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A special phase transformation phenomenon in high-nitrogen austenite
September 2001
Materials Letters 50 Ž2001. 225–229 www.elsevier.comrlocatermatlet
A special phase transformation phenomenon in high-nitrogen austenite Mingjuan Hu a , Jiangsheng Pan a,) , Zuchang Zhu b, Qiu Chungcheng a a
Key Laboratory of High Temperature Materials and Tests of the Ministry of Education, Shanghai Jiao Tong UniÕersity, Shanghai 200030, People’s Republic of China b Department of Materials Science and Engineering, Shanghai UniÕersity of Engineering Science, Shanghai 200336, People’s Republic of China Received 25 March 2000; received in revised form 5 October 2000; accepted 5 October 2000
Abstract A special phenomenon of media temperature phase transformation is observed in high-nitrogen-containing austenite. The fine nuclei of decomposition form in the austenite grains and grain boundaries firstly, and the finer decomposition products can be found in the nucleus. Then the decomposition nucleus grows very slowly while the amount of the new phase nucleus increases constantly. Finally, the regions containing the decomposition nucleus are connected to one another and a high-dispersive morphology is formed. This structure is different not only from martensite but also from lower bainite with plate or needle structure. The hardness of the decomposition product reaches to 900 HV, which is higher than that of tempered martensite. The isothermal decomposition has a long incubation period. The amount of decomposition products increases with time and finally reaches to 100%, which is different from the bainite transformation. The observation shows that structural morphology and dynamics of the phase transformation are significantly different from bainite and martensite. q 2001 Elsevier Science B.V. All rights reserved. Keywords: High-nitrogen supercooled austenite; Phase transformation; Isothermal decomposition
1. Introduction An austenite layer can be seen in the nitrided case of pure iron or slow-carbon steel after nitriding or nitrocarburizing at 600–7008C w1x. From Fe–N or Fe–N–C phase diagram, it can be seen that the maximum content of nitrogen in the austenite layer is up to 2.8 wt.%. The optical microscopy and scanning electron microscopy were employed to study the tempering behavior of the high-nitrogen ) Corresponding author. Tel.: q86-21-62932563; fax: q86-2162932587. E-mail address: [email protected] ŽJ. Pan..
austenite at 200–2508C. The results show that the isothermal transformation process and result are different from those of bainite w1x. The first-stage of our research is reported in this paper.
2. The conditions of obtaining high-nitrogen austenite layer Fig. 1 shows the nitrogen chemical potential in gaseous phase, Dm N as a function of temperature in the Fe–N system. It was plotted according to the data tested by Darken and Gurry w2x, who adopted the method offered by Fueki w3x. These data were
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 2 9 - 4
M. Hu et al.r Materials Letters 50 (2001) 225–229
226
Fig. 1. Dm N as a function of temperature in the Fe–N system.
tested and verified by Hu and Pan w4x. It clearly shows the range of nitrogen chemical potential in Fe–N system in the state of equilibrium. If the atmosphere containing nitrogen is heated, the nitrogen chemical potential in gaseous phase is Dm N s RT 1n a n . If P s 1 atm is chosen as the standard condition, Dm N can be expressed as: D m N s RT ln K p
PNH 3 P H1.52
.
Ž 1.
Table 1 shows the equilibrium condition of gaseous phase in the Fe–N system, which was calculated on the basis of Fig. 1 and formula Ž1. when nitriding in the atmosphere of NH 3 and H 2 in the temperature range of 640–6508C. It can be seen that when nitriding at 6408C and 6508C, ammonia partial pressure inside the furnace,
Fig. 2. The schematic of the device for nitriding or nitrocarburizing. 1 – adjusting valve, 2 – flow meter, 3 – furnace for predecomposition of ammonia, 4 – mixer, 5 – tube, 6 – centrifugal fan, 7 – guide tank, 8 – nitriding tank, 9 – stabilizer and absorber, 10 – measurer for decomposition of ammonia.
P NH 3 is greater than 15%, the compound layer consisting of gX or ´ will appear at the surface of work-piece, and the subsurface is an austenite layer. The maximum content of nitrogen in the austenite, which is up to 2.8 wt.% N Žnitrided at 6508C., is located in the region that is adjacent to the compound layer, and the minimum content of nitrogen in the austenite is in the region that is adjacent to a-Fe. 3. Experimental procedures The nitriding or nitrocarburizing was processed in a small-scale well furnace shown in Fig. 2, whose size is B 300 = 350 mm. The rate of decomposition of ammonia, PNH 3 , is controlled by the flow meter, and the P NH 3 in the furnace is kept at about 15%.
Table 1 Ammonia partial pressure equilibrated to the phases in Fe–N system when the gaseous mixture of ammonia and its decomposition products are heated in the furnace Phase
a-Fe g-Fe X g ŽFe 4 N. ´-Fe 2 – 3 N
P NH 3rPH3r2 2
D m N ŽkJrmol.
PNH 3 Ž%.
6408C
6508C
6408C
6508C
6408C
6508C
- 31.5 31.5–35.3 35.3–45.2 ) 45.2
- 31.8 31.8–36.3 36.3–45.4 ) 45.4
- 0.0707 0.117–0.0707 0.433–0.117 ) 0.433
- 0.0658 0.118–0.0658 0.385–0.118 ) 0.385
- 4.3 6.8–4.3 20.1–6.8 ) 20.1
- 4.0 6.9–4.0 18.4–6.9 6.9 and ) 18.4
M. Hu et al.r Materials Letters 50 (2001) 225–229
When nitrocarburizing, the mole proportion of ammonia and methanol was 12:1, and the temperature was kept in the range of 640–6508C. From the provided equilibrium conditions, the compound layer can be obtained at the surface of specimen after nitriding or nitrocarburizing. It can be ensured that the maximum content of nitrogen is located in the outer austenite layer. The pure iron or 20 steel specimens were nitrided or nitorcarburized for 2 h, and then quenched directly into oil at room temperature and then transferred to a nitric acid salt for isothermal quenching for different times at 2258C, finally, taken out for air cooling. The metallographs were taken at Neophot II and SEM observation was performed with Hitachi S520. The observation samples were etched with nital.
4. Experimental results and discussion 4.1. The isothermal decomposition process of highnitrogen austenite Fig. 3 illustrates a typical micrograph of the quenched layer of austenite Ž20 steel is nitrocarburized for 3.5 h at 6508C.. It is shown that, in the austenite close to the matrix with low nitrogen content, martensite transformation takes place when quenching, and black needle tempered martensite appears after tempering. However, in the austenite close to the compound layer, a large amount of austenite can be preserved because its Ms is below
Fig. 3. The metallograph of nitrided layer of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2258C for 1 h.
227
Fig. 4. The metallograph of nitrided layer of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2008C for 8 h.
the room temperature. A small amount is decomposed after tempering for 1 h at 2258C, and the decomposition takes place firstly in the region close to the compound layer, where the nitrogen content is the maximum. With the continuing of tempering, the amount of decomposed austenite increases, and the decomposition range extends gradually to the inside of the austenite ŽFig. 4.. Moreover, the austenite can decompose completely after tempering for long enough time. 4.2. The characteristics of decomposition kinetics of high-nitrogen austenite The experimental result shows that the characteristics of decomposition kinetics of high-nitrogen austenite are as follows: 1. An apparent incubation period exists before high-nitrogen austenite decomposing. In the range of 200–2508C, the lower the temperature, the longer the incubation period ŽTable 2.. 2. The decomposition starts in the region with the maximum nitrogen content after the incubation period, and the amount of decomposition increases with the time. 3. The higher the nitrogen content in the austenite, the shorter the incubation period, and the faster the decomposition. 4. The decomposition can complete 100% after long enough time.
M. Hu et al.r Materials Letters 50 (2001) 225–229
228
Table 2 The incubation period and the ending time of isothermal decomposition at different temperature using 20 steel nitrocarburized at 6508C and quenched in oil Isothermal temperature Ž8C.
Incubation period Žh.
Finish time of decomposition Žh.
200 225 250
2 1.2 0.5
untested 10 5
4.3. The structural pattern of the decomposition product of the high-nitrogen austenite With the observation of optical microscopy ŽFig. 4. and scanning electron microscopy ŽFigs. 5 and 6., the characteristics of the isothermal decomposition product of the high-nitrogen austenite in the range of 200–2508C can be described as follows. Ž1. The decomposition product has the irregular shape, not like lath or needle . It is different from that of bainite or martensite in steel ŽFigs. 5 and 6.. Ž2. The decomposition product is very small, with the magnitude of about 0.1 mm, far less than that of normal martensite or bainite. The very small precipitates exist inside the decomposition product, and there is a certain orientation relationship between them and the matrix ŽFigs. 5 and 6.. Ž3. In the process of isothermal decomposition, the nuclei of decomposition product are formed near
Fig. 6. SEM microstructure of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2258C for 4 h. A: the decomposition product of austenite; B: tempered martensite; C: the residual austenite; D: the isolated decomposition product Žmarked with an arrow..
the austenite grain boundaries and inside the crystal grains. Inside the residual austenite, many fine decomposition products exist isolative, in which tiny precipitates with orientation can be observed. Ž4. With the increase of time, the size of single decomposition product increases difficulty, but the amounts of nuclei augments constantly. Ž5. The hardness of the decomposition product can be up to 900 HV, which is apparently higher than that of tempered martensite for the same steel. Ž6. This phenomenon can be seen in the nitrided layer of nitrided or nitrocarburized steel specimens at the temperature above the eutectoid point in the Fe–N–C ternary system.
5. Discussion
Fig. 5. SEM microstructure of pure iron nitrided at 6408C for 2 h and tempered at 2258C for 3.5 h. A: the isothermal decomposition product of the high-nitrogen austenite; B: the tempered martensite.
The structure has a promising application and should be studied deeply due to its high degree of dispersion and high hardness by the isothermal decomposition of high-nitrogen austenite. Bell et al. w5x observed the decomposition product tempered at 310–3508C after nitrocarburizing of
M. Hu et al.r Materials Letters 50 (2001) 225–229
low-carbon steel containing manganese ŽEn32. by TEM, and found that high-nitrogen austenite transformed to tiny Fe 4 N and a-Fe structure in the discrete way. Han and Song w6x found that the nitrogen atoms segregated during the tempering at 100– 2008C in the high-nitrogen austenite with 8.6 at.% N, and in which the short-range ordering of Fe 4 N-type or N–Fe–N²100: type was formed. Chen and Hu w7x studied the tempering transformation of quenched martensite of Fe–N alloy, but the problem of highnitrogen austenite decomposition was not involved. In summary, the isothermal decomposition process and structure pattern had not been studied deeply in the range of 200–3008C in high-nitrogen austenite. The observed phenomenon showed that the structure morphology and the transformation kinetics are apparently special during isothermal decomposition in the range of 200–2508C. It can be presumed from Fig. 5 that the segregation of nitrogen atoms or the precipitates of Fe 4 N may appear firstly in the austenite, then the transformation from g to a takes place near N-depleted region. This transformation can only be confined to very small range because of the limitation of nitrogen diffusion capacity at low temperature. As a result, the decomposition product cannot coarse significantly. It is probably because the decomposition process can only be continued by the increase of the decomposition nuclei. At present, the TEM observation and the study of transformation mechanism about the decomposition of high-nitrogen austenite are being carried out.
6. Conclusions The austenite layer can be obtained by nitriding above A 1 in the Fe–N system or nitrocarburizing above the eutectoid point in the Fe–N–C ternary system. The maximum nitrogen content in austenite close to the compound layer is up to 2.8 wt.% N. The initial investigation shows that after long-enough incubation period, the nuclei of decomposition product appear firstly at the juncture of compound and
229
austenite, where the nitrogen content is maximum. Afterwards, the tiny nuclei of decomposition product are formed at austenite grain boundaries. Inside the grain, the amount of nuclei of decomposition product increases with the time, but the sizes of those single nuclei are very small, and then a large amount of decomposed regions is formed. Then the decomposition regions connect with one another and the highly dispersed structure is formed. This structure is different from the needle shape of lower bainite and martensite. Moreover, its hardness is much higher than that of tempered martensite. With the increase of temperature, the incubation period is shortened and the whole process is quickened. Finally, the austenite can be decomposed completely. The structure pattern and kinetic characteristics of high-nitrogen austenite during the isothermal decomposition at medium temperature are different from those of normal lower bainite and martensite.
Acknowledgements This study was sponsored by the National Natural Science Foundation of the People’s Republic of China.
References w1x J. Pan, W. Zhu, M. Hu, L. Fang, Transactions of Metal Heat Treatment 12 Ž4. Ž1991. 9–16 Žin Chinese.. w2x L.S. Darken, R.W. Gurry, Physical Chemistry of Metals, MeGraw-Hiu Publishing, London, 1953. w3x K. Fueki, Journal of Institute of Metals 14 Ž2. Ž1995. 125–134. w4x M. Hu, J. Pan, The Principle of Chemical Heat Treatment of Iron and Steel, Shanghai Jiao Tong University Publisher, Shanghai, People’s Republic of China, 1996 Žin Chinese., March, 33 pp. w5x T. Bell, M. Kinal, G. Munstermann, Heat Treatment of Metals 14 Ž2. Ž1987. 47–51. w6x K.H. Han, Y.K. Song, Materials Science and Engineering A 260 Ž1999. 246–251. w7x S. Chen, C. Hu, Transactions of Metal Heat Treatment 20 Ž2. Ž1999. 8–13 Žin Chinese..
Materials Letters 50 Ž2001. 225–229 www.elsevier.comrlocatermatlet
A special phase transformation phenomenon in high-nitrogen austenite Mingjuan Hu a , Jiangsheng Pan a,) , Zuchang Zhu b, Qiu Chungcheng a a
Key Laboratory of High Temperature Materials and Tests of the Ministry of Education, Shanghai Jiao Tong UniÕersity, Shanghai 200030, People’s Republic of China b Department of Materials Science and Engineering, Shanghai UniÕersity of Engineering Science, Shanghai 200336, People’s Republic of China Received 25 March 2000; received in revised form 5 October 2000; accepted 5 October 2000
Abstract A special phenomenon of media temperature phase transformation is observed in high-nitrogen-containing austenite. The fine nuclei of decomposition form in the austenite grains and grain boundaries firstly, and the finer decomposition products can be found in the nucleus. Then the decomposition nucleus grows very slowly while the amount of the new phase nucleus increases constantly. Finally, the regions containing the decomposition nucleus are connected to one another and a high-dispersive morphology is formed. This structure is different not only from martensite but also from lower bainite with plate or needle structure. The hardness of the decomposition product reaches to 900 HV, which is higher than that of tempered martensite. The isothermal decomposition has a long incubation period. The amount of decomposition products increases with time and finally reaches to 100%, which is different from the bainite transformation. The observation shows that structural morphology and dynamics of the phase transformation are significantly different from bainite and martensite. q 2001 Elsevier Science B.V. All rights reserved. Keywords: High-nitrogen supercooled austenite; Phase transformation; Isothermal decomposition
1. Introduction An austenite layer can be seen in the nitrided case of pure iron or slow-carbon steel after nitriding or nitrocarburizing at 600–7008C w1x. From Fe–N or Fe–N–C phase diagram, it can be seen that the maximum content of nitrogen in the austenite layer is up to 2.8 wt.%. The optical microscopy and scanning electron microscopy were employed to study the tempering behavior of the high-nitrogen ) Corresponding author. Tel.: q86-21-62932563; fax: q86-2162932587. E-mail address: [email protected] ŽJ. Pan..
austenite at 200–2508C. The results show that the isothermal transformation process and result are different from those of bainite w1x. The first-stage of our research is reported in this paper.
2. The conditions of obtaining high-nitrogen austenite layer Fig. 1 shows the nitrogen chemical potential in gaseous phase, Dm N as a function of temperature in the Fe–N system. It was plotted according to the data tested by Darken and Gurry w2x, who adopted the method offered by Fueki w3x. These data were
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 2 2 9 - 4
M. Hu et al.r Materials Letters 50 (2001) 225–229
226
Fig. 1. Dm N as a function of temperature in the Fe–N system.
tested and verified by Hu and Pan w4x. It clearly shows the range of nitrogen chemical potential in Fe–N system in the state of equilibrium. If the atmosphere containing nitrogen is heated, the nitrogen chemical potential in gaseous phase is Dm N s RT 1n a n . If P s 1 atm is chosen as the standard condition, Dm N can be expressed as: D m N s RT ln K p
PNH 3 P H1.52
.
Ž 1.
Table 1 shows the equilibrium condition of gaseous phase in the Fe–N system, which was calculated on the basis of Fig. 1 and formula Ž1. when nitriding in the atmosphere of NH 3 and H 2 in the temperature range of 640–6508C. It can be seen that when nitriding at 6408C and 6508C, ammonia partial pressure inside the furnace,
Fig. 2. The schematic of the device for nitriding or nitrocarburizing. 1 – adjusting valve, 2 – flow meter, 3 – furnace for predecomposition of ammonia, 4 – mixer, 5 – tube, 6 – centrifugal fan, 7 – guide tank, 8 – nitriding tank, 9 – stabilizer and absorber, 10 – measurer for decomposition of ammonia.
P NH 3 is greater than 15%, the compound layer consisting of gX or ´ will appear at the surface of work-piece, and the subsurface is an austenite layer. The maximum content of nitrogen in the austenite, which is up to 2.8 wt.% N Žnitrided at 6508C., is located in the region that is adjacent to the compound layer, and the minimum content of nitrogen in the austenite is in the region that is adjacent to a-Fe. 3. Experimental procedures The nitriding or nitrocarburizing was processed in a small-scale well furnace shown in Fig. 2, whose size is B 300 = 350 mm. The rate of decomposition of ammonia, PNH 3 , is controlled by the flow meter, and the P NH 3 in the furnace is kept at about 15%.
Table 1 Ammonia partial pressure equilibrated to the phases in Fe–N system when the gaseous mixture of ammonia and its decomposition products are heated in the furnace Phase
a-Fe g-Fe X g ŽFe 4 N. ´-Fe 2 – 3 N
P NH 3rPH3r2 2
D m N ŽkJrmol.
PNH 3 Ž%.
6408C
6508C
6408C
6508C
6408C
6508C
- 31.5 31.5–35.3 35.3–45.2 ) 45.2
- 31.8 31.8–36.3 36.3–45.4 ) 45.4
- 0.0707 0.117–0.0707 0.433–0.117 ) 0.433
- 0.0658 0.118–0.0658 0.385–0.118 ) 0.385
- 4.3 6.8–4.3 20.1–6.8 ) 20.1
- 4.0 6.9–4.0 18.4–6.9 6.9 and ) 18.4
M. Hu et al.r Materials Letters 50 (2001) 225–229
When nitrocarburizing, the mole proportion of ammonia and methanol was 12:1, and the temperature was kept in the range of 640–6508C. From the provided equilibrium conditions, the compound layer can be obtained at the surface of specimen after nitriding or nitrocarburizing. It can be ensured that the maximum content of nitrogen is located in the outer austenite layer. The pure iron or 20 steel specimens were nitrided or nitorcarburized for 2 h, and then quenched directly into oil at room temperature and then transferred to a nitric acid salt for isothermal quenching for different times at 2258C, finally, taken out for air cooling. The metallographs were taken at Neophot II and SEM observation was performed with Hitachi S520. The observation samples were etched with nital.
4. Experimental results and discussion 4.1. The isothermal decomposition process of highnitrogen austenite Fig. 3 illustrates a typical micrograph of the quenched layer of austenite Ž20 steel is nitrocarburized for 3.5 h at 6508C.. It is shown that, in the austenite close to the matrix with low nitrogen content, martensite transformation takes place when quenching, and black needle tempered martensite appears after tempering. However, in the austenite close to the compound layer, a large amount of austenite can be preserved because its Ms is below
Fig. 3. The metallograph of nitrided layer of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2258C for 1 h.
227
Fig. 4. The metallograph of nitrided layer of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2008C for 8 h.
the room temperature. A small amount is decomposed after tempering for 1 h at 2258C, and the decomposition takes place firstly in the region close to the compound layer, where the nitrogen content is the maximum. With the continuing of tempering, the amount of decomposed austenite increases, and the decomposition range extends gradually to the inside of the austenite ŽFig. 4.. Moreover, the austenite can decompose completely after tempering for long enough time. 4.2. The characteristics of decomposition kinetics of high-nitrogen austenite The experimental result shows that the characteristics of decomposition kinetics of high-nitrogen austenite are as follows: 1. An apparent incubation period exists before high-nitrogen austenite decomposing. In the range of 200–2508C, the lower the temperature, the longer the incubation period ŽTable 2.. 2. The decomposition starts in the region with the maximum nitrogen content after the incubation period, and the amount of decomposition increases with the time. 3. The higher the nitrogen content in the austenite, the shorter the incubation period, and the faster the decomposition. 4. The decomposition can complete 100% after long enough time.
M. Hu et al.r Materials Letters 50 (2001) 225–229
228
Table 2 The incubation period and the ending time of isothermal decomposition at different temperature using 20 steel nitrocarburized at 6508C and quenched in oil Isothermal temperature Ž8C.
Incubation period Žh.
Finish time of decomposition Žh.
200 225 250
2 1.2 0.5
untested 10 5
4.3. The structural pattern of the decomposition product of the high-nitrogen austenite With the observation of optical microscopy ŽFig. 4. and scanning electron microscopy ŽFigs. 5 and 6., the characteristics of the isothermal decomposition product of the high-nitrogen austenite in the range of 200–2508C can be described as follows. Ž1. The decomposition product has the irregular shape, not like lath or needle . It is different from that of bainite or martensite in steel ŽFigs. 5 and 6.. Ž2. The decomposition product is very small, with the magnitude of about 0.1 mm, far less than that of normal martensite or bainite. The very small precipitates exist inside the decomposition product, and there is a certain orientation relationship between them and the matrix ŽFigs. 5 and 6.. Ž3. In the process of isothermal decomposition, the nuclei of decomposition product are formed near
Fig. 6. SEM microstructure of 20 steel nitrocarburized at 6508C for 3.5 h and tempered at 2258C for 4 h. A: the decomposition product of austenite; B: tempered martensite; C: the residual austenite; D: the isolated decomposition product Žmarked with an arrow..
the austenite grain boundaries and inside the crystal grains. Inside the residual austenite, many fine decomposition products exist isolative, in which tiny precipitates with orientation can be observed. Ž4. With the increase of time, the size of single decomposition product increases difficulty, but the amounts of nuclei augments constantly. Ž5. The hardness of the decomposition product can be up to 900 HV, which is apparently higher than that of tempered martensite for the same steel. Ž6. This phenomenon can be seen in the nitrided layer of nitrided or nitrocarburized steel specimens at the temperature above the eutectoid point in the Fe–N–C ternary system.
5. Discussion
Fig. 5. SEM microstructure of pure iron nitrided at 6408C for 2 h and tempered at 2258C for 3.5 h. A: the isothermal decomposition product of the high-nitrogen austenite; B: the tempered martensite.
The structure has a promising application and should be studied deeply due to its high degree of dispersion and high hardness by the isothermal decomposition of high-nitrogen austenite. Bell et al. w5x observed the decomposition product tempered at 310–3508C after nitrocarburizing of
M. Hu et al.r Materials Letters 50 (2001) 225–229
low-carbon steel containing manganese ŽEn32. by TEM, and found that high-nitrogen austenite transformed to tiny Fe 4 N and a-Fe structure in the discrete way. Han and Song w6x found that the nitrogen atoms segregated during the tempering at 100– 2008C in the high-nitrogen austenite with 8.6 at.% N, and in which the short-range ordering of Fe 4 N-type or N–Fe–N²100: type was formed. Chen and Hu w7x studied the tempering transformation of quenched martensite of Fe–N alloy, but the problem of highnitrogen austenite decomposition was not involved. In summary, the isothermal decomposition process and structure pattern had not been studied deeply in the range of 200–3008C in high-nitrogen austenite. The observed phenomenon showed that the structure morphology and the transformation kinetics are apparently special during isothermal decomposition in the range of 200–2508C. It can be presumed from Fig. 5 that the segregation of nitrogen atoms or the precipitates of Fe 4 N may appear firstly in the austenite, then the transformation from g to a takes place near N-depleted region. This transformation can only be confined to very small range because of the limitation of nitrogen diffusion capacity at low temperature. As a result, the decomposition product cannot coarse significantly. It is probably because the decomposition process can only be continued by the increase of the decomposition nuclei. At present, the TEM observation and the study of transformation mechanism about the decomposition of high-nitrogen austenite are being carried out.
6. Conclusions The austenite layer can be obtained by nitriding above A 1 in the Fe–N system or nitrocarburizing above the eutectoid point in the Fe–N–C ternary system. The maximum nitrogen content in austenite close to the compound layer is up to 2.8 wt.% N. The initial investigation shows that after long-enough incubation period, the nuclei of decomposition product appear firstly at the juncture of compound and
229
austenite, where the nitrogen content is maximum. Afterwards, the tiny nuclei of decomposition product are formed at austenite grain boundaries. Inside the grain, the amount of nuclei of decomposition product increases with the time, but the sizes of those single nuclei are very small, and then a large amount of decomposed regions is formed. Then the decomposition regions connect with one another and the highly dispersed structure is formed. This structure is different from the needle shape of lower bainite and martensite. Moreover, its hardness is much higher than that of tempered martensite. With the increase of temperature, the incubation period is shortened and the whole process is quickened. Finally, the austenite can be decomposed completely. The structure pattern and kinetic characteristics of high-nitrogen austenite during the isothermal decomposition at medium temperature are different from those of normal lower bainite and martensite.
Acknowledgements This study was sponsored by the National Natural Science Foundation of the People’s Republic of China.
References w1x J. Pan, W. Zhu, M. Hu, L. Fang, Transactions of Metal Heat Treatment 12 Ž4. Ž1991. 9–16 Žin Chinese.. w2x L.S. Darken, R.W. Gurry, Physical Chemistry of Metals, MeGraw-Hiu Publishing, London, 1953. w3x K. Fueki, Journal of Institute of Metals 14 Ž2. Ž1995. 125–134. w4x M. Hu, J. Pan, The Principle of Chemical Heat Treatment of Iron and Steel, Shanghai Jiao Tong University Publisher, Shanghai, People’s Republic of China, 1996 Žin Chinese., March, 33 pp. w5x T. Bell, M. Kinal, G. Munstermann, Heat Treatment of Metals 14 Ž2. Ž1987. 47–51. w6x K.H. Han, Y.K. Song, Materials Science and Engineering A 260 Ž1999. 246–251. w7x S. Chen, C. Hu, Transactions of Metal Heat Treatment 20 Ž2. Ž1999. 8–13 Žin Chinese..