CA2031281A1 - Permanent magnet having improved corrosion resistance and method for producing the same - Google Patents

Abstract

ABSTRACT

A permanent magnet of the neodymium-iron-boron type having improved corrosion resistance imparted by a combination of oxygen, carbon and nitrogen. Oxygen is provided in an amount equal to or greater than 0.6 weight percent in combination with carbon of 0.05-0.15 weight percent and nitrogen 0.15 weight percent maximum.
Preferably, oxygen is within the range of 0.6-1.2% with carbon of 0.05-0.1% and nitrogen 0.02-0.15 weight percent or more preferably 0.04-0.08 weight percent. The magnet may be heated in an argon atmosphere and thereafter quenched in an atmosphere of either argon or nitrogen to further improve the corrosion resistance of the magnet.

Description

I ¦ ~3ACR(~RQUN~) Q~ T~
i¦ Field of the Inv~ntion This invention relatt~tY to a permanent maqnet having improved corroStion resi~tance and to a method for producing the ~ame.
Description of the Prior Art It i8 known to produce permanent magnetSt of a rare earth elemenT~iron-~oron compo ition to achieve high ~nergy product at a lower cost than samarium cobalt magnetY. TheYe magn4ts do, however, exhibit ~ev~re corro~ion by oxidation in air, particularly under humid condition~. Thi~ results in degradation of ths magnetic proporties during u3e of ~ho m~gnet.
Efforts have be~n made to improve the corrosion re~istancR of these magnets, ~uch as by applying metallic plating~ thereto, us-ing aluminum-ion vapor depo~ition coatin~, organic resin coat-ing~, ~ynthetic resin coating~, metal-resin double layer coatings, as well a~ combinations of these coating sy~tems. In addition, chemical ~urface treatments have been employed with these magnets in an attempt to improve the corro310n resLstance thereof.
I Met~llic plating0, applied by electro or electroless plating j practicea, provlde plat~ngs of nickel, copper, tin and cobalt.
~he~e practice~ h~v~ been ~omewhat succe~sful in improving the l corrosion re~istance of these magnet~. Problem~ m~y result with k~ I thls plating practice from the acldic or alk.~line ~olutions u~ed j70 ! in ehe pretreatment employed prior to the plating operation.
The~e ~olutions may remain in the porou~ ~urface of the magnet or may react with neodymium-rich phases thereof to form un~table ~wO,,,c~. compounds. The~e un~table compound3 roact during or after plating FINNECAN, HENDERSON
FAR~90W, C~RR8TT

N W
~511~NOTON, OC 20005 202 - 40tl. 4000 .

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~ ~o cau8~ 10~9 of plating adhe~ion. Nith motallic platings, it is ¦ co~mon for the plating to exhibit microporosity which tends to accelerate reaction of unstable pha~e~. For exc~mple, if there is a reac~ive media, such as a halide, in the en~ironment, such a.~ is the case with ~alt water, a galvanic reaction may result between the metallic platins and the unstable phases of the magnet.
WL~h aluminum-ion vapor depo~ition no pretreatment i~
required and thu the problems of mstallic platings in thi~ regard i ~re avoided. Coatings of this type~however, ar~ characteri2ed by -j significant microporo~'ty because of the nonuniform deposition of the coating on ~he eurface of the magnet. In addition, thiY
¦ practica i9 not amenabla to ma~et production proce~sa~ and thus is '1 too expensive for commercial application.
The u3e o~ re~in coating~ su~fer from poor adhesion to result j in the gradual removal of the coating followed by oxidation of the I magnet ~urfac~ at the removed coating portion thereof.
! Metallic-resin double layered coatings if not applied in a I continuous fa~hion result in accelerated, spreading corrosion from ! any area0 of coatlng discontinuity.
Chemic~l ~u~ace treatments, including chromic acid, hydro~luorlc ~cid, oxalic acid or pho~phate treatments, all suffer o ~S~ F~æ ~ ~ tho dls~dvantage of requiring expensive equipment to comply /3/~ wlth envlronment~l regulations. Consequently, the~e practices are not commercially fe~ible from the cost standpoint.
All of tha conventional methods for improving the corrosion resistanc0 of permanent magnets of thi~ type suffer from ~he same ~wo~ce~ ¦ deficiency in that the corro~ion protection i~ o~talned by a EC~N, HNDER50N:, FARA90W, GARRETT ! ¦
~ D~,NNER N
~00 ~ 5TRCLr, ~I W
.c~or~, oc ~0005 ! ~ -- 2 ~o~ ~ol ~ooo 'I .

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¦ surfac~ treatment of th0 magnet. Accordingly, the m~gne~ per 4e I i~ not ~tabilized with re~pect to corro3ion by any of th~e ' -~urface-traatment practices.
I It i~ known to vary the compo~ition of the magnet to improve : the corrosion re3istance thereof. Alloy modification4 of thi type are disclosed in Nara~imhan et al., U.S. P~eent No. 4,588,439 wherein a~ oxygen addition i3 added to lmprove corrosion re~i~t2nc2 by reducing the di~integration of the magnet in humid hLgh-temperatur~ condition~. A. Rim, and J. Jacob~on: lEEE Tran : on Mag. ~ag-23, No. 5, 1987 disclo~e the addition o~ aluminum and dyspro~ium or dysprosium oxide for thi~ purpose. Thi~ publication al~o recognize~ that chlorine cont~mination o~ the magnet result~
in deterioratLon of the corrosion resistanc~ both in humld and in dry air at elevated temperature. Sagaw~ et al., Japanece Patent No. 63-38555, 1986 di3cloae the addition of cobalt and aluminum to ; improve corro~ion resistance. These alloying additions are combined with reduced carbon and oxygen content~. TakQshita, and W~tanabe~ Proceeding~ of 10th Int~l WorXshop on ~E ma~nets and theLr ~ppllc~tion ~ yoto, Japan, 1989 di~close the addition of j oxideJ of chromium, yttrium, v~nadium and ~luminum for purpose~ of . corro~ion re~iDt~nce in these alloys. H. N~kamur~, A. Fukumo and Yon~yaa~mal Procaedings of 10th Int~l Workshop on RE Magnets and . Thelr Application (II) ~yoto, Japan, 1989, di~clo3e8 the substitu-tion of a portion of iron with cobalt and zirconium for thi~
purpose. A. Hasabe, E. Otsuki and Y. Umetsu~ Proceedings of the 10th Int~l Workshop on ~E M~gnet~ and their Appllcation (II), ~w O~-C~
FINNECAN, HENDER50N
FAR.~30~. CI~RRETT
~ D~NNER
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¦ ~yoto, Japan, 1989, disclo~e various anodic polarization tQCh31iqUa8 for improving corroilon re~i~tance.
All of these practic~s may re~ult in improved corro~ion rQsistance but otherwicle provide problem-q, such as increa~ed coRt or degradation of m~gn~tic properties. For example, the addition of cob lt incra~ses the Curie temperature but cnus3s a decrease in ` coercive force. The addition of the aforemention~d oxides ; degrades the energy product of the magnet~.
~LY~
It i~ accordingly a pri~ary ob~ect o~ the pra ent inven~ion to provide a permanent magnet and a method ~or producing the ~ame wherein improved corro~ion rQsi~tance may be achievsd while !I minimizinq ad~er~e effact~, such a~ degradation of the magnetic propertle~ and increa~ed co~t.
In accordance with the invention there i8 pro~ided a permanent magnet having improved corrosion re2~istance, which magnet con0ists os~entlally of Nd2-Fel4-~ with oxygen being equal to or gre~ter than 0.6 weight ~, carbon 0.05 to 0.15 weight ~ and nitrogen 0.15 weight ~ mnx~mum. Preforably, oxygen may be 0.6 to 1.~ weight ~, c~rbon 0.05 to 0.1 weight ~ and nitrogen 0.02 to 0.15 or mor~ proforably 0.04 to 0.08 welght ~.
¦ In ~ccord~nco wlth the method of the invention the a~or~montloned magnet composLtions may be heated in an argon at~o~phero and there~fter quenched in a nitrogen atmosphere to further improve the corroslon re~istance thereof. The hea~ing in the argon atmo~phera may be conducted at a temperature of about ~AW O~C~ S50C.
: FINNEG~N, HENDERSON ~, FAR~3GW, G~RRETT
~ Di.NNER
l~oO 1 5Tl~t~T, N. W
w.~5~NG~ON, OC 20005 . ~ _ 4 202~0~000 ' ' . `

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All percentage3 are in weight percent unles~ otherwise indicated.
~RIEF DESCRIPTION OF THE DR~WINGS
F$g. l i~ a graph ~howing the weight lo~s of Fe-33.5~ Nd~
B-0.1%C-(O.OS to 0.15S)N magnets made from atomLzed powder after expo~ure in an autoclave at 5-10 psL for 96 hour~, a3 a function of the oxygen content of the magnet sample~;
Fig. 2 i~ a ~imilar graph ~howing the weight 108~ of a magnet of the ~ame CompOBitiOn a~ Fig. 1, except hAving O.014 to O.025%
N, aftar 96 hours expo~ure in an autoclave t 5-lO p~l, a3 a funs-j tion of the oxygen content;
Fig. 3 is a ~imllar graph showing the weiqht lo~s after j 96 hour~ exposure in an autoclava at 5-10 p91 as a function of the oxygen content of magnets having the compo~ition~ in weight ;percent li~ted on thi~ figure;
Fig. 4 i~ a ~imilar graph show$ng weight los~ aftar exposure in an autocl~ve at 5-10 p8i a~ a functlon of carbon content of magnets having the composition~ in wolght percent listed on thi3 flgure7 j Fig. S i~ ~ ~imilar graph showing the weight los~ of Fe-33.9 ¦ Nd-l~lS~ B-0,46~ 0-0.055~ N magnets after exposure in an autoclave at S-10 p~l a~ a functlon of carbon content, exposure time and I sur~ace treatment;
I ¦ Fig. 6 i9 a ~imllar gr~ph showing welght 109~ of Fe-33.9% Nd-I 1.15%B-0.33~ 0-0.024~ N magnets after autoclave testing for 40 i hours at 5-10 p8i a~ a function of the carbon content and ~urface ~w orrlc~ treatment;
F~RAEOW, GARRErr .;
~ D~NNER
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W~5~1NGTON, OC 2000~
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,7 Fig. 7 i~ a similar graph ~howing weight 1088 of Fe-Nd-~-0.45% O-0.10 ~o 0.16~ C magnet~ aftar expo~ure ln an autoclave for 40 hour3 and 96 hour~ at 5-10 p~i as a functLon of the nitrogen content; and Fig. 8 is a ~imilar graph showing wsight 1088 of ~e-34.2~ Nd-1.13~ B-0.55q 0-0.06~ C magnets after expo~ure in an autoclave for 40 hour3 at 5-10 p31 as a function oA~ ni~rogan Contant.
D~CRIPTIQN Ql~T~ PREFERRE12 E~BQ~I7~ ~S
To demon~trate the Ln~ention permanen~ magn~t alloy~ and magnets made therefrom were produced by convention 1 powder metal-lurgy teChlliqUE~8. The permanent magnet alloy from which the ma~net s~mples were produced contained one or more of the rare earth el~ment~, Nd and Dy, in combination with iron and boron.
The m~terial wa~ produced by vacuum inductlon melting of a pre-~lloyed charge to produce a molten mas~ of the desired perm~Anent magnet alloy composition. The molten mas3 wa~ either poured into a mold or ~tomized to form ~ine powder ~y the uqe of argon gas. The alloy RNA~l w~As atomized with a mixture of argon and nltrogon ga~. Wlth the molten materlal poured into a mold, the reaul~ing aolidl~ied ingot cas~ing wa~ crushed and pulverized to ~or~ coaLu~ powderJ. These powders, as well as the atomized powdors, were ground to form fine po~der by ~et ~illing. The ,1 , !i .

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FINI`:EC~N. HENDERSON i FAR,~9OW, GARRETr I !
a DUNNER
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j averag0 p~rticle qizes of the~e ~Llled powder~ were in the range 1! to 4 mlcron~.
The oxygen content of the alloys wa3 controlled by introduc-ing a controlled amount of air during ~et milling or altern~tely blending the powder~ in air a~ter the milling operation. The i ni~rogen content was usually controlled by introducing a controlled amount of nitrogen during ~et milling, but nitrogen was al30 introduced durlng atomiz~tion. The latter practice usually producad a high nitrogen content alloy. With high nitrsgan ~, content alloy~, tho nitrogen conten~ W2~ controlled by blending low and high n$trogqn alloy powder~. This pr~ctlco wa~ used to ~S~ f~ produce tha ~a~ples report~d~Table 11 hereinafter. The ca~bon iqD ,I content waJ controlled by introducing a controlled ~mount of carbon into the alloy~ during melting and/or by blending high 1 carbon alloy powder and low carbon alloy powder to achieve the I deqired carbon content.
The alloy powders wero placed in A rubber beg, aligned in a magnetic ~ield and compacted by cold iso3tatic pre~ing. The ~peci~ic ~lloy compo~Ltion~ uaed in the experimantal work reported hereln aro li~t~d ln Table 1.

l c~ltlcn (wt.%) I nl_ ~ N~ ~ _ C N ~RE
I Alloy 3 (A) 64.3~ 3~.0 1.15 0.0~
! Allcy 3C~1 ~C1 ~ 33.~ 1.13 0.~ 3~.0 Al~oy 3C-2 (~ D~l 33.~ 1.15 0.~ 34.0 All~y 3C-3 (A~ ~ 13.~ 1.10 0.10 3~.0 ; RN~-l (A) 63.9 34.~ 1.0 0.05 0.40 33.1 CR~1 (C) ~ 32.~ 1.1 0.0~ 33.2 CR~ (C) ~ 3~.3 1.~ 0.05 ~2.9 ~ o~r~c~- ~A) d~x~ ~bo ~t~ pa~hr FAR~30W GARRE1T ¦ ~C) ~t~ t~ a~
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The cold pre~sed compacts were sin~ered to sub~tantially full theoretical density in a vacuum furnace at a tsmperature of 1030C
for ons hour. A portion of ~he qintered or qinterad pluq heat I tre~ted magne~ was than ground to a ds~irQd ~hapa. Some of tha ground magnet~ were further heat treated in variou~ environmentq a~ different ~ampera~ure3, as w011 a~ being sub~ected to surface treatment, such a~ with chromic acid.
The 3~mples werQ testQd with re~pect to corrosion behavior using an ~utocl~ve operated at 5-10 psi in a team environment at a temperatura of 110-115QC ~or 18, 40 or 96 hours. After autoclave testing, the weight 10~8 of th~ samples was measured with a balance after removing the corrosion productq therefrom.
The weight 10~8 per unit area of tha sample was plotted as a func-tion of the oxygen, nitrogen or carbon content. The contents of oxygen, nitrogen and carbon in tha m~gna~ were analyzed with a Leco oxygen-nitrogen an~lyzer and carbon-sulfur an~lyzer. The corro3ion product w~.s identified by the uae of X-ray diffraction.
It has been determined from the work reported herein that the corrosion rate o~' Nd-~e-B magnets is af~ected by the oxygen, carbon and nitrogen content~ of the magnet alloy compo~iition and the heat trea~ient cycle of the m~gnet.

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FARAaOW, G~RRErT
a D~l'NNER
~oo t ~r~c~r, N. W -- 8 w~5r~ GrO~ C 2000!~
20~ 40~1 4000 ., .

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I Figure~ 1-3 and Table~ 2-5 report the waight los3 for the ¦ raportad magnet compo3itions after expo~ura in an autoclave at S-10 p~i within the temperature range of 110-115C for 40 and 96 hour3, a~ a function of the oxygen content. The weight 1053 of the magnet w~ mea~ured per unit area of the sample during autoclave te~ting to provido an indication of the corrosion rate of the magnet in the autoclavo environment. As hown in Figure 1 . and ~able 2, the corro~ion rato of the magnet decrea3es rapidly as the oxygen content increase~ from 0.2 to about 0.6~, and reache~ a minimum when the oxygen content is between 0.6 and l.OS. With the ~ -minimum corrosion rate, the weight lo~ i3 le95 than 1 mg/cm2 and ~ . .
the corros$on products arQ barely ob~ervable on the 3urfaca of the . 1, magnet aample aftar expo~ure in the autoclava environment for tha ; te~t perlod. The oxygen content required to achieve the minimum ; corrosion rate variQs depending upon the carbon and nitrogen content~ with the corrosion rate decrea3ing rapldly as the oxygen content incre~ses up to about 0.6~. As shown in Figure 2 and Tabla 3, the corro~ion rate of the reported alloy al~o decrease~
rapidly with oxygen content lncreases from 0.2 to 0.6~ and reaches the mini~um ~t an oxygen content o~ 1.2~. In thi~ regard a may ' be ~een fro~ Figure3 1 and 2, the beneficial affect of oxygen on tho CO~OBion r~te 3hift3 from a relatively high oxygen content of about 1.0~ to A relc~tively low oxygen content of about 0.6~ a~ the nitrogen contant i6 varied from A range of 0.014-0.025~ to 0.05-.' 0.15~ with a carbon content of 0.10. ~ence, at these oxygen and .. carbon contents, the corrosion rate decreaEles as the nitrogen con-~WO~IC........ tent increa~es from about 0.020 to between 0.05 to O.lS0. This . FINNEG~!;. HeNDER50N
F.~R.~3GW, G~RRETT
a DL'NNER
'~00 1 ~-IICCT, 1~. W. ` g w~ 145rO~, OC 2000 202-'~0~ ~000 . ~

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¦I data ~hows the significance of nitrogen and that nitroqen is beneficial in improving corrosion resi~tance within the oxygen content limits of tho invention, including the preferred oxygen limit of 0.6 to 1.2~.
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T~Q 2. ~3~ loa- o~ ~b~ .lC-~O.G~-O.l~)N ~
~e,~ po~æ ~ ~ ~n ~ ~ ~10 p-l tor ~0 ., ~ Gn~ H.T. ~ NzQ

: , 0.27 0. ~i 0.081 5~ 27~i 40.9 130 0.~3 O.OJ9 0.10 ~.9 93 . ~.3 9~
1l 0.~ 0. ~ 0.09~ 3,~i 3,~ ~7.0 il 0.5~ 0.~1 0.~1- 0.9t~3.11 0.9æ 6.0 ,j 0.6;t5 O.~g 0.~51 0.3S0.33 0.4S 1.2~
O.~i 0.0~ 0.10 0.~9 3.~ ~.2~. 2.S7 0.111!1 0.~ 0.093 0.~40.4;2 ~.05i 0,45 ,1 O.t~ 0.~4 0.~,0 0.1110.07 0.45 0.0~ .
O.ôgi O.~ .10 0.~ 0.03 0.~12 0.~7 0.91!~i 0.11 .O.t19:1 0.3~30.33 0.~0 0.22 0 . 99~i O. L'I O. O~i 0. 6~1i 1. 7;~ 0. ~ 5 : .
5~1~ 3. ff~t la~ ot ~3t.!52~1.11!~-O.lC-~0.016~0.02~)N ~
~aG~ p~x~v-ly, ~ a ~x~cn o~ Q nd ~ oo~ur~.
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0.2~ 0.0~ 0.10 92.9 l~D 6a.a 36 0.3~0 0.0~ 0.10 3~.6 l66 l.~a 224 i 0.~ 0.0~ 0.10 23.~ ~04 10.4 1~6 0.30 0.0~ 0.~0 12.fl 11~ lOS
I o.~ o.~ 0.~0 3.8~ 72.~ o.al ?0.9 0.60 0.0~ 0.~0 ~.l 14S ~.l 12~
0.~3 0.01~ O.lO l~.g ~2.~ a.l~ ~6.s 0.~2~ O.Old O.lO 2.43 2~.0 0.~ -17.3 0.92 0.01~ 0.10 0.39 6~9a 0.1~ 1.3 0.01~ o.~0 0.~ 1.~3 o.~ o.a ~w o~ce~
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~! The corro~ion rates of the identical alloy compo~ition u~ed in obtaining the d~t~ reported in Figur~s 1 and 2 except with ~:
varying nitrogen contene~ were compared a3 a function of the oxygsn cont0nt. As ~hown in Figure 3 and Table 4, ths corro~ion ; rates of both m~gne~ having low ni~rogen (0.038g) and with higher nitrogan (0.064~) decrea~ed r~pidly a~ the oxygon con~snt . !
increa~ed. It m~y be .~een, however, that th~ corro~ion rate progres~es downwardly as tho nitrogQn content increa~e~ from 0.038 ' to 0.064~ at th~ reported r~nge of oxygen 60ntont with a c~rbon ! content o~ 0.13~.
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4. Na~t lc~s ot gK~nd Far3~.~k~1.15~ ~*~ 5~ m~ p~r ~n~ ~U~X~A~O ~ at 5-10 pi ~o ~ fl~lGn o~ g, ~ ~n~ C co~nt~

Q ~ _5~ Q ~r g6 Hr O 4~ 0 Od~ O.~ 69.2 13 0.~0 0.0~ 0.
0.~ 0.~4 0.13 O.
.' 0.~.~ 0.03t 0.~ 1.~ 2~6 0.~7 0.03~ 0.13 ~.~ 92.~. ~32 0.~5 0.039 O.L~ 0.1 30.~ g3 I

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FAR.~80W. G.~RRE1T : !
aD~ER
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Tabl~ 5 ~how~ th~ corro~ion rat~ o~ th~ roport~d alloy compooitlon as a ~unction o~ tho oxyg~n contant. rh~ corrosion rate decrQa~as a~ tho oxy~en contont increas~. It i~ notQd, ¦ howevgr, that t~ corro~ion Or thi~ alloy i3 highor than that o~' the alloy F~-33.9Nd-1.15B-0.064N-0.14C alloy 3hown in Tabls 4 at a ' ~imilar oxygon content rang~. This indicate~ ~hat the corro~ion rat~ i9 al~o af~ct~d by th~ carbon content. Fron th~sa r~sult3, it may ~o ~eon that ths corro~ion rat~ i3 a~ ct~d not only by the oxygen contont but al80 by t~o c~rbon and nitrogen content~.

h4h~ n~n~ ~r3~N~ 5a D~ 0~ D~z-d I eaR ~t ~0 ~1 ~ ~ ~ o~ -, Q, ~, a~ C Qo~u~.

', 0.3 0.0~4 o.o~n 23.0 57 ~ 39S
; 0.51~ 0. ~ 0.0~ 1.9 3~ 1 207 , o.~n O.Ogl 0.001 4.~ ~9.~ 191 ~Flgur~- 4-6 imd Tabl~u 6-9.shsw tho w~ight lo-s o~ Nd-Fe-B
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magnet~ a~t~r expo~uro in an autoclavQ environmant at 5-10 psi at a t~mp~ri~turH o~ 110-115'C a~ a ~unction o~ tho ci~r~on content.

C. W~t~ lo~ o~ For3~ 1.15~ os~ b ~x~ 2~
po~br an~oe ~p~r~ in ~u~x~JNa ~ ae S-10 p i ~a a x*1On o~ 9, ~, and C oo~rt~ and ~n~ac~ tff~t~
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33.9 l.L~ 0.~2 0.056 0.~ 6.4 29.5 ~.8 ~5.
, ~4.0 1.15 o.0a o.o~o 0.06~ 1.3 0.~ 1.1 0.1 I 3J~9 1.13 0.~2 O.Ofl~ 0.11 1.~ 0.~ 0.8 0 ~
., 33.7 l.lS 0.~ 0.05~ 0.~1 0.1 0.1 0.~ 0 :
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FINNEC~W . HEND~R50N:
F~R.~90W GARRETT , ~ D~NNER -- 12 -- :
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32.~ 1.1 0.~5 0.02~ 0.~3~ 9.~ 39.4 32.3 1.1 0.~5 0.023 O.~g6 O.S~4.~3 3~.t 1.1 ~.~65 0.021 0.014 31.8 1~2 32.7 1.~ 0.93 0.02~ 0,0~7 ~ 8~.5 32.5 ~.~ 0.~ 0.02~ 0.03~ 15.4 32.~ o.~a 0.02~ 0.0~ 1.090.49 32.3 1.1 1.~ O.Q2~. 0.0~ 2.6~0.22 32.5 1.~. 1.0~ 0.033 0.0~35 0.~ 0. 9 a~ ~o S~ ~10 pl a~ ~ ~ o~ C
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' 34.0 l.~S O.q- O.Og~ 0.0~ ,.3 78.11 0.12 7.2 46.3 33.9 1.~ 0~ 0.0~2 0.10~3.9 l~,.t 5~.-- 0.~ 2. ~, 16~0 3~.9 l.lg 0.~ 0.0~ 0.140~,.2 3-.- 71.~ 0.212~9 10.3` 33.8 1.15 0.~ 0.0~ 0.1~ 2!~.~ 62.S~.~ 9.1 19.4 3~.~ l.lg O.~g 0.0~1 o.a~20.~ 9~.S 20~ o.

T~- 9. ~e la# o~ 3~.s~1.15~0.33Q-o.Oa~ ~ ~ ~ .
p~ a~ ~ t~t at S-10 pi ~ ~ h~ti o~' C ~o~cenc arc~
~ er~t~ne.

O~ltlcn G~ H.S. H2C~04 0 N ~ 1~ ~ ~C 13 H~ 4Q H~a ~ 0 H~
~4.0 1.1~ 0.3~ 0.029 0.0~ 3.~ 0.9 290.4 23 33.9 1.13 0.34 0.0~1 O.OB9 0.2 ~3.1 0.~ 29o.a 27 ,1 33.9 1.1~ 0.33 0.0~ 0.110 0.1 60 0.~ 200.~ 29 I I 33 . h 1. ~g 0. 33 0 . 023 0.130 ~ . 0 910 . 2 28 0 . 7 48 ~ `
il 3~-8 1.1~ 0.~2 0.022 O.~g~ 0.7 94 0.1 231.~ ~8 ' 33.~ o.a9 o~o~9 o.~oo 19.6 Llg 1.~ .7 '12 ~ AJ may b~ ~can f~o~ thi~ dat~, when the oxygon contont i~
'groater than 0.6~ and the nitrogen content i~ about 0.025~, the ~w O~'C~-F INNEG~N. HENDER50N . ¦
F.~RABOW, G-~RRErT
~ DENNER
00 1 5~ , N W l -- 13 w~ O-O~ ~ OC 2000 202 011`000 .. . ~ . . .
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I¦ corro~ion rate of the magnet decreases rapidly a3 the carbon content i~ increa~ed up to about 0.05% and then reache~ the i minLmum corro~ion rate at about 0.064 carbon, a~ 3hown in Figure 4 -! and Table 6 and 7. When the oxygen content i~ greater than 0.6~, the nitrogen content i9 0.05-0.08% and the carbon content is : within the range of 0.06-O.lS~, the corrosion rate i~ a~ the minimum level. I~ the oxygen content is about 0.7~, and the : -carbon contenT~ exceeds O.lS~, ths corro~ion rate bagin~ to increase. I the oxygen content i~ greater than 0.8~, ~hen the minLmum corro~ion rate ~ontinue~ until the carbon content reaches about 0.2~. This data indicatea that carbon is an important alement in affecting the corrosion rate even in the presence of relatively high oxygen contents. The 3ignificant car~on content for the minimum corrosion rate i~ about 0.05%, ~nd the maximum carbon content for the minimum corro~ion rate..i~ about 0.15%.
Therefore, when the oxygen content i~ in the r~nge 0.6-1.2~, thi~
carbon range results ln the minimum corro3ion rate. :
Figure S and ~able 8 ~how that th~ corro~ion rates of Nd-Fe-8 magnet~ cont~ining 0.46~ oxygen and 0.055~ nitrogon decreases to their lowe~t level~ when the carbon content is increased up to . abou~ 0.113 ~nd then rise~ wlth further increa~e~ in the carbon I conten~.
It is noted that although the corrosion rate decreases to its lowe~t l~vel when the carbon content i9 within the above-stated range of the invention, the corro~Lon rate i9 still relatively high with an oxygen content of 0.46~, whlch i3 lower than the 0.6 , lower limit for oxygen in ~ccordance with the invantion. This ~w O~'IC-~ , FINNECAN, HENDERSON
F~R~OW, Cr~RRETT
9 Dl,'NNER
~rRttr~ ~ W _ 14 ~5~ .1G101~, OC 2000~ 1 ~02 . 0~1 ~000 ,`: ` . ",' , ~ ,.
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indicatQs that carbon reduces the corrosion rate but does not achleve thls alone but only in combination with oxygen within the llmit of the invention. Therefore, the minimum corrosion raee can be obtained by eontrolling both oxygen and carbon, as -~hown in Figure 4.
Further reduc~ion in the oxygen content c~ ~ell as in the nitrogen content increase~ the overall corro~ion rate, as shown in Figure 6 and Table 9. The corro~ion r~te of Nd-~e-B magnet containing 0.33~ oxygan and 0.024% nitrogen decrea3es to its low-e~t value when the c~rhon content is incrsased up to about 0.1~
and then incre~3es with further incre~se~ in the carbon content.
The corrosion rate of thi~ magnet as a function of the carbon content exhibits a much higher corrosion rate than th~ oÇ the maqnet containlng higher oxygen. Thi3 indicate~ that ths magnet containing relatlvely low oxygen i~ much more ea~ily oxidized.
Prom thiY data, it wss determlned that the carbon content to achieve de,Yired low corrosion rates i~ within the range of 0.05-0.15~.
Figures 7 ~nd 8 and Tables 10 and 11 show the weight loss of Nd-Fe-B m~gnet~ ter exposure in an autoclave environment at 5-10 p~i at a temperature of llO-llSC a~ a function of the nitrogen content.
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33.81.13 0.~ 0.~41 0.1~ 32.3 193 ~.3 lO0 33.81.~,g 0.~ 48 0.1~ 4~.5 ~i~ 3.7 g~
33 81 15 0.6~ 0.0~ O.~S a~.g ~.~ 9.1 19.
0.~6~ 0.~ 0.1~ aa.o 12~ 3.9 7~.3 33 91 2S 0-~.5 0-0~ 31.5 1~ ~ 32 33 912~ O.U 0.07~ 0.10 ao.2 103 l.~ n.~ -~ ~;
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34.0~.~i 0.43 o.a37 0.00~ .5 12.6 ~4 1l.~d 0.~1 O.llXI 0.0~ ~a.l 11.2 24 3~ 3 0.~ 0.~ 0.060 1~ 40 34.J1.13 0.~ o.all o.os~ ~1113 1~ lC4 34.~ 9 0,3~ 0.0!~ ~ 1.2 il ,~
I

A~ ~ho~n in Figure 7, when tha carbon content i~ relatively . high (0.10-0.163C), the corro~ion rate decreaso3 ~8 the nitrogan 'I

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F~R~OW. GARRETT
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contant increAse~ from about 0.04 to about 0.07~. Similar I bahav~or was al90 ob~erved with re3pect to the data reported in Figures 1 and 2. When the nitroqen COn~Qnt increases from 0.014-0.0254 to 0.05-0.15~ in the Fe-33.5Nd-1.13-O.lC alloy, the corro-: sion rate dscrea eR ~ub~tantially at a -~imilar oxygen content.
When, however, the carbon content i9 relatively low (about 0.06%), the effect of tho nitrogan content on the corroqion rate i9 adversa. Figure 8 and Table 11 ~how the weight los-q of th~
P~ f~e ~ reported magnets made from blends ofAat ~ zed powdor (RNA-1) and D ~ rgon atomized powder (Alloy 3), a-~ a function of tha nitrogan content. Since the nitrogen atomi~ed powder ~RN~-l) contains a high nitrogen content (0.4~)l a low nitrogen content alloy powder ' (Alloy 3) was blended in a proper r~tio to control the nitroqen I content of tha ~lloy. A~ shown in Fiquxe 9, the corro~ on rate-of ! low c~rbon content alloyB increases slowly up to 0.1~ nitrogen and then increases with further increase~ in the nitrogen content.
There}ore, a high nitrogen content exceedLng 0.15~ nitrogen i~
I detrimnnt41 to the corrosion re~istance of low carbon Nd-Fe-B
.I magnet~ with nitroqen contents being bene~icial withLn the range I of 0.05-0.15~ wlth carbon contents withln the range of the inven-I tion. Thl~ d~t~4 indicate~ that the carbon and nitroqen content~

may advernely a~fect the coxrosion re3istance imparted by each if they ~re not e~ch within the limlts of the invention. Thi~ data also ~how~ that the corrosion rate reachea a minimum even though , the nitrogen content is a8 low ~g 0.025~ when the oxygen and carbon contents ~re within the limits of the invention, as ~hown in Table 7 And Figure 4. From these rQsults, the proper ~w Or,,c~.
INNECAN, HENDERSON .;
FAR~BOW, GARRETT

~10O I 5TaccT~ N W " -- 17 IIGTON, OC 2000, 202.40~.~000 ~ .

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¦ nitrogen content for a minimum corro ion rata i3 O.lS~ maximum, preferably 0.02-O.lS~, and more pre~erably 0.04-0.08~.
Heat trestment in an argon atmosphere ~ollowed by a nitrogen i qusnch aub~tantially reduce~ the eorrosion rate, as shown in Figure 8.
A~ ~hown in F$gure~ 5, 6 and 8, m~gnetY heat treat~d in an argon atmo~phere followad by nltrogan quenching exhibit a corro-~ion rate much low~r than untroat~d magn~t~. Thia indicate~ that the corrosion re~ist~nce c~n be improv~d by this he~t ~reatment but that the corro~ion r~ tance cannot b~ i~prov~d to the extent achieved wLthin the oxygen, carbon and nitrogen li~t~ in ac-cordanco with the inventlon. The improv~mont in oorrosion re~istAnc~ achieved through th~a heat tr~at~nt ~ay re~ult from the modl~lcation of the magnet 3urface by forming a protective ; ~ layer thereon.
Tablea 12, 13 and 14 ~how the weight lo~ of various Nd-Fe-3 m~gnets ater autoclave t~ting, a~ a function o~ the surface treatment or haat trcatmant.

~ a~p~ lc~o o~ ~N~.9~rl.~ n~.07C
! tr~t.
i ~ S~ (~) I ~
oo~3~ 2.1 2.9 5~P~ ~n ~r~ CbD~h 0.~ 0.6 ln N~- ~ 2.9lO.l P~ iR 17 ~ ~ ~/lAr N~ ~nxh 1.1 9.6 i ~xP~ ~n Ar - ~ 6.
~xP~ in V ~ ~ ~Jl~ N~ Cu~xh ~1.2 7 ';
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-I~NECAN, HENDER50N
F~R~aW. GARRETT
a D~NNER
1~00 1 ~TRttr, N. W -- 18 IOTO~I, OC ZOOO~ i ~o~ ~oa ~ooo `~' 'I
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at 5~10 pl a~ a ~lan o~ a~aa~ ~e~.

2~.S 23.~ ~9.1 55ac ~2 ~ ~.2 1.~ 1.4 S50~ ~n VQ~ ~ 5/~S - Na Q~ 31~1 ~.5 6.9 20~ 2~ 5~ . 6 ao~ ~ .3 ~9.0 61.5 ~ ~y B ~9 :j ~ a~. s A.~30 . Y 3 - ~
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FINNEC~N, HENDERSON ~;
F~RA80W. GARRaTT
~ DUNNER ~ -- 19 --1~00 1 ~rR~T, N. W.
W~5-1~NOTON, OC 20005 202'400''~000 ~I
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A~ ~hown in T~ble 12, the magnet heat traated at 550C in an argon a~mosphere followed by nitrogen quenching exhibLted a corro-~ion rate lower than that of the control sample (a ground and untreated magnet), while magnets heat treated at 550C in nitrogen or heated at 900C in vacuum, argon or nitrogen exhibited corro-sion rate~ higher than that of the control sample. Thi~ data ~hows that haae treatments other than at about 550C in argon fol-lowed by nitrogen quenching form a non-protective laysr and thu~
increase the corro~ion rate of the magnet. Table 13 al-qo ~hows the weight 1093 of variou~ magnet3 after ~utoclave testing as a function of he~t tre~tment. A~ 3hown in Table 13, heat treatment at 550C in argon follow~d by a nitrogen quench ~ub~tantially reduce~ the corro~ion r~te from that of the control sample, while heat treatment at 550C in nitrogen and argon followed by nitrogen quenching increa3es the corro3ion rate. As shown in this table, preheating the sample at 200C in air or nitrogen lncreases the corro~ion rate over that of the control sample. However, the magnet heat treated at 550C in argon followed by a nitrogen quench exhibit~ ~ further decrease in the corrosion rate after haating ~t 200C in ~ir. Improved corro~ion re~istance iq al~o achiev~d by he4t tr~ating in vacuum at 5500C followed by argon quenching. A~ ~hown in Table 14 a heat treatment in a vacuum at SS0C or 900C ~ubatantially reduce~ the corroYion rate from the contxol sample, while heat treatments at 550C in nitrogen or oxygen containing en~ironments or in argon followed by air quench-ing increa~es th~ corro~ion rate ,. , ~
~A~ O~IC~-FIN!:ECA!;, HENDERSON, ¦
F~R~OW. CARRETT
a D~NNER
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~ignificantly. H~A~ treatment at 550C under argon ~lightly improveR the corrosion re~i~tance.
Table 1$ shows tho~e pha~es identified ~y X-ray diffraction . formed on the 4urf~ce of the magnets after various heat treatment~.

~o~3~ nA~) N~2F~14~ N~
A~/5~ N2 ~ o-~ x ~ ~d) - V ~ 5~ AX Q~ o~ Nd~Pol~8, y (~n~ Yd) A~5~P~ - A~ Q~h l.J3Na ~ 2J3Ar~90Pe r i~ ~ c ~ 2F~
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I Tablo 16, 17 and 18 ~how ~agn-tic prop rtl~ o~ various Nd-:: . Fe-B ~agn~ts as ~ ~unction o~ t~o carbon, nitrogen and oxygen contents.

. M~*l~ pr~p=~lf~ o~ 3~ F~ ~lloy uff~r ~J~ t ~n~t~d 5~ ~P~ ~or ~ ~r ~ a ~u~*~An cg c, N, an~ 0 o~Tu~
~, 0.0~ 0.0~ O.U ~.1 11.~ ~.3 ~J.6 0.0~7 ~.0~3 0.93 ~.~ ~0.9 0.1 34.8 0.03~ o.oaa 0.7~ 9.~ 3~.2 ! 0.03~ 0.0~1 O.J1 ~.S 1~.1 9.~ 36.6 ; 0.05~ 0.003 0.~ ~.0 9.~ 33.6 i 0. ~ 0.~4 0.~2 ~.~ ~.1 9.3 36.
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~ FINNEG.~N, HENDER50N
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0.0!~13 0.~71/0~7~ 0-9 3S.9 0.~1 000'73 0-6~ a~U-3 10-~5 35.9 O.lS 0-G~ 0- ~.9 ~ Ss.a ~3.7 o-al o-o~ o~ ~.99-0 ~3.7 . ' .
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~d ~e 5~4C ~c~ 2 hr ~- ~ ~ o~ C, N, a~ O ocr~nts.
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! o.o~l o.o~ 0.42 12.1 ~1.3 9.5 34.g '! o.lo o.o~ 0.50 12.~ ~Ø3~.9 ~7.S
0.0~ ~.0~ 0.5~ L~.O ~.4~,0.~ 34.6 0.1.0 0.073 ` ~ 1 12.2 10.3 t.~ 34.9 0.1~ O.Og~ 0.~ .6 9.~ 6.~ 36.0 o.~o o.o~a o.~o L~.l o.~ 3.~ 31.9 0.05~ 0.~ 0.~9 l2.a ~ 9.2 35.7 0.~.0 0.0~9 0.~ 12.3 9.~ 8.0 35.0 0.13 0.~4~ 0.-1 la.l 9.0 6.0 33.0 .
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~^.R~Ea~, C~RRETT
a D~NNER --:2 2 ~oo I ~cer. N W , . INOTON, OC ~OOO~
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i As ~hown in rable 16 wi~h fixed carbon and nitrogen contents, the higher oxygen content gives lightly higher remanenca (Br) and slightly lower intrin3ic coercivity (iHC) than at a lower oxygen t~ .o~y rLe ~ content. A~ the carbon content incr~ases from 0~ 4 to 3.056~, tha remanenca remains the s~me and the intrinsic coercivity increases sub~tantially from 11.4 to 13.0 T~Oe. Thi~ indicate~ that ~he magnetic propartie~ generally improve as the carbon contant increa~es up to abou~ 0.064. WLth highar carbon contants, both rem~nence and intrin3Lc coercivity remaLn ths s~me with carbon contTant increT~Ase~ from 0.0?0 to 0.11% and begin to decrease with further increa~aQ in the carbon content, as ~hown by the da~a presented in Table 17. It ~hould be noted, however, that the ~quarensss and Hk value decrease as the carbon content increase~.
An additLonal example of the effect3 of high carbon are shown in the data presented in Table 18. Unlike the data presen~ed in Table 17, in the tests reported in this table the intrinsic coercivity of the magnet decreased as the carbon content increased ~rom about 0.06~. The remanence slightly increased up to about 0.1~ carbon and then decreased with fuxther increases in ~he carbon content. ~he s~uareness and Hk value al~o decreased as carbon content increAsed. These result~ indicate that the magnetlc propertles as a function of the carbon content vary '! dependlng upon ~he alloy composition. In general, as the carbon .": ,1 content increase~ up to about 0.064, the magnetic propert es may lmprove. When the carbon content lncreases from 0.06 to about 0.11~, the magnetic propert1es may remain the same or decrease sl1ghtly. Further increa~e~ in the carbon content may _educe ~ha ~A~ o~lct~
rlNNEC~, HE~DER5C~N
: F~R.~aC~lV, G~RRE~T
a D~NNER
,,OO,,,Rc~r~w - 23 -~ 5~115T0~, DC ~000 '. 202'-011'~000 '`

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`' ' : , ' ~ ~ ' ' ' , . ' ¦ magnetlc proper~ie~ ~ubstantially. When the nitxOgQn content i3 ¦ ralativaly low ( le59 than 0.08%), the magnatic prop~rties do not ,¦ chAngs ~ignifi~ntly. Howevor, if the nitrogen content i~ high (greater than 0.15~) it form~ NdN by con~uming the neodymium-rich phase, which deteriorate~ th~ magnetic propartie-q, den~ification and corrosion resi3tance.
~ may be ~ean from the data reported and di~cu~ed above in accordance wlth the invention, the corro~ion rate of the magnet~
; decrease~ wlth increaaing oxygen content and reache~ a mini~um with an oxygan content wi~hin the range of 0.6 to 1.2~ with the - maximum carbon ~ontent being 0.15~. The effacT. of oxygsn on cor-I rosion rssi3tance is dependent upon the carbon and nitrogen ,I content~, which muYt ~e maintained within the limits of the inven-, 1l tion.
Tha corroslon resistance i3 al~o improved with proper heat treatment to form a protective oxid~tion resistant layer on the magnet surface.
The ~agnetic propertie~ also vary with the oxygen, carbon and nitrog~n contents.

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~AW O~IC~ I i ! .
F I~NECAN, HENDeRSON
F~AR~U~OW~ GAARETT
a D~NNER
1500 I 5r /~, N w. I ~ 24 ~51~1NCT0ll, DC 2000 .1. .
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Claims (13)

1. A permanent magnet having improved corrosion resistance consisting essentially of Nd2-Fe14-B with oxygen equal to or greater than 0.6 weight %, carbon 0.05 to 0.15 weight %, and nitrogen 0.15 weight % maximum.

2. The permanent magnet of claim 1 with oxygen 0.6 to 1.2 weight %, carbon 0.05 to 0.1 weight % and nitrogen 0.02 to 0.15 weight %.

3. The permanent magnet of claim 2 with nitrogen 0.04 to 0.08 weight %.

4. The permanent magnet of claim 1 with oxygen 0.6 to 1.2 weight %.

5. A method for producing a permanent magnet having improved corrosion resistance, said method comprising producing a permanent magnet consisting essentially of Nd2-Fe14-B with oxygen equal to or greater than 0.6 weight %, carbon 0.06 to 0.15 weight %, and nitrogen 0.15 weight % maximum, heating said permanent magnet in an argon atmosphere and thereafter quenching said permanent magnet in an atmosphere selected from the group consist-ing of argon and nitrogen.

6. The method of claim 5 with oxygen 0.6 to 1.2 weight %, carbon 0.05 to 0.1 weight % and nitrogen 0.02 to 0.15 weight %.

7. The permanent magnet of claim 6 with nitrogen 0.04 to 0.08 weight %.

8. The method of claim 5 with oxygen 0.6 to 1.2 weight %.

9. The method of claims 5, or 6, or 7 or 8 wherein said heating in an argon atmosphere is conducted at a temperature of about 550°C.

10. A method for producing a permanent magnet having improved corrosion resistance, said method comprising producing a permanent magnet consisting essentially of Nd2-Fe14-B with oxygen equal to or greater than 0.6 weight %, carbon 0.05 to 0.15 weight %, and nitrogen 0.15 weight % maximum, heating said permanent magnet in a vacuum at a temperature within the range of 550 to 900°C and thereafter quenching said permanent magnet in an atmosphere selected from the group consisting of argon and nitrogen.

11. The method of claim 10 with oxygen 0.6 to 1.2 weight %, carbon 0.05 to 0.10 weight % and nitrogen 0.02 to 0.15 weight %.

12. The method of claim 10 with oxygen 0.6 to 1.2 weight %.

13. The method of claim 10, or 11, or 12 wherein said heat-ing in an argon atmosphere is conducted at a temperature of about 550°C.