Mechanical Property Potential of Iron Base Infiltrated Parts

Transcription

1 Mechanical Property Potential of Iron Base Infiltrated Parts F. J. Semel Hoeganaes Corporation, Cinnaminson, NJ ABSTRACT The effects of composition and processing on the transverse rupture and tensile properties of iron base infiltrated specimens are presented. It is shown that a wide variety of properties are available based on either simple alloy modifications and/or heat treatment of a standardized base compact composition. The observed properties compare favorably with those generally reported for the Compacted Graphite cast irons and the plain Ductile irons in both the as-cast and heat treated conditions. INTRODUCTION Starting in late 2001, research efforts in this laboratory were directed to developing iron base infiltration as a viable parts making process. The initial results of these efforts were reported in two papers that were presented in 2004 [1, 2]. The focus in both cases was to establish a basic understanding of the process and to define the conditions needed to implement it as a practical matter. It was shown that infiltration to near theoretical densities was possible in several Fe-C based alloy systems. The necessary processing conditions generally included temperatures below 1200 o C (2190 o F), times of less than ½ hour and the use of standard hydrogen-nitrogen atmospheres with modest methane additions (typically less than 0.5 v/o) to control the carbon potential. The required infiltrant compositions were at or near the corresponding eutectic liquidus values of the selected alloy system and the required base compact compositions were likewise at or near the eutectic solidus values of the selected system. The possibility to combine an infiltrant composition of one alloy system with a base compact composition of another system was also demonstrated. In addition, it was shown that base compact densities of 6.8 g/cm 3 or less were sufficient to obtain a virtually pore free density after infiltration. Thus, a particular advantage of the technology as compared with the traditional high pressure / high density processes [3, 4] is that the low starting densities essentially provided the potential to press larger parts that can later be infiltrated to the same or even higher final densities. The as-infiltrated carbon contents of the alloy systems that were studied typically ranged from about 2 % to about 2.35 %. In an otherwise un-alloyed Fe-C composition, carbon contents in this range normally result in an as-infiltrated microstructure which consists of pearlite in a network of hyper-eutectoid grain boundary carbides. Such microstructures are inherently brittle and have limited potential for structural applications. Consequently, the possibility to use alloy additions to graphitize the hyper-eutectoid carbon

2 and produce cast-iron like microstructures was investigated. It was found that modest additions of either silicon or nickel were effective in this regard. However, based on what was generally known of their alloying effects as well as the results of a series of trials with pre-alloyed nickel compositions [2], it was concluded that the silicon was the better choice for future development. Thus, subsequent work was directed towards obtaining a sufficient understanding of its effects on both the infiltration and graphitization processes to facilitate the design of silicon containing infiltrant and base compact compositions. The ultimate objective was to normalize the development of the technology in terms of one such alloy in each case as standard iron base infiltration compositions. As it turned out, the defining studies indicated silicon contents that nominally averaged 0.18 % for the infiltrant composition and 0.75 % for the base compact composition. Based on the eutectic equilibrium of the Fe-C-Si system as indicated by the ThermoCalc program [5], the corresponding carbon contents were 4.28 % for the infiltrant and 1.91 % for the base compact. The infiltrant weight to full density in this alloy system is about 15 % of the base compact weight. Thus, based on the infiltrant and base compact compositions in each case, its easily shown that the respective carbon and silicon contents to be expected after infiltration are about 2.21 % and 0.68 %. Likewise, assuming that all of the hyper-eutectoid carbon is graphitized during the process and that the balance forms pearlite, it can also be shown that the final density to be expected is about 7.53 g/cm 3. The attendant studies of the systems reasonably confirmed each of these expectations. Following this, a series of trials was conducted to determine the resultant transverse rupture and tensile properties as well as the effects on properties of minor alloy modifications of the standard base compact composition in both the as-infiltrated and heat treated condition. The alloy modifications in the survey included one or more of copper, nickel, manganese and molybdenum. Based on the graphite morphology that was observed during the defining studies, the heat treatments included stress relieving, light to full annealing, and normalizing. Regrettably, due to a lack of knowledge regarding the relevant transformation characteristics of these alloys, the Q&T and austempering heat treatments, as generally applied to the Ductile cast irons [6], were not attempted. The purpose of the present report is to communicate the results of this survey as well as to discuss their significance relative to the potential of the iron base infiltration technology versus both the cast irons and traditional P/M. EXPERIMENTAL PROCEDURE The alloy modifications that were included in the survey were made exclusively to the standard base compact composition (i.e. the infiltrant composition was not modified). In all, there were nine such modifications. These are shown overleaf in Table 1 along with the alloy designations that are used to identify them through the balance of the paper. The carbon contents shown in the table correspond to the eutectic solidus values of the various alloys. Each of the compositions was subsequently infiltrated with the standard infiltrant composition (4.28% C, 0.18% Si, bal. Fe and residual impurities). The infiltrant weight was nominally 13.5 % of the base compact weight. Thus, allowing for the contributions of the residual impurities typical of the iron base powders used in making the infiltrant and base compact compositions, the final infiltrated alloy contents in each case were about 90 % of the values shown in the table. Two iron base powders were used in making the several mixes including: Hoeganaes Corporation Ancorsteels 1000 B and 50 HP. The Ancorsteel 50 HP was used in making the molybdenum containing

3 compositions. The Ancorsteel 1000 B was used in making the balance of the compositions, the infiltrant composition and in diluting the Ancorsteel 50 HP to make the two 0.30% Mo containing compositions. Table 1 - Base Compact Compositions Used in the Study Alloy Carbon Manganese Copper Nickel Molybdenum Silicon ID % % % % % % Si Base Base Mn Base + 1 Cu Base + 1 Ni Base + 2 Cu Base + 1 Cu + 1 Ni Base Mo Base Mo + 1 Cu Base Mo Base Mo + 2 Cu The indicated carbon units in each case were admixed as Asbury grade 3203 HS graphite. Additional graphite in the amount of ~ 0.06 % for the base compact mixes and ~ 0.20 % for the infiltrant mix was added to allow for the inevitable dusting and sintering losses that occur in the process. The admix copper additions were made in the form of Acupowder grade 8081 copper. The admix nickel additions were added as International Nickel grade 123 nickel. The admix manganese additions were added as a proprietary Hoeganaes Corporation ferro-manganese alloy. The admix silicon additions were likewise added in the form of a proprietary ferro-silicon alloy. Each of the base compact mixes was lubricated with 0.45 % Lonza Acrawax C and 0.10% Baer Locher zinc stearate. The infiltrant mix was lubricated with 0.10% of the same grade zinc stearate. All of the mixes were submitted to binder treatment processing in accordance with the Ancorbond process [7]. The base compact mixes were compacted into standard transverse rupture strength (ASTM B 528) and dog bone tensile (ASTM E 8) specimens at a nominal density of 6.7 g/cm 3. The infiltrant slugs were compacted to the same geometries but at a constant compaction pressure of ~ 550 MPa (40 tsi). As mentioned, the infiltrant weight was nominally 13.5 % of the base compact weight. Testing after infiltration was in accordance with the indicated specifications. Density checks of the tensile specimens were typically limited to one or two specimens per composition and were conducted prior to testing using the water immersion method (ASTM B 328). TRS testing was performed on a Tinius Olsen compression testing machine at a crosshead speed of 2.5 mm/min (0.1 /min). Tensile testing was performed on a Zwick/Roell Z-100 tensile machine at a crosshead speed of mm /min (0.025 /min). The machine was equipped with a 2.5 cm (1 ) extensometer and provided automated readouts of the elastic modulus, the 0.2 % offset yield strength, the ultimate tensile strength and the percent elongation value. The reported TRS and tensile properties in each case represent the average of tests on from three to five specimens per condition. The specimens were processed in an high temperature production belt furnace. Infiltration was at 1185 o C (2165 o F) for ½ hour at temperature. The furnace atmosphere was nominally 90 % N 2 and 10 % H 2 by volume and was additionally treated with 0.25 v/o of CH 4 to minimize carbon losses to oxygen impurities. To insure complete graphitization of the hyper-eutectoid carbon contents of the specimens, normal cooling subsequent to infiltration was interrupted at temperatures just below the lower critical temperature of the 0.75% Si Base composition at about 760 o C (1400 o F) with a slow cooling step of from 10 to 15 minutes duration.

4 The TRS specimens were tested in the as-infiltrated condition. The tensile specimens were tested in both the as-infiltrated and heat treated conditions. Four different heat treatments were investigated including: a stress relief, a sub-critical anneal, a normalizing treatment and, a partial ferritizing anneal. The stress relief was at 200 o C (400 o F) for 1 hour in N 2. The sub-critical anneal was at 755 o C (1390 o F) for ½ hour in N 2. The normalizing treatment consisted of austenitizing at 870 o C (1600 o F) for ½ hour in synthetic DA followed by normal cooling in the cold zone of the furnace to ambient temperatures [i.e. at about 100 o C/min (3 o F/sec) in the range from 845 to 315 o C (1550 to 600 o F)].The partial ferritizing anneal consisted of austenitizing at 925 o C (1700 o F) for ½ hour in synthetic DA, furnace cooling to 715 o C (1320 o F) and holding for 1.5 hours in N 2 followed by normal cooling to ambient temperatures. As a matter of interest, in the case of Ductile iron a full ferritizing anneal reportedly requires a hold of about 5 hours at the sub-critical temperature plus slow cooling thereafter to 345 o C (650 o F) [8]. RESULTS AND DISCUSSION In general, the study of the indicated compositions and conditions of heat treatment was the cumulative outcome of a series of three smaller studies. All had the explicit objective to determine the mechanical property potential of the iron base infiltration technology. However, each addressed different interests in terms of the alloy additions, the heat treatments and the particular properties that were examined. Consequently, its appropriate to present and discuss the results in terms of these smaller studies. Tensile Properties of the Silicon Base Composition and Alloy Effects of Copper, Nickel and Manganese The objective of first of the three studies was to determine the tensile properties of the 0.75 % Si Base composition and to get an indication of the effects of modest additions (~ 1 % or less) of copper, nickel and manganese. The alloy effects of copper and nickel were of interest because they are well known P/M additives. The manganese composition was included because there was also an interest to compare the results with the properties of the cast irons and other than carbon and silicon, all of the several cast iron grades that exist contain some manganese [9]. The tensile properties of the Si Base and of four alloy modifications of the Base were determined in the as-infiltrated condition and both the stress relieved and sub-critically annealed conditions. The asinfiltrated properties of the five compositions are shown below in Table 2. Table 2 As-Infiltrated Properties of the Si Base and Cu, Ni and Mn Modified Mixes Alloy ID Yield Strength Ultimate Strength Elongation Hardness Density MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA g/cm 3 Si Base (51.3) (70.0) Base + 1 Cu (61.2) (86.6) Base + 1 Ni (54.6) (73.1) Base + 1 Cu + 1 Ni (62.5) (84.5) Base Mn (55.8) (74.8) The final carbon contents of the specimens ranged from ~ 2.0 to 2.1 %. The individual values generally varied with the eutectic solidus values of the base compact compositions as indicated in the earlier Table 1 and otherwise appeared to have been effected by modest dusting and sintering losses during processing in advance of infiltration. Assuming complete graphitization of the hyper-eutectoid carbon contents involved, the corresponding pore free densities of the specimens would be expected to range from ~ 7.52 to 7.54 g/cm 3. Thus, the densities of the first three compositions listed in Table 2 were all

5 upwards of 98.5 % of the pore free value. In contrast, the densities of the last two compositions listed were obviously low in comparison with the first three. Nevertheless, both were upwards of 96 % of the pore free value. In the case of the Base + 1Cu + 1Ni specimens, the low density is thought to indicate an unfavorable effect of the combination of the copper and nickel on one or more of the densification mechanisms involved in the process. Other than infiltration which, of course, is the principle mechanism, the process that was used in the study was specifically designed to effect about 20 to 25 % of the densification by liquid phase sintering. Thus, the low density, in this case, was very probably the result of a poor liquid phase sintering response due possibly to an adverse effect of the alloy additives on the dihedral angle of the system. Liquid phase sintering may have also contributed to the low density of the Base Mn specimens as well. However, in this case, there were clear indications that the effect was due chiefly to gas formation, probably the result of on-going MnO reduction, subsequent to infiltration. As expected, the tensile and hardness results in the table showed that the Si Base composition had the lowest overall properties. In comparison, each of the alloy compositions exhibited substantially higher strength values and in most cases, modestly higher ductility and hardness values as well. The greatest increases in all four properties were in the specimens of the Base + 1Cu and the Base + 1Cu + 1Ni compositions. In view of the apparently poor liquid phase sintering response of the latter composition, the general implication of the findings was that copper was the single most effective alloy addition. The stress relief anneal generally led to very modest improvements in the ultimate strength and ductility values but to little or no change in the yield strength, hardness or density values. In contrast, the subcritical anneal led to fairly substantial ductility increases but to equally significant decreases in strength and hardness. The density, in most cases, was unaffected by this treatment. The results of the sub-critical anneal are shown below in Table 3. Table 3 Sub-Critically Annealed Properties of the Si Base and Cu, Ni and Mn Modified Mixes Alloy ID Yield Strength Ultimate Strength Elongation Hardness Density MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA g/cm 3 Si Base (45.8) (69.0) Base + 1 Cu (52.7) (81.5) Base + 1 Ni (47.4) (71.1) Base + 1 Cu + 1 Ni (48.5) (73.4) Base Mn (47.0) (69.0) The presence of free graphite in these compositions makes them similar in many respects to the cast irons. Apart from alloy content and the microstructure of the iron base matrix, it is the morphology of the graphite precipitates that largely determines the properties of the cast irons. In general, there are four different morphologies which comprise the predominant types in each of the four principal cast iron grades. In order of decreasing symmetry and correspondingly, of decreasing potential in terms of mechanical properties, these include: 1) the nodular or spheroidal type of the Ductile cast irons; 2) the temper carbon type of the Malleable cast irons; 3) the vermicular or compacted type of the Compacted Graphite cast irons; and, 4) the flake type of the Grey cast irons [10]. The predominant graphite morphology of the present compositions including those that have yet to be discussed was the vermicular or compacted type. Thus, it is of interest to compare the properties of the present compositions with those of the Compacted Graphite (or CG) cast irons. For the record, a micrograph showing the vermicular or compacted graphite morphology that typified the compositions of the study is shown overleaf in Figure 1.

6 Figure 1 Typical Graphite Morphology of Iron Base Infiltrated Specimens For purposes of the indicated comparison, the mechanical properties of two grades of the CG cast iron as reported in the open literature are presented in Table 4 [11]. These data include the as-cast condition as well as two conditions of heat treatment: the fully ferritized; and, the normalized conditions. Based on the similarities of the microstructures of the iron based matrices involved, the as-cast and normalized conditions are reasonably comparable to the present as-infiltrated condition. The ferritizing condition, however, is specifically designed to increase ductility and is not comparable with either the as-infiltrated condition or the sub-critically annealed condition. These results were included here primarily as general information and to provide a point of comparison with findings that will be introduced later. A brief review of the data in Table 4 will show that the highest strength and hardness properties are associated with the normalized condition and the highest ductilities are, of course, those of the ferritized condition. Predictably, the strength and hardness values of the nickel containing grade were generally better than those of the un-alloyed grade. Table 4 - Typical Tensile Properties Of Compacted Graphite Cast Irons Iron Yield Strength Tensile Strength Elongation Hardness Nickel Condition Matrix (a) MPa (ksi) MPa (ksi) % HRA (e) % As-Cast 60% F 263 (38.1) 325 (47.1) Ferritized (b) 100% F 231 (33.5) 294 (42.6) Normalized (c) 90% P 307 (44.5) 423 (61.3) As-Cast 328 (46.7) 427 (61.9) Ferritized (b) 100% F 287 (41.6) 333 (48.3) Normalized (c) 90% P 375 (54.4) 503 (73.0) (a) F, ferrite; P, pearlite. (b) Annealed, 2 hr. at 900 o C (1650 o F), furnace cooled to 690 o C (1275 o F), held 12 hr., cooled in air. (c) Austenitized 2 hr. at 900 o C (1650 o F), cooled in air. (e) Converted from Brinell values.

7 Comparison of the data in this table with those in the earlier Table 2 will show that the strength and hardness of all of the infiltrated compositions were significantly better than those of both the CG irons in the as cast condition and of the plain CG iron in the normalized condition. Otherwise, the strength and hardness of the Si Base composition closely approached those of the nickel containing grade in the normalized condition while those of the remaining four compositions were either equivalent or substantially better than those of the nickel grade in the normalized condition. In contrast, the ductilities of the infiltrated compositions were generally not as good as the cast irons. However, the differences in the as-cast and normalized conditions were not great and may have simply been a natural consequence of the greater strengths and hardnesses of the infiltrated compositions. In addition, the geometries of the test specimens that were used in each case were significantly different and may have also contributed to the ductility differences. For example, the properties of the cast irons were reportedly based on 25 mm (1 ) diameter rounds whereas the infiltrated compositions were, as previously indicated, based on the standard P/M dog bone geometry. The fact that the strengths and hardnesses of the infiltrated compositions were generally better than those of the CG cast irons is thought to be attributable to the inherently lower densities of the cast irons. For example, compared with the present compositions, the cast irons generally have both higher carbon and higher silicon contents and each lead to significantly lower pore free densities. In the case of the CG irons, the carbon reportedly ranges from 2.5 to 4.0 % and the silicon from 1.0 to 3.0 % [9]. At a midrange carbon of 3.25 % and a mid-range silicon of 2.0 %, quantitative estimates indicate that their pore free density ranges from about 7.25 g/cm 3 in the normalized condition (~ 90 % pearlite) to about 7.17 g/cm 3 in the fully ferritized condition (100 % ferrite). In comparison, at a mid-range carbon of 2.05 % and a silicon of 0.68 %, the same method of estimation indicates that the pore free density of the present Si Base composition ranges from 7.53 g/cm 3 in the normalized condition (~ 95 % pearlite) to about 7.45 g/cm 3 in the fully ferritized condition. It is also of interest to compare the properties of the present compositions with those of the standard P/M grades. Since the microstructures of the infiltrated compositions are predominantly pearlitic, the most realistic comparison is with the predominantly pearlitic P/M grades or, in effect, with the 0.6 to 0.9 % carbon containing grades in the as-sintered condition. MPIF Standard 35 lists three such grades which are otherwise plausibly comparable in terms of their total second alloy contents. These include the following: F-0008, FC-0208 and FN The highest tensile properties that the Standard lists for each are shown below in Table 5. Table 5 Tensile Properties of Comparable Standard P/M Grades MPIF Yield Strength Ultimate Strength Elongation Hardness Density Grade Designation MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA * g/cm 3 F (40.0) (57.0) FC (65.0) (75.0) < FN (55.0) (90.0) * Converted from HRB values. Limiting the considerations to the Si Base and Base + 1Cu compositions, comparison of the data in this table with those in the earlier Table 2 will show that the properties of the Si Base were far superior to those of the F-0008 grade and otherwise approached those of the FC-0208 grade. Similarly, the properties of the Base + 1Cu composition were generally superior to those of the FC-0208 grade and rivaled those of the FN-0208 grade. Here again, the higher density of the infiltrated compositions is almost certainly the major underlying cause of the indicated differences.

8 Alloy Effects of Molybdenum and Molybdenum Plus Copper The second in the series of the three smaller studies mentioned had the objective to determine the alloy effects of modest additions of molybdenum and molybdenum and copper to the Si Base composition. It consisted essentially of infiltrating the four molybdenum containing compositions that are indicated in the earlier Table 1 and of determining their tensile properties in each of three conditions including: the asinfiltrated, the stress relieved, and the sub-critically annealed conditions. As is generally well known, molybdenum is more frequently combined with nickel in P/M applications than with copper. Thus, it may be of interest to note that the preference for copper in this instance was largely based on its relatively better performance than nickel in the earlier study. As in the earlier study, the properties in the stress relieved condition were in most cases very similar to those of the as-infiltrated condition. However, in the case of the Base Mo + 2 Cu composition, the stress relief effected significant increases in both the yield and ultimate strength values of the order of 70 Mpa (10,000 psi) each. Thus, in this instance, it is appropriate to present and discuss the stress relieved properties rather than the as-infiltrated ones. These are shown below in Table 6. Table 6 -Tensile Properties of the Molybdenum Containing Alloys in the Stress Relieved Condition Elastic Yield Ultimate Alloy ID Modulus Strength Strength Elongation Hard. Den. GPa (10 6 psi) MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm RHA g/cm 3 Base Mo (23.5) (65.3) (94.0) Base Mo (23.6) (82.7) (99.4) Base Mo + 1 Cu (24.8) (85.3) (108.5) Base Mo + 2 Cu (23.3) (80.9) (109.3) Comparison with the data in Table 2 will show that the infiltrated densities of these compositions were a little higher than those of the earlier study. The two highest values listed (7.53 g/cm 3 ), in fact, equaled the best available estimates of the corresponding pore free densities. As a matter of interest, assuming no graphitization of the hyper-eutectoid carbon, estimates place the limiting pore free density of the iron base infiltration process at about 7.64 g/cm 3. Thus, higher infiltrated densities than the present values are possible and are sometimes observed. However, their occurrence indicates incomplete graphitization of the hyper-eutectoid carbon and the likely presence of coarse grain boundary carbides which are almost certain to have adverse effects on ductility and ductility related properties (e.g., ultimate strength, toughness and machinability). In addition to the strength and elongation values, the table also lists the elastic modulus values that were observed in the tests. These data were included because this property is chiefly affected by density and the high densities of the present compositions were expected to manifest as increased modulus values. In fact, the values shown in the table are generally higher than those quoted in MPIF Standard 35 for the majority of P/M grades which, of course, typically involve lower densities. However, there was also a lot of scatter in the present data and there is reason to suspect that somewhat higher values may have been observed if the method of testing had been more reliable. For example, in a study of the effects of carbon content on the properties of Ductile iron, the observed modulus values for carbon contents in the present range (2.0 to 2.1 %) averaged about 10 % higher than the above values at ~182 GPa (26.5 x 10 6 psi), [12]. A review of the remaining data in Table 6 will show that strength and hardness generally increased and ductility decreased with increasing alloy content. Thus, the Base Mo composition had the lowest ultimate strength and hardness and the highest elongation values whereas the Base Mo + 2 Cu composition had the highest strength and hardness and the lowest elongation values. The correlation with

9 alloy content, however, was not perfect. It broke down somewhat in the case of the yield strength. For example, the yield strength increased with increasing alloy content up to the Base Mo + 1 Cu composition but decreased again in the case of the Base Mo + 2 Cu specimens. This and the fact that the ultimate strength and elongation differences between these compositions were not large (i.e. almost certainly not statistically different) suggested that the leaner of the two may actually be closer to the optimum in terms of alloy content. As will be seen, the properties in the sub-critically annealed condition tended to support this idea but also showed an interesting aspect of the more highly alloyed composition as well. These findings are shown below in Table 7. Table 7 - Sub-Critically Annealed Properties of the Molybdenum Containing Compositions Alloy ID Elastic Yield Ultimate Elong. Hard. Den. Modulus Strength Strength GPa (10 6 psi) MPa (10 3 MPa (103 % in 2.5 psi) RHA g/cm 3 psi) cm Base Mo (25.0) (49.9) (82.4) Base Mo (24.8) (55.6) (85.4) Base Mo + 1 Cu (27.0) (54.1) (85.3) Base Mo + 2 Cu (23.6) (71.2) (88.3) Comparison of these results with those in Table 6 will show that the anneal in this case had the same general effects as seen in the previous study (Table 2 vs. Table 3). The strength and hardness values decreased and the elongation values increased. However, in contrast with the previous study, the elongation improvements in this instance were substantially larger than the earlier ones both on a percentage basis and, in most cases, in terms of the final absolute values. Undoubtedly, the best example of the latter is the elongation value of the Base Mo + 1 Cu composition which at 5.1% rivaled those of the CG irons in the fully ferritized condition (Table 4). The reasons underlying the greater elongations in this instance, however, were uncertain. Since the processing in the two studies was nominally the same, its reasonable to speculate that they were some how related to the compositional differences (e.g., possibly to an effect of the molybdenum on carbide spheroidization kinetics). However, whatever the reason, it will be clear that its investigation was generally beyond the scope of the study. The results in Tables 6 and 7 also show that both the density and the elastic modulus properties were essentially unaffected by the anneal. The constancy of these values and especially of the density is an indication that carbide spheroidization rather than graphitization was the primary metallurgical effect of the anneal. If significant graphitization had occurred during the process, then the density and, at least in theory, the modulus values would have decreased. As a matter of interest, metallographic examinations confirmed the existence of significant carbide spheroidization in connection with the anneal. Returning to the general question of alloy effects, comparisons showed that the strength and hardness properties of the molybdenum containing compositions were substantially higher than those of the compositions of the earlier study in both the as-infiltrated and stress relieved conditions. This is indicated by the data in Tables 2 and 6 but, of course, the comparison in this instance in not direct because these tables refer to different conditions (as-infiltrated vs. stress relieved). In the case of the data in Tables 3 and 7, however, the comparison is direct and clearly show the indicated superiority of the molybdenum containing compositions in the sub-critically annealed condition. Copper and molybdenum are commonly added to the Compacted Graphite cast irons and here again, where reasonably direct comparisons were possible, the indications were that the properties of the present iron base infiltrated compositions were generally superior in strengths and hardness and reasonably comparable in ductility [13, 14]. For example, based on a correlation of the effects of molybdenum on the

10 tensile properties of the CG irons, the ultimate strength to be expected in the predominantly pearlitic iron is reportedly ~ 475 MPa (68,900 psi) at a molybdenum content of 0.3 % and ~ 522 MPa (75,600 psi) at a content of 0.5 % [15]. As will be evident, both values are significantly inferior to the ultimate strength values shown in each of the foregoing tables for the Base Mo and Base Mo compositions. More interesting, perhaps, is a comparison of the properties of the present compositions with those of the well known P/M grades that contain molybdenum. The properties of three such grades as selected from the Low Alloy Steel and Diffusion Alloyed Steel categories of MPIF Standard 35 are shown below in Table 8. In each case, the data correspond to steels in the as-sintered condition with carbon contents in the eutectoid range from 0.4 to 0.9 %. Significantly, the strength, hardness and modulus values that are shown in the table are essentially the highest values listed in the Standard for steels in the as-sintered condition. Table 8 Properties of Selected Molybdenum Containing P/M Steels Elastic Yield Ultimate MPIF Elong. Hard. Den. Modulus Strength Strength Grade Designation GPa (10 6 psi) MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm RHA g/cm 3 FLN (23.5) (70.0) (100.0) FD (23.0) (73.0) (103.0) FD (24.5) (71.0) (125.0) As those familiar with the MPIF system of grade designation will appreciate, the alloy contents of the indicated grades are generally both different and in particular, higher than those of the present infiltrated compositions. All three of the P/M grades contain at least 0.5 % molybdenum and 1.75 % nickel. In addition, each of the diffusion alloyed (FD pre-fixed) grades nominally contain 1.5 % copper. However, despite these differences, comparison of the data in this table with those in the earlier Table 6 will show that the strength, hardness and modulus values of each of the two copper containing variants of the infiltrated compositions were superior to the those of the FLN and FD-0208 grades and were either superior to or closely approached those of the FD-0408 grade. Otherwise, the ductility values of the two data sets were reasonably comparable. Alloy Effects of Copper in the Si Base and Two Additional Heat Treatments The objective of the third and last of the three smaller studies mentioned was to examine the effects on properties of copper modifications of up to 2 % of the Si Base composition in five conditions as follows: as-infiltrated, stress relieved, sub-critically annealed, normalized and partially ferritized. The study also included determinations of the TRS properties in the as-infiltrated condition. These results are shown below in Table 9. Table 9 - TRS Properties of the Si Base and Copper Modified Base in the As-infiltrated Condition Alloy ID Transverse Rupture Strength Dimensional Change Hardness Density MPa (10 3 psi) % RHA g/cm 3 Si Base (152.5) Base + 1 Cu (159.6) Base + 2 Cu (166.6) The TRS properties were chiefly of interest because target applications include Grey cast iron components and rupture strength is a commonly cited property of the Grey irons. However, the latter is typically quoted in terms of the breaking load of a standard test specimen and is not directly comparable

11 with the usual P/M values. For comparison, standard P/M specimens were prepared from a Grey iron component of interest. They exhibited an average TRS value of 465 MPa (67,500 psi), a hardness of 53 RHA, and a density of 7.23 g/cm 3. The tensile properties of the subject compositions in each of the three process conditions that have so far been cited in the paper are shown in Table 10. Table 10 - Tensile Properties of the Si Base and Two Copper Containing Compositions in the As- Infiltrated, Stress Relieved and Sub-Critically Annealed Conditions Alloy ID Yield Strength Ultimate Strength Elongation Hardness Density MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA g/cm 3 As-Infiltrated Si Base (53.1) (72.9) Base + 1 Cu (59.5) (89.0) Base + 2 Cu (69.8) (87.7) Stress Relieved Si Base (51.4) (76.7) Base + 1 Cu (59.5) (87.9) Base + 2 Cu (70.2) (84.2) Sub-critically Annealed Si Base (43.3) (76.1) Base + 1 Cu (46.7) (77.6) Base + 2 Cu (55.4) (77.7) A review of these data will show that the densities of the Si Base and Base + 1 Cu compositions were a little lower than earlier (Tables 2 & 3) and that the density of the Base + 2 Cu composition was essentially intermediate of these two. All of the values were upwards of 98 % of the pore free value (~7.53 g/cm 3 ). Comparison of the tensile properties of the Si Base and Base + 1 Cu compositions with those of the earlier study will show that the two data sets were generally similar. In many cases, the ultimate strength and elongation values in the present data were marginally higher while the yield strength and hardness values were either the same or marginally lower. The general trends in the data with respect to the effects of the heat treatments were also similar. Relative to the as-infiltrated condition, the stress relief either slightly increased the ultimate strength and elongation values or had no effect while the sub-critical anneal generally decreased the strength and hardness values and increased the elongations. In the case of the Base + 2 Cu composition, the major effects of the additional copper appeared to be to increase the yield strength and hardness and decrease the elongation values relative to both the Si Base and Base + 1Cu compositions. The ultimate strength values of the higher copper composition were either the same or marginally lower than those of the Base + 1 Cu composition. The indicated increases in the yield strength were in all cases fairly substantial ( 70 MPa 10,000 psi) whereas the hardness increases were generally marginal. There was also a similar indication in the case of the yield strength of the high Cu variant of the earlier molybdenum containing compositions but only in the sub-critically annealed condition (Table 7). The general indication of the study as a whole with respect to copper was that its most consistent effects were in increasing the yield strength and, to a lesser degree, the hardness properties. For example, at 1 % copper, the yield strength and hardness increases were attended by similar or greater increases in the ultimate strength and by increased ductilities. However, at 2 % copper, whereas the yield strength

12 continued to increase, the effects on the ultimate strength and hardness were both small and mixed and were otherwise accompanied in all cases by decreased ductilities. The properties of the subject compositions in the normalized condition are shown below in Table 11. Table 11 Properties of the Si Base and Copper Modified Bases in the Normalized Condition Alloy ID Yield Strength Ultimate Strength Elongation Hardness Density MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA g/cm 3 Si Base (84.7) (113.3) Base + 1 Cu (92.6) (120.8) Base + 2 Cu (106.3) (121.2) A review of these data will show that the heat treatment had virtually no effect on density but very substantial effects on each of the other properties listed. For example, a general comparison of the tensile and hardness values in the table with those in the balance of the paper will show that in most cases, they surpassed the best of the latter in strength and hardness and were otherwise similar in ductility in the asinfiltrated and stress relieved conditions. Both these properties and the as-infiltrated and stress relieved properties in the earlier Table 10 are generally much higher than those of any of the known CG irons. However, they are comparable with the properties of the Ductile irons in the plain or essentially un-alloyed condition (i.e. containing from 3 to 4 % C, 0.1 to 1.0 % Mn, and 1.8 to 2.8 % Si) [9]. Remarkably, in fact, although marginal in ductility, the strength and hardness values indicated by the present findings significantly exceed those of the latter in most instances. For example, the minimum requirements of the three highest strength grades of plain Ductile iron in accordance with ASTM A 536 are shown below in Table 12. Table 12 Minimum Tensile Property Requirements of Ductile Iron According to ASTM A 536 ASTM Yield Strength Ultimate Strength Elongation Typically Recommended Grade Designation MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm Process Condition (60.0) (80.0) 3.0 As Cast (70.0) (100.0) 3.0 Normalized (90.0) (120.0) 2.0 Oil Quenched & Tempered A cursory comparison of the present findings with these data will show the following: 1) the requirements of the grade were closely approached by the properties of the Base + 1 Cu composition in the as-infiltrated and stress relieved conditions; 2) the strength and hardness requirements of the grade were significantly exceeded by the properties of the Si Base composition in the normalized condition; and, 3) the requirements of the oil quenched and tempered grade were met by the properties of the Base + 1 Cu composition in the normalized condition. To be fair, what is being called normalizing here may be more akin to sinter hardening. As is generally well known, the normalizing heat treatment derives its name from the fact that it consists of austenitizing, usually at a low temperature above the upper critical, followed by normal cooling in still air. As applied to cast irons, the general aims are to eliminate hyper-eutectoid carbides if they exist, refine the grain size and produce an iron base matrix that has a predominantly pearlitic microstructure. The cooling rate of the process naturally depends on the mass of the casting and may be less than ~10 o C/min (0.3 o F/sec). In comparison, the findings in the present case derive from specimens that were normally cooled in the cooling zone of a P/M furnace. As mentioned in the procedure section, the average cooling rate was estimated to be ~100 o C/min (3 o F/sec) in the temperature range from 845 to 315 o C (1550 to 600 o F).

13 This, of course, is essentially an intermediate cooling rate in the sinter hardening range and, at the very least, would be expected to produce a finer grain size and a smaller interlamillar spacing than would be typical of a normally cooled large casting. In fact, metallographic examinations showed the microstructures of the resulting iron base matrices of all three compositions to be 100% pearlitic. In each case, the grain size was refined relative to the asinfiltrated condition and the interlamillar spacing of the pearlite was exceedingly fine; being completely irresolvable at 1000X. Thus, in view of the fact that the pore free density of the Ductile irons is about the same as that of the CG irons, the excellent properties of the present compositions in comparison with the Ductile iron requirements were thought to be largely attributable to the combined effects of their higher densities and the relatively higher cooling rates that typify normal P/M processing. The properties resulting from the partial ferritizing anneal of the subject compositions are shown below in Table 13. Table 13 Response of the Si Base and Copper Modified Bases to the Partial Ferritizing Anneal Alloy ID Yield Strength Ultimate Strength Elongation Hardness Density MPa (10 3 psi) MPa (10 3 psi) % in 2.5 cm HRA g/cm 3 Si Base (34.7) (64.2) Base + 1 Cu (45.7) (72.1) Base + 2 Cu (63.5) (80.0) The aim in attempting to partially ferritize the subject compositions was to get some idea of what the iron base infiltration technology has to offer in terms of ductility. Unfortunately, however, the particular treatment that was used here was not sufficiently ferritizing to do this. Accordingly, a comparison of the above results with those in the earlier Table 10 will show that while the treatment did result in increased ductility values relative to the as-infiltrated and stress relieved conditions, it was generally not as effective in this regard as the simpler sub-critical anneal. In cast irons, ferritizing is normally accomplished by a treatment that consists of first austenitizing at a temperature that is sufficiently above the critical to dissolve any hyper-eutectoid carbides that may exist followed by furnace cooling to a lower temperature that is just below the critical. The actual ferritizing of the structure occurs at the lower temperature and typically requires minimal holding times of several hours (cf., the ferritizing treatment that was noted in the case of the CG irons in Table 4). The specific details of the process differ according to the type of cast iron being treated [8]. In the present case, limited dilatometric studies of the transformation characteristics of the Si Base composition had suggested the particular treatment that was used here. As it turned out, analysis of the findings combined with the results of metallographic examinations of the broken tensile specimens in each case indicated that the treatment fell short of producing the desired effects for two reasons as follows. First and foremost, the hold time (1.5 hrs) at the low temperature (715 o C) was simply too short; and second, the low temperature itself may have been marginally high with respect to the Base + 1 Cu composition and was almost certainly too high for the Base + 2 Cu composition. Thus, for now, the ductility potential of the iron base infiltration technology remains for future studies to demonstrate. Summary and Conclusions The mechanical property potential of iron base infiltration as a novel P/M technology was presented in terms of the tensile and transverse rupture properties and property improvements that are obtainable by

14 simple alloy modifications and/or heat treatment of a standardized silicon containing base composition. Nine different alloy modifications of the Si Base composition involving modest additions of one or two of copper, nickel, molybdenum and manganese (Table 1) as well as limited investigations of four different heat treatments versus the as-infiltrated condition were included in the study. The latter included a low temperature stress relief, a sub-critical anneal, normalizing and a partial ferritizing anneal. Initially, the as-infiltrated tensile properties of the Si Base and four alloy modifications of the base were presented (Table 2) and discussed. The alloy modifications involved additions of copper, nickel, copper plus nickel and manganese. The as-infiltrated densities varied from a low of 7.25 g/cm 3 to a high of 7.47 g/cm 3 versus a pore free value of ~ 7.53 g/cm 3. The properties of all four of the alloy modifications were superior to those of the Si Base composition. The Base + 1 Cu composition exhibited the best overall properties. Based on this and the infiltrated densities of the various compositions, nickel and manganese were eliminated from further consideration in the study. The five compositions were also tested in the stress relieved and sub-critically annealed conditions. In most cases, the stress relief led to slight increases in ultimate strength and elongation but had little to no effect on the yield strength, hardness or density. The sub-critical anneal generally decreased both strength and hardness and increased elongation (Table 3). There was no significant effect of the anneal on density. It was explained that most of the carbon in the silicon containing compositions of the study is in the form of graphite precipitates of the vermicular or compacted variety which are also common to the so-called Compacted Graphite or CG cast irons. Based on this similarity, the as-infiltrated properties of the Si Base and Base + 1 Cu compositions were compared with those of a plain and an alloyed CG iron in both the ascast and normalized conditions (Table 4). The comparison generally showed that the properties of the Si Base composition were superior to those of the plain CG iron and similar to those of the alloyed one and that the properties of the Base + 1Cu composition were superior to both. The as-infiltrated properties of the Si Base and Base + 1 Cu compositions were also compared with those of various well known P/M grades with somewhat similar results (Table 5). In general, the alloy contents of both the P/M grades and the CG irons in these comparisons were higher than those of the infiltrated compositions but their densities were appreciably lower. Thus, the generally better properties of the infiltrated compositions in the two cases were attributed to their higher densities. The results of a similar study of the alloy effects of molybdenum and molybdenum plus copper on the Si Base composition were next presented (Table 6) and discussed. The data in this case included the elastic modulus values that were observed as well as the usual tensile properties. It was noted that the modulus values were generally higher than those of most of the standard P/M steels and that this was expected in view of the higher densities of the infiltrated compositions. Otherwise, the data showed that strength and hardness increased and elongation decreased with increasing alloy content. Relative to the earlier study, the strengths and hardnesses of the these compositions were all substantially higher than those of the Si Base composition and with the exception of the leanest alloyed one of the present series, higher than those of the Base + 1 Cu composition as well. The properties of the molybdenum containing compositions in the sub-critically annealed condition were also presented (Table 7) and discussed. As previously, the general effect of the anneal was to decrease strength and hardness and increase elongation. Interestingly, however, in spite of the fact that the strengths of these compositions were generally higher than those of the earlier study, their elongation values were also generally higher as well. Metallographic examinations indicated that the primary microstructural result of the anneal was carbide spheroidization and it was speculated that the better ductilities in this case may be an effect of the molybdenum additions on spheroidization kinetics.