Methods to Improve the Fatigue Life of Sinter-Hardened Components

Transcription

1 Methods to Improve the Fatigue Life of Sinter-Hardened Components Alan Taylor GKN Sintered Metals Salem, Indiana Francis Hanejko Hoeganaes Corporation Cinnaminson, NJ Abstract: Previous experimental work showed that fatigue performance is affected by the alloy system, heat treatment method, and microstructural features of test specimens. The present study will present information concerning the effects of varying the sinter-hardening cooling rate (and subsequent microstructure features) on the mechanical properties sinter-harden steels and the Ancorloy MDCL material system. Emphasis will be given to the rotating bending fatigue performance of these systems and how this experimental data correlates with the fatigue performance of the actual component in accelerated life testing. Introduction: This work is a continuation of a previous study evaluating a number of sinter-hardening materials based on MPIF standard grades: FLNC-4408 and FLN plus a proprietary silicon containing alloy, Ancorloy MDCL. [1] The previous work experimental focused on the core carbon of the P/M alloys and the effect that case carburizing had on developing beneficial residual compressive stresses in the surface with a concomitant increase in fatigue endurance limit. The aim of the work was to improve the fatigue performance over traditional FLC and FLNC P/M sinter hardening grades. The results of this work showed that there were two principle options to improve the fatigue performance of sinter-hardening steels. The first option required both a material and process change utilizing a silicon alloyed sinterhardening steel coupled with high temperature sintering. This option developed fatigue strength improvements of up to 30% over standard sinter-hardening grades. The addition of 0.35% silicon to the P/M steel alloy promoted an increase in hardenability but more importantly dramatically increased the strength of the sinter-hardening material with resulting tensile strengths approaching 180,000 psi (1240 MPa) and a corresponding increase in the fatigue endurance limit.

2 The second option investigated the integration of a secondary carburizing cycle after sintering or sizing; in particular, the use of vacuum or gas carburizing followed by an accelerated gas quench similar to conventional sinter-hardening practices. The benefits of a secondary carburizing cycle (or potentially in line with conventional sintering) are:! Leaner alloy systems can be utilized promoting higher part densities with the potential to use a smaller compaction press for the same size part! Sizing of the part for improved dimensional precision! Eliminating oil quenching and the associated oil entrapment problems and the greater variability in dimensions associated with oil quenching! The development of a carburized case and the resulting compressive surface stresses with the corresponding improvement in mechanical properties (notably up to 15% improvement in fatigue endurance limits)! Development of a favorable acicular tempered martensitic microstructure often required in high performance components An approach not investigated in the previous work was a method(s) to facilitate higher green densities and corresponding sintered densities. The use of advanced compaction methods such as heated die techniques and advanced lubricant / binder systems offer the potential for improved density with improved fatigue endurance limits. This report will focus on methods to improve the green density of a FLNC-4408 sinter-hardening material. Experimental Procedure: The focus of this evaluation was to improve the mechanical properties (in particular the fatigue endurance) of sinter hardening materials. Although changing the alloy system to the silicon alloy steel produced dramatic increases in fatigue endurance; however, the associated process and equipment changes are more of a long-term solution. As a concurrent approach, higher sintered densities have a beneficial effect on fatigue performance. [2] Thus a primary focus of this work was to develop viable methods to increase the green and sintered density without exceeding 45 tsi (620 MPa), a realistic upper limit on the compaction pressure considering the potential fragile tooling members. Shown in Table 1 are the premixes compositions evaluated in this study. The materials studied varied the amount and type of lubricant added plus how the premix was prepared (conventional double cone mixer vs. ANCORBOND processing). Once the premixes were prepared, the compressibility of the various material options was developed over a range of compaction pressures from 30 tsi to 50 tsi (415 to 690 MPa). Additionally, the compressibility studies were extended to temperatures of 120 F (49 C) and 140 F (60 C).

3 Table 1 Premix Compositions Evaluated Mix ID Premix Method MPIF Designation Lube and Amount A Double Cone FLNC %, 0.625%, 0.50% Acrawax B Double Cone FLNC % & 0.625% Ferrolube C Double Cone FLNC % Caplube D ANCORBOND FLNC %, 0.625%, 0.50% Acrawax E AncorMax D FLNC % AncorMax D lube Standard compressibility and transverse rupture strength samples (TRS) were prepared for the various materials and compaction conditions. Additionally, rotating bending fatigue samples were prepared by first compacting 0.45 inch x 0.45 inch x 3.5 inch (1.15 x 1.15 x 8.9 cm) sample pieces to typical densities of 7.1 g/cm³ and 7.2 g/cm³. All samples containing the 0.8% graphite addition were sintered at 2050 F (1120 C) in a 90 v/o nitrogen and 10 v/o hydrogen atmosphere for 20 minutes at temperature followed by accelerated cooling via a Varicool cooling unit. After sinter hardening, all samples were tempered at 400 F (205 C) for 1 hour. The TRS and tensile specimens were evaluated in this condition. The rotating bending fatigue samples were subsequently machined to final size. Rotating bending fatigue samples to study the effect of carburizing were initially slow cooled after sintering, rough machined inch (0.13 mm) oversize and then vacuum carburized as follows: 1700 F (925 C) for 30 minutes followed by a 30 minute diffuse cycle. After the diffuse cycle, the samples were rapid gas quenched in nitrogen to room temperature and then tempered at 400 F (205 C) for 1 hour. The rough machined samples were then ground to final dimensions. Testing of the rotating bending fatigue samples was done via a staircase method in accordance to ASTM E 468. A run out was defined as 10 7 cycles no failures. Testing was done at 8000 RPM. Results: A prerequisite to improving the fatigue performance of commercially available sinter-hardening materials is attaining high green and sintered densities. The test matrix shown in Table 1 was evaluated to assess the effect of different lubricants and compaction temperatures on the ability to achieve higher densities. Attaining higher densities at compaction pressures at or below 45 tsi (620 MPa) is vital to insure acceptable tool life because of the fragile tooling.

4 Green Density, g/cm³ % Acrawax.625% Acrawax 0.75% Acrawax 0.5% Ferrolube 0.625% Ferrolube 0.625% Caplube Compaction Pressure, tsi Figure 1: data collection Compressibility curves various lubricant types and additions using TRS bars for Static Ejection Force, lbs % Acrawax.625% Acrawax 0.75% Acrawax 0.5% Ferrolube 0.625% Ferrolube 0.625% Caplube Compaction Pressure, tsi Figure 2: collection Static stripping force of various lubricant additions using TRS bars for data

5 Figures 1 and 2 present the data generated with TRS samples on the effects of different lubricants and amounts (conventionally blended) on the green density and static stripping force required for ejection. Decreasing the lubricant addition from 0.75% to 0.50% resulted in a ~0.05 g/cm³ increase in green density with a 5% to 10% increase in static strip load at compaction pressures of 45 tsi (620 MPa) and 50 tsi (690 MPa). The trade off of density vs. ejection load needs to considered for practical production. Shown in Figures 3 are compressibility curves evaluating the effect of compaction temperature for the 0.75% Acrawax blend and the AncorMax D variant of the FLNC-4408 premix. Because of the similar response of the conventional blended and ANCORBOND materials, only the data for the 0.75% Acrawax containing premix is shown. Somewhat surprising, the Acrawax containing premix showed little improvement in green density with raising the die temperature from room temperature to 140 F (60 C). However, the AncorMax D premix containing 0.625% total lubricant showed approximately a 0.10 g/cm³ increase in green density relative to the conventional blends. It is also interesting to note that the green density of the AncorMax D material at 40 tsi (550 MPa) is nearly the same as the conventional material at 50 tsi (690 MPa). Thus, the issue of attaining higher density with fragile tooling is facilitated with this proprietary lubricant / binder system. Green Density, g/cm³ R T, Mix A 120F, Mix A 140F, Mix A 140F, Mix E Compaction Pressure, tsi Figure 3: Compressibility curves as a function of die temperature and premix composition. The green expansion and green strength of the materials are presented as Figures 4 and 5. Heated die compaction does result in greater green expansion, with the AncorMax showing having the largest green pop-out. Despite the larger green expansion of the AncorMax D premix, this material showed the highest green strength of the materials evaluated. The green strength improvement of the AncorMax D premix compacted at 140 F (60 C) shows a >25% improvement over the conventional materials evaluated.

6 Green Expansion, % Room Temperature, MIx A F, Mix A 140F, Mix A F, Mix E Compaction Pressure, tsi Figure 4: Green expansion after compaction as a function of the die temperature 2500 Green Strength, psi % Acrawax, RT Ancormax D, 140 F Compaction Pressure, tsi Figure 5: Green strength of a conventionally blended material and the AncorMax D processed material. Prior experimental work on these alloy systems documented the tensile properties as a function of sintered density and sintering temperature, these results are shown in Tables 2 and 3. Table 2 presents the tensile properties of the two materials that are considered through hardening;

7 that is, a case and core carbon of approximately 0.8%. Table 3 presents the tensile properties of the 0.5% graphite materials, which are characteristic of the core properties of the carburized materials. Table 2 Tensile Properties of High Carbon Sinter-Hardened Materials Material ID FLNC Ancorloy MDCL Mod Sintering Temp F (1120 C) MPIF Referenc e FLC Converted from HRA Density, g/cm³ YS, 10³ psi (MPa) UTS, 10³ psi (MPa) Elong. % Hdness, HRA/ HRC* (550) 92 (635) / (625) 103 (710) / (677) 114 (790) / (745) 124 (855) / (850) 141 (975) / (880) 150 (1035) / (730) 136 (940) / (785) 157 (1085) / (835) 176 (1215) / (545) < / (690) < / (630) < / 38 Despite the lower hardness values achieved at 2050 F (1120 C) the FLNC-4408 had tensile and yield strength values exceeding the typical values listed in MPIF Standard 35. Sintering the FLNC-4408 at resulted in complete nickel diffusion, thus increasing the hardenability and producing hardness values more consistent with the MPIF standard and giving tensile properties exceeding the typical values listed in Standard 35. The modified Ancorloy MDCL material with its higher hardenability produced hardness values in excess of the MPIF reference standard. Tensile properties of the modified Ancorloy MDC material showed ultimate tensile strengths approaching 180,000 psi (1240 MPa) at a 7.1g/cm³ sintered density. In addition to the high strength, the silicon containing material exhibited tensile elongation of approximately 2%. These results are superior to the commercially available sinter-hardening materials.

8 Table 3 Tensile Properties of 0.5% Graphite Materials Material ID FLNC Ancorloy MDCL Sintering Temp F (1120 C) 2150 F (1177 C) * Converted from HRA Density, g/cm³ YS, 10³ psi (MPa) UTS, 10³ psi (MPa) Elong. % Hdness, HRA / RC* (510) 92 (635) / 88Rb (565) 107 (740) / 93Rb (615) 115 (795) / 97Rb (655) 109 (755) / 97Rb (745) 135 (933) / (780) 142 (980) / (635) 117 (810) / (725) 142 (980) / (765) 153 (1060) / (750) 140 (970) / (850) 164 (1135) / (915) 181 (1250) / 35

9 Fatigue Data Rotating bending fatigue samples were prepared and tested, the results of this work is shown in Table 4. Material Table 4 Rotating Bending Fatigue Properties of Sinter Hardened Materials FLNC-4408 MDCL with 0.8% GR FLNC-4405 Carburized MDCL Carburized Density, g/cm³ Sinter Condition 2050 F (1120 C) 2050 F (1120 C) 2150 F (1175 C) 2050 F (1120 C) 2050 F (1120 C) UTS, 10 3 psi (MPa) 90% FEL10 3 psi (MPa) 120 (828) 44.5 (307) 140 (965) 46.1 (318) 155 (1070) 54.0 (372) 150 (1034) 60.9 (420) 190 (1310) 61.1 (421) 150 (1034) 59.9 (413) 107 (738) 53.3 (368) 116 (800) 53.8 (371) 165 (1138) 72.3 (499) 178 (1228) 69.5 (479) MPIF FLC (690) 33.0 (228) 4608 Std (828) 42.0 (290) The trends of the data presented in Table 4 are as follows:! As expected, higher densities promote higher fatigue endurance limits! The choice of alloy FLNC-440x vs. the MDLC material shows that the silicon containing material has higher fatigue life at all density levels and sintering conditions! Sintering the MDCL at a temperature considered not optimum for this alloy 2150 F (1175 C) showed superior fatigue life relative to the FLNC-440x material.! Carburizing has a dramatic effect on the fatigue life, promoting an approximate 15% increase compared to the 0.80% carbon material.! High temperature sintering of the FLNC-4408 showed an improvement in fatigue endurance life compared to the fatigue results for sintering at 2050 F (1120 C). This result is conflicts with other reported fatigue results of elemental nickel additions in P/M alloys. [3] This discrepancy in performance is a result of two competing phenomena: the

10 crack arresting nature of nickel rich areas competing with the greater hardenability and the nearly 100% tempered martensitic microstructure of the high temperature sintered FLNC Tempered martensite is the preferred microstructure for fatigue applications. Thus the greater amount of transformation in the high temperature sintered samples promotes higher fatigue endurance. All previous work reported in the literature was done in the as-sintered condition, thus enhancing the crack arresting characteristics of the nickel rich regions. Discussion: The objective of this work was to achieve improved fatigue resistance of automotive hydraulic pump components. This work demonstrated increasing the density of sinter-hardened or carburized test samples had the desired beneficial effect on the fatigue performance. Methods investigated to increase the density included the use of lesser amounts of lubricant; however, this option results in only a minor increase in density with higher ejection forces with the potential for increased die wear and part scoring. A second method to increase the density involved the use of heated die techniques. Using a heated die (60 C) with a standard blend does not result in any significant improvement in green density. Utilizing the AncorMax D premixing technology resulted in higher green densities, specifically, the green density at 40 tsi (550 MPa) with heated dies is nearly equivalent to 50 tsi (690 MPa) with a standard Acrawax blend at room temperature. Thus, the higher density can be achieved with a specially formulated lubricant and engineered binder. This lubricant technology has been shown to give equivalent ejection characteristics as standard lube amounts provided the height of the part is less than 1 inch (2.5 cm). [4] Along with the increase in green density a 25% improvement in green strength was also observed. As an extension of the experimental work performed last year, fatigue testing of a standard FLNC-4408 and Ancorloy MDCL showed the silicon-containing alloy had nearly a 30% improvement in fatigue life. This improvement in fatigue endurance was observed even when the sintering temperature of the silicon containing steel was not optimal. As expected the carburizing did show improvements over the standard sinter-hardening processing. Fatigue life improvements up to 50% over the standard FLNC-4408 were achieved by optimizing both the material and processing.

11 Carbon Content, % FL-4200 FLN C M in FLN C M in Distance from Surface, inches Figure 7: carburizing. Sintercarb carbon gradient plus the carbon gradient developed during vacuum One potential that was discussed was incorporating a carburizing cycle either during or immediately after the sintering step. This would give the necessary case / core carbon relationship promoting desirable residual compressive surface stresses. An experiment was performed on the actual hydraulic components in which the parts were sintered in a Sintercarb furnace. The Sintercarb process has a specially designed furnace that has traditional sintering high heats and secondary heating zones at typical steel hardening temperatures (at 1800 F {985 C}) with high carbon potential of typically 0.9%. [5] Oil pump components were manufactured from am FL-420x material with a 0.27% graphite addition. The objective was to carburize the part similarly to the vacuum carburizing cycles in previous work. However the SinterCarb furnace has no accelerated convective cooling capability. Consequently, a second sinter cycle was therefore undertaken in a traditional fast cool convective sinter furnace to achieve a sinter harden structure. Results of the carbon gradient during this test are shown in Figure 7. The results of this initial trial on carburizing in line with sintering proved successful compared to previous results from vacuum carburizing done in earlier testing. To determine fatigue performance the toroidal shaped component was fatigue tested using a servo hydraulic test machine. Comparison was made to current production parts using the existing sinter-hardening process. The results on an actual component showed a 15% improvement in axial fatigue. The improvement in fatigue endurance of actual parts is nearly the same (percentage) as the results achieved with rotating bending fatigue testing, verifying the validity of the data developed. Thus, the potential to sinter and carburize was verified but more importantly, the fatigue testing demonstrated a 15% improvement.

12 References: 1. F. Hanejko, A. Taylor, Advanced Sintering Materials and Practices, Advances in Powder Metallurgy and Particulate Materials 2002, Part 13, pp 13-29, Metal Powder Industries Federation, Princeton, NJ, Material Standards for P/M Structural Parts, 2000 Edition, Published by Metal Powder Industries Federation, K. Narasimhan, S. Polasik, and N. Chawla, Surface Replication as Means of Monitoring Fatigue Crack Initiation and Propagation in Ferrous P/M Alloys, Advances in Powder Metallurgy and Particulate Materials, Part 12, pp , Metal Powder Industries Federation, Princeton, NJ, I. Donaldson, S. Luk, G. Poszmik, Processing of Hybrid Alloys to High Densities, Advances in Powder Metallurgy and Particulate Materials 2002, Part 8, pp , Metal Powder Industries Federation, Princeton, NJ, J.S. Adams, D. Geis, D. Glover, etal, SinterCarb Processing, Federal Mogul Bearing Group Research, Internal Report, July 1978.