Compacted/Vermicular Graphite Cast Irons in the Fe-C-AI System

Transcription

1 Compacted/Vermicular Graphite Cast Irons in the FeCAI System D. M. Stefanescu F. Martinez The University Alabama University, Alabama INTRODUCTION The idea trying completely to substitute aluminum for silicon in cast irons was first conceived by Defrancq, Van Eeghem and DeSy, 1 ' 2 and resulted in the development a new family gray cast irons in the FeCAl system. Several advantages were claimed to derive from this substitution, the most notable being the following ones: higher tensile strengths ranging from 2.7 ksi to 78.2 ksi, higher deflection at rupture, excellent resistance to thermal shock, good thermal conductivity at high temperature, high graphitization tendency and low chilling tendency. By increasing the manganese and copper contents the hardenability was Shown to increase continuously, until selfhardening cast irons in the ascast state were obtained. A more recent study performed by R. P. Walson 3 concluded that cast irons in the FeCAl system will find specific applications in construction and agricultural equipment for both highstrength lightweight components having thin walls and operating at or near room temperature, and as an alternate material to supplement higher alloyed metals for use at moderate or high temperatures. Many applications and examples were given as, for example, diesel engine exhaust manifolds, diesel engine turbocharger housings, automobile disk brake rotors and brake drums, heat treatment fixtures, engine heads, engine cylinder liners, camshafts, turbine exhaust diffusers, gear case housings and piston rings. While producing regular compacted/vermicular graphite cast iron ( iron) in the FeCSi system, it is difficult to cast thin sections, due to the high chilling tendency this particular iron. When increased amounts ferrosilicon are added as postinoculant in order to counterbalance this chilling tendency, usually high nodularity irons (more than 2% nodularity) will result. Because this high nodularity some the basic properties irons might be lost. It was sought that by completely replacing silicon with aluminum, irons with a lower tendency toward developing high nodularity will be possible to cast, since it is recognized that aluminum, as opposed to silicon, is a strong antinodularizer. PROCEDU A total 17 heats were melted in a 5 lb acidlined induction furnace. The standard meltingtreatment procedure for all heats was as follows. First, 5 lb high purity pig iron (low silicon, low sulfur) were melted. After melt down the iron was superheated at 15C (2732F) and then solid aluminum was added and stirred in the melt. The melt was then superheated at 155O16OOC ( F) and taped into a 5lb ladle. Rare earth () silicides (3.92% total, 9.28% La, 15.6% Ce, 33.2% Si) or lanthanum ferrosilicon (La) (29.3% total, 26.9% La, 2.9% Ce, 3% Si) were added on the bottom the empty furnace, after which the iron from the ladle was transferred back in the furnace. After this melt treatment the iron was held in the furnace for an additional two minutes at low power to allow for complete dissolution the compactizing alloy and then the iron was ready for pouring. From each heat the following castings were poured: 1) Spectrographic analysis samples (copper mold); 2) Eutectometer thermal analysis samples; 3) Chill wedge, No. 2 (ASTM A 7) (air set); ) Chill pins (air set): 32 mm (.125 in.),.8 mm (.188 in.), 6.5 mm (.25 in.), 7.9 mm (13 in.), 9.5 mm AFS Transactions AFS Library Copy: Page 1 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

2 Table 1. Summary Aluminum Additions, Melt Treatment and Chemical Analysis for the Experimental Heats Heat No. Al A2 A3 Aluminum Added,% Melt Compactizing Treatment, Postinoculation C Chemical Analysis, % Si 3 3. Al Mn P S.6.5 Ce La Bl Bl Cll C12 C13 C21 C22 C23 C p.3, FeSl Dll D12 D13 D21 D La La La La La : Not determined. (75 in.) and mm ( in.) diameter; 5) Keel block for mechanical properties evaluation. The eutectometer cups were cut and used for metallographic and scanning electron microscopy (SEM) examination. Brinell hardness was measured on all keel blocks and tensile strength and elongation were determined on all machinable samples. Four series experimental heats were produced as follows: 1) Series A: in order to determine the optimum amount silicides to be used for iron treatment at a constant level aluminum addition; 2) Series B: study the influence the amount aluminum added over the structure and properties irons at a constant level addition; 3) Series C: study the influence various levels ferrosilicon () postinoculation over the structure and properties FeCAl irons; ) Series D: in order to check if a beneficial graphitizing influence lanthanum, as reported for FeCSi irons 5 can be proved to exist also for FeCAl irons. SULTS A DISCUSSIONS Aluminum additions, melt treatment procedures and chemical analysis for all the experimental heats are given in Table 1, while cooling curve data, description microstructures, chill tests data and mechanical properties are given in Table 2. Influence the Amount Rare Earth Silicides Addition In series A heats, increasing amounts rare earth () silicides, i. e., and % were added to the liquid cast iron, after a constant 3.17% aluminum addition. No postinoculation was performed. The influence the amount silicides added over the structure, cooling curves and hardness cast irons in the FeCAl system, was similar to the one observed when were added to typical FeCSi cast irons, as described in reference. A % silicides resulted in a flaketype D graphite structure exempt carbides (Fig. la). At % silicides addition compacted graphite formed with practically no spheroidal graphite and some % carbides (Fig. lb). As the amount added was further increased to %, the carbide content increased to 21%, but the graphite structure was still 1% compacted/vermicular (Fig. lc). The Brinell hardness raised accordingly from 121 HB for % to HB for % (Fig. 2). As expected, both the temperature eutectic recalescence (TER) and the temperature eutectic undercooling (TEU) decreased sharply when the graphite structure changed from flake to compacted, and continued to decrease as the carbide content in the structure increased (Fig. 2). The AT = TER TEU interval jumped from to 7 C at the flakecompacted transition, and decreased at 6 C when a higher silicides addition resulted in higher carbide content. As already mentioned, this influence was similar to that one observed on FeCSi irons. Since it was evident that the optimum range silicides addition lays in the range % to 5, and that the tendency toward carbides formation increased with the addition, it was decided not to run any heats with additions more than %. Influence Aluminum Content Two heats were run in series B with different aluminum additions. By using data previously obtained for heat A2, three heats with various aluminum additions were made available for discussion, as follows: heat Bl 3.6% Al, AFS Transactions AFS Library Copy: Page 2 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

3 Table 2. Summary Cooling Curve Characteristics, Micro structure, Chill Data and Mechanical Properties for the Experimental Heats Heat No. Al A2 A3 TL Cooling Curve Data, C TEU TER AT 7 6 MicroStructure Graphite Matrix Flake F+Pe Pe+F+%Fe 3 C Pe+F+21%Fe 3 C Maximum Diameter Chill the White Pin Depth Bar, mm mm Mechanical Properties Tensile Strength,psi 15,995 Brinell Hardness Elongation % 1. Bl B %SG Pe+F+%Fe 3 C Pe+F+9%Fe 3 C 12, Cll C12 C13 C21 C22 C23 C Pe+F+9%Fe 3 C Pe+F+2%Fe 3 C Pe+F+9%Fe 3 C F+Pe+Fe 3 C* F+Pe+Fe3C* F+Pe+Fe 3 C* F+Pe+Fe 3 C* ,398 5,217,77 7, Dll D12 D %SG Pe+F+Fe 3 C* Pe+F+Fe 3 C* Pe+F+Fe 3 C* ,31 7, D21 D Pe+F+%Fe 3 C Pe+F+9%Fe 3 C 27 : compacted/vermicular graphite S : spheroidal graphite F: ferrite Pe: pearlite Fe 3 C: carbides Fe^C*: less than 2% intercellular carbides heat A2 3.17% Al, heat B2 2.5% A1. As evident from Table 2 and Fig. 3, as the amount Al added decreased, the carbide content increased from % to 9%, while the hardness increased from 176 HB to 179 HB. It was also observed that as the amount Al decreased, the carbides were segregated around the interconnected graphite, which in turn was embedded in a ferritic matrix (Fig. a). Still the graphite did not show the typical tendency to transform into irregular graphite, as observed for Fe CSi irons, but persisted as wellformed, interconnected compacted graphite, as shown in Figs. b and c. Influence Postinoculation The influence increasing amounts ferrosilicon added as postinoculant was tested on FeCAl irons at two different levels aluminum additions, i. e. 2.1% (heats Cll to C13) and 2.5%(heats C21 to C2). It was evident that the low aluminum content (e. g. 1.22% Al for heat Cll) resulting from a 2.1% Al addition combined with a higher silicides addition (%) increased the chilling tendency the iron to such an extent that postinoculation with up to.7% ferrosilicon was inefficient in promoting a structure free carbides. The Brinell hardness was rather high, ranging from to 21 HB, and the chill wedges were completely white ( mm chill depth), as shown in Table 2 for heats Cl 1, C12, C13. Although the quality compacted graphite was very good, the metallographic structure showing very short and thick (Fig. 5), the presence 2 to 9% carbides in the structure ruined the machinability all samples from these heats. At higher levels aluminum additions 2.5% (1.3 to 1.56% Al content), postinoculation worked much better. As shown in Fig. 6, both the hardness and the chill depth dropped considerably when the iron was postinoculated with %. Surprisingly, an increase in the amount postinoculant added did not result, as expected, in a further decrease either Brinell hardness or chill. Good mechanical properties were obtained for the iron postinoculated with %, i. e. 5 to 7 ksi and 3 to % elongation (heats C21 and C22). Although graphite shape was very good, some persisting intercellular carbides (Fig. 7) prevented higher elongations from being obtained, even though the total amount carbides was below 2%. Increasing the amount postinoculation did not bring an improvement in elongation. On the contrary, higher amounts intergranular carbides, probably due to the higher residual cerium from heat C23, resulted in a decrease in elongation. Generally, it was observed that graphite shape was somehow better in FeCAl irons than in FeCSi irons. The average thickness the graphite was the order 3 fxm and many graphite terminations a aggregate were spheroidal (Fig. 6). On the other hand, neither the increase the amount silicides added (up to %), nor the increase in the amount postinoculation (up to.7% ), altered significantly the graphite shape, i. e. the AFS Transactions 1 AFS Library Copy: Page 3 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

4 » ' * ' * ', *.. ^., v,* *?,, * & r V X } I. *" r ' ' l'» * : * >. > a) c) 119 t I»>**»1 # H, y,>,> b) % RA EARTH SUICIDE ADDITION,'/ Fig. 1. Structures FeCAl irons (3.7% Al added) after various additions silicides 1X, 2% Nital etched. a) % silicides; bj % silicides; c) % silicides. Fig. 2. Influence silicides additions over cooling curves, structure and properties FeCAl irons. 2 AFS Transactions AFS Library Copy: Page 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

5 iv > *s. W C) Fig.. Structure a FeCAl iron containing 1.68% Al (2.5% Al added) after treatment with % silicides (heat B2). a) metallographic micro structure, nital etched, 1X; b) SEM microstructure, deep etching, 1X; cj SEM microstructure, deep etching, 11X. b) 2 / SIL. ADDED 15 \ HB 19 \ \ Q 1 m EC V. CARBIDES S \ 18 5 Fig. J. Influence aluminum added over the structure and Brinell hardness FeCAl irons. I I ALUMINUM ADDED, /. AFS Transactions 3 AFS Library Copy: Page 5 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

6 . ', *. *. ',. * rf / Al ADDED 3 19 CHILL / ADDED AS POSTINOCUL ANT, /. Fig. 6. Influence the amount postinoculant on Brinell hardness and chill depth, for heats with 2.5% Al and % silicides additions. Fig. 5. Metallographic structure no C12 iron (2.1% Al addition + % silicides + % ). Nitaletched, 1X. y* f! <&, > ' a f a) b) Fig. 7. Structures FeCAl irons (2.5% Al added +.6% silicides) after a % postinoculation (heat C22). a) metallographic microstructure, nital etched, 1X; b) SEM microstructure, deep etching, 15X; c) SEM microstructure, deep etching, X. AFS Transactions AFS Library Copy: Page 6 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

7 6 u o _ Ul I CE Ul I La \ / LaFeSI 1 2 Z La 1 Fig. 7. (continued) c) / ADDED AS POSTINOCU LANT,/ Fig. 8. Influence the amount postinoculant on the AT = TERTEU interval and on the chill depth, for heats with 2.5% Al addition and treated with La. structure was 1% compacted. This can be definitely claimed as an advantage over the FeCSi compacted graphite cast irons. Influence Lanthanum Ferrosilicon Since lanthanum was proved to reduce the chilling tendency irons as compared with cerium, 5 five melts marked as series D were treated with % and.6% lanthanum ferrosilicon (La). For each level La addition, increased amounts postinoculants were used, ranging from % to.7%. As evident from Fig. 8, the increase postinoculation level resulted in a decrease the AT interval for both heats treated with % and.6% La. For heats DU, D12 and D13 this might be due to a slight increase in nodularity, while for heats D21 and D22 the decrease might be explained by an increase in the carbide content. A slight decrease in chill depth was observed for the heats treated with % La (Fig. 8), while all chill tests were completely white at all levels postinoculation when the irons were treated with.6% La. This confirms findings for the FeCSi irons 5 that lanthanum can be used for iron production in a narrower range contents than cerium. Tensile strengths were again in the range 6 to 7 ksi (Table 2), but elongation was poor, i. e. 1.5%, although graphite shape was good, and very few intercellular carbides were apparent in the structure (Fig. 9). By comparing Fig. 9 and Fig. 7 it looks like La will promote less intercellular carbides than silicides, but on the other hand the pearlite content will be higher, resulting in an increase in hardness (see Table 2). CONCLUSIONS Compacted/vermicular graphite cast irons in the FeCAl system were produced by treating the melt with % to % silicides or to.6% La, with and without postinoculation. The aluminum content ranged from 1.2 to 2.3% (2.1 to 3.6% addition) while the silicon content was 1 to.95%, the higher figure corresponding to.7% postinoculation. Generally, the structures exhibited very good quality compacted graphite, but some intercellular carbides persisting even at less than 2% total amount carbides prevented high elongation from being obtained. Nevertheless, when a 1.56% Alcontaining cast iron was treated with % silicides and postinoculated with %, tensile strengths 5 to 7 ksi with elongation 3 to 5% were obtained. Although not much difference was observed between the chilling tendencies irons in the FeCAl and FeCSi systems, increased postinoculation up to.7% did not result in higher nodularity for FeCAl irons, which is certainly an advantage. At this point, it is believed that elimination intercellular carbides, and subsequently higher elongations, can be achieved by increasing the aluminum level at higher values than those used in this work. ACKNOWLEDGMENTS The authors would like to express their gratitude to the Research Grant Committee and the Chemical and Metallurgical Engineering Department at The University Alabama for financial support this research. AFS Transactions 5 AFS Library Copy: Page 7 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.

8 dm \ a) b) The assistance Cast South Inc., Marion, Alabama, in providing the chemical analysis is also acknowledged. Also, special thanks are due to Mr. Thomas Prucha, Technical Director Cast South Inc. for his permanent support this research. Thanks are further extended to Deborah Clayton, Electron Microscopist, Department Biology for her help with the SEM study, In Gan Chen, Research Assistant, and Dennis Moore, Graduate Teaching Assistant, both at The University Alabama, for their help and suggestions.»* v FENCES 1. Defrancq C, Van Eeghem J., A. DeSy, "Study the Inoculation Gray Cast Irons from the FeCAl System; Development a New Flake Graphite Cast Iron with Very High Strength," th International Foundry Congress, Beograd (1969) (in French). 2. C. Defrancq, J. Van Eeghem, A. DeSy, "Further Development Aluminum Cast Iron, Inoculated with High Amounts Calcium," th International Foundry Congress, Moscow (1973) (in Fren ch). 3. R. P. Walson, "Aluminum Alloyed Cast Iron Properties used in Design," AFS Transactions, vol 85, pp 5158 (1977).. D. M. Stefanescu, C. R. Loper, Jr., "Effect Lanthanum and Cerium on the Structure Eutectic Cast Irons," AFS Transactions, vol 89, pp 25 (1981). 5. D. M. Stefanescu, R. C. Voigt, C. R. Loper, Jr., "The Importance the Lanthanum/Rare Earth Ratio in the Production Compacted Graphite Cast Irons," AFS Transactions, vol. 89, pp (1981). Fig, 9, Metallographic structures FeCAl irons treated with % La at various levels postionoculation. Nital etch. 1X. a) % (heat Dll); b) % (heat D12); c).7% (heat D13). 6 AFS Transactions AFS Library Copy: Page 8 8 Pages, Provided to User for Internal Use and Not Public Redistribution or Resale.