Carpenter Acube 100 is a

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1 Beryllium-free Cobalt Alloy for High-load Bushings Richard B. Frank*, Karl A. Heck*, Joseph Stravinskas* Carpenter Technology Corporation Wyomissing, Pennsylvania Co-Cr-Mo alloy offers an excellent combination of strength, ductility, and resistance to corrosion and wear. It is hardened by warm or cold working, and by thermal treatment. *Member of ASM International Carpenter is a beryllium-free Co-28Cr-6Mo alloy designed for aerospace bushing and bearing applications. These applications require minimum yield strength of 140 ksi (965 MPa) for bars having diameters between two and four inches. To develop these strength levels, a series of inch (66.7-mm) diameter bars were produced by rolling and by forging to compare warmworking capabilities. Bars were rolled on hand rolling mills, and were forged in a rotary forge. For both practices, billets were strain-hardened by warm working during the final reductions. Table 1 compares the room-temperature tensile properties Lockheed Martin is evaluating at the center, mid-radius, and sub-surface locations of the bar. beryllium-free alloys for the F-35 Lightning II Joint Strike Fighter. As the table shows, rotary forging resulted in higher For Lockheed Martin Aerospacedesigned F-35 parts, Lockheed strength and was more effective in developing fine, uniform grain structures. The rotary-forged bar exceeded 140 ksi (965 intends to complete a statistically MPa) yield strength even at the center location. Similar based qualification of Co-Cr-Mo rotary-forging practices were applied to larger bar diameters alloy to one-to-one replace CuBe for bushings greater than 2.5 inch of 3.085, 3.50, and inches (78, 89, and 105 mm), with (63.5 mm) OD for new design. similar results. Table 2 shows the typical room-temperature tensile properties developed in 0.5 to 1.5-inch ( mm) diameter bars after annealing, hot working, and warm work- TABLE 1 ROOM-TEMPERATURE TENSILE PROPERTIES AT VARIOUS LOCATIONS IN THE BAR Warm-working Test 0.2% yield Ultimate tensile Elong. Reduction of method location strength, ksi (MPa) strength, ksi (MPa) in 4D, % area, % Rolled Center 133 (917) 195 (1345) Rotary forged Center 145 (1000) 200 (1379) Rolled Mid-radius 139 (958) 196 (1351) Rotary forged Mid-radius 155 (1069) 207 (1427) Rolled Subsurface 152 (1048) 204 (1407) Rotary forged Subsurface 177 (1220) 217 (1496) Warm-worked inch (66.7-mm) diameter bar TABLE 2 TYPICAL ROOM-TEMPERATURE TENSILE PROPERTIES 0.2% yield Ultimate tensile Elong. Red.of HRC Condition strength, ksi (MPA) strength, ksi (MPa) in 4D, % area,% hardness Annealed 85 (585) 150 (1035) Hot-worked 110 (760) 160 (1105) Warm-worked 142 (979) 193 (1331) inch ( mm) diameter bar product ADVANCED MATERIALS & PROCESSES MARCH

2 ing. Common practice is to warm or cold work smaller-diameter bars to meet the 120 ksi (827 MPa) minimum 0.2% yield strength requirement of the ASTM F medical implant specifications. Phase Transformations For critical applications such as aerospace bearings, Acube100 alloy is premium melted. This consists of primary melting in vacuum-induction furnaces (VIM), followed by electro-slag remelting (ESR). Premium melting provides high purity, excellent cleanliness, and a sound ingot structure. Ingots are typically homogenized to minimize microsegregation, and then forged to an intermediate billet size. Billets are then re-forged to large-diameter bars, or hot-rolled to smaller-diameter bars. Cobalt alloys transform from a hightemperature, face-centered cubic (fcc) phase to a lower-temperature, hexagonalclose-packed (hcp) phase during deformation and thermal processing. However, the transformation is sluggish, even for pure cobalt. Alloying additions such as carbon, nickel, iron, and manganese stabilize the fcc structure. Additions of chromium, molybdenum, tungsten, and silicon tend to stabilize the hcp structure. Carbon, chromium, and especially molybdenum provide solid-solution strengthening of Co-Cr-Mo alloys. Another major boost in strength results from the transformation from fcc to hcp during warm- or cold-working operations, or during a thermal aging treatment, Fig. 3. In addition, Carpenter conducted studies to evaluate the effects of higher nickel and iron contents, in an effort to reduce alloying cost. However, experiments showed that both nickel and iron additions in the 0 to 15 wt% range reduced strengthening capability by reducing the amount of hcp phase formed during mechanical working. A composition with 5 wt% Fe and 10 wt% Ni was also found to have lower wear resistance in simulated bushing tests. For these reasons, nickel and iron contents are limited to 1 wt% maximum. (Table 3) Thermal Aging Warm-worked samples representing bar sizes of to 3.5 inches (67 to 89 mm) were aged for four hours at temperatures ranging from 1300 to 1550 F (704 to 843 C) to determine hardening response. Samples were treated in electric (air) furnaces and then were air cooled to room temperature. Figure 1 illustrates the significant increase in hardness during aging, with maximum hardness developed during aging in the 1350 to 1400 F (732 to 760 C) range. Room-temperature tensile specimens (0.250-inch ground gauge sections) were Hardness, HRC inch radius inch radius 3.5 inch radius 40 As-worked Aging temperature, o F/4 hr Fig. 1 Hardness increases significantly during aging. TABLE 3 CHEMICAL COMPOSITION (WT%) OF ACUBE 100 ALLOY C Mn Si Cr Mo Ni Fe N Co max 1.0 max 0.17 Balance 20 ADVANCED MATERIALS & PROCESSES MARCH 2010

3 machined from the mid-radius location of aged bar sections. Tests show that it is possible to further increase the yield strength of warm-worked material by up to 40 ksi (276 MPa) by aging in the range of 1325 to 1375 F (718 to 746 C), but tensile elongation is significantly reduced. The minimum tensile elongation value of 5% is comparable to that of the CuBe alloy for bushing applications. However, the best combination of strength and ductility results when high strength (140 ksi) is developed by warm working only, as it also provides elongation values exceeding 15%. Bushing Wear Test The subscale bushing wear test setup consists of an un-lubricated, constrained bushing fitted around a hardened 440C steel pin that is rotated to simulate inservice conditions. For the threshold wear test, the bushing is progressively loaded from 2000 to 10,000 pounds, with the load applied downwards, creating contact between the pin and the 12 o clock top dead As-Forged Aged 1375 F (746 C) Nitronic 60 (cold-worked to 145 ksi min YS) Cu-Be (aged to 145 ksi min YS) (aged to 157 ksi YS) (warm-worked to 130 ksi YS) (warm-worked to 140 ksi YS) Fig. 2 has much better resistance to bushing wear than Cu-Be and Nitronic alloys. center bushing position. The test is run for 2000 cycles, with the load increased by 500 pounds every 100 cycles after initial loading to 2000 pounds (Fig. 2). Wall flattening Inside diameter wear Dimensional change, inches One cycle consists of travel from 0 to +25 degrees, then back through 0 to -25 degrees, and then returning back to 0 degrees. Displacement is measured from the For pricing and access details, contact: Denise Smith ASM International denise.smith@asminternational.org or call Alloy Phase Diagrams Center We just added ANOTHER 1,500 new diagrams to the Alloy Phase Diagrams Center! This addition brings the Center to 31,000 online diagrams, and puts the full breadth and scope of the available data on binary and ternary phase diagrams on your desktop. The update includes: industrial and heat-resistant category, including solder, brazes and copper alloys category, including Pb, Bi, Ag, Au, Sb, In, Ga, Cd, Zn, Cu and Sn ADVANCED MATERIALS & PROCESSES MARCH

4 TABLE 4 BUSHING WEAR-TEST RESULTS Bushing Wear Test Data Average Dimensional Change Yield Strength Bar Dia. (2-3 tests) Alloy Condition ksi (MPa) in. (mm) ID Wear, in. (mm) Wall Flattening, in. (mm) ACUBE 100 Alloy WW 144 (993) 1.0 (25) (-0.064) (+0.165) ACUBE 100 Alloy WW 130 (896) 3.5 (89) (+0.229) ACUBE 100 Alloy WW+Aged 157 (1083) 3.5 (89) (+0.064) Cu-Be CW+Aged (TH04) (-0.396) * (+1.372) * (UNS C17200) 145 (1000) min Nitronic 60 CW (Grade D) 1.75 (44) (-0.432)** (+3.429)** (UNS S21800) 145 (1000) min WW = warm-worked. CW = cold-worked. * One of two tests stopped at 1726 cycles due to excessive force and metal build-up on arbor pin. ** Grinding started at 875 cycles; arbor pin grooved. Nitronic is a U.S. registered trademark of AK Steel. 12 o clock position each cycle, with increasing negative values corresponding to wear at the bushing- to-pin interface. Failure is defined as extensive wear and/or galling between the bushing and pin, resulting in the test fixture being unable to maintain the required load. Bushing and pin wear surfaces are examined after testing and changes in dimensions are recorded. Testing was done at Cradin Aerospace in a fixture that applied high loads at low rotational oscillatory speeds, according to Lockheed Martin procedure LHM-010. Duplicate 0.9-in. OD x 0.7-in. ID (22.9 x 17.8-mm) bushing specimens were machined from the center region of 1.0-in. (25-mm) and 3.5-in. (89-mm) radius alloy bars warm-worked to yield strength levels of 130 to 135 ksi (896 to 931 MPa). Duplicate specimens were No event brings it all together like AeroMat. Optimizing Performance and Affordability of Aerospace Materials June 20-24, 2010 Meydenbauer Center Bellevue, WA USA The latest advances in materials and processes for aerospace applications are coming to the world s No. 1 aerospace industry location Bellevue, Washington. For 2010, AeroMat organizers will focus on developing, manufacturing, and applying advanced materials and processes in our ever-changing global economy. Learn from industry and government experts sharing their research, findings and views on how current conditions will affect the way the aerospace industry and its suppliers do business in the future. Take an education short course. Our courses are designed to meet the needs of the aerospace materials community. Stay up-to-date and competitive. Be part of the dynamic exposition reach aerospace decision makers. Reserve your booth space and your sponsorship today. Learn, teach, network advance your career and the industry. Plan today for AeroMat ASM International 9639 Kinsman Road Materials Park, Ohio ADVANCED MATERIALS & PROCESSES MARCH 2010

5 a b c d Fig. 3 This illustrates the microstructure of warm-worked 3.5 inch (89 mm) bar before and after aging at 1375 F (746 C). a: The fcc phase in cobalt alloys is metastable in the solution treated condition. b: The fcc phase transforms to hcp during warm or cold working and during thermal treatment (aging) in the range of F ( C). In the warm-worked condition, intragranular striations of the hcp phase are apparent. c: Another significant change in the microstructure is shown: preferential precipitation of carbides along hcp phase platelets. d: The carbides are enriched in Cr, Si, and Mo and are believed to be M23C6 carbides. Several other investigators have researched the aging of this type of alloy. also machined after age hardening to a higher yield strength level of 157 ksi (1083 MPa). Table 4 shows the bushing wear test results, including comparisons for Cu-Be (UNS C17200) and Nitronic 60* (UNS S21800) alloys tested by Lockheed Martin during a Metals Affordability Initiative project. The test results in Table 4 indicate that has much better resistance to bushing wear than the Cu-Be and Nitronic 60 alloys. shows minimal wear or distortion, as evidenced by the low dimensional changes for ID wear and wall flattening of the bushing. Also, the improvement was apparent at a lower yield strength level of 130 ksi (896 MPa), as well as a higher age-hardened strength level of 157 ksi (1083 MPa). Based on these test results, the ACUBE alloy appears to be a suitable wear-resistant replacement for Cu-Be alloy in the TH04 condition (CW + aged to 145 ksi minimum). * Nitronic is a U.S. registered trademark of AK Steel Inc. For more information: Carol Aulenbach, Carpenter Technology Corp., Wyomissing, Pa.; caulenbach@cartech.com; com. Joseph Stravinskas is R&D Program Manager, Aerospace; Richard B. Frank is an R&D staff specialist; Karl A. Heck is a process engineer at Carpenter Technology Corp. Acknowledgments The authors thank Jeanne Treasurer of Carpenter Technology and Chad Henry of Lockheed Martin Aerospace for their helpful assistance during development of this product. The medical industry has taken advantage of the high work-hardening characteristics of Co-Cr-Mo alloys to develop high tensile-strength properties at room temperature. The ASTM F-1537 specification for Co-28Cr-6Mo alloys for surgical implants requires a minimum 0.2% offset yield strength of 120 ksi (827 MPa) and a minimum tensile strength of 170 ksi (1172 MPa). In smaller bar diameters under 2 inches (51 mm), it is common for suppliers of low-carbon wrought Co-Cr-Mo alloy to produce material with higher minimum strength levels of 140 ksi YS (965 MPa) and 190 ksi UTS (1310 MPa). ADVANCED MATERIALS & PROCESSES MARCH