In magnetic bearing or flywheel technologies, there is a demand for high induction soft magnetic alloys for forged components rotating at high speed (50,000-80,000 rpm). Hiperco ® Alloy 27 (Co27-Fe) is ductile and has high magnetic saturation (24 kG) but with low yield stress (35-55 ksi). This work reports a carbon strengthened Co27Fe-C0.23 alloy with high yield stress (75-85 ksi), high induction (B=20 and 21 kG at H=100 and 150 Oe, respectively) and good ductility (1" elongation =21%). Post-forge annealing is critical to achieve desired combination of high magnetic induction, high yield stress, and good ductility.
This article is published in "Journal of Applied Physics Vol 87 No 9, (May 2000), PP6523-6524."
With the advantages of low friction and freedom from flammable lubricants, magnetic bearings have been replacing ball bearings in high speed and high power devices such as compressors, engines, and generators, especially in aircraft and other high-end applications. Fe-Co materials, with high saturation moments, are candidates for meeting high thrust/weight ratio requirements. Co49Fe49V2 (Hiperco® Alloy 50) type strip showing excellent soft magnetic properties at a nearly ordered state, can be considered for laminations used as rotors/stators for radial magnetic bearings [1-2]. However, thrust magnetic bearing discs, journals or shafts have to use forged bulk material with high induction, good ductility and high strength. Co27Fe (Hiperco Alloy 27, no ordering tendency) has good ductility and high induction, but its yield stress (35-55 ksi) is too low. Mild steels or marage steels, with good mechanical strength but much lower induction, were the only available materials for the above applications; however these resulted in heavier and less-compact devices undesirable for aircraft applications. We have discovered that carbon effectively increases the yield stress of Co27Fe alloy while retaining good ductility and high induction. The material could be considered as a better choice for thrust bearing applications. This paper presents the detail studies on Co27-Fe-CX (x=0-1.2 wt %) alloy.
The alloys were prepared by vacuum induction melting (30-400 lbs). The cast ingots were heated to 1200 °C and then forged into billets. The alloys have excellent hot workability suitable for forging. We used ring samples for DC magnetic tests. The tensile specimens were machined from the forged slab with gauge length 1" and diameter 0.25". Heat treatments were done in a dry hydrogen atmosphere. Presumably vacuum anneal can also be used if a bright surface is not a concern. Magnetic and mechanical tests were done at room temperature.
Results and Discussion
The Hiperco Alloy 27 (carbon trace 0.0005 wt %) annealed at 900 °C/24 hr shows the highest magnetic properties, as the reference point, in Co27-Fe alloy but low strength (yield and ultimate tensile stresses are 34.8 ksi and 56.8 ksi, respectively). By increasing carbon content, the yield strength can be raised from 35-54 ksi to 75-85 ksi and the ultimate tensile strength from 57-93 ksi to 116-139 ksi, depending on anneal temperature. However, too much carbon addition ( >0.3 wt %) reduces the ductility (smaller elongation observed) while yield strength is not further increased. The microstructure study indicated that graphite particles formed in those high carbon ( > 0.6wt %) alloys. It is evident that carbon's solubility has been greatly reduced by 27 wt % Co in Fe.
To achieve better mechanical strength, the lower annealing temperature (730-760 °C) should be used in spite of slightly lower magnetic performance. A higher temperature (800-900°C) anneal resulted in higher permeability but lower yield strength. Carbon is an austenite former so that it will reduce the g-phase transformation temperature. The annealing temperature for Co27-Fe-Cx is preferred to be kept below 800 °C to avoid g-phase precipitation. Cooling rate seems to have an effect on the yield strength. The slower cooling rate after annealing tends to result in higher yield strength. We strongly recommend the following heat treatment parameters: anneal forged and machined components at 760 °C for 2 hours and slow cool at 100 °C/hr in dry hydrogen.
Magnetic induction at lower testing fields (e.g. at 30 Oe) shows more sensitive change as carbon level increases. In contrast, the saturation induction (high field test point) is less affected by small carbon additions. This means that a small carbon addition decreases the permeability effectively but only dilutes saturation moment to a much less degree in single phase alloys (carbon < 0.23 wt %). In alloys with two-phase structure (carbon > 0.6 wt %), the graphite phase is detrimental in reducing magnetic induction and worsens permeability significantly.
Based on the above observations, we had concluded that carbon level 0.23 wt % is optimum for the best combination of yield strength, ductility and magnetic induction. Hiperco Alloy 27 HS is a commercially available alloy with composition Co27-Fe-C0.23 . The magnetic induction of Hiperco Alloy 27 HS, listed in table I as Co27-Fe-C0.225 alloy, still far exceeds ASTM A801 standard requirement for Hiperco Alloy 27 (type-2 alloy) bar products.
Raised yield points appeared in stress-strain curves of Co27-Fe-Cx alloys, indicating that carbon enters the interstitial site of the Co-Fe lattice to form a "Cottrell atmosphere". Carbon also reduces the grain size from ASTM 4-5 in Hiperco Alloy 27 to ASTM 8-9 in Hiperco Alloy 27 HS. Because of the smaller grain size, the coercivity of Co27Fe-C0.23 is 12 Oe compared with 5.1 Oe for the Co27Fe alloy. Solid solution and grain refining are identified as two major strengthening mechanisms for this alloy.
B. Properties of as-forged alloy before anneal
As-forged Co27-Fe-Cx alloys are brittle. "Carbon bands" (commonly found in carbon steels) formed after heating at 1200 °C followed by forging. This high/low carbon phase structure is responsible for the brittleness of as-forged material. Post-forge annealing can homogenize the carbon distribution, being confirmed through the carbon mappings on annealed and un-annealed Hiperco Alloy 27 HS samples by Electron Microprobe Analysis, thereby eliminating the "carbon bands" and enhancing the ductility.
After forging and machining, components made with Hiperco Alloy 27 HS must be annealed to ensure safe operation and to reach best magnetic properties.
0.23 wt % carbon addition to Co27Fe alloy significantly enhances the mechanical strength while providing high induction and good ductility. A post-forge anneal is critical for getting the necessary combination of magnetic and mechanical properties.
David A. Englehart provided extensive technical assistance during this research. The author also acknowledges that Dr. M. Scott Masteller had provided many useful discussions.
1 M.S. Masteller, D.W. Dietrich: US patent 5,501,747 (1996)
2 M.S. Masteller, J.W. Bowman, Lin Li: IEEE Trans. on MAG, 32 (5) (1996) pp 4839-41
3 Lin Li: US patent 5,741,374 (1998)
By Lin Li
Carpenter Technology Corporation