Advanced Binder Treated FY Francis Hanejko & William Tambussi. Hoeganaes Corporation Cinnaminson, NJ USA

Transcription

1 Advanced Binder Treated FY-4500 Francis Hanejko & William Tambussi Hoeganaes Corporation Cinnaminson, NJ USA Abstract: The benefits of utilizing phosphorous as an alloying element in pure iron are well documented. These include improved DC magnetic performance, higher sintered densities, and good mechanical properties. A major disadvantage of alloying phosphorus in iron is the greater tool wear experienced with the using of ferro-phosphorous additives. Utilizing an experimental binder treatment process, a reduction in the strip and slide was observed during laboratory testing of both traditional binder treated premixes at sintered densities less than 7.0 g/cm³ and with warm materials at sintered densities of >7.2 g/cm³. This reduction in strip and slide has the potential to minimize wear. Both the mechanical and magnetic properties of the new material will be presented. Introduction: Advantages of using phosphorus in PM steels include good magnetic properties, higher magnetic resistivity, and good mechanical properties with excellent elongation and impact toughness. [1,2] Typical applications for phosphorous containing materials (FY-4500) include automotive speed sensors, flux rings for DC electric motors, and electromagnetic actuators. [3] Additionally, phosphorous is added in FC-0205 type steels utilized in automotive main bearing caps because of the unique combination of strength, ductility, and good sintering response. [4] The major disadvantage of phosphorous containing materials is that phosphorous is added as a ferro-phosphorous inter-metallic. Whether Fe 2 P or Fe 3 P, these inter-metallic compounds possess high hardness, which then results in a powder premix consisting of a relatively soft iron matrix incorporating a high hardness second phase particle that can cause significant wear. The hardness values of Fe 2 P and Fe 3 P are ~1050 HV and 1000 HV, respectively; this is compared to an iron particle hardness of ~100 BHN (no conversion to HV is realistic at this low hardness). [5] These high hardness particles result in abrasive wear of the punches and often the actual tool material. Frayman has reported that the hard particle additions abrade the metallic binder of

2 carbide inserts. [6] The consequence of these hard particles is excessive tool wear with subsequent more frequent tool repair and replacement. Utilizing advanced coatings has lessened tool wear but not eliminated it. [7] Thus for the mainstream users of FY-4500, they have to simply live with the inherent high tool wear and frequent repair associated with ferrophosphorus containing materials. Hoeganaes Corporation produces FY-4500 materials as a binder treated premix (ANCORBOND ). Although this binder treatment does not reduce the abrasive nature of ferrophosphorus, it minimizes segregation thus preventing potential localized excessive tool wear. The experimental work described in this report focuses on an experimental enhanced binder treatment system designed to counteract the abrasive characteristic of ferro-phosphorus particles. Although the exact nature of this new binder system is proprietary, it does function with both conventional PM lubricants designed for both conventional and with modifications can be utilized with warm processes. The work discussed in this report will document the mechanical properties, laboratory measured ejection characteristics of this experimental binder treatment system. Additionally, this paper will report on the use of a 0.85% molybdenum prealloy material that is premixed with an equivalent amount of ferro-phosphorus to yield a 0.45% phosphorus material. This 0.85% molybdenum iron powder premixed with ferro-phosphorus has shown increases in both the yield and tensile strengths of a material with minimal decreases in magnetic response. [8] Combining this advanced binder treatment of traditional FY-4500 materials with a 0.85% molybdenum prealloyed steel can produce mechanical properties higher than FY-4500 materials with the added advantage of reduced and punch wear. Experimental Procedure: In this study, six laboratory premixes were prepared and evaluated; these premixes are listed in Table 1. All premixes were prepared by adding 2.9% Fe 3 P plus the appropriate powder lubricant; thus producing a final sintered phosphorus content of ~0.45%. The premixes were evaluated for compressibility, transverse rupture strength, green strength, tensile and impact properties. Compaction conditions for the cold conditions were a temperature of ~25 C (75 F) and pressures of 415, 550, and 690 MPa. For the warm premixes, the temperature was ~93 C (200 F) with pressures of 550, 690, and 830 MPa. In addition to the mechanical property specimens magnetic toroids were pressed from mixes 1 and 2 to a density of 6.9 g/cm³ and from mixes 3 through 6 to a density of 7.25 g/cm³. After, all samples were sintered in a laboratory belt furnace on ceramic trays. Sintering conditions were 1120 C (2050 F) in a 90% nitrogen / 10% hydrogen atmosphere, with a cooling rate after sintering of ~0.6 C per second. The time above 1095 C (2000 F) was approximately 20 minutes.

3 Table 1 Premixes Evaluated in this Investigation Premix ID Base Iron Type Premix Type 1 High Conventional (ANCORBOND Compressibility Binder treated, FY-4500) Iron Powder cold 2 3 (AncorMax 200 FY-4500) 4 5 (AncorMax 200 FL-4400 with 0.45% Phosphorus) 6 High Compressibility Iron Powder High Compressibility Iron Powder High Compressibility Iron Powder 0.85% prealloyed Molybdenum 0.85% prealloyed Molybdenum Enhanced binder treated, cold Warm, conventional Enhanced binder treated, warm Warm, conventional Enhanced binder treated, warm Lubricant added % Lubricant Added Acrawax 0.75% Proprietary 0.75% Proprietary 0.40% Proprietary 0.40% Proprietary 0.40% Proprietary 0.40% After sintering, the samples were tested via the appropriate MPIF test standard. [9] In addition to the mechanical property test samples, ejection characteristics were quantified. The methodology employed was to compact a green strength bar at 690 MPa, then measure two key characteristics, strip and slide. Strip is defined as the initial break free load required to initiate movement of the test specimen and slide is defined as the load immediately before the specimen is ejected from the. Strip and slides pressures are then calculated by dividing the measured force by the area of the bar in contact with the surface. Results: Shown as Table 2 are the powder properties of the six premixes evaluated in this study. The advanced binder treatment utilized in combination with either high compressibility iron or the 0.85% molybdenum prealloyed material does not affect the apparent density (AD) or flow characteristics relative to conventional processing. The data presented in Table 2 was collected on 250 kg premixes. No data is presented for the FL-4400 material because the premix size was only 10 kg. However, it is assumed that comparable AD and flow to the FY-4500 will be observed with this material.

4 Table 2 Powder Properties of Materials Evaluated Flow, sec / Premix ID Base Iron Type Premix Type AD, g/cm³ 50 g 1 FY-4500 Conventional, cold FY-4500 Enhanced, cold FY-4500 Warm, conventional FY-4500 Enhanced, warm FL-4400 Conventional, warm 6 FL-4400 Enhanced, warm NA 7.50 Green Density, g/cm³ Cold compacted FY-4500 Warm Die FY-4500 Warm Die, FLN Compaction Pressure, MPa Figure 1: Compressibility data for six materials evaluated, all premixes with 2.9% added ferro phosphorus Compressibility information is presented in Figure 1. As expected the warm premixes show ~0.10 to 0.20 g/cm³ improvement in compressibility compared to the conventional premixes. Although not clearly shown, compressibility of FY-4500 for the standard and experimental enhanced binder systems are identical for both cold and warm grades. A similar trend was noted for the FL4400 with 0.45% sintered phosphorus. Thus, the

5 experimental binder system had no detrimental effects on the compressibility of the FY-4500 alloy system Green Strength, MPa ANCORBOND FY-4500 EXP BInder RT Compaction AncorMax 200 FY-4500 Exp Binder, warm FY Green Density, g/cm³ Green Strength, psi Figure 2: Green Strength of Tested Materials Figure 2 presents the green strength of the FY-4500 materials compacted at both room temperature and at 93 C (200 F). The experimental binder system has no detrimental effect on the green strength of FY-4500 regardless of conditions. It is noteworthy; warm gives an approximate 75% increase in green strength over the range of green densities evaluated. This higher green strength can result in a potential reduction in green part damage during part handling prior to sintering. Table 3 presents the mechanical properties developed for the six alloys investigated. Transverse ruptures test samples were prepared for each of the materials; however, the ductility of the six materials invalidated these data. The TRS samples were utilized to quantify the DC after sintering. Referring to Table 3, premixes 1, 3, and 5 represent commercially available material with ferro phosphorus additives. Premixes 2, 4, and 6 are the experimental binder system materials. Comparing the commercial premix to the experimentally prepared premix, there is no difference in either the measured tensile or impact properties. In Table 3, utilizing a 0.85% molybdenum prealloyed steel with 0.45% sintered phosphorus, a yield strength of ~ 275 MPa (40,000 psi) with an ultimate tensile strength of 413 MPa (60,000 psi) was achieved. Elongation values of >11% with impact energies of ~90 joules (70 ft.lbf) were achieved. This combination of mechanical properties combined with the inherently high modulus (because of the high sintered density) offers a unique combination of mechanical properties.

6 Table 3 Sintered Mechanical Properties of the Six Premixes Evaluated Mix ID Std FY cold Exp FY cold Warm FY-4500 Exp warm FY-4500 Warm FL-4400 Exp warm FL-4400 Sintered Density, g/cm³ Yield Strength, MPa (psi) 218 (31,700) 245 (35,700) 267 (38,800) 216 (31,400) 241 (35,000) 258 (37,500) 232 (33,700) 258 (37,500) 269 (39,100) 229 (33,300) 261 (37,900) 273 (39,700) 240 (34,900) 273 (39,600) 290 (42,100) 235 (34,100) 271 (39,400) 286 (41,500) UTS, MPa (psi) Elongation, % Impact, Joules (ft.lbf) 280 (40.600) (7) (47,100) (17) (51,700) (22) (39,600) (10) (46,800) (14) (51,300) (20) (47,500) (13) (53,400) (25) (55,600) (39) (47,100) (11) (53,900) (22) (56,700) (42) (49,300) (22) (56,800) (43) (60,000) (70) (47,900) (22) (56,100) (35) 39 Hardness, HRA 409 (59,400) (64) 42 Figure 3 presents the dimensional change data for the six materials after sintering at 1120 C (2050 F) in a 90% nitrogen / 10% hydrogen atmosphere. The experimental binder system for the cold showed slightly higher shrinkage by approximately 0.02% at the 6.8 g/cm³ sintered density compared to the commercial material (due to gage R&R this is essentially a zero difference). The sintered dimensional change for the warm FL-4500 was the

7 same for both the conventional material and the experimental binder system. The same trend was determined for the FL-4400 with added ferro phosphorus Sintered DC, % AM 200 FL-4400 Cold FY-4500 conv. Cold, FY-4500 Experimental Warm FY-4500, conv Green Density, g/cm³ Figure 3: Dimensional change of the premixes, premixes 3 and 4 are excluded because of carbon pick up during sintering. Table 4 presents the DC magnetic data developed for each premix; two trends are apparent. First, the experimental binder system does not affect the magnetic performance of the either FY-4500 or the prealloyed FL-4400 material at either 6.9 or 7.25 g/cm³ sintered densities. Second, the prealloyed 0.85% molybdenum steel premixed with 0.45% sintered phosphorus has nearly identical magnetic properties to the standard FY-4500 material. The data presented in Table 4 appears to suggest that higher density does not improve the magnetic properties of PM materials. However, MPIF standard 35 shows that higher part density produces higher magnetic performance. The reason for the discrepancy in the data shown is as follows: premix 1 and 2 were sintered at a different time relative to premixes 3 through 6. Chemical testing showed that premixes 3 through 6 had higher sintered carbon thus the lower permeability and higher coercive force. This higher sintered carbon was not a result of lubricant burn off issues but rather a result of improper furnace conditioning.

8 Table 4 Sintered DC Magnetic Data Mix ID Sintered Density, g/cm³ Applied Field, Oe Max DC Perm Max Induction, kgauss Hc, Oersteds Br, kgauss Std FY cold Exp FY cold Warm FY Exp warm FY-4500 Warm FL Exp warm FL Ejection characteristics as measured by strip and slide are presented in Table 5. The data shown is for the FY-4500 material only. At ambient conditions, the experimental binder system shows a 10% lower stripping pressure and ~5% lower sliding pressure compared to the standard ANCORBOND treated material. At warm conditions, the standard material has a significantly higher strip and slide compared to the lower density green strength bars. However, the modified experimental binder produces a dramatic decrease in both the strip and slide. Table 5 Ejection Characteristics of FY-4500 Premix ID Std FY-4500 cold Exp FY-4500 cold Warm FY-4500 Exp warm FL-4400 Green Density, g/cm³ Stripping Pressure, MPa (psi) Sliding Pressure, MPa (psi) (5630) 19.8 (2870) (5150) 19.1 (2780) (8700) 48.2 (7000) (4800) 29.6 (4300)

9 Discussion: The experimental binder treatment described in this report showed reduced ejection forces for FY-4500 materials as measured by strip and slide stress. In addition to the reduced ejection forces, mechanical property testing demonstrated no deleterious effect on the compressibility or sintered mechanical properties of the FY-4500 material system. To verify the usefulness of the laboratory data, preliminary testing was performed on production PM components. Beta site testing showed reduced ejection pressures, reduced wear, and an overall improvement in the characteristics of FY-4500 material at green densities ranging from 6.8 to 7.0 g/cm³. A qualitative observation was reduced press adjustments during extended production runs. The fewer number of press adjustments was attributed to reduced entrapment of powder between the punches and s thus enabling the press to return to the fill position with less powder drag between the punches and assembly. In terms of wear, measurement of the top punch showed no measurable wear on the top punch after a production run of ~50,000 parts. This represented a considerable improvement from the commercially available material. It was also noted that no differences were observed concerning powder compressibility or sintered dimensional change response. This experimental binder system for the FY-4500 material system has demonstrated that equivalent powder compressibility with identical sintered mechanical properties can be achieved. The implication of this data is reduced wear and potentially lower repair and replacement cost. This new development can potentially lead to further utilization of ferro-phosphorus containing premixes in new magnetic applications. Utilizing a 0.85% molybdenum prealloyed steel base material in combination with ferrosteel phosphorus gives magnetic properties equivalent to standard FY The molybdenum material does show an approximate 10% increase in yield strength and ultimate tensile properties with no loss in elongation. The other interesting aspect of this data is significantly higher impact energies found with this material system. Conclusions: From the results presented in this paper, the enhanced binder treatment of ferro-phosphorus containing materials has demonstrated the following: Warm of FY-4500 type materials gives a 0.1 to 0.2 g/cm³ increase in green density at equivalent pressures. Warm results in an increase in green strength of ~ 75% at equivalent green densities. The experimental binder system reduces the stripping and sliding stresses of both cold compacted and warm compacted FY This experimental binder system has identical compressibility to the ANCORBOND and AncorMax 200 materials. Utilizing cold conditions, the experimental binder system does give greater shrinkage over a density range of ~6.8 to 7.1 g/cm³. No difference in DC was observed with the warm compacted material. Magnetic performance was unaffected, as anticipated the higher the sintered density the better the magnetic performance. Mechanical properties are identical.

10 Utilizing an 0.85% prealloyed molybdenum steel in combination with the ferro phosphorus addition increased the yield and tensile strengths and gave a significant increase in the impact energy. The experimental binder system described in this report is still undergoing laboratory development and is not currently commercially available. References: 1. Material Standard for PM Structural Parts (MPIF Standard 35), 2009 Edition, Published by Metal Powders Industries Federation, Princeton, NJ, H. Rutz, F. Hanejko, C. Oliver, Effects of Processing and Materials on Soft Magnetic Performance of Powder Metallurgy Parts, Advances in Powder Metallurgy and Particulate Materials 1992, Vol. 6, pp , Metal Powder Industries Federation, Princeton, NJ, H. Rutz, F. Hanejko, G. Ellis, The Manufacture of Electromagnetic Components by the Powder Metallurgy Process, International Conference on Powder Metallurgy and Particulate Materials, Metal Powders Industry Federation, Princeton NJ, Donald White, Auto Industry Boosts PM Parts, Heat Treat Magazine, January 1993, p J. Nowacki, Phosphorus in iron alloys surface engineering, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 24, Issue 1, September 2007, pp to Leonid Freyman, Advanced Cemented Carbides for PM Tooling Applications, Presented at 2009 PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 28, 2009, Sponsored in Cooperation with the Powder Metallurgy Equipment Association. 7. Robert Jacoby, Coating Technology for Metal Forming Operations, Presented at 2009 PM Parts Compacting/Tooling Seminar, Cleveland OH, Oct 27 28, 2009, Sponsored in Cooperation with the Powder Metallurgy Equipment Association. 8. I. Gabrielov, C. Wilson, etal, P/M High Strength Magnetic Alloys, Advances in Powder Metallurgy and Particulate Materials , Edited by William B. Eisen and Shiz Kassam, Vol. 7, pp , Metal Powders Industry Federation, Princeton NJ, Standard Test Methods for Metal Powders and Powder Metallurgy Products, 2008 Edition, Published by Metal Powders Industries Federation, Princeton, NJ, 2008.