A Comparison of FC-0208 to a 0.3% Molybdenum Prealloyed Low-Alloy Powder with 0.8% Graphite

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1 A Comparison of FC-0208 to a 0.3% Molybdenum Prealloyed Low-Alloy Powder with 0.8% Graphite Francis Hanejko Manager, Customer Applications Hoeganaes Corporation Cinnaminson, NJ Abstract Iron copper steels are the most widely used structural PM materials. Although the elemental copper addition melts during initial sintering, complete homogenization of the copper is not achieved at conventional sintering temperatures and sintering times. This lack of microstructural homogeneity coupled with the potential for large pores can often lead to reduced strength and greater dimensional variability during sintering and subsequent heat treatments, if required. A proposed alternative to the FC-02XX materials is a 0.3% molybdenum prealloyed material. This material has nearly identical compressibility compared with the iron copper materials but has the added benefit of a homogeneous microstructure. This paper will focus on a comparison of FC0208 to a 0.3% molybdenum material with 0.8% added graphite. The materials will be compared in both the as-sintered and heat-treated conditions. Additional testing will evaluate the dimensional stability and variability of these two materials in both the as-sintered and heat-treated conditions. Introduction: Iron-copper steels (FC-02XX) are the most widely used structural PM materials. Current applications include main bearing caps, powder forged connecting rods, carrier assemblies, etc. Although FC 02XX series are most frequently used in the as-sintered condition, they are occasionally heat-treated to increase strength or improve wear resistance. One intrinsic drawback of the iron-copper-carbon system is that copper is added elementally. This elemental copper results in part growth during sintering (positive dimensional change, DC) coupled with a potential for large voids after sintering. [1] A review of the iron copper phase diagram shows that the room temperature solubility of copper in iron is <2% [2]. Nevertheless, production experience shows that large free copper particles are rarely seen in 2% copper steels. Copper contents above 2% show copper precipitates in either the grain boundaries or at prior particle boundaries.

2 Although copper melts at 1083 C (1981 F), the diffusivity of copper into iron is such that complete alloy homogenization is not achieved unless long sintering times or high sintering temperatures are employed. Experimental work by Baran [3] and Lindsley [4] illustrated high copper concentrations adjacent to prior particle boundaries with the copper level in the center of the iron particle decreasing to almost zero, see Figure 1. This copper gradient leads to variations in microstructure and a potential for variations in dimensional change (DC) after sintering and heat-treating. Lindsley [4] demonstrated that this localized gradient causes excessive retained austenite in sinter-hardening grades that can transform to un-tempered martensite upon exposure to low temperature treatments (or low climatic temperatures); thus resulting in additional growth of the component. a. SEM image of 0.50% molybdenum steel with 2% copper added, sintered at 1120 C (2050 F) for 20 minutes at temperature. The line illustrates where both copper and nickel EDAX traces were taken. b. Copper and nickel EDAX traces along scan line in (a.). Note high concentration of copper adjacent to prior particle boundary. Copper trace is shown in green. No nickel was added to this material Figure 1: Copper gradient in a pre-alloyed molybdenum steel, sintered at 1120 C (2050 F) for 20 minutes at temperature The primary advantage of the FC-02XX materials is their unique combination of low material cost coupled with ease of processing and good mechanical properties. [5] Elemental copper additions do not degrade the base iron compressibility and compacted densities of greater than 7.0 g/cm³ attainable. However, recent developments in the manufacture of low molybdenum content steel powders presents the possibility of substituting a prealloyed 0.3% molybdenum steel powder for the FC02XX materials. With the recent downward trend of molybdenum pricing, the cost penalty of using prealloyed molybdenum steels is lessened. If reductions in part-to-part dimensional variation are possible, then any raw material cost differential will be offset by potential reductions in out of specification parts. This paper will focus on comparing the mechanical properties of an FC-0208 material vs. 0.3% molybdenum steel in both the as-sintered and heat-treated condition. As a second part of this study, the effects of

3 variations in copper and graphite additions were studied to assess the impact of these variables on dimensional change stability and sintered strength. Experimental Procedure: This study was divided into two segments. Part one of the study investigated an FC0208 vs. a prealloyed 0.30% molybdenum steel with 0.8% graphite addition. Part two investigated the sensitivity of variations in copper and graphite additions on the dimensional change of the FC-0208 in relation to variations of graphite additions on the prealloyed 0.3% molybdenum steel. Part 1: Comparison of FC-0208 to 0.3% molybdenum with 0.8% graphite Four premixes were prepared as shown in Table 1. The mixes containing atomized Acrawax C were conventional premixes prepared via double cone blending. The premixes containing the warm die lubricant (AncorMax 200 ) were prepared via a proprietary premixing process. Table 1 Composition of Premixes Prepared for Part 1 Mix Base Iron % Copper % Graphite % Lube & Type 1 Unalloyed iron powder % Acrawax 2 Unalloyed iron powder % warm die lubricant 3 Prealloyed 0.3% % Acrawax Molybdenum 4 Prealloyed 0.3% Molybdenum NA % warm die lubricant The following test specimens were compacted from the four premixes listed in Table 1. Compressibility test specimens at 415, 550, and 690 MPa (30, 40, and 50 tsi). For the warm-die premixes additional compressibility data was generated at 830 MPa (60 tsi). The conventional premixes were compacted in dies at room temperature; whereas, the AncorMax 200 premixes were compacted at 93 C (200 F), no powder preheating. Transverse rupture strength bars for both as-sintered and heat treated strength at the various compaction pressures. MPIF standard dog-bone type tensile specimens for both as sintered and heattreat conditions (MPIF standard 10). Sintering of all samples was done at 1120 C (2050 F) in a continuous belt furnace in an atmosphere of 90% nitrogen / 10% hydrogen for ~20 minutes at temperature; the cooling rate after sintering was ~0.6 C per second (no accelerated cooling). All samples were sintered on ceramic trays. Heat-treated samples were re-austenitized at 815 C (1500 F) in a 75% hydrogen / 25% nitrogen atmosphere for 1 hour at temperature. They were oil quenched in 65 C (150 F) oil and subsequently tempered at 205 C (400 F) for 1 hour in a nitrogen atmosphere.

4 These premixes were utilized to evaluate the dimensional change variability during the heat-treat response of a prototype gear. The gear geometry used is shown in Figure 1. Approximately 1,000 gears were made from each premix, the two conventional doublecone premixes were run in unheated tooling with a target part density of 7.1 g/cm³ and the warm-die lubricant premixes were run in a die heated to 93 C without top punch heating and a target part density of ~7.1 g/cm³. The compacted gears were sintered at 1120 C (2050 F) for 20 minutes at temperature in a 90% nitrogen / 10% hydrogen atmosphere in a 6-inch ceramic belt furnace without accelerated cooling. From these sintered gears 50 were randomly chosen and sent to a commercial heat-treat facility for a quenching and tempering operation. The heat treat cycle used was to austenitize at 870 C (1600 C) for 1 hour in a 0.8% carbon atmosphere followed by oil quenching and tempering. From these 50 heat-treated gears ~20 were then measured for part-to-part dimensional variation via a measurement over wire (MOW) of the gear form. Spur Gear for Evaluation Major OD 1.10 in (28.3 mm) Minor OD 0.85 in (21.6 mm) Pitch Diameter 0.96 in (24.5 mm) Pressure angle 20 ID 0.38 in (9.5 mm) # Teeth 16 Module 1.66 Figure 2: Spur gear produced to evaluate part-to-part consistency in the heattreated condition Part 2: Effect of copper and graphite variations on FC0208 and effect of graphite variations on 0.3% molybdenum prealloy Shown in Table 2 is a list of premixes prepared with the objective of determining the effects of copper and graphite variations on an FC-0208 material and the effects of graphite variations on a 0.3% molybdenum prealloyed steel powder.

5 Table 2 Material Combinations Evaluated to Determine Effects of Copper and Graphite Variations Base Iron w/o Copper w/o Graphite Un-alloyed iron powder Prealloyed 0.3% molybdenum N/A The range shown in Table 2 was selected to cover standard variations in alloy additions that are traditionally used within the FC0208 specification range. All premixes shown in Table 2 were prepared using conventional premixing techniques with 0.75% added Acrawax C. From these premixes, standard transverse rupture strength bars were compacted at 415, 550, and 690 MPa (30, 40, and 50 tsi). Sintering was done under the conditions previously listed. The TRS bars were evaluated for dimensional change and as-sintered strength. Results and Discussion: A. Property comparison of FC-0208 vs. 0.3% prealloyed molybdenum steel with 0.8% graphite Figure 3 presents the compressibility curves of the four materials listed in Table 1. As previously noted, compaction of the Acrawax containing premixes took place in unheated dies at room temperature; whereas, compaction conditions of the warm-die powder premixes was in dies heated to 93 C (200 F) without any prior powder heating. The compressibility of the prealloyed 0.3% molybdenum steel is identical to the FC-0208 material at both room temperature and at die temperatures of 93 C. Utilizing warm die processing results in a 0.05 to 0.10-g/cm³ increase in green density relative to conventional lubricants for both material options. The optimal benefit with warm-die compaction is observed at compaction pressures greater than 550 MPa (40 tsi). Although not shown, warm die processing also promotes higher green strength with reduced green expansion and reduced ejection forces. [6] The as-sintered and heat-treated transverse rupture strengths as a function of sintered density for the FC0208 and 0.3% molybdenum prealloy with 0.8% added graphite are presented in Figure 4. Warm-die processing did not affect the sintered or heat-treated strength. As anticipated, in the as-sintered condition, the FC-0208 material possesses higher strengths when compared to the prealloyed 0.3% molybdenum material. However, once heat-treated the strengths of the two materials are nearly equivalent.

6 Green Density, g/cm³ FC0208 AM 200 FC % Mo with.8% Gr AM % Mo with.8% Gr Compaction Pressure, MPa Figure 3: Compressibility of the four initial premixes TR Strength, MPa FC0208 as sintered FC 0208 HT A30 HP as sintered A30 HP HT Sintered Density, g/cm³ Figure 4: As-sintered and heat-treated transverse rupture strengths

7 As-sintered tensile properties and heat-treated tensile properties for the 0.8% graphite additions are presented in Table 3. Similar to the trend observed with transverse rupture strength testing, the as-sintered yield and ultimate tensile strengths of the FC-0208 are greater than the prealloyed 0.3% molybdenum steel. Elongation values are nearly the same. Presented in Table 3 are the ultimate tensile strengths of the FC-0208 vs. the 0.3% molybdenum steel in the quenched and tempered condition. Similar to what was found in TRS testing, in the heat-treated condition the prealloyed 0.3% molybdenum steel with 0.8% graphite and heat-treated FC-0208 have nearly identical ultimate tensile strengths. The data presented Table 3 represent compaction at 415, 550, and 690 MPa. The prealloyed 0.3% molybdenum material has lower growth relative to the FC-0208, thus the sintered densities at the same compaction pressure are higher for the prealloyed molybdenum material. Table 3 Summary of Tensile Properties of FC0208 and Prealloyed 0.3% Molybdenum Steel Material FC % Mo Pre-alloy with 0.80% Graphite Sintered Density, g/cm³ Heat Treated 0.2% YS (MPa) UTS, (MPa) Total Elongation (%) Hardness, (HRA) No Yes NA 771 < No Yes NA 805 < B. Dimensional Change and How it is Affected by Variations in Chemistry The dimensional change variations resulting from variations in graphite additions to the two materials studied are studied in Figures 5 and 6. Lowering the graphite addition had an opposite effect for the two materials evaluated. For the FC0208, lowering the graphite to 0.7% had little effect on the DC, remaining approximately the same as the 0.8% additions. However, the 0.30% molybdenum pre-alloyed material with a 0.7% graphite addition showed reduced the DC after sintering. Increasing the graphite to 0.9% resulted in less growth for the FC-0208 relative to 0.8% graphite and approximately the same growth as the 0.8% graphite for 0.3% molybdenum prealloyed steel. The trend observed for the FC-0208 material is consistent with data reported in the literature. [7] Higher carbon contents in FC0208 material decrease the solubility of copper in iron, thus reducing the growth and increasing the amount of fine copper precipitates. [1] Conversely, graphite additions above 0.7% result in greater growth for the 0.3% molybdenum prealloyed steel (greater volume expansion of the iron lattice).

8 Sintered DC, % % Cu, 0.80% Gr 2% Cu, 0.70% Gr 2% Cu, 0.9% Gr Green Density, g/cm³ Figure 5: Effect of Graphite additions on the DC of FC Sintered DC, % % Graphite 0.70% Graphite 0.90% Graphite Green Density, g/cm³ Figure 6: Effect of Graphite additions on the DC of 0.3% Molybdenum steel

9 The decrease in growth for FC-0208 with 0.9% graphite was approximately 0.07% to 0.08% over the green density range of 6.75 to 7.10 g/cm³ compared with the material that contained 0.7 or 0.8% graphite. Reviewing the data for the 0.3% molybdenum prealloyed steel, the increase in growth with varying the graphite addition from 0.7% / 0.8% to 0.9% was ~0.05% over the same green density range. Along with reduced scatter, the molybdenum prealloyed steel exhibits reduced growth relative to the FC-0208 material. Another feature of the prealloyed steel material was the reduced growth as a function of increasing green density. The molybdenum steel showed about a 0.05% increase in DC as the density increased from 6.75 to 7.10 g/cm³; while, the FC-0208 material showed ~0.07% increase in growth over the same green density range. Implications of these data are that the molybdenum steel is potentially less sensitive to variations in green density and carbon variations part-to-part. This reduced sensitivity can give reduced part-to-part dimensional scatter and lower heat-treat variations. Interesting for both materials is the overlap of DC for the 0.7% and 0.8% graphite additions for F-C0208 and 0.8% and 0.9% graphite for the 0.3% molybdenum steel. The inference of this is that reduced growth variations can be achieved by maintaining graphite levels in specific (albeit different) ranges for both materials. Sintered DC, % Figure 7: Green Density, g/cm³ Effect of Copper Variations on DC of FC % Cu, 0.8% Gr 2.2% Cu, 0.80% Gr 1.5% Cu, 0.80% Gr 2.5% Cu, 0.80% Gr 2.0% Cu, 0.8% Gr A30HP, 0.8% Gr Shown in Figure 7 is the effect of copper variations on the FC-0208 again comparing the data with the 0.8% graphite molybdenum steel. The data presented in Figure 7 suggest that variations in copper content result in dimensional change variations up to 0.07% over the copper range evaluated. As anticipated variations in both graphite and copper additions result in variability in growth of the FC-0208 material. The absolute magnitude of these variations is larger than the DC variation of the 0.3% molybdenum steel with the potential variability of graphite only.

10 Table 4 Summary of DC Variations for FC-0208 and Pre-alloyed 0.3% Molybdenum Material FC-02XX with 2% Copper 0.3% Mo Prealloy Sintered Density, g/cm³ Heat Treated 0.8% gr 0.9% gr 0.7% gr No Yes No Yes Table 4 presents the maximum variation in DC with variations in graphite content for the alloys tested. A maximum DC variation of ~0.09% can be expected at a given green density with the as-sintered FC-0208 material and the range of graphite additions evaluated in this study. A variation of up to 0.12% was observed for the FC-0208 material in the heat-treated condition. The greatest effect on DC for FC0208 materials results from variations in graphite additions. For the molybdenum pre-alloyed steel at a given density level; potential variations in carbon content result in a maximum DC variation in the as-sintered condition of ~0.08%. Heat-treating the molybdenum steel reduces this maximum DC variation to ~0.04% (approximately one third of the variation observed with the FC-0208). Sintered and heat treated TRS testing of materials with the possible variations in copper / graphite for the FC-0208 and for graphite only for the 0.3% molybdenum steel are shown in Table 5. Focus is given to TRS results only. Copper has the strongest influence on TRS properties for copper containing steels. Higher copper contents increase TRS strength; this results from additional precipitation hardening effects. Results for the 0.3% molybdenum steel show the highest heat-treated strength with a graphite addition of 0.7%. Additional graphite decreases the strength. A possible explanation for these phenomena is that molybdenum is a ferrite stabilizer and it decreases the eutectoid carbon level. At graphite additions of 0.8% and 0.9% the occurrence of grain boundary carbides are possible.

11 Table 5 Range of Sintered and Heat-Treated TRS Test Results for Copper and Graphite Variation Iron Base % Cu % Gr Unalloyed Iron 0.30% molybdenum prealloy steel NA Sintered Density, g/cm³ Sintered TRS, MPa (psi) Heat Treated TRS, MPa (psi) (133,700) 1100 (160,000) (150,500) 1305 (189,600) (167,600) 1325 (192,600) (123,600) 1100 (160,000) (153,100) 1340 (194,600) (170,500) 1450 (210,500) (113,000) 990 (143,800) (139,300) 1195 (173,500) (156,100) 1240 (180,500) (97,900) 1055 (153,100) (119,000) 1370 (198,900) (132,200) 1480 (215,350) (98,000) 1120 (163,000) (119,000) 1205 (175,300) (132,000) 1295 (187,900) (104,600) 940 (136,100) (124,700) 1130 (163,800) (134,100) 1215 (176,900) C. Results of heating treating gear shown in Figure 2. Prototype gears were produced using the four materials discussed in part one of this study. They were made on a Dorst 140 metric ton compaction press at a rate of 10 per minute. The gears were sintered in a continuous belt sintering furnace at 1120 C (2050 F) in a 90% nitrogen / 10% hydrogen atmosphere with a time at temperature of ~20 minutes. After sintering approximately 50 gears from each material grouping were heattreated by quenching and tempering utilizing a commercial heat-treat cycle for FC0208. After quenching, the gears were then tempered at 205 C (400 F). The gears were checked for dimensional variation using mm ( inch) diameter pins. Measurements were taken at four locations per gear starting with the front of the gear (as compacted) and then measuring three additional locations around the outside diameter. The results of the measurement over wires (MOW) are presented in Table 6.

12 Table 6 Dimensional Variations of Heat Treated Gears Material FC-0208 (regular premix) 0.3% Mo with 0.8% gr (regular premix) FC-0208 (bonded premix 0.3% Mo with 0.8% gr (bonded premix) Av. MOW Inches (mm) (30.72) (30.71) (30.73) (30.69) Max MOW Inches (mm) (30.78) (30.72) (30.78) (30.71) Min MOW Inches (mm) (30.67) (30.69) (30.69) (30.67) Max Min Inches (mm) (0.11) (0.04) (0.09) (0.04) St Dev MOW (0.03) (0.01) (0.02) (0.01) Table 7 Apparent Hardness Values of Gears in the Heat-Treated Condition Material FC-0208 (regular premix) 0.3-0% Mo with 0.8% gr (regular premix) FC-0208 (bonded premix 0.3% Mo with 0.8% gr (bonded premix) Sintered Density, g/cm³ Average Hardness, HRA Range of Hardness, HRA / / / / 73.5 The data presented in Table 6 clearly demonstrate the reduced scatter realized with the prealloyed 0.3% molybdenum steel relative to a standard FC-0208 material. The absolute magnitude of the MOW is different for the two material combinations; this is a result of the absolute DC differences between an FC-0208 and a 0.3% molybdenum prealloyed steel with 0.8% graphite. Thus, it can be clearly seen that a molybdenum steel gives greater dimensional stability in the heat-treated condition. No green or sintered gears were measured because the focus of this report was to investigate heattreated results. One additional observation is the comparison of the warm-die powder premixes vs. regular premixes. For the FC-0208 material, the binder treated warm-die powder gave a reduced overall scatter as measured by the maximum measurement minus the minimum measurement and also the standard deviation of the measurement differences. Thus, the binder treatment does result in reduced part-to-part scatter even

13 for the copper-containing steel. No difference in dimensional variation was observed for the binder treated 0.3% molybdenum steel. Apparent hardness data collected on the gears are presented in Table7. The gears were initially compacted to a 7.1 g/cm³ green density; the densities given in the table are heat-treated densities. Another benefit of the low alloy steel powder is the greater uniformity of heat-treated hardness as measured by the range of hardness readings. One potential benefit of eliminating the copper may be a reduced tendency for cracking during induction hardening. Induction hardening of copper containing PM steels has shown sensitivity to cracking. A possible explanation is the non-uniform microstructure inherent with copper containing PM steels. Eliminating the copper would eliminate one potential for crack initiation during induction hardening of the PM materials. Discussion: The objective of this study was to evaluate the feasibility of using a 0.3% molybdenum steel in lieu of FC In the as-sintered condition, the TRS and tensile properties of the 0.3% molybdenum steel with 0.8% graphite are inferior to the FC-0208 steel. The molybdenum steel does experience less growth during sintering compared with the FC Additionally, the molybdenum prealloyed steel has reduced DC variation resulting from variations in compacted density and premixed graphite variations when compared to the copper steel. Once heat-treated the 0.3% molybdenum steel with 0.8% graphite has nearly equivalent TRS and ultimate tensile strength compared to the copper steel. Dimensional change of the heat-treated TR specimens using the molybdenum steel exhibited reduced dimensional change variation with variations in density and variations in graphite additions. An implication of this data suggests that reduced scatter is possible in actual components that required a heat-treatment. Measurement over wire measurements of compacted, sintered, and heat-treated gears showed a significant reduction in the variability of the MOW within a gear and gear measurements made over 20 measured gears. This reduced variation as measured by the standard deviation of the MOW was greater than 50%. One possible implication of this data is the potential to use a 0.3% molybdenum prealloyed steel in those applications possessing tight dimensional control coupled with a heat-treated microstructure. PM is continually being challenged to produce evertighter DC along with higher densities. Using a prealloyed molybdenum steel reduces variations in DC with variations in density and graphite levels yet provides heat-treated strengths equivalent to heat-treated FC Additionally, the uniform microstructure promotes greater uniformity of hardness with the potential for reduced cracking if induction hardening is required. Conclusions: Results from this study showed the following trends: 1. In the as-sintered condition an FC-0208 has superior mechanical properties when compared with a 0.3% molybdenum prealloyed steel with 0.8% added graphite.

14 2. In the heat-treated condition, the 0.3% molybdenum prealloyed steel with 0.8% added graphite has nearly identical strength to a heat treated FC Variations in graphite additions have opposite effects on both material. Higher graphite additions result in less growth for the FC-0208 and approximately the same growth for a 0.3% molybdenum prealloyed steel. 4. Reducing the graphite addition to 0.7% results in more growth for the FC-0208 and less growth for the 0.3% molybdenum prealloyed steel. 5. Carbon has the greatest effect on DC variation for the copper steels. 6. Variations of part density result in lower DC variations for the 0.3% molybdenum pre-alloyed material when compared with the FC Heat treatment of gears made from both materials showed greater dimensional scatter for a FC-0208 relative to a 0.3% molybdenum prealloyed steel. 8. Using a binder treated FC-0208 resulted in reduced scatter relative to a regular premix. The rationale for this is the reduced potential for graphite segregation. References: 1. ASM Metals Handbook, Volume 7 Ninth Edition, published by ASM International, Materials Park, Ohio, 1998, pp ASM Metals Handbook, Volume 8 Eight Edition, published by ASM International, Materials Park, Ohio, 1973, p T. Murphy, M. Baran, An Investigation into the Effect of Copper and Graphite Additions to Sintering Hardening Grades Advances in Powder Metallurgy and Particulate Materials --, Compiled by W. Brian James and Russell A. Chernenkoff, Metal Powders Industries Federation, 2004, pp. 4. B. Lindsley, T. Murphy, Effect of Post Sintering Thermal Treatments on Dimensional Precision and Mechanical Properties of Sinter Hardening PM Steels, Advances in Powder Metallurgy and Particulate Materials , Compiled by John Engquist and Thomas F. Murphy, Metal Powders Industries Federation, 2007, pp.. 5. Materials Standards for PM Structural Parts, MPIF standard 35, 2007 Edition, published by Metal Powders Industry Federation, Princeton, NJ, 2007, pp F. Hanejko, High Density via Single Pressing / Single Sintering, Presented at Euro PM Hoeganaes Corporation Technical Literature at product, unalloyed steels.