace at a rate of 50–70 d/min (drops per minute) in the boost stage and 30–40 d/min in the diffusion stage. The carburizing atmosphere pressure was kept positive in the range of 14–20 mbar during carburizing. Approximate composition of the gas is listed in Table 2. After carburizing, some samples were directly water-quenched and the others directly air-cooled.
* Corresponding author. Tel.: +44-1509-263171; fax: +44-1509223949. E-mail address: [email protected] (S.-H. Song)
0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 0 6 1 - 1
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166 Table 1 Chemical composition of the experimental steels (wt.%)
163
There is no apparent difference in microstructure between RE-free and RE-containing samples.
Steel
C
Si
Mn
P
S
RE
1 2 3 4
0.20 0.19 0.18 0.19
0.28 0.26 0.27 0.28
0.48 0.49 0.49 0.48
0.009 0.013 0.005 0.014
0.011 0.010 0.004 0.002
– 0.024 0.032 0.130
3.2. Carburizing kinetics
Table 2 Approximate composition of the carburizing gas (vol.%) CnH2n+2
CnH2n
CO
CO2
H2
O2
N2
10–15
0.6
10–20
50.4
60–70
50.4
5
The distance from the sample surface to the 50% pearlite position was taken as the case depth. The case depth was measured by optical microscopy for the as-air-cooled samples mechanically polished and etched by an ethanol solution of 4% nitric acid. Fig. 1 gives an example for its determination. In the measurement of case depths, five samples were employed for each condition and the arithmetic mean of data points acquired was taken as the measured result. The microhardness distribution from the sample surface to the centre was measured by an HX-500 microhardness testing machine under constant load (100 g) and loading time (15 s) with five water-quenched samples for each condition. In the measurement of case depth, the arithmetic mean of data points obtained was taken as the measured result.
3. Results
3.1. Microstructure For the sample air-cooled directly after carburization, microstructure in the carburized layer, as shown in Fig. 1, is pearlite plus ferrite in the inner part and pearlite plus cementite in the outer part. This is because, after carburization, the carbon concentration increases on going from the matrix to the surface.
Fig. 2 represents the carburizing kinetics of the four experimental steels, with and without RE oxide in carburizer, at 850 and 910oC, respectively. Clearly, the kinetic curves are parabolic. They may be divided into two stages. The first stage is from the beginning to 1 h and the second one is from 1 to 4 h. The curve slope in the first stage is larger than that in the second stage. RE in steel and RE in carburizer both have an enhancing effect on carburization. The case depth increases with increasing RE content in steel until 0.032 wt.%. When the RE content is greater than 0.032 wt.%, the case depth no longer increases. The enhancing effect of RE in carburizer is stronger than that of RE in steel. The combined enhancing effect of RE in steel and RE in carburizer is stronger than their individual effect.
3.3. Potency of enhancement The potencies of carburization enhancement caused by RE in steel and RE in carburizer individually are represented in Table 3 and Table 4, respectively, and their combined potency in Table 5. Here the potency of enhancement, P, is given by P= xi − xo
(1)
where xo is the case depth of the RE-free sample without RE in carburizer for evaluation of the enhancement potency of RE in steel or the case depth of the RE-free and RE-doped samples without RE in carburizer for evaluation of the enhancement potency of RE in carburizer; and xi is the case depth of the RE-free and RE-doped samples, either without RE in carburizer for evaluation of the enhancement potency of RE in steel or with RE in carburizer for evaluation of the enhancement potency of RE in carburizer. Table 3, Table 4 and Table 5 may be summarised as follows. When the RE content is about 0.032 wt.% (Steel 3) in steel, the enhancing effect is the strongest. In general, RE in carburizer is more effective than RE in steel in enhancing the carburization and the combined effect of RE in steel and RE in carburizer at 850oC is nearly the same as that at 910oC except for the 1-h carburized sample.
3.4. Microhardness
Fig. 1. Typical optical micrograph showing how to determine the case depth.
Microhardness of the as-quenched 4-h-carburized samples for Steels 1 and 3 are illustrated in Fig. 3 as a function of distance from the sample surface. Clearly, the microhardness of RE-doped Steel 3 is somewhat higher than that of RE-free steel at the same distance to
164
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
Fig. 2. The case depth as a function of carburizing time for (a) Steel 1, (b) Steel 2, (c) Steel 3, and (d) Steel 4 air-cooled directly after carburizing at 850 and 910oC (C1 and C3: 850 and 910oC without RE in carburizer, respectively; C2 and C4: 850 and 910oC with RE in carburizer, respectively).
the surface. The difference in microhardness between Steels 1 and 3 in the scenario without RE in carburizer is smaller for the 850oC-carburized samples than for the 950oC-carburized ones (see Fig. 3a). This is because the difference in the potency of carburization enhancement is smaller for the 850oC-carburized samples than for the 950oC-carburized ones (see Table 3 and Table 4). Also, the microhardness is more enhanced by RE in carburizer than by RE in steel because of the same reason as in enhancing the case depth, i.e. RE in carburizer is more effective than RE in steel in enhancing the carburization, which will be discussed in Section 4. It may be seen from Fig. 2 and Fig. 3 that, at the same distance from the surface, the microhardness increases generally with increasing case depth. This is reasonable because, at the same distance from the surface, the carbon concentration in as-quenched martensite within the carburized layer would increase with increasing case depth.
4. Discussion It is well known [7] that thermochemical treatment of
the steel may be divided into four steps. They are reaction in medium, external diffusion, interface reaction, and internal diffusion. Of these four steps, the slowest step determines the rate of thermochemical treatment. In general, the reaction in medium and the external diffusion are quite fast. As a consequence, the rate of thermochemical treatment is controlled by either interface reaction or internal diffusion. It may be seen from Fig. 2 that the kinetic curves are parabolic, which follow the diffusion distance-time relationship, x= K(Dt) 1/2, where x is the case depth, D is the diffusion coefficient of carbon, t is the carburizing time, and K is a material constant. The microhardness distribution presented in Fig. 3 corresponds to the carbon distribution from the surface to the matrix. It may be concluded that RE in steel and RE in carburizer both facilitate the carbon supply on the surface so as to have more carbon diffusing into the samples, i.e. RE in steel and RE in carburizer may both be able to speed up the interface reaction on the sample surface, but RE in carburizer is more effective than RE in steel due to the larger quantity adsorbed on the sample surface.
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
In general, the RE oxide is very stable and difficult to dissociate at carburizing temperatures and in carburizing environments. As a result, the RE oxide itself cannot penetrate into the sample. RE involved in RE oxides can however activate the sample surface and enhance transport of carbon atoms from the carburizer into the sample [8], i.e. RE in carburizer can play a catalytic role in carburization. As is well known [9,10], the RE oxide has been employed as a catalyst in the petrochemical and automotive industries for quite a long time. It can accelerate transformation of CO to CO2. In our case, kerosene is a hydrocarbon and can decompose into CO and CnH2n + 2 during carburizing. CO and CnH2n + 2 have the following reactions on the medium-sample interface: 2CO[C]+CO2
(2)
CnH2n + 2 n[C]+(n + 1)H2
(3)
Since RE elements are very high electropositive and reactive, RE in steel may easily combine with oxygen adsorbed on the sample surface to form RE oxides so as to activate the surface. This, as described above, is
165
beneficial to carburization. In addition, RE and their oxides may be able to enhance transformation of CO2 (adsorbed on the surface) to CO and decomposition of CO or CnH2n + 2 (adsorbed on the surface) to active carbon atoms ([C]). The above two processes are both beneficial to carburization. RE elements are very active to free surfaces and grain boundaries in steels and segregate strongly at these surfaces and boundaries [11–16]. At carburizing temperatures, RE atoms segregated at grain boundaries diffuse to the surface through fast grain-boundary diffusion and then proceed to diffuse to all over the sample surface through even faster surface diffusion. RE on the surface can enhance the interface reaction as RE in carburizer. However, if the RE content is too high, RE and Fe may form brittle RE-Fe intermetallic compounds which do not contribute to RE segregation at grain boundaries. As a consequence, excessive RE in steel cannot contribute any more to the enhancement of carburization. Although there is no experimental evidence about RE distribution from the sample surface to the matrix, the preceding discussion should be reasonable from the standpoint of diffusion theories.
Table 3 Potency of enhancement caused by rare earths (RE) for carburization at 850 oC, P (mm) Time (h)
1 2 3 4
P for RE in steel
P for RE in carburizer
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0 0 0 0
0.03 0.05 0.08 0.09
0.06 0.08 0.11 0.14
0.05 0.09 0.09 0.10
0.08 0.13 0.20 0.27
0.21 0.25 0.27 0.27
0.29 0.37 0.38 0.41
0.28 0.34 0.34 0.38
Table 4 Potency of enhancement caused by rare earths (RE) for carburization at 910oC, P (mm) Time (h)
1 2 3 4
P for RE in steel
P for RE in carburizer
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0 0 0 0
0.04 0.02 0.03 0.06
0.13 0.11 0.13 0.17
0.11 0.05 0.09 0.22
0.14 0.11 0.14 0.13
0.30 0.31 0.33 0.32
0.35 0.39 0.39 0.37
0.34 0.37 0.35 0.24
Table 5 Combined potency of enhancement caused by rare earths (RE) both in steel and in carburizer for carburization at 850 and 910oC, P (mm) Time (h)
1 2 3 4
P at 850oC
P at 910oC
Steel 1
Steel 2
Steel 3
Steel 4
Steel 1
Steel 2
Steel 3
Steel 4
0.08 0.13 0.20 0.27
0.24 0.30 0.35 0.35
0.35 0.45 0.49 0.55
0.33 0.43 0.43 0.48
0.14 0.11 0.14 0.13
0.34 0.33 0.36 0.38
0.48 0.50 0.52 0.54
0.45 0.42 0.44 0.46
166
Z.-X. Yuan et al. / Materials Science and Engineering A267 (1999) 162–166
5. Summary Both RE in carburizer and RE in steel accelerate the gas carburizing process at 850 and 910oC. The catalytic mechanism is that the RE oxide enhances the mediumsample interface reaction during carburizing. The most appropriate RE content in 0.2%C steel may be about 0.032 wt.%. The catalytic effect of RE in steel is weaker than that of RE in carburizer.
References
Fig. 3. Microhardness distribution from the surface to the matrix in the as-water-quenched carburized samples for Steels 1 and 3: (a) carburizing temperature = 850oC and (b) carburizing temperature = 910 oC (C1 and C2: Steels 1 and 3 without RE in carburizer, respectively; C3 and C4: Steels 1 and 3 with RE in carburizer, respectively; carburizing time = 4 h).
In terms of the above discussion, that the difference in microhardness between Steels 1 and 3 in the case without RE in carburizer is smaller for the 850oC-4-hcarburized samples than for the 950oC-4-h-carburized ones (see Fig. 3a) may be attributed to a lower diffusion rate of RE atoms at 850oC leading the potency of carburization enhancement to be smaller for the 850oCcarburized samples than for the 950oC-carburized ones (see Table 3 and Table 4).
.
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