Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Fe~Cr-Co PERMANENT r1AGNErr ALLOY AND Al.LOY PROCESSJNG
T hnical Field_
The invention is concerned with Fe-Cr-Co
5 magnetic materials.
Background of the Invention
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~ agnetic materials suitable for use in relays,
ringers, and electro-acoustic transducers such as
loudspeakers and telephone receivers characteristically
10 exhibit high values of magnetic coercivity, remanence, and
energy product.
Among established alloys having suitable magnetic
properties are Al-Wi-Co-~e and Cu-Ni-Fe alloys which are
members of a group of alloys considered to undergo spinodal
15 decomposition resulting in a fine-scale two~phase
microstructure. Recently, alloys containing Fe) Cr and Co
have been investigated with regard to potential suitability
in the manufacture of permanent magnets. SpeciEically,
certain ternary Fe-Cr-Co alloys are disclosed in H. Kaneko
2~ et al, "New Ductile Permanent Magnet of Fe-Cr-Co Systems",
AIP Conference Proceedin~s No. 5, 197~, p. 1088, and in
U. S. patent 3,806,336, "Magnetic Alloys". Quaternary
alloys containing ferrite forming elements such as, e.g.,
Ti, Al, Si, Nb or Ta in addition to Fe, Cr and Co are
25 disclosed in U. S. patent 3,954,519, "Iron-Chromium-Cobalt
Spinodal Decomposition Type Magnetic Alloy Comprising
Niobium and/or Tantalum", in U. S. patent 3,989,556,
"Semihard Magnetic Alloy and a Process for the Production
Thereof", in U. S. patent 3,982,972, "Semihard Magnetic
30 Alloy and a Process for the Production Thereof", and in
U. S. patent 4,075,437, "Composition, Processing, and
Devices Including Magnetic Alloy".
The use of ferrite forming elements such as,
e.g., Ti, Al, Si, Nb or Ta in quaternary alloys has been
35 advocated, especially at higher Co levels or in the
presence of impurities such as, e.g., C, N or O, to
facilitate production of a preliminary fine-grained alpha
phase structure by low-temperature annealing.
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Summary of the Invention
According to one aspect o~ the invention there is
provided a method for producing a magnetic element compris-
ing a body of an alloy consisting of 25-29 weight percent
Cr, 7-12 weight percent Co, and remainder essentially Fe
characterized in that said method comprises the steps of
(1) subjecting said body to an annealing temperature which
does not exceed 1000 degrees C and which is in the range
of 650-950 degrees C when said alloy contains 25 weight
percent Cr and 7 weight percent Co, in the range of 650-375
degrees C when said alloy contains 25 weight percent Cr,
and 12 weight percent Co, in the range of 650-1100 degrees
C when said alloy contains 29 weight percent Cr and 7
weight percent Co, in the range of 650-975 degrees C when
said alloy contains 29 weight percent Cr and 12 weight
percent Co, and in a range whose limits are obtained by
approximate linear interpolation at intermediate levels of
Cr and Co, whereby an average grain size not exceeding 70
micrometers is obtained in said alloy, (2) forming said
body into a desired shape at a temperature not exceeding
100 degrees C either by wire drawing or deep drawing by
an amount corresponding to a cross-sectional area reduc-
tion of at least 50 percent or by deep drawing or bending
so as to result in a change of direction of at least 30
degrees, the resulting radius of curvature being such that
it does not exceed a value which is proportional to the
change in direction, which for a 30 degree change in
direction is equal to the thickness of the part being
bent, and which for a 90 degree change of direction is
equal to 4 times the thickness of the part being bent, and
(3) aging said alloy.
In accordance with another aspect of the invention
there is provided an article of manufacture comprising a
body of a magnetic alloy consisting of 25-29 weight per-
cent Cr, 7-12 weight percent Co, and remainder essentially
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Fe and having at least 3000 grains per mm3 and a coercive
force in the range of 300-600 Oersted, a remanence in the
range of 8000-13000 Gauss, and a magnetic energy product
in the range ~f 1-6 MGOe.
The invention is an essentially ternary Fe-Cr-Co
magnetic alloy whose grain size is suf~iciently fine to
result in at least 3000 grains per mm3 and which has a
coercive force in the range of 300-600 Oersted, a remanence
in the range of 8000-13000 Gauss, and a maximum magnetic
energy product in the range of 1-6 MGOe. Thus, the alloy
consists essentially of 25-29 weight percent Cr, 7-12
weight percent Co, and remainder Fe and may be conveniently
produced, e.g., by a process involving solution annealing
at a temperature in the range of 650-1000 degrees C to
produce a fine-grained, essentially single phase alpha
structure, followed by cold forming and aging. Magnets
made from such alloys may be used, e.g., in electro-
acoustic transducers such as loudspeakers and telephone
receivers, in relays, and in ringers.
Brief Description of the Drawing
In the drawing:
FIG. l shows phase diagrams of two Fe-Cr-Co alloy
systems containing 9 weight percent Co and ll weight
percent Coj respectively;
FIG. 2 is a photomicrograph showing grain struc-
ture, magnifie~ 100 times, of an Fe-Cr-Co magnetic alloy
; containing 28 percent Cr and ll weight percent Co which
was solution annealed at 900 degrees C; and
FIG. 3 is a photomicrograph showing grain
-structure, magnified 100 times, of an Fe-Cr-Co magnetic
alloy containing 28 weight percent Cr and~ll weight
` percent Co which was solution annealed at 1300 degrees C.
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Detailed Description
In accordance with the invention it has been
realized that Fe-Cr-Co alloys containing Cr in a preferred
range of 25-29 weight percent, Co in a preferred range
of 7-12 weight percent, and remainder essentially Fe
can be produced so as to simultaneously have a maximum
energy product in the range of 1-6 MGOe and a grain size
corresponding to at least 3000 grans per mm3, such grain
structure being particularly beneficial when the alloy is
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to be cold shaped. A more narrow range of Cr content may
be preferred and, specifically, in the interest of
optimizing alloy formability, an upper limit of 28 weight
percent and, in the interest of optimizing magnetic
5 properties, a lower limit of 26 weight percent Cr may be
preferred.
Alloys of the invention may be prepared, e.g., by
casting from a melt of constituent elements Fe, Cr and Co
or their alloys in a crucible or furnace such as, e.g., an
10 induction furnace. Alternatively, a metallic body having a
composition within the specified range may be prepared by
powder metallurgy. Preparation of an alloy and, in
particular, preparation by casting from a melt calls for
care to guard against inclusion of excessive amounts of
15 impurities as may originate from raw materials, from the
furnace, or from the atmosphere above the melt. If such
care is taken and, in particular, if sufficient care is
taken to minimize the presence of impu~ities such as, e.g.,
nitrogen, addition of ferrite forming elements may be
~0 dispensed with. To minimize oxidation or excessive
inclusion of nitrogen, it is desirable to prepare a melt
with slag protection, in a vacuum, or in an inert
atmosphere such as, e.g., an argon atmosphere. Levels of
specific impurities are preferably kept below 0.05 weight
25 percent C, ~.05 weight percent N, 0.2 weight percent Si,
0.5 weight percent Mg, 0.1 weight percent Ti, 0.5 weight
percent Ca, 0.1 weight percent Al, 0.5 weight percent Pln,
a. 05 weight percent S, and 0.05 weight percent O.
Typical processing of the alloy after casting is
30 as follows. The alloy is soaked at a temperature at which
the allo~ is in a two-phase, alpha plus gamma state for a
period of 1-10 hours, temperatures in the range of 1100-
1300 degrees C being generally appropriate for this
purpose. More specific preferred limits on such
35 temperature corresponding to alloys containing~
respectively, 9 weight percent Co and 11 weight percent Co
can be obtained from FIG. 1. The alloy is then hot worked
in such two-phase state, e.g., by hot rolling, forging, or
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extruding to break down the as-cast structure and, if
desired, the alloy may be shaped by cold working. In order
to develop a uniformly fine grain structure, the alloy is
then solution annealed at a temperature at which the alloy
is in an essentially single-phase alpha state and which
generally is in the range of 650-1000 degrees C. Preferred
upper limits on annealing temperature for specific alloys
may be conveniently obtained by approximate linear
interpolation between the following values: 950 degrees C
for an alloy containing 25 weight percent Cr and 7 weight
percent Co, a75 degrees C for an alloy containing 25 weight
percent Cr and 12 weight percent Co, 1100 degrees C for an
alloy containing 29 weight percent Cr, and 7 weight
percent Co, and 975 degrees C for an alloy containing
29 weight percent Cr and 12 weight percent Co and are
further required not to exceed 1000 degrees C in the
interest of minimization of grain growth. In the interest
of improved kinetics, a lower limit of 800 degrees C is
preferred and, in the interest of minimizing gamma phase,
preferred upper limits are obtained by approximate linear
interpolation between respective values of 925 degrees C,
850 degrees C, }075 degrees C, and 950 degrees C and also
under the further provision that annealing temperature not
exceed 1000 degrees C.
If the alloy has been cold worked, solution
annealing so as to substantially recrystallize and
homogenize the alloy may take from 10 minutes to 2 hours
depending on annealing temperature and size of ingot. ~ore
typically, time required is in the range of 30-90 minutes.
Solution annealing may be performed in air or, in the
interest of minimizing surface oxidation, under exclusion
of oxygen~
Solution annealing is terminated by rapid
quenching, e.g., by water or brine quenching, or, in the
case of thin strips, by air quenching and preferably so as
to result in a cooling rate of at least 1000 degrees C/min.
throughout the alloy. At this point, the alloy is at or
near room temperature, i.e., at a temperature which does
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not exceed 100 degrees C, and has an essentially unifo mly
fine grain size not exceeding 70 micrometers (corresponding
to at least 3000 grains per mm3). Such grain structure is
lllustrated by FIG. 2 and may be contrasted with the coarse
5 structure obtained by annealing at elevated temperature as
illustrated by ~IG. 3.
At a temperature not exceeding 100 degrees C, the
alloy may then be cold formed, e.g., by bending, wire
drawing, deep drawing, or swagging. Particular benefits
10 are derived from the fine-grained structure if the alloy is
to be cold formed by wire drawing, deep drawing, or
bending, i.e., by a technique which causes at least local
tensile deformation. On account of the uniformly fine
grain structure of the alloy as annealed and quenched,
15 drawing may be by an amount corresponding to an essentially
cross-sectional area reduction of at least 50 percent.
Similarly, bending may result in a change of direction of
at least 30 degrees, the resulting radius of curvature
being such that it does not exceed a value which is
20 proportional to the change in direction, which
for a 30 degree change of direction is equal to the
thickness of the part being bent, and which for a 90 degree
change of direction is equal to 4 times the thickness of
the part being bent.
Processing as described above characteristically
comprises a step of maintaining the alloy at a temperature
corresponding to an essentially single phase alpha state.
Alternate processing so characterized may be, e.g., by hot
working with finishing temperature in an essentially single
30 phase alpha range, cooling, and forming. Moreover, forming
may be carried out in stages with intermediary additional
solution anneàling and quenching. Additional processing
steps such as e.g., machining by drilling, turning, or
milling before or after forming are not precluded.
The shaped alloy is finally subjected to an aging
treatment to develop magnetic hardening. Such aging
treatment may follow any of a variety of schedules, for
example as disclosed in U.S~ patent No. 4,075,437~
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and in U.S. Patent 4,174,983 issued November 20, 1979, which
allows the production of magnets having magnetic remanence
of 8000-13000 Gauss, magnetic coercivity of 300-600 Oersted,
and magnetic energy product o~ million Gauss-Oersted.
Accordingly, such alloys may serve, upon magnetization in a
magnetic field, as magnets in relays, ringers, and electro-
acoustic transducers such as loudspeakers and telephone
receivers.
In the following examples, phase structure and
grain size were determined by X-ray dif~raction analysis,
hardness measurements, and metallographic analysis of
microstructure after solution annealing and quenching, but
before cold shaping. Average grain size was in the range of
25-40 micrometers as shown in Table I. Also shown in Table
I are magnetic remanence Br, coercivity H3, and energy
product (BH)maX determined after aging oE the alloys.
Example _
An ingot of an alloy containing 26.8 weight percent
Cr, 9.4 weight percent Co, and balance essentially Fe was
cast from a melt. Ingot dimensions were a thickness of 1.25
inches (31.8 mm.), a width of 5 inches (127 mm.), and a
length of 12 inches (304.8 mm.). The cast ingot was heated
to a temperature of 1250 degrees C, hot rolled into a quarter
inch (6.4 mm.) plate, and water cooled. Sections of the
plate were cold rolled at room temperature into strips having
a thickness of 0.1 inches 12.5 mm.) and width of 0.625 inches~
(15.9 mm.). The strips were annealed at 900 degrees C for
30 minutes and water cooled. The strips were reheated to
630 degrees C, maintained at this temperature for 1 hour,
cooled at an essentially constant rate of 15 degrees C/h to
a temperature of 555 degreec C, maintained at 540 degrees C
for 3 hours, and maintained at 525 degrees C for 4 hours.
Example 2
Strips of an alloy containing 27.7 weight percent
Cr, 10.9 weight percent Co, and balance essentially Fe were
prepared by casti~ng, hot working, quenching, solution
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annealing, cooling, and rolling as described in Example lo
The strips were reheated to 635 degrees C, maintained at
this temperature for 3 minutes, cooled at an essentially
constant rate of 15 degrees C/h to 555 degrees C, maintained
at 540 degrees C for 3 hours ancl maintained at 525 degrees C
for 4 hours.
Example 3
Strips of an alloy containing 27.3 weight percent
Cr, 7.2 weight percent Co, and balance essentially Fe, were
prepared as described in Example 1. The strips were reheated
to 620 degrees C, maintained at this temperature for 1 hour,
cooled at an essentially constant rate of 15 degrees C/h to
555 degrees C, maintained at 555 degrees C for 2 hours, at
540 degrees C for 3 hours, and at 525 degrees C for 16 hours.
Example 4
Strips of an alloy containing 26.8 weight percent
Cr, 10.6 weight percent Co, and balance essentially Fe were
prepared as described in Example 1. The strips were soft
and ductile and could readily be bent in any direction by
90 degrees over a sharp edge having a radius of curvature
of 1/32 of an inch (0.08 mm.) or drawn so as to result in
99 percent area reduction. Strips were aged according to
a schedule disclosed in U.S. Patent 4,174,983 by maintaining
the alloy at a temperature of 680 degrees C for 30 minutes,
rapidly cooling at a first rate of 140 degrees C/h to 615
degrees C, and then cooling at exponentially decreasing
rates of from 20 to 2 degrees C/h to a temperature of from
525 degrees C.
Example 5
0.7 inch (17.8 mm.) diameter rods of an alloy
containing 27.9 weight percent Cr, 10.7 weight percent
Co, and balance Fe were prepared by casting, hot working,
solution annealing, and quenching. The rods were cold
drawn to 0.07 inch (1.78 mm.~ diameter wire ~having 99
percent reduced cross-sectional area), solution annealed
at 930 degrees C for 30 minutes, and cooled to room
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temperature. An aging heat treatment was carried out by
maintaining the drawn wire for 30 minutes at 700 degrees
C, cooling to 615 degrees C at a rate of 30 degrees C/h
in a magnetic field of 1000 Oersted, and cooling to a
5 temperature of 480 degrees C at exponentially decreasing
rates of from 20 to 2 degrees C/h"
TABLE I
Grain Br Hc (BH)
Cr Co Size
Ex. ~t.~ Wt.~ ~m G Oe MGOe
_ _ _, _ _ _ _
15 1 26.8 9.4 30 10010 380 1.55
227.7 10.9 25 9750 400 1.72
327.3 7.2 40 9280 300 1.10
426.8 10.6 40 10010 370 1.76
527.9 10.7 30 12750 570 5.03
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