Atomized Low Apparent Density (AD) Iron Powder For Advanced PM Applications

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1 Atomized Low Apparent Density (AD) Iron Powder For Advanced PM Applications Peter Sokolowski and Francis Hanejko Hoeganaes Corporation Cinnaminson, NJ ABSTRACT A low apparent density atomized iron powder, Ancorsteel AMH, was developed to meet market demands. The primary usages for this material are lower density applications where green and sintered strengths are critical to the performance of the PM part. This paper will focus on the green and sintered comparisons between Ancorsteel AMH and traditional sponge iron in both straight graphite and copper graphite premixes. It will be demonstrated that this material can be utilized as an alternative to traditional sponge iron powder with increased micro-cleanliness and no loss in mechanical properties. INTRODUCTION The application of sponge iron in the marketplace has been successful for years, largely due to its attractive features including favorable green strength at low part densities and high specific surface area. These characteristics enabled sponge iron to be commonplace in products ranging from shock absorbers to serving as catalysts for prolonged low temperature heating in heat packs. In an effort to continually supply these areas of application and potentially expand into new markets that require higher quality material at reduced costs, an alternative sponge like iron powder was developed, Ancorsteel AMH. The capability to atomize an iron powder with characteristics approaching that of traditional sponge has facilitated its incorporation into the PM industry as an alternate to sponge. While for some applications the porous structure of sponge is ideal, the fine pore network inhibits its ability to reach higher densities at reasonable compaction pressures, preventing it from being useful in parts requiring improved sintered strengths. Therefore, this paper discusses the relevant properties of a water atomized iron powder alternative for reduced sponge iron, highlighting its usefulness and benefits. The fabrication method used to produce traditional sponge iron versus that of water atomized iron powder, as discussed here [1], form similar yet notably different particle morphologies, which set them apart from one another. Table 1 lists a few key properties of a conventional water atomized powder (steel), the newly developed low apparent density powder (AMH), and that of a traditional direct reduced sponge powder (sponge). These properties were measured on commercially available products. As seen in the table, the apparent density (AD) of AMH at 2.55 g/cm 3 lies between that of steel, 2.92 g/cm 3, and the sponge value, 2.50 g/cm 3. The powder flow characteristics are similar among the powders tested, suggesting comparable behavior during die fill. As already mentioned, the surface area of sponge makes it ideal for applications sensitive to chemical reactivity, such as controlled low temperature heat packs. The specific surface areas were measured using the BET method with krypton gas over multiple partial pressures as opposed to a single point analysis, providing a more accurate result. It is evident that AMH has lower surface area than sponge, however it shows an increase over steel powders at comparable particle size distributions and its already successful inception in heat pack applications has proven it as a viable alternative to sponge. Ancorsteel is a registered trademark of Hoeganaes Corporation

2 ID Table 1: Iron powder properties Material AD (g/cm 3 ) Flow (s/50g) BET surface area (m 2 /g, ± ) Steel Ancorsteel 1000B AMH Ancorsteel AMH Sponge Traditional Sponge Iron Shown in Figure 1 are representative SEM (top row) photomicrographs of a sponge particle (A1), steel particle (B1), and an AMH particle (C1). The sponge and AMH particles share similar surface irregularities greater than steel powder, where AMH is a hybrid of surface characteristics between sponge and steel particles. Surface irregularities increase the total surface area, Table 1, capable of mechanically locking with another particle surface during compaction; thus promoting higher green strengths. The light optical photos (bottom row) illustrate differences in porosity between the three materials at a density of 6.8 g/cm 3. In (A2), the sponge material exhibits widespread fine internal porosity with few large pores. Steel particles (B2) typically have little or no internal porosity and thus the porosity is largely between particle surfaces. As a result of the proprietary processing method, AMH contains a greater amount of internal porosity over steel powder and yet exhibits similar, larger inter-particle surface pores (C2). The reduced amount of internal porosity is one characteristic that allows steel and AMH powders to achieve higher green densities at lower compaction pressures. Another distinction can be made concerning micro-cleanliness of the powder, which has a strong affect on the sintered properties and consequently the structural performance. As an integral processing step prior to water atomization, the liquid metal refining enables increased control over cleanliness of the final product via slag formation. Typical sponge production does not allow for similar processing methods to reduce impurities in the charged material. To highlight the difference in cleanliness between sponge and AMH, representative powder samples of each were sintered, forged, cross-sectioned, and analyzed using image analysis software to determine the level of contamination. Figure 2 is an un-etched photomicrograph that illustrates the non-metallic inclusion content inherent with reduced iron powder. The dark areas are non-metallic particles and the light gray area is fully dense sponge with 0.6 w/o graphite. Owing to the source of raw material and nature of the processing route, sponge typically contains a significant amount of non-metallic inclusions such as silicon or titanium oxides. These inclusions, some quite large, can negatively impact properties and lead to machining difficulties. In contrast, Figure 3 displays the cross-section view of fully dense AMH with 0.6 w/o graphite. It is evident there are few inclusions present, providing a greater degree of cleanliness.

3 A1 B1 C1 A2 B2 C2 Figure 1: Photomicrographs of (A1,2) top: sponge particle; bottom: cross-section sponge powder at 6.8 g/cm3, (B1,2) top: steel particle; bottom: cross-section steel powder at 6.8 g/cm3, and (C1,2) top: AMH particle; bottom: cross-section of AMH powder at 6.8 g/cm3; dark areas in bottom pictures are porosity Figure 2: Cross-sectional view of forged sponge with 0.6 w/o graphite; dark areas are inclusions; light optical, as polished Figure 3: Cross-sectional view of forged AMH with 0.6 w/o graphite; dark areas are inclusions; light optical, as polished To quantify the non-metallic inclusion contents in each material, the powder-forged samples were metallographically prepared and analyzed using an automated image analysis system. The samples were evaluated twice, measuring first the number of inclusions without any nearest neighbor joining and secondly using a smaller near neighbor distance of approximately 6µm. The results are displayed in Table 2. As a result of this analysis, the area percent of the detected inclusions for sponge is 2.80% compared with 0.03% for AMH. These frequencies are calculated as the number of inclusions per 100 mm². Ancorsteel AMH has nearly 100 times fewer inclusions relative to sponge.

4 Table 2: Non-metallic inclusion analysis results Number of Inclusions (No Joining) Number of Inclusions ( ~6 µm Joined) Inclusion Size Sponge AMH Sponge AMH >30 µm >75 µm >100 µm >150 µm >200 µm ALLOY PREPARATION AND TESTING Standard laboratory premixes using AMH, sponge, and rolled sponge powders were prepared. These powders were mixed with Acrawax C and varying amounts of Asbury type 3203H graphite. Admixed copper was used to produce alloys with 1 w/o and 2 w/o Cu. Green densities were measured on 25.4 mm diameter cylinders with 0.75 w/o Acrawax C, following MPIF Standard 45. Green strength was measured using 12.7 mm thick bars in accordance with MPIF Standard 15. [2] Uniaxial compaction of transverse rupture strength (TRS) bars were pressed at 275 MPa, 415 MPa, and 550 MPa. Dogbone tensile bars were compacted at 415 MPa, 550 MPa, and 690 MPa. All samples were sintered concurrently in a mixed atmosphere of 90 v/o nitrogen and 10 v/o hydrogen (90/10) at 1120 C for 15 minutes in a continuous-belt furnace with a cooling rate of 0.7 C/s. Prior to mechanical testing, sintered density, dimensional change (DC), and apparent hardness, were determined on the TRS samples. The densities of the green and sintered steels were determined in accordance with MPIF Standard 42. Tensile testing adhered to MPIF Standard 10 [2]. Sintered carbon values were measured using a Leco 200 carbon-sulfur combustion gas analyzer with standards run before and after samples. All designated graphite contents were within 0.03 w/o sintered carbon of each other. RESULTS AND DISCUSSION Figure 4 displays the green strength versus green density of three iron powders in an FC premix with 0.75 w/o Acrawax C lubricant. All other premix configurations reveal a similar trend. The sponge material does exhibit higher green strength relative to AMH; nevertheless AMH demonstrates equivalent strength as rolled sponge. There are alternatives to further increase the green strength of AMH through judicious selection of innovative lubricants or the use of warm die compaction; therefore potential issues during part handling relating to lower green strength can be overcome.

5 Figure 4: Green strength vs. Green Density of AMH, sponge, and rolled sponge in an FC-0208 premix with 0.75 w/o Acrawax C lubricant Figure 5 shows the transverse strength trends in both a F-0008 and FC-0208 premix for AMH and sponge. The sintered strength of AMH is comparable to sponge at a given density. If comparing sintered strength values for a given compaction tonnage, the atomized product actually out-performs sponge due to the increased compressibility. These results indicate that a low AD powder with attractive green strength can provide improved sintered strengths for applications with more demanding structural requirements. Figure 5: Sintered TR strength of AMH and sponge in a F-0008 and FC-0208 composition The sintered dimensional change versus green density for premixes F-0008 and FC-0208 based on AMH and sponge is given in Figure 6. The DC values for AMH exhibit more positive sintered dimensional change over sponge for a given green density.

6 Figure 6: Sintered DC vs. Green Density in F-0008 and FC-0208 compositions Figure 7 presents the sintered dimensional change for AMH versus graphite addition as a function of admixed copper. When no copper is present, increasing the graphite results in increased growth of the sintered part. At 1 w/o copper, diffusion of copper into the iron creates growth, over-riding the effect of carbon in solution; yet dimensional precision of AMH is fairly stable at 0.30% over the range in graphite additions studied. In mixes containing 2 w/o Cu, there is significant growth at low graphite, however the interaction between Cu and graphite at higher graphite content leads to less growth. This phenomenon is common in ferrous PM alloys and has been studied in numerous alloy systems [3]. Figure 7: Sintered DC vs. graphite content for AMH with 0, 1, and 2 w/o Cu Table 3 presents the tensile properties of AMH with 0.6 w/o and 0.8 w/o graphite with or without 2.0 w/o copper. The compositions without copper reveal a lower strength, yet greater ductility when compared to the compositions containing copper. These results show that AMH can be tailored to meet specific mechanical properties while maintaining improved

7 compressibility, sintered properties, and comparable green strength to that of traditional sponge iron. Table 3: Tensile properties of AMH Compaction Graphite Copper SD 0.2% YS UTS Elongation Hardness MPa w/o w/o g/cm 3 MPa MPa % HRB CONCLUSION An atomized iron powder, Ancorsteel AMH, with characteristics approaching that of traditional direct reduced iron sponge has been successfully developed. This atomized sponge like powder with low apparent density and higher compressibility compared with sponge makes it attractive for many applications. The micro-cleanliness has been shown to be higher in AMH than in sponge as a result of the difference in processing methods employed for each. The green strength, while lower than traditional sponge, is equivalent to rolled sponge and can be further tailored to meet specific needs. The sintered dimensional change, while generally exhibiting more positive growth over sponge, can be modified to demonstrate similar growth as sponge thus making it possible to prevent re-tooling. As a result of increased compressibility, comparable and even higher sintered strengths can be attained without having to increase compaction tonnage. The benefits of AMH provide a promising substitute to sponge in applications demanding similar powder qualities with improved performance. ACKNOWLEDGEMENTS The authors wish to thank Cynthia VanDuser and Thomas Murphy for their contributions in collecting the data presented in the manuscript. REFERENCES 1. F. Hanejko and H. Rutz, A New Atomized Low Apparent Density Sponge Like Iron Powder, in EURO PM2009 Proceedings of the International Powder Metallurgy Congress & Exhibition, Vol. 2, Copenhagen, Denmark, October 2009, EPMA, pp MPIF Standard Test Methods for Metal Powders and Powder Metallurgy Products, B. Lindsley, T. Murphy, Dimensional Control in Cu-Ni Containing Ferrous PM Alloys, Advances in Powder Metallurgy & Particulate Materials, compiled by W. Gasbarre, and J. von Arx, MPIF, Princeton, NJ, 2006, part 10, p