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Precipitation in high strength low alloy (HSLA) steel: a TEM study
Materials Science and Engineering A323 (2002) 285– 292 www.elsevier.com/locate/msea
Precipitation in high strength low alloy (HSLA) steel: a TEM study S.K. Mishra(Pathak) *, S. Das, S. Ranganathan National Metallurgical Laboratory, Jamshedpur 831007, India Received 22 March 2000; received in revised form 28 March 2001
Abstract The HSLA-100 steels contain various alloying additions such as Cr, Mn, Mo, Cu and Ni apart from niobium and carbon. Precipitation of carbonitrides in these steels is complex in nature due to several elements with affinity for carbon and nitrogen. This phenomenon plays a significant role in the microstructure evolution of these steels during thermomechanical processing. The precipitates formed in a HSLA-100 steel containing Cr0.58, Mn0.87, Mo0.57, Nb0.032, Ni3.54, Cu1.98, C0.02 at different temperatures were studied using transmission electron microscope. The selected area diffraction and EDS analysis were used to identify the precipitates. The investigation showed that several complex precipitates were present in the steel. The type of precipitates and their morphology and the relevance of these precipitates to the design of HSLA steels are discussed in this communication. © 2002 Elsevier Science B.V. All rights reserved. Keywords: HSLA steel; Precipitation; TEM
1. Introduction Precipitation of carbonitrides is an important phenomenon influencing the microstructure of high strength low alloy (HSLA) steels and, therefore, the mechanical properties. Considerable efforts are expended the world over to understand the precipitation phenomenon in these steels. This is crucial for a successful design of the alloys and of the thermomechanical treatment to be adopted in order to achieve the desired mechanical strength. Several studies have been reported in literature on the prediction of the chemistry and quantity of the precipitates formed in these steels at different temperature [1 – 8]. Attempts have also been made to characterise these precipitates using optical microscopy and other techniques [9 –13]. Ti, Nb, and V are most commonly used as the alloying
* Corresponding author. E-mail address: [email protected] (S.K. Mishra(Pathak)).
elements to precipitate as carbonitrides and induce grain refinement in these steels. Apart from these other elements such as Mo, Cu, Ni are added to these steels making the chemistry complex. In addition to these, Al is present in these steels resulting from the steel making process. The presence of these alloying elements makes the precipitation behaviour very complex. A comprehensive understanding of the precipitation behaviour is essential to achieve the desired properties. HSLA-80 and 100 steels are being developed applications in naval structures. These steels contain various alloying elements such as Cr, Mn, Mo, Cu and Ni apart from Nb and C and other trace elements. The copper is added for precipitation strengthening whereas few have strong affinity for carbon and nitrogen and to form their carbonitrides. Hence, the precipitation in these steels is complex in nature. Microstructural evolution during austenite decomposition in HSLA-80 steels at different temperature has been reported in literature [14,15]. The complex precipitation in HSLA-100 has not been explored much.
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 8 2 - X
S.K. Mishra(Pathak) et al. / Materials Science and Engineering A323 (2002) 285–292
286 Table 1 Composition of steel C
Mn
P
S
Si
Cu
Ni
Cr
Mo
Al
Nb
0.02
0.87
0.003
0.006
0.25
1.98
3.54
0.58
0.57
0.038
0.032
Fig. 1. TEM of extraction replica of as-received sample, showing rectangular rod type precipitates containing Al, Nb, Mo.
1.1. Complex precipitation The HSLA steels contain several alloying elements, which can form various types of precipitates. The elements, Mo, Ti, V and Nb have an affinity for the
formation of carbonitrides. Therefore, the cationic sublattice of the carbonitride precipitates will contain more than one alloying element. However, the studies predicting the precipitation phenomena in these steels have usually adopted a simple picture of the cationic sublattice containing only one or two of the major alloying elements, usually chosen from Ti, V and Nb. It will be very useful to actually characterise these precipitates and analyse the chemistry of these and to explore the possibility of the other alloying elements occurring in the cationic sublattice. Formation of intermetallics, the precipitation of aluminium nitride and the presence of Fe in the carbonitride phase are other related phenomena, which can influence the precipitation characteristics of these steels. Hence an analysis of these also is important in this context. Studies reported in the literature [9– 13] have usually described the precipitation behaviour in samples aged at low temperatures prepared from hot rolled steel. These studies show that the precipitates are complex in chemistry containing several alloying elements in the cationic sublattice. These studies used samples treated at the respective temperature for a short interval of time. The present investigation was carried out to study the chemistry of the precipitates at equilibrium with the matrix at different temperature from 1000–1200 °C.
Fig. 2. TEM of extraction replica of as-received sample showing (a) AlN precipitates and (b) corresponding SAED pattern.
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287
Fig. 4. TEM of extraction replica of 1000 °C treated sample showing different precipitates.
Fig. 3. TEM of thin foil of as-received sample showing (a) lath type structure and (b) precipitates.
2. Experimental details The steel under study was obtained from the office of Naval Research, USA. The steel used in the present investigation was melted at 1700 °C. It was vacuum degassed, followed by argon stinting and calcium silicide injection. The steel was bottom poured from the degas ladle and cast into ingots. The slabs were reheated and soaked for 1 h at 1150 °C and taken to a roughening mill at 1230– 1260 °C. The 2 × 96×340 in. size pieces were austenised at 900 °C for 2 h and water quenched. The plates were shipped as water quenched and were used in this study. The composition of the steel is given in Table 1. Specimens were cut from bulk into 15× 10 ×8 mm sizes for heat treatment. The samples were equilibrated for 1 h each at different temperatures ranging from 1000
Fig. 5. TEM of extraction replica of 1000 °C treated sample showing (a) MoTiC2 precipitates and (b) corresponding SAED pattern.
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were carried out at 200 kV using a Philips CM200 TEM with EDAX DX-4 EDS spectrometer for elemental analysis. The microstructural characterisation was carried out in bright field mode. The selected area diffraction and in-situ EDS analysis were carried out to identify the precipitates. EDS analysis obtained were the representative of the elements present and were qualitative.
3. Results and discussion
Fig. 6. TEM of thin foil of 1100 °C treated sample showing lathboundary precipitates.
to 1200 °C under argon (IOLAR-1) atmosphere and were quenched in water. The samples were prepared by the carbon extraction replica method on metallographic samples for analysing the precipitates. Replicas were taken out carefully on copper grid by deep etching. A few thin foil specimens were also made. Thin foil specimens for Transmission Electron Microscope (TEM) were prepared by punching 3 mm discs from the wafers cut from heat treated samples. Wafers were mechanically ground by hand to around 100 mm before punching. These discs were twin jet electropolished to electron transparency in a mixture of perchloric and glacial acetic acid. TEM studies
The received samples showed three major types of precipitates. EDS indicated that the rod shaped precipitates (Fig. 1) in these samples contained Al, Nb, Mo and Cr in the ratio of 55:17:13:3.5, approximately. In these studies, the analysis of the anionic sublattice could not be established directly since the EDS is not sensitive to the analysis of C and N, which are the primary constituents of the anionic sublattice in these precipitates. SAED analysis of these precipitates could not confirm the identity of the particle. It is possible that these are mixture of (Al, Nb) and (Al, Mo) carbides or the complex carbides containing Al, Nb, Mo. Presence of AlN precipitates of sizes in the range of 0.2–0.3 mm were found and could be confirmed by SAED and EDS analysis.The SAED pattern analysis show the hexagonal phase of AlN with lattice parameter as a=3.110 A, b/a= 1.0, c/a= 1.6 and k= 120. EDS show presence of Al only. The received samples show the presence of AlN precipitates in circular and rectangular morphologies. The microstructure and SAED pattern of one such precipitate are given in Fig. 2. In addition to these, particles rich in Nb were also detected.
Fig. 7. TEM of extraction replica of 1100 °C treated sample showing (a) Al5Fe2 precipitates and (b) corresponding SAED pattern.
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289
Fig. 9. TEM of extraction replica of 1150 °C treated sample showing (a) Al – Fe precipitates and (b) corresponding SAED pattern.
Fig. 8. TEM of thin foil specimen of 1100 °C treated sample showing (a) lath structure and (b) fine precipitate and dislocations.
These contained Fe and Ti also. Though not reported in Table 1, trace amounts of Ti were present in this steel. Some precipitates containing Nb as major constituent along with Mo were also observed. The SAED of the Nb rich precipitates matched well with that of Niobium carbide (NbC). Mo may be forming complex carbide and is in solution of NbC and thereby forming a (Nb, Mo)C. The close similarity between the patterns of carbide and carbonitride preclude any conclusive assessment of the chemistry of the precipitate. The microstructure of the thin foil specimen of the received samples is shown in Fig. 3. A lath-type structure is seen and also rectangular and circular type precipitates are clearly visible. Fig. 4 show the microstructure of the precipitates for the sample quenched from 1000 °C. The precipi-
tates were found to be in the size range of 0.1–0.3 mm. It was observed that the sample quenched from 1000 °C has different precipitates. (MoTi)C2 precipitate, cubic a= 4.313 A, was confirmed through SAED pattern analysis. The EDS show the composition as Al0.4Nb0.4Mo47.7Ti42.9Cr1.5Mn1.3Fe0.7, all weights are given in atom percent. In all the investigations EDS results were taken a qualitative one. Mo and Ti were nearly in the ratio 1:1 in EDS analysis. This is consistent with the analysis of the SAED pattern which showed these precipitates to be (MoTi)C2. (MoTi)C2 precipitates were in the range of 0.5– 0.6 mm in size and were rectangular and distorted circular in shape. Fig. 5 shows the typical microstructure and indexed SAED pattern of a (MoTi)C2 precipitate. Many precipitates of AlN in the range of 0.2–0.25 mm were also seen in the extraction replica. The EDS analysis of these precipitates shows the presence of Al only.
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Fig. 10. TEM of extraction replica of 1200 °C treated sample showing fine precipitates (a, b).
The extraction replica for the samples quenched from 1100 °C showed the presence of AlN. Fine precipitates of the order of 100 nm near the lath boundary were observed Fig. 6. They were very fine and hence no SAED pattern of these could be obtained. The EDS analysis of the cluster of precipitates showed the presence of Mo, Ti, Nb, Al, Cr, Fe. It is possible that the precipitate containing Mo and Ti which were observed at 1000 °C are gradually going into solution and hence they are becoming finer too. Beside these, precipitates of intermetallic phase Al5Fe2, tetragonal a = 7.675 A, b/a =0.834, c/a = 0.547, were found. One such precipitate along with the SAED pattern is shown in Fig. 7. These were about 0.3 mm in size and have elliptical and rectangular morphology.
Fig. 11. TEM of extraction replica of 1200 °C treated sample showing (a) Mo3N2 precipitates and (b) corresponding SAED pattern.
The EDS analysis showed the composition as Al88.6 Nb0.6Mo3.2Ti0.5Cr1.5Mn0.5Fe4.9Ni0.3. Lath martensite type microstructure was seen in thin foil specimens. Very fine precipitates and high density of dislocations were also seen and are shown in the Fig. 8. Al –Fe precipitates, cubic a=5.8 A, were found in the samples prepared from 1150 °C. Fig. 9 shows the microstructure and SAED pattern of the precipitate. EDS analysis shows the qualitative composition as Al50.6Nb0.5Mo1.8Cr0.6Fe41.4Ni2.0. Similar compositions were also obtained for different precipitates whose SAED patterns were matching with AlFe phase. SAED analysis and EDS analysis in terms of Al and Fe ratio were very near. Hence it was concluded that these were AlFe phase precipitates. These precipitates were found to
S.K. Mishra(Pathak) et al. / Materials Science and Engineering A323 (2002) 285–292
Fig. 12. TEM of thin foil of 1200 °C treated sample showing (a) lath structure and (b) very fine precipitates and dislocation.
be of the order of 0.5– 1.0 mm in size. A few precipitates with Fe as major constituent were also found whose SAED pattern matches with Fe7C3. These particles contained Fe and Cr in the ration of 67:9. It may be noted that formation of similar precipitates with varying ratio in the cationic sublattice in the FeCrC system has been already reported. Some AlN precipitates of 0.1–0.2 mm sizes were also observed. Replicas prepared from the samples quenched from 1200 °C showed fewer precipitation. AlN precipitates were not observed in this case. Very fine precipitates of size ranging between 10– 100 nm were seen (Fig. 10). Again SAED pattern could not be obtained from these particles. The EDS analysis showed the presence of
291
Al6.6Nb5.1Mo2.9Ti4.5Cr48.4Mn10.6Fe23.8Ni6.1. Precipitates of Mo3N2 phase, cubic a= 4.650 A were found in these samples. The SAED pattern matched well with the cubic Mo3N2 phase but it also nearly matched with hexagonal Mo7Nb3C4. The EDS analysis showed the presence of Mo79Nb5.1Al3.6Ti0.5Cr7.4Mn0.9Fe2.9Ni0.7. On the basis of SAED better matching it was deduced that these precipitates are cubic Molybdenum nitride with niobium as solid solution in the matrix. However, the presence MoNb complex carbonitrides or carbides can not be ruled out. Fig. 11 shows the microstructure and corresponding SAED pattern. Thin foil specimen showed the finer precipitates and presence of dislocations (Fig. 12). It is clear that precipitates are fewer for the sample prepared from 1200 °C compared with 1100 °C or as received samples. The TEM studies reported here show that the precipitates formed are complex in nature containing several alloying elements in the cationic sublattice of the precipitates. Since Mo and Cr have an affinity to form carbides and nitrides, their presence in the precipitates studied is not surprising. It is also apparent that different types of the precipitates with different chemistry and crystallographic structure are formed in these steels. In addition to these, intermetallic compounds and simple precipitates such as Fe7C3 have been detected. Therefore, a simple picture of precipitation in these steels focussing only on the niobium or titanium carbonitrides is not adequate to represent the precipitation behaviour in this class of steel. Any realistic modelling and prediction of the precipitation behaviour during thermomechanical treatment has to take into consideration the complex precipitation scenario as reported here. The Nb rich precipitates detected in the as received sample have not been detected in the samples quenched from higher temperatures. This is consistent with the predictions made on the solubility of this precipitate in the steel. AlN has been detected in all the samples except for the samples quenched from 1200 °C AlN precipitates are expected to dissolve in the matrix at about 900 °C. Their presence in these steels even at higher temperature calls for further investigation. These samples were soaked at prescribed temperature only for 1 h. A longer soaking time can possibly ensure that these particles were dissolved completely. It is also possible that these particles were precipitated during quenching from high temperatures.
4. Conclusion Precipitates formed in a HSLA steel quenched from various temperatures were studied using TEM. The precipitates formed are complex in nature containing several alloying elements in the cationic sublattice. In addition to carbonitrides, intermetallic compounds were also detected in the steel. The studies show that a simple
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picture of the precipitation behaviour is not adequate to describe the precipitation phenomena in these steels. The complex precipitation behaviour has to be taken into consideration while designing the alloy and the thermomechanical treatment.
Acknowledgements The authors are grateful to the office of the Naval Research, USA for financial assistance to carry out this work under grant number N00014-95-1-0015.
References [1] M Grujicic, A.M Sarosiek, L Kaufmann, W.S. Owen, Calphad 9 (1985) 117 – 128. [2] M Grujicic, L Kaufmann, W.S. Owen, Calphad 10 (1986) 37 – 47.
[3] M Grujicic, I.J. Wang, W.S. Owen, Calphad 12 (1988) 261 – 275. [4] P.R. Rios, Mat. Sci. Technol. 4 (1988) 324 – 327. [5] P.R. Rios, Mat. Sci. Eng. 142A (1991) 87 – 94. [6] Heilong Zou, J.S. Kirkaldy, Metall. Trans. 22A (1991) 1511 – 1524. [7] Satoshi Akamatsu, Takehide Senuma, Mitsuhiro Hasebe, ISIJ Int. 32 (1992) 275 – 282. [8] Shuji Okaguchi, Tamotsu Hashimoto, ISIJ Int. 32 (1992) 283 – 290. [9] R. Varughese, P.R. Howell, Mat. Character. 30 (1993) 261 – 267. [10] M. Mujahid, A.K. Lis, C.I. Garcia, A.J. de Ardo, Key Eng. Mat, vols. 84 – 85, Trans. Tech. Publications, Switzerland, 1993, pp. 209 – 236. [11] K.A. el-Fawakhy, Md.F. Mekkawy, M.L. Mishreky, M.M. Eissa, ISIJ Int. 31 (9) (1991) 1020 – 1025. [12] Wenpu Dong, Zuebo Fu, Lang Yang, Mat. Character. 37 (1996) 169 – 175. [13] C.O.I. Emenike, J.C. Billington, Mat. Sci. Tech. 5 (1989) 566 – 574. [14] S.W. Thompson, D.J. Colvin, G. Krauss, Metal. Meter. Trans. 27A (1996) 1557 – 1571. [15] S.W. Thompson, G. Krauss, Metal. Meter. Trans. 27A (1996) 1573 – 1588.
Precipitation in high strength low alloy (HSLA) steel: a TEM study S.K. Mishra(Pathak) *, S. Das, S. Ranganathan National Metallurgical Laboratory, Jamshedpur 831007, India Received 22 March 2000; received in revised form 28 March 2001
Abstract The HSLA-100 steels contain various alloying additions such as Cr, Mn, Mo, Cu and Ni apart from niobium and carbon. Precipitation of carbonitrides in these steels is complex in nature due to several elements with affinity for carbon and nitrogen. This phenomenon plays a significant role in the microstructure evolution of these steels during thermomechanical processing. The precipitates formed in a HSLA-100 steel containing Cr0.58, Mn0.87, Mo0.57, Nb0.032, Ni3.54, Cu1.98, C0.02 at different temperatures were studied using transmission electron microscope. The selected area diffraction and EDS analysis were used to identify the precipitates. The investigation showed that several complex precipitates were present in the steel. The type of precipitates and their morphology and the relevance of these precipitates to the design of HSLA steels are discussed in this communication. © 2002 Elsevier Science B.V. All rights reserved. Keywords: HSLA steel; Precipitation; TEM
1. Introduction Precipitation of carbonitrides is an important phenomenon influencing the microstructure of high strength low alloy (HSLA) steels and, therefore, the mechanical properties. Considerable efforts are expended the world over to understand the precipitation phenomenon in these steels. This is crucial for a successful design of the alloys and of the thermomechanical treatment to be adopted in order to achieve the desired mechanical strength. Several studies have been reported in literature on the prediction of the chemistry and quantity of the precipitates formed in these steels at different temperature [1 – 8]. Attempts have also been made to characterise these precipitates using optical microscopy and other techniques [9 –13]. Ti, Nb, and V are most commonly used as the alloying
* Corresponding author. E-mail address: [email protected] (S.K. Mishra(Pathak)).
elements to precipitate as carbonitrides and induce grain refinement in these steels. Apart from these other elements such as Mo, Cu, Ni are added to these steels making the chemistry complex. In addition to these, Al is present in these steels resulting from the steel making process. The presence of these alloying elements makes the precipitation behaviour very complex. A comprehensive understanding of the precipitation behaviour is essential to achieve the desired properties. HSLA-80 and 100 steels are being developed applications in naval structures. These steels contain various alloying elements such as Cr, Mn, Mo, Cu and Ni apart from Nb and C and other trace elements. The copper is added for precipitation strengthening whereas few have strong affinity for carbon and nitrogen and to form their carbonitrides. Hence, the precipitation in these steels is complex in nature. Microstructural evolution during austenite decomposition in HSLA-80 steels at different temperature has been reported in literature [14,15]. The complex precipitation in HSLA-100 has not been explored much.
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 8 2 - X
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286 Table 1 Composition of steel C
Mn
P
S
Si
Cu
Ni
Cr
Mo
Al
Nb
0.02
0.87
0.003
0.006
0.25
1.98
3.54
0.58
0.57
0.038
0.032
Fig. 1. TEM of extraction replica of as-received sample, showing rectangular rod type precipitates containing Al, Nb, Mo.
1.1. Complex precipitation The HSLA steels contain several alloying elements, which can form various types of precipitates. The elements, Mo, Ti, V and Nb have an affinity for the
formation of carbonitrides. Therefore, the cationic sublattice of the carbonitride precipitates will contain more than one alloying element. However, the studies predicting the precipitation phenomena in these steels have usually adopted a simple picture of the cationic sublattice containing only one or two of the major alloying elements, usually chosen from Ti, V and Nb. It will be very useful to actually characterise these precipitates and analyse the chemistry of these and to explore the possibility of the other alloying elements occurring in the cationic sublattice. Formation of intermetallics, the precipitation of aluminium nitride and the presence of Fe in the carbonitride phase are other related phenomena, which can influence the precipitation characteristics of these steels. Hence an analysis of these also is important in this context. Studies reported in the literature [9– 13] have usually described the precipitation behaviour in samples aged at low temperatures prepared from hot rolled steel. These studies show that the precipitates are complex in chemistry containing several alloying elements in the cationic sublattice. These studies used samples treated at the respective temperature for a short interval of time. The present investigation was carried out to study the chemistry of the precipitates at equilibrium with the matrix at different temperature from 1000–1200 °C.
Fig. 2. TEM of extraction replica of as-received sample showing (a) AlN precipitates and (b) corresponding SAED pattern.
S.K. Mishra(Pathak) et al. / Materials Science and Engineering A323 (2002) 285–292
287
Fig. 4. TEM of extraction replica of 1000 °C treated sample showing different precipitates.
Fig. 3. TEM of thin foil of as-received sample showing (a) lath type structure and (b) precipitates.
2. Experimental details The steel under study was obtained from the office of Naval Research, USA. The steel used in the present investigation was melted at 1700 °C. It was vacuum degassed, followed by argon stinting and calcium silicide injection. The steel was bottom poured from the degas ladle and cast into ingots. The slabs were reheated and soaked for 1 h at 1150 °C and taken to a roughening mill at 1230– 1260 °C. The 2 × 96×340 in. size pieces were austenised at 900 °C for 2 h and water quenched. The plates were shipped as water quenched and were used in this study. The composition of the steel is given in Table 1. Specimens were cut from bulk into 15× 10 ×8 mm sizes for heat treatment. The samples were equilibrated for 1 h each at different temperatures ranging from 1000
Fig. 5. TEM of extraction replica of 1000 °C treated sample showing (a) MoTiC2 precipitates and (b) corresponding SAED pattern.
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were carried out at 200 kV using a Philips CM200 TEM with EDAX DX-4 EDS spectrometer for elemental analysis. The microstructural characterisation was carried out in bright field mode. The selected area diffraction and in-situ EDS analysis were carried out to identify the precipitates. EDS analysis obtained were the representative of the elements present and were qualitative.
3. Results and discussion
Fig. 6. TEM of thin foil of 1100 °C treated sample showing lathboundary precipitates.
to 1200 °C under argon (IOLAR-1) atmosphere and were quenched in water. The samples were prepared by the carbon extraction replica method on metallographic samples for analysing the precipitates. Replicas were taken out carefully on copper grid by deep etching. A few thin foil specimens were also made. Thin foil specimens for Transmission Electron Microscope (TEM) were prepared by punching 3 mm discs from the wafers cut from heat treated samples. Wafers were mechanically ground by hand to around 100 mm before punching. These discs were twin jet electropolished to electron transparency in a mixture of perchloric and glacial acetic acid. TEM studies
The received samples showed three major types of precipitates. EDS indicated that the rod shaped precipitates (Fig. 1) in these samples contained Al, Nb, Mo and Cr in the ratio of 55:17:13:3.5, approximately. In these studies, the analysis of the anionic sublattice could not be established directly since the EDS is not sensitive to the analysis of C and N, which are the primary constituents of the anionic sublattice in these precipitates. SAED analysis of these precipitates could not confirm the identity of the particle. It is possible that these are mixture of (Al, Nb) and (Al, Mo) carbides or the complex carbides containing Al, Nb, Mo. Presence of AlN precipitates of sizes in the range of 0.2–0.3 mm were found and could be confirmed by SAED and EDS analysis.The SAED pattern analysis show the hexagonal phase of AlN with lattice parameter as a=3.110 A, b/a= 1.0, c/a= 1.6 and k= 120. EDS show presence of Al only. The received samples show the presence of AlN precipitates in circular and rectangular morphologies. The microstructure and SAED pattern of one such precipitate are given in Fig. 2. In addition to these, particles rich in Nb were also detected.
Fig. 7. TEM of extraction replica of 1100 °C treated sample showing (a) Al5Fe2 precipitates and (b) corresponding SAED pattern.
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289
Fig. 9. TEM of extraction replica of 1150 °C treated sample showing (a) Al – Fe precipitates and (b) corresponding SAED pattern.
Fig. 8. TEM of thin foil specimen of 1100 °C treated sample showing (a) lath structure and (b) fine precipitate and dislocations.
These contained Fe and Ti also. Though not reported in Table 1, trace amounts of Ti were present in this steel. Some precipitates containing Nb as major constituent along with Mo were also observed. The SAED of the Nb rich precipitates matched well with that of Niobium carbide (NbC). Mo may be forming complex carbide and is in solution of NbC and thereby forming a (Nb, Mo)C. The close similarity between the patterns of carbide and carbonitride preclude any conclusive assessment of the chemistry of the precipitate. The microstructure of the thin foil specimen of the received samples is shown in Fig. 3. A lath-type structure is seen and also rectangular and circular type precipitates are clearly visible. Fig. 4 show the microstructure of the precipitates for the sample quenched from 1000 °C. The precipi-
tates were found to be in the size range of 0.1–0.3 mm. It was observed that the sample quenched from 1000 °C has different precipitates. (MoTi)C2 precipitate, cubic a= 4.313 A, was confirmed through SAED pattern analysis. The EDS show the composition as Al0.4Nb0.4Mo47.7Ti42.9Cr1.5Mn1.3Fe0.7, all weights are given in atom percent. In all the investigations EDS results were taken a qualitative one. Mo and Ti were nearly in the ratio 1:1 in EDS analysis. This is consistent with the analysis of the SAED pattern which showed these precipitates to be (MoTi)C2. (MoTi)C2 precipitates were in the range of 0.5– 0.6 mm in size and were rectangular and distorted circular in shape. Fig. 5 shows the typical microstructure and indexed SAED pattern of a (MoTi)C2 precipitate. Many precipitates of AlN in the range of 0.2–0.25 mm were also seen in the extraction replica. The EDS analysis of these precipitates shows the presence of Al only.
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Fig. 10. TEM of extraction replica of 1200 °C treated sample showing fine precipitates (a, b).
The extraction replica for the samples quenched from 1100 °C showed the presence of AlN. Fine precipitates of the order of 100 nm near the lath boundary were observed Fig. 6. They were very fine and hence no SAED pattern of these could be obtained. The EDS analysis of the cluster of precipitates showed the presence of Mo, Ti, Nb, Al, Cr, Fe. It is possible that the precipitate containing Mo and Ti which were observed at 1000 °C are gradually going into solution and hence they are becoming finer too. Beside these, precipitates of intermetallic phase Al5Fe2, tetragonal a = 7.675 A, b/a =0.834, c/a = 0.547, were found. One such precipitate along with the SAED pattern is shown in Fig. 7. These were about 0.3 mm in size and have elliptical and rectangular morphology.
Fig. 11. TEM of extraction replica of 1200 °C treated sample showing (a) Mo3N2 precipitates and (b) corresponding SAED pattern.
The EDS analysis showed the composition as Al88.6 Nb0.6Mo3.2Ti0.5Cr1.5Mn0.5Fe4.9Ni0.3. Lath martensite type microstructure was seen in thin foil specimens. Very fine precipitates and high density of dislocations were also seen and are shown in the Fig. 8. Al –Fe precipitates, cubic a=5.8 A, were found in the samples prepared from 1150 °C. Fig. 9 shows the microstructure and SAED pattern of the precipitate. EDS analysis shows the qualitative composition as Al50.6Nb0.5Mo1.8Cr0.6Fe41.4Ni2.0. Similar compositions were also obtained for different precipitates whose SAED patterns were matching with AlFe phase. SAED analysis and EDS analysis in terms of Al and Fe ratio were very near. Hence it was concluded that these were AlFe phase precipitates. These precipitates were found to
S.K. Mishra(Pathak) et al. / Materials Science and Engineering A323 (2002) 285–292
Fig. 12. TEM of thin foil of 1200 °C treated sample showing (a) lath structure and (b) very fine precipitates and dislocation.
be of the order of 0.5– 1.0 mm in size. A few precipitates with Fe as major constituent were also found whose SAED pattern matches with Fe7C3. These particles contained Fe and Cr in the ration of 67:9. It may be noted that formation of similar precipitates with varying ratio in the cationic sublattice in the FeCrC system has been already reported. Some AlN precipitates of 0.1–0.2 mm sizes were also observed. Replicas prepared from the samples quenched from 1200 °C showed fewer precipitation. AlN precipitates were not observed in this case. Very fine precipitates of size ranging between 10– 100 nm were seen (Fig. 10). Again SAED pattern could not be obtained from these particles. The EDS analysis showed the presence of
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Al6.6Nb5.1Mo2.9Ti4.5Cr48.4Mn10.6Fe23.8Ni6.1. Precipitates of Mo3N2 phase, cubic a= 4.650 A were found in these samples. The SAED pattern matched well with the cubic Mo3N2 phase but it also nearly matched with hexagonal Mo7Nb3C4. The EDS analysis showed the presence of Mo79Nb5.1Al3.6Ti0.5Cr7.4Mn0.9Fe2.9Ni0.7. On the basis of SAED better matching it was deduced that these precipitates are cubic Molybdenum nitride with niobium as solid solution in the matrix. However, the presence MoNb complex carbonitrides or carbides can not be ruled out. Fig. 11 shows the microstructure and corresponding SAED pattern. Thin foil specimen showed the finer precipitates and presence of dislocations (Fig. 12). It is clear that precipitates are fewer for the sample prepared from 1200 °C compared with 1100 °C or as received samples. The TEM studies reported here show that the precipitates formed are complex in nature containing several alloying elements in the cationic sublattice of the precipitates. Since Mo and Cr have an affinity to form carbides and nitrides, their presence in the precipitates studied is not surprising. It is also apparent that different types of the precipitates with different chemistry and crystallographic structure are formed in these steels. In addition to these, intermetallic compounds and simple precipitates such as Fe7C3 have been detected. Therefore, a simple picture of precipitation in these steels focussing only on the niobium or titanium carbonitrides is not adequate to represent the precipitation behaviour in this class of steel. Any realistic modelling and prediction of the precipitation behaviour during thermomechanical treatment has to take into consideration the complex precipitation scenario as reported here. The Nb rich precipitates detected in the as received sample have not been detected in the samples quenched from higher temperatures. This is consistent with the predictions made on the solubility of this precipitate in the steel. AlN has been detected in all the samples except for the samples quenched from 1200 °C AlN precipitates are expected to dissolve in the matrix at about 900 °C. Their presence in these steels even at higher temperature calls for further investigation. These samples were soaked at prescribed temperature only for 1 h. A longer soaking time can possibly ensure that these particles were dissolved completely. It is also possible that these particles were precipitated during quenching from high temperatures.
4. Conclusion Precipitates formed in a HSLA steel quenched from various temperatures were studied using TEM. The precipitates formed are complex in nature containing several alloying elements in the cationic sublattice. In addition to carbonitrides, intermetallic compounds were also detected in the steel. The studies show that a simple
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picture of the precipitation behaviour is not adequate to describe the precipitation phenomena in these steels. The complex precipitation behaviour has to be taken into consideration while designing the alloy and the thermomechanical treatment.
Acknowledgements The authors are grateful to the office of the Naval Research, USA for financial assistance to carry out this work under grant number N00014-95-1-0015.
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