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In situ technique for synthesizing Fe-TiC composites
Suipta Metallurgica ei Materialia, Vol. 32, No. 4, pp. 577-5851995 Copyright 0 1994 Ekevier Science Ud Printed in the USA. AU rights reserved 0956-716X/95 $9.50 + .OIl
IN SITU TECHNIQUE FOR SYNTHESIZING
Fe-Tic COMPOSITES
C. Raghunath*, M. S. Bhat’ and P. K. Rohatgi** *Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721 **Materials Department, University of Wisconsin - Milwaukee Milwaukee, WI 53201 (Received August 9,1994) (Revised September 23,1994)
Introduction The addition of ceramic particles such as carbides to a ferrous matrix can dramatically change the properties of the alloy and consequently its behavior in services(l,2). By careful selection of hard carbides, it is possible to design MMCs which have the potential to withstand high abrasive wear environments(3).Therefore these MMCs are being considered as a potential replacement for WC-Co mechanical seals, which often fail during service due to .thermal shock(4). Moreover these MMCs retain the properties even at high temperatures with the required low friction for both static and dynamic components. In this paper we highlight a novel technique of producing Tic reinforcements in an iron matrix by an in situ technique. In situ processing of composites has been in use over three decades. Table 1 summa rizes previous work on in situ synthesis by various workers(5-lo), in both ferrous and non-ferrous matrices. Composites containing reinforcements generated in situ are likely to have the advantages of virgin uncontaminated interfaces and can be further tailored during solidification processing to promote adequate bondingbetween the matrix and the reinforcements. In addition, this route of synthesis of composites may be more economical as the reinforcements are not manufactured and handled separately. Theoretical Back~ound Successful incorporation of solid ceramic particles in castings requires that the melt should wet the solid ceramic phase. Work by Ramquistfll) has shown that TiC particles easily wet iron at high temperatures, and as time progresses wetting further improves because of chemical wetting. He has also shown that metals with an unfilled d-band (transition metals) wet carbides much more easily than metals with a filled d-band. The in situ formation of TiC is highly exothermlc and is moderated by the carbon available in the molten metal, leading to a fine and homogeneous distribution of stoichiometric carbide(l2). A detailed analysis of the system is necessary to find out the reaction products, reaction paths, the rate limiting step in the formation of Tic and the solidification rate. It is our notion that the formation of the carbide occurs through diffusion of titanium to a carbon site, where it precipitates as carbide by chemical reaction. To start with, the rate limiting step is the availability of carbon in the molten metal. However, towards the end, as the reaction proceeds the titanium is depleted, diffusion of titanium across the boundary layer becomes the rate limiting step. Thermodynamic considerations indicate that TiC is stable during solidification. In addition, reaction time and temperature may influence carbide size, distribution and volume fraction in the matrix. In the multicomponent system(Fe-Ti-C), it is possible that a variety of compounds may form depending on the processing conditions. Reactions and the accompanying free energy changes are as follows: Ti(s) = Ti(1) AG = 18842 (1940 - T) /1940, J/mole (I) Ti(l) + C(s)
=
Tic(s)
AG = -186,606 + 13.22 T, J/mole
577
(2)
578
In
Vol. 32, No. 4
situsyathesis
In the ternary Fe-TX, as titanium depletion occurs due to the formation of Tic, the activity of iron increases and it may lead to the formation of Fe.& when titanium concentration falls below a certain critical level. This composition should be known in order to maintain the desired level of titanium so as to prevent the formation of Fe&. Hence the following reaction is considered. 3FeW + Tic(s)
= Fe&(s)
+ Ti(1)
(3)
The relative rates of these reactions depend on the carbon in the melt, and the processing temperature. By using a shrinking core model031 which takes into account both the diffusion and the chemical reactions, one can estimate the rate of reaction of carbon and titanium with time. Also, depending on the temperature of the melt, time taken for the completion of the above reactions can be estimated. Exuerlmental
Procedure
The composition of the raw materials used for the synthesis of composites is shown in Table 2. Ductile iron and ferro titanium (average size > 6Opm ) were procured from Grede Foundry and Galt Alloys, Inc., respectively. The synthesis of the composite was carried out in an MgO-lined induction furnace, with a blanket of nitrogen gas over the melt. Initially the ductile iron was melted and upon reaching the processing temperature of 1450 “C, ferro titanium was added. Since induction melting has the inherent advantage of stirring, only gentle stirring of the melt with a graphite rod was sufficient for proper mixing of the additions to the alloy. Table 2 Compositions Material
C
Ductile iron
3.7
Ferro Titanium
0.09
Ti -70.65
of the raw materials used in the synthesis of Fe-Tic
composites
S
P
Si
Al
V
Mn
Cu
Fe
2.5
0.21
__-
-_-
__I
0.38
0.1
balance
0.0134
0.012
0.112
0.014
0.88
---
---
balance
Low volume fraction Tic composites produced by this technique were statically cast. However molten metal containing a large volume fraction of Tic could not be poured, hence it was squeezed under a pressure of 1000 psi and allowed to solidify in the crucible within the furnace. This was followed by remelting and solidification under vacuum (10” torr) in a resistance furnace. Samples were removed for observation under optical and scanning electron microscopy. Microstructures were examined to determine the carbide content and distribution. Diffraction analysis was done to identify the nature of the carbide formed.
Initial experiments showed considerable foaming from the carbon boil in the melt, and a rapid rise in temperature due to the exothermic reaction of the formation of Tic. High viscosity of the melt compounded this problem and often led to high porosity in the ingot. However, close control of the temperature, time and mode of addition resulted in relatively defect free ingots. Figure 1 shows the composite produced under controlled processing conditions. Microscopic examination of the sections taken at the bottom, middle and top portions of the castings revealed tmiform distribution of Tic particles. Figure 2 is the optical micrograph taken at the middle of the casting. As shown in the figure, TiC particles produced in situ are nearly spherical in nature. Upon holding at a predetermined temperature, however these particles assumed spheroidal shape by a process of ripeningflU. Inspite of the nitrogen blanket over the solidifying melt, the castings showed defects due to the entrapped gasses. These defects were partly reduced by solidifying
Vol. 32, No. 4
In Situ Synthesis
519
under pressure even though vacuum remelting was necessary to obtain defect-free castings. The variation of density as a function of the type of process, shown in Figure 3 substantiates it. The sphericity and the uniform distribution of the titanium carbide produced in situ are confirmed by SEM mapping (figure 41. That the particles were TiC was later confirmed by X-ray diffraction analysis. Figure 5 shows the x-ray diffraction pattern, indicating prominent TiC peaks, along with Fe& peaks. Currently, experiments are underway to study the coarsening mechanism of TiC produced in situ in the matrix. Conclusions From the present study of synthesizing in situ composites, the following conclusions can be drawn: 1.
It is possible to synthesize composites upto 30 ~01% of TiC in ferrous matrix. The in situ carbide formed has been identified as TiC by both EDS and X-ray diffraction analyses.
2.
Composites containing upto 10 ~01% TiC can be statically cast, while higher ~01% TiC melts require application of pressure during solidification to produce relatively sound casting.
3.
To avoid the problem of foaming, vacuum synthesis of the composites may offer a viable route for synthesizing Tic composites.
4.
Microstructures of the composite show that the carbides are spheruletic and distributed uniformly in the ferrous matrix.
5.
At ~01% of TiC greater than 0.45 in the melt, the distribution of the carbide is adversely affected due to the high viscosity of the two-phase mixture and the inability to exercise close control over the temperature of the melt. Acknowledeement
The authors wish to acknowledge stimulating discussions with Dr. S. Bhattacharya, Dr. S. Ray, Dr. B.N. Keshavaram and Mr. Jagadeesh at the University of Wisconsin-Milwaukee. Authors also wish to acknowledge Mr. G. Simanson of Stainless Steel Foundries, Milwaukee for help in successfully carrying out the heats. Two of the authors (C.R and M.S.B) are thankful to Prof. D. Poirier for critically evaluating the manuscript. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13.
P. Chesney, Metals and Materials, 373 (1990). J.R. Tinklepaugh and W.B. Crandall, eds., Cermets, Reinfold Publishing Corporation, NY, 146 (1960). M. Epner and E. Gregory, Transactions of the Metallurgical Society of AlME, 218, 117 (1960).
H.D. Lamping, W. Galliers and S.W. Wolosin, Automobile Engineers Meeting, Toronto, Canada, Paper # 741043, SAE (1974). S.R. Tittagala, P.R. Beeley and A.N. Branley, Mef. Technol., 10, 257 (1983). M. Chen, T.Z. Kattamis, B.V. Chambers and J.A. Comie, TMS Conf. Proc., F.A. Smidt and PJ. Blau, eds., ASM International, Gaithersburg, Maryland, p.63 (1988). M.J. Kouak and KS. Kumar, US Patent 4808372 (1989). B.S. Terry and O.S. Chinyamakobvu, J. Mat. Sci. Letters, 10, 628 (1991). Y. Lin, R.H. Zee and B.A. Chin, Metall. Trans., 22A, 859 (1991). BCockran, M. Saqib, R. Omlor, R. Srinivasan, L.E. Matson and I. Weiss, Scripta Metallurgica et Materialia, 25,393 (1991). L. Ramqvist, ht. J. Powder Met., 4, 1 (1965). P. Sahoo and M.J. Koczak, Materials Science and Engineering, A131, 69 (1991).
K.N. Han, Private Communications.
580
14.
In Situ Synthesis
B.V. Chambers, MSc. Thesis, Department Institute of Technology (1987).
of Meterials Science and Engineering,
Vol. 32, No. 4
Massachussek
Table 1.
Summary of work to date by various workers on in situ processing
0.4 - 0.5 Tic
0.1 - 0.3 TiC
Vol. 32, No. 4
581
In Situ Synthesis
Figure 1. Fe-30 ~01% Tic composite ingot produced in situ.
-.Ascasl
Figure 2. Optical micrograph of Fe-30 ~01% TiC composite.
tsQllemcest
v-
remelted
Figure 3. Variation of density as a function of casting processes.
582
Vol. 32,No. 4
In situ syntlleaia
(4 Figure
(b)
4. SEM photomicrograph
of Fe-Tic composite (a) Distribution
of TiC paekh,
7 Standard data collected on Nicolet Diffractometer
Fe&
30 t 40 Two-theta 50 Figure 5. X-ray diffraction pattern of Fe-30 ~01% Tic composite.
(b) Ti map.
IN SITU TECHNIQUE FOR SYNTHESIZING
Fe-Tic COMPOSITES
C. Raghunath*, M. S. Bhat’ and P. K. Rohatgi** *Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721 **Materials Department, University of Wisconsin - Milwaukee Milwaukee, WI 53201 (Received August 9,1994) (Revised September 23,1994)
Introduction The addition of ceramic particles such as carbides to a ferrous matrix can dramatically change the properties of the alloy and consequently its behavior in services(l,2). By careful selection of hard carbides, it is possible to design MMCs which have the potential to withstand high abrasive wear environments(3).Therefore these MMCs are being considered as a potential replacement for WC-Co mechanical seals, which often fail during service due to .thermal shock(4). Moreover these MMCs retain the properties even at high temperatures with the required low friction for both static and dynamic components. In this paper we highlight a novel technique of producing Tic reinforcements in an iron matrix by an in situ technique. In situ processing of composites has been in use over three decades. Table 1 summa rizes previous work on in situ synthesis by various workers(5-lo), in both ferrous and non-ferrous matrices. Composites containing reinforcements generated in situ are likely to have the advantages of virgin uncontaminated interfaces and can be further tailored during solidification processing to promote adequate bondingbetween the matrix and the reinforcements. In addition, this route of synthesis of composites may be more economical as the reinforcements are not manufactured and handled separately. Theoretical Back~ound Successful incorporation of solid ceramic particles in castings requires that the melt should wet the solid ceramic phase. Work by Ramquistfll) has shown that TiC particles easily wet iron at high temperatures, and as time progresses wetting further improves because of chemical wetting. He has also shown that metals with an unfilled d-band (transition metals) wet carbides much more easily than metals with a filled d-band. The in situ formation of TiC is highly exothermlc and is moderated by the carbon available in the molten metal, leading to a fine and homogeneous distribution of stoichiometric carbide(l2). A detailed analysis of the system is necessary to find out the reaction products, reaction paths, the rate limiting step in the formation of Tic and the solidification rate. It is our notion that the formation of the carbide occurs through diffusion of titanium to a carbon site, where it precipitates as carbide by chemical reaction. To start with, the rate limiting step is the availability of carbon in the molten metal. However, towards the end, as the reaction proceeds the titanium is depleted, diffusion of titanium across the boundary layer becomes the rate limiting step. Thermodynamic considerations indicate that TiC is stable during solidification. In addition, reaction time and temperature may influence carbide size, distribution and volume fraction in the matrix. In the multicomponent system(Fe-Ti-C), it is possible that a variety of compounds may form depending on the processing conditions. Reactions and the accompanying free energy changes are as follows: Ti(s) = Ti(1) AG = 18842 (1940 - T) /1940, J/mole (I) Ti(l) + C(s)
=
Tic(s)
AG = -186,606 + 13.22 T, J/mole
577
(2)
578
In
Vol. 32, No. 4
situsyathesis
In the ternary Fe-TX, as titanium depletion occurs due to the formation of Tic, the activity of iron increases and it may lead to the formation of Fe.& when titanium concentration falls below a certain critical level. This composition should be known in order to maintain the desired level of titanium so as to prevent the formation of Fe&. Hence the following reaction is considered. 3FeW + Tic(s)
= Fe&(s)
+ Ti(1)
(3)
The relative rates of these reactions depend on the carbon in the melt, and the processing temperature. By using a shrinking core model031 which takes into account both the diffusion and the chemical reactions, one can estimate the rate of reaction of carbon and titanium with time. Also, depending on the temperature of the melt, time taken for the completion of the above reactions can be estimated. Exuerlmental
Procedure
The composition of the raw materials used for the synthesis of composites is shown in Table 2. Ductile iron and ferro titanium (average size > 6Opm ) were procured from Grede Foundry and Galt Alloys, Inc., respectively. The synthesis of the composite was carried out in an MgO-lined induction furnace, with a blanket of nitrogen gas over the melt. Initially the ductile iron was melted and upon reaching the processing temperature of 1450 “C, ferro titanium was added. Since induction melting has the inherent advantage of stirring, only gentle stirring of the melt with a graphite rod was sufficient for proper mixing of the additions to the alloy. Table 2 Compositions Material
C
Ductile iron
3.7
Ferro Titanium
0.09
Ti -70.65
of the raw materials used in the synthesis of Fe-Tic
composites
S
P
Si
Al
V
Mn
Cu
Fe
2.5
0.21
__-
-_-
__I
0.38
0.1
balance
0.0134
0.012
0.112
0.014
0.88
---
---
balance
Low volume fraction Tic composites produced by this technique were statically cast. However molten metal containing a large volume fraction of Tic could not be poured, hence it was squeezed under a pressure of 1000 psi and allowed to solidify in the crucible within the furnace. This was followed by remelting and solidification under vacuum (10” torr) in a resistance furnace. Samples were removed for observation under optical and scanning electron microscopy. Microstructures were examined to determine the carbide content and distribution. Diffraction analysis was done to identify the nature of the carbide formed.
Initial experiments showed considerable foaming from the carbon boil in the melt, and a rapid rise in temperature due to the exothermic reaction of the formation of Tic. High viscosity of the melt compounded this problem and often led to high porosity in the ingot. However, close control of the temperature, time and mode of addition resulted in relatively defect free ingots. Figure 1 shows the composite produced under controlled processing conditions. Microscopic examination of the sections taken at the bottom, middle and top portions of the castings revealed tmiform distribution of Tic particles. Figure 2 is the optical micrograph taken at the middle of the casting. As shown in the figure, TiC particles produced in situ are nearly spherical in nature. Upon holding at a predetermined temperature, however these particles assumed spheroidal shape by a process of ripeningflU. Inspite of the nitrogen blanket over the solidifying melt, the castings showed defects due to the entrapped gasses. These defects were partly reduced by solidifying
Vol. 32, No. 4
In Situ Synthesis
519
under pressure even though vacuum remelting was necessary to obtain defect-free castings. The variation of density as a function of the type of process, shown in Figure 3 substantiates it. The sphericity and the uniform distribution of the titanium carbide produced in situ are confirmed by SEM mapping (figure 41. That the particles were TiC was later confirmed by X-ray diffraction analysis. Figure 5 shows the x-ray diffraction pattern, indicating prominent TiC peaks, along with Fe& peaks. Currently, experiments are underway to study the coarsening mechanism of TiC produced in situ in the matrix. Conclusions From the present study of synthesizing in situ composites, the following conclusions can be drawn: 1.
It is possible to synthesize composites upto 30 ~01% of TiC in ferrous matrix. The in situ carbide formed has been identified as TiC by both EDS and X-ray diffraction analyses.
2.
Composites containing upto 10 ~01% TiC can be statically cast, while higher ~01% TiC melts require application of pressure during solidification to produce relatively sound casting.
3.
To avoid the problem of foaming, vacuum synthesis of the composites may offer a viable route for synthesizing Tic composites.
4.
Microstructures of the composite show that the carbides are spheruletic and distributed uniformly in the ferrous matrix.
5.
At ~01% of TiC greater than 0.45 in the melt, the distribution of the carbide is adversely affected due to the high viscosity of the two-phase mixture and the inability to exercise close control over the temperature of the melt. Acknowledeement
The authors wish to acknowledge stimulating discussions with Dr. S. Bhattacharya, Dr. S. Ray, Dr. B.N. Keshavaram and Mr. Jagadeesh at the University of Wisconsin-Milwaukee. Authors also wish to acknowledge Mr. G. Simanson of Stainless Steel Foundries, Milwaukee for help in successfully carrying out the heats. Two of the authors (C.R and M.S.B) are thankful to Prof. D. Poirier for critically evaluating the manuscript. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13.
P. Chesney, Metals and Materials, 373 (1990). J.R. Tinklepaugh and W.B. Crandall, eds., Cermets, Reinfold Publishing Corporation, NY, 146 (1960). M. Epner and E. Gregory, Transactions of the Metallurgical Society of AlME, 218, 117 (1960).
H.D. Lamping, W. Galliers and S.W. Wolosin, Automobile Engineers Meeting, Toronto, Canada, Paper # 741043, SAE (1974). S.R. Tittagala, P.R. Beeley and A.N. Branley, Mef. Technol., 10, 257 (1983). M. Chen, T.Z. Kattamis, B.V. Chambers and J.A. Comie, TMS Conf. Proc., F.A. Smidt and PJ. Blau, eds., ASM International, Gaithersburg, Maryland, p.63 (1988). M.J. Kouak and KS. Kumar, US Patent 4808372 (1989). B.S. Terry and O.S. Chinyamakobvu, J. Mat. Sci. Letters, 10, 628 (1991). Y. Lin, R.H. Zee and B.A. Chin, Metall. Trans., 22A, 859 (1991). BCockran, M. Saqib, R. Omlor, R. Srinivasan, L.E. Matson and I. Weiss, Scripta Metallurgica et Materialia, 25,393 (1991). L. Ramqvist, ht. J. Powder Met., 4, 1 (1965). P. Sahoo and M.J. Koczak, Materials Science and Engineering, A131, 69 (1991).
K.N. Han, Private Communications.
580
14.
In Situ Synthesis
B.V. Chambers, MSc. Thesis, Department Institute of Technology (1987).
of Meterials Science and Engineering,
Vol. 32, No. 4
Massachussek
Table 1.
Summary of work to date by various workers on in situ processing
0.4 - 0.5 Tic
0.1 - 0.3 TiC
Vol. 32, No. 4
581
In Situ Synthesis
Figure 1. Fe-30 ~01% Tic composite ingot produced in situ.
-.Ascasl
Figure 2. Optical micrograph of Fe-30 ~01% TiC composite.
tsQllemcest
v-
remelted
Figure 3. Variation of density as a function of casting processes.
582
Vol. 32,No. 4
In situ syntlleaia
(4 Figure
(b)
4. SEM photomicrograph
of Fe-Tic composite (a) Distribution
of TiC paekh,
7 Standard data collected on Nicolet Diffractometer
Fe&
30 t 40 Two-theta 50 Figure 5. X-ray diffraction pattern of Fe-30 ~01% Tic composite.
(b) Ti map.