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An investigation of the precipitation process during ageing of a sintered low-carbon copper steel
Pon der Technology-Elsevier Sequoia SA .. Lausanne-Printed in the Netherlands
237
An Investigation of the Precipitation Process During Ageing of a Sintered Low-Carbon Copper Steel A- STERN and L . LEVIN Department of Materials Engineering, Technion-Israel Institute
of Technology, Haifa (Israel)
(Received April !9,1971 ; revised August 16, 1971)
Summary
EXPERIMENTAL
Tire precipitation process during ageing (at 500°C) of a low-carbon copper steel containing 2 % Cu and 0.03 % C produced by sintering was studied by means of transmission electron microscopy. The precipitates, consisting of a solid solution of iron in copper, appear in two orientations relative to the iron matrix, and their growth is controlled by the diffusion of copper atoms within the matrix.
(a) Material The study was made on 8 x 6 mm sintered specimens prepared from electrolytic iron, electrolytic copper (?.0 % by weight) and synthetic graphite (0.03 %), with a CTC solution of stearic acid (0 .5 %) as lubricant_ The powder mixture was compacted by unidirectional cold-pressing using a floating die at a pressure of 95 kgf/mm 2 _ The sintering process (in an argon atmosphere free of oxygen and moisture) consisted of two stages : (1) Presintering-heating at 400 ° C for 10 minutes in order to eliminate the lubricant. (2) Sintering proper-heating at 1150 ° C for 3 hours_ The specimens were quenched in brine from the sintering temperature to room temperature, and were then aged at 500° C for various time intervals, after which they were again brine-quenched to room temperature.
INTRODUCTION Machine parts made of iron-copper alloys and iron-copper--carbon alloys are widely used' . The copper contributes to improved alloy strength with relatively little detriment to its ductility and makes the alloy more corrosion resistant', while the carbon admixture increases hardenability 3. There are two techniques of producing copper steel : casting and sintering . In both techniques, the mechanical properties of the product can be improved by heat treatment The ageing process in cast alloys has been thoroughly investigated - '- But the sintering technique is widely used in mass production, and the subsequent heat treatment is largely based on empirical data . As noted by Jones9, very little information of value is available concerning the results of heat-treating alloyed sintered steel materials . Elliot' o, in a study on a sintered Fe-Cu product containing 19 % Cu, found that the influence of heat treatment differs occasionally when compared with a cast alloy treated under the same conditions . The object of this study was to investigate the precipitation that accompanies the ageing process which takes place in a sintered Fe-Cu-C alloy when subjected to the appropriate heat treatment
0
(b) Electron-microscopic examination The structure of the product in the course of the ageing process was examined by means of transmission electron microscopy, using a JEM-7-JOEL instrument- The acceleration voltage was 100 kV_ The specimens were prepared in two stages : (1) Jet polishing with hydrochloric acid (75 HCI-T 25 % H2 0)(2) Final electrolytic polishing with 10 % perchloric-x-90% acetic acid .
RESULTS AND DISCUSSION The density attained after sintering was 7 .3 g/cm 3, and the hardness was 32 R„_ A previous study" Ponder
TecnnoL. 5 (1971/72)
238
A . STERN, L . LEVIN
Fig- 1 . Precipitates in sintered copper steel (2% Cu, 0 .03%V C) aged at 500°C for 96 hours_
4000
3000
2000
1000
50
100
Fig . 2. Mean particle radius vs. aging time_
showed that the hardness of the sintered specimens can be increased by heat treatment The optimal temperature-time combination for specimens of the composition in question was 500' C and 2 hours, which yielded a hardness of 49 RA _ The structure of the material after ageing t 500°C was investigated by electron microscopy by the bright and dark field techniques . The examination revealed the precipitates that formed, whose electron micrograph (in bright field) is shown in Fig . 1 . The contrast that was obtained is characteristic of a difference in the crystalline structure of the precipitates and matrix". From the figure it can be seen that the precipitates are near-spherical . The cube of the mean radius of the precipitate versus ageing time is plotted in Fig . 2, which shows the linear relationship : 0 = cons[ + kt The slope found by the least square method is k=1_13 x 10 -23 cm3 sec- ' with a standard deviation of 2_2 x 10 -25 cm 3 sec- ' . The maximum standard deviation in r was found to be 4 .6 A_ The above relationship indicates that the precipitation reaction is controlled by diffusion of the copper atoms within the iron matrix '3-'5. The precipitate orientations were studied by their electron diffraction patterns (Figs. 3 and 4)_ The area selected for the diffraction shown in Fig . 3 was a part of a single Fe crystal in the initial stages of the precipitation process, when the precipitates are still very small . The fact that the precipitates are very small explains why the diffraction points in Fig. 3 are broken up_ The high intensity points in
h
Cons Elm] Cu Fe
Fig 3. Diffraction pattern--specimen aged at 500° C for 2 hours.
(hkl) Cu 0
Fig 4_ Analysis of diffraction pattern in Fig. 3.
Powder TechnoL, 5 (1971(2)
239
AGEING OF A SINTERED LOW-CARBON STEEL
Figs. 3 and 4 are dispersed throughout a rectangle corresponding to plane (011) in the b .c.c- structure of iron, while most of the other points are dispersed in pairs which form two hexagons corresponding to plane (111) in the f.c.c. structure of copper (a-phase) . The mutual orientation of the points may be interpreted as follows : Precipitation of copper in an iron matrix obeys the Kurdimov-Sachs relationships ', that is, {111}f« II {011}b., and the parallel directions in both structures are the densest The matrix has two dense directions in plane (011), namely [111] and [111], while the precipitates have three dense directions in plane (111) : [101], [110] and [01 -1] -As the orientational notation of the precipitates is determined by that of the matrix, the following correspondence should hold [0T 1] co II 111]FC , [110]c, II [111] FC i'_e- the precipitates appear in two distinct orientations (the hexagons in Fig . 4) . In addition to the array of points of the iron lattice and the two arrays of the copper, Fig . 3 shows additional diffraction points, which may represent either cementite or oxide lattices . Definite identification was not possible since the points were not arranged in a symmetric lattice in the diffraction pattern in questionCONCLUSIONS
Sintered low-carbon copper steel may be aged by heat treatment. As in cast alloys, the ageing procrcs gives rise to a-phase precipitation . It was found that the diffraction pattern corresponding to plane (011)
of the iron lattice is related to two distinct orientations of the precipitates . The cube of the average radius of the precipitates is directly proportional to the ageing time . This indicates that diffusion is the controlling mechanism of precipitation. REFERENCES
I J_ Ljungberg and H_ Shula . Some new applications of sintered low-alloy steels, Powder Met., 5 (1962) 265. 2 D_ K_ Bullens, Steel and its Heat Treatment, VoL 1, Wiley, New York, 1954, pp. 262-5. 3 E. A. Wilson, Copper maraging steels, J. Iron Steel Inst., 206 (1968)1644 E. Hornbogen and R C_ Glenn, A metallographic study of precipitation of copper from alpha-iron, Trans_ AIME, 218 (1960) 1064. 5 F_ Hornbogen,Ageingand plastic deformation ofan Fe-0 .9 Cu Alloy, Trans. Am. Soc. Metals, 57 (1964) 120. 6 G. R Speich and R A . Orani, The rate of coarsening of copper precipitate in an alpha-iron matrix, Trans. AI3lE, 233 (1965) 623. 7 A R Cox, Metallography and strengthening mechanisms of 0_3% C-1S % Cu steels, J . Iron Steel Inst., 205 (1967) 51 . 8 M. R Kirshnadev and I_ Lc May, Microstructure and mechanical properties of a commercial low copper-bearing steel, J. Iron Steel Irsz., 208 (1970) 4589 W. D. Jones, Fundamental Principles of Ponder Metallurgy, Arnold, London, I st edn .,1960, pp . 871-2. 10 J. E_ Elliot, Characteristics of copper-infiltered porous iron, Aferallurgia, 52 (1955) 22611 A. Stern, L. Lenin and S . F_ Dirnfeld, Sintering and ageing of low-carbon copper steel, Intern. J. Powder 3Mer., in press. 12 P. B_ Hirsch et al- Electron Microscopy of Thir. Cr}srals. Buttensorths, London. 1st edn.. 1965, p. 337. 13 I_ M . Lifshic and V. V. Slyosov, The kinetics of precipitation from supersaturated solid solutions, PhDs. Chem. Solids, 19 (1961)3514 C_ Wagner, Theorie der Alterung von Niederschlagen durch Umlosen (Ostwald-Reifung), Z_ Eleczrochent . 65 (1961) 3515 A J_ Ardell, The mechanisms of phase transformation in crystalline solids, Monograph No. 33, Inst. Metals. London, 1969.
Powder Technol., 5 (1971 /72)
237
An Investigation of the Precipitation Process During Ageing of a Sintered Low-Carbon Copper Steel A- STERN and L . LEVIN Department of Materials Engineering, Technion-Israel Institute
of Technology, Haifa (Israel)
(Received April !9,1971 ; revised August 16, 1971)
Summary
EXPERIMENTAL
Tire precipitation process during ageing (at 500°C) of a low-carbon copper steel containing 2 % Cu and 0.03 % C produced by sintering was studied by means of transmission electron microscopy. The precipitates, consisting of a solid solution of iron in copper, appear in two orientations relative to the iron matrix, and their growth is controlled by the diffusion of copper atoms within the matrix.
(a) Material The study was made on 8 x 6 mm sintered specimens prepared from electrolytic iron, electrolytic copper (?.0 % by weight) and synthetic graphite (0.03 %), with a CTC solution of stearic acid (0 .5 %) as lubricant_ The powder mixture was compacted by unidirectional cold-pressing using a floating die at a pressure of 95 kgf/mm 2 _ The sintering process (in an argon atmosphere free of oxygen and moisture) consisted of two stages : (1) Presintering-heating at 400 ° C for 10 minutes in order to eliminate the lubricant. (2) Sintering proper-heating at 1150 ° C for 3 hours_ The specimens were quenched in brine from the sintering temperature to room temperature, and were then aged at 500° C for various time intervals, after which they were again brine-quenched to room temperature.
INTRODUCTION Machine parts made of iron-copper alloys and iron-copper--carbon alloys are widely used' . The copper contributes to improved alloy strength with relatively little detriment to its ductility and makes the alloy more corrosion resistant', while the carbon admixture increases hardenability 3. There are two techniques of producing copper steel : casting and sintering . In both techniques, the mechanical properties of the product can be improved by heat treatment The ageing process in cast alloys has been thoroughly investigated - '- But the sintering technique is widely used in mass production, and the subsequent heat treatment is largely based on empirical data . As noted by Jones9, very little information of value is available concerning the results of heat-treating alloyed sintered steel materials . Elliot' o, in a study on a sintered Fe-Cu product containing 19 % Cu, found that the influence of heat treatment differs occasionally when compared with a cast alloy treated under the same conditions . The object of this study was to investigate the precipitation that accompanies the ageing process which takes place in a sintered Fe-Cu-C alloy when subjected to the appropriate heat treatment
0
(b) Electron-microscopic examination The structure of the product in the course of the ageing process was examined by means of transmission electron microscopy, using a JEM-7-JOEL instrument- The acceleration voltage was 100 kV_ The specimens were prepared in two stages : (1) Jet polishing with hydrochloric acid (75 HCI-T 25 % H2 0)(2) Final electrolytic polishing with 10 % perchloric-x-90% acetic acid .
RESULTS AND DISCUSSION The density attained after sintering was 7 .3 g/cm 3, and the hardness was 32 R„_ A previous study" Ponder
TecnnoL. 5 (1971/72)
238
A . STERN, L . LEVIN
Fig- 1 . Precipitates in sintered copper steel (2% Cu, 0 .03%V C) aged at 500°C for 96 hours_
4000
3000
2000
1000
50
100
Fig . 2. Mean particle radius vs. aging time_
showed that the hardness of the sintered specimens can be increased by heat treatment The optimal temperature-time combination for specimens of the composition in question was 500' C and 2 hours, which yielded a hardness of 49 RA _ The structure of the material after ageing t 500°C was investigated by electron microscopy by the bright and dark field techniques . The examination revealed the precipitates that formed, whose electron micrograph (in bright field) is shown in Fig . 1 . The contrast that was obtained is characteristic of a difference in the crystalline structure of the precipitates and matrix". From the figure it can be seen that the precipitates are near-spherical . The cube of the mean radius of the precipitate versus ageing time is plotted in Fig . 2, which shows the linear relationship : 0 = cons[ + kt The slope found by the least square method is k=1_13 x 10 -23 cm3 sec- ' with a standard deviation of 2_2 x 10 -25 cm 3 sec- ' . The maximum standard deviation in r was found to be 4 .6 A_ The above relationship indicates that the precipitation reaction is controlled by diffusion of the copper atoms within the iron matrix '3-'5. The precipitate orientations were studied by their electron diffraction patterns (Figs. 3 and 4)_ The area selected for the diffraction shown in Fig . 3 was a part of a single Fe crystal in the initial stages of the precipitation process, when the precipitates are still very small . The fact that the precipitates are very small explains why the diffraction points in Fig. 3 are broken up_ The high intensity points in
h
Cons Elm] Cu Fe
Fig 3. Diffraction pattern--specimen aged at 500° C for 2 hours.
(hkl) Cu 0
Fig 4_ Analysis of diffraction pattern in Fig. 3.
Powder TechnoL, 5 (1971(2)
239
AGEING OF A SINTERED LOW-CARBON STEEL
Figs. 3 and 4 are dispersed throughout a rectangle corresponding to plane (011) in the b .c.c- structure of iron, while most of the other points are dispersed in pairs which form two hexagons corresponding to plane (111) in the f.c.c. structure of copper (a-phase) . The mutual orientation of the points may be interpreted as follows : Precipitation of copper in an iron matrix obeys the Kurdimov-Sachs relationships ', that is, {111}f« II {011}b., and the parallel directions in both structures are the densest The matrix has two dense directions in plane (011), namely [111] and [111], while the precipitates have three dense directions in plane (111) : [101], [110] and [01 -1] -As the orientational notation of the precipitates is determined by that of the matrix, the following correspondence should hold [0T 1] co II 111]FC , [110]c, II [111] FC i'_e- the precipitates appear in two distinct orientations (the hexagons in Fig . 4) . In addition to the array of points of the iron lattice and the two arrays of the copper, Fig . 3 shows additional diffraction points, which may represent either cementite or oxide lattices . Definite identification was not possible since the points were not arranged in a symmetric lattice in the diffraction pattern in questionCONCLUSIONS
Sintered low-carbon copper steel may be aged by heat treatment. As in cast alloys, the ageing procrcs gives rise to a-phase precipitation . It was found that the diffraction pattern corresponding to plane (011)
of the iron lattice is related to two distinct orientations of the precipitates . The cube of the average radius of the precipitates is directly proportional to the ageing time . This indicates that diffusion is the controlling mechanism of precipitation. REFERENCES
I J_ Ljungberg and H_ Shula . Some new applications of sintered low-alloy steels, Powder Met., 5 (1962) 265. 2 D_ K_ Bullens, Steel and its Heat Treatment, VoL 1, Wiley, New York, 1954, pp. 262-5. 3 E. A. Wilson, Copper maraging steels, J. Iron Steel Inst., 206 (1968)1644 E. Hornbogen and R C_ Glenn, A metallographic study of precipitation of copper from alpha-iron, Trans_ AIME, 218 (1960) 1064. 5 F_ Hornbogen,Ageingand plastic deformation ofan Fe-0 .9 Cu Alloy, Trans. Am. Soc. Metals, 57 (1964) 120. 6 G. R Speich and R A . Orani, The rate of coarsening of copper precipitate in an alpha-iron matrix, Trans. AI3lE, 233 (1965) 623. 7 A R Cox, Metallography and strengthening mechanisms of 0_3% C-1S % Cu steels, J . Iron Steel Inst., 205 (1967) 51 . 8 M. R Kirshnadev and I_ Lc May, Microstructure and mechanical properties of a commercial low copper-bearing steel, J. Iron Steel Irsz., 208 (1970) 4589 W. D. Jones, Fundamental Principles of Ponder Metallurgy, Arnold, London, I st edn .,1960, pp . 871-2. 10 J. E_ Elliot, Characteristics of copper-infiltered porous iron, Aferallurgia, 52 (1955) 22611 A. Stern, L. Lenin and S . F_ Dirnfeld, Sintering and ageing of low-carbon copper steel, Intern. J. Powder 3Mer., in press. 12 P. B_ Hirsch et al- Electron Microscopy of Thir. Cr}srals. Buttensorths, London. 1st edn.. 1965, p. 337. 13 I_ M . Lifshic and V. V. Slyosov, The kinetics of precipitation from supersaturated solid solutions, PhDs. Chem. Solids, 19 (1961)3514 C_ Wagner, Theorie der Alterung von Niederschlagen durch Umlosen (Ostwald-Reifung), Z_ Eleczrochent . 65 (1961) 3515 A J_ Ardell, The mechanisms of phase transformation in crystalline solids, Monograph No. 33, Inst. Metals. London, 1969.
Powder Technol., 5 (1971 /72)