US3081164A - Nonmagnetic iron-base alloys - Google Patents

Description

March 12, 1963 J.H.SCHRAM1M 3,081,164

NONMAGNETIC IRON-BASE ALLOYS Filed Nov. 4, 1959 WITNESSES INVENTOR 2% v Joggb H. Schromm WA ATTiNEY United States Patent Ofiice 3,081,164 Patented Mar. 12, 1963 3,081,164 NONMAGNETIC IRON-BASE ALLOYS Jacob H. Schranim, Bayonne, Ni, assignor to Westinghouse Electric Corporation, East iittsburgh, Pin, a corporation of Pennsylvania Fiied Nov. 4, 1959, 'Ser. No. 850,822 2 Claims. (Cl. 75-123) This invention relates to improved nonmagnetic ironbase alloys, and members prepared therefrom.

Certain austenitic alloys of iron are particularly suitable for applications demanding strong, nonmagnetic material. In addition, these alloys are valued for the excellent resistance to corrosion, and for the high temperature properties, which may be imparted to them by appropriate additives. Nickel is ordinarily specified for inclusion in these alloys to establish the austenitic structure. While nickel is excellent for this purpose, because of its strategic need and relatively high cost, the replacementof nickel with a readily availableand less expensive additive is highly desirable.

One group of alloys of the austenitic type is the stainless steels, the most common being 18% chromium, 8% nickel, balance iron with a small amount of carbon (from .08% to 20%). The ordinary austenitic stainless steels are not susceptible to precipitation hardening by heat treatment and this is disadvantageous where high hardness and strength is required. In order to provide alloys which are hardenable by heat treatment, alloys have been made 'wherein the carbon content is higher than the carbon content in stainless steel, and wherein the chromium content is lower than that in stainless steel (resistance to corrosion not being critical in the proposed application). The additional hardness which is obtainable in such alloys is secured by the precipitation of carbides during heat treatment. The particular carbide which is precipitated is probably chromium carbide, although other carbides may also be involved.

In the construction of large inner-cooled tu-rbogenerators, the retaining ringsand wedges used are preferably manufactured from a high strength, substantially nonmagnetic material to resist the strains placed upon the members, and to avoid paths of magnetic leakage which would decrease the efiiciency of the generators. The function of the retaining rings and wedges is tohold in position on the generator rotor the heavy conductors which carry the excitation field current. At the normal speed of a two pole generator, 3600 r.p.m., an intense centrifugal force is developed which tends to throw the conductors oh? the rotor, and the retaining rings and wedges that secure the conductors in place against this great force must necessarily be formed from a material having high strength. In general, a 02% yield strength of 100,000 pounds per square inch (p.s.i.) is a minimum requirement for this application. This strength level is not particularly .diflicult to reach in low alloy magnetic steels, but the use of such magnetic materials is undesirable because they limit the efficiency of the generators to whichthey are applied. The eddy currents induced in magnetic retaining rings due to their proximity to conductors carrying 'heavy'currents, represent wasted energy which latter, on continued operation, takes the form of heat generated in the rings. Thus, not only is there a .loss of energy, but in heating, the rings reach excessive temperature and the operating conditions of the generator are adversely affected. With nonmagnetic rings this energy loss'is negligible and the rings will remain relatively cool.

The steels which are presently employed for generator retainer rings and wedges are of the type to which reference has been made above; that is, they are austenitic in structure. andirely at least in, part, on the precipitation hardening phenomenon characterized by the precipitation of carbides to achieve desired strength and hardness levels. It has been found, in actual practice, that the ordinary precipitation hardening procedure, i.e., formation of a solid solution, and cooling to the aging temperature and holding at this temperature to cause precipitation of hardening components, is not suffioient to produce the required combination of strength and ductility (elongation). It has been found necessary to cold work the alloys before or during aging to attain the desired level of mechanical properties. In these alloys, work hardening and precipitation hardening occur substantially simultaneously. Thus, the alloys harden so readily during the cold working that they become excessively hard and the amount of reduction which can be eifected is severely limited. Therefore, even though high capacity tools are employed to perform the cold working, the high hardness of the alloy as it is being worked prevents thorough working of the alloy member, consequently, the amount of deformation varies widely over the cross section of the worked member, and hence, the mechanical properties similarly show wide variation and nonuniformity within the individual member.

Accordingly, it is a principal object of this invention to provide a readily workable nonmagnetic austenitic iron-base alloy having high strength and hardness, which is precipitation hardenable and employs a minimum amount of nickel.

It is another object of this invention to provide a precipitation hardenable nonmagnetic iron-base alloy having predetermined amounts of manganese and titanium therein.

It is still another object of this invention to provide precipitation hardenable, substantially nonmagnetic, ironbase alloys having predetermined amounts of nickel, manganese, and titanium therein.

It is a further object of this invention to provide generator retaining rings and wedges having high strength and hardness, from an alloy including a minimum of carbon and nickel, which may be discretely work hardened and precipitation hardened.

Other objects of this invention will, in part, be obvious, and will, in part, appear hereinafter.

A clearer understanding of the function of the members formed from the alloys of this invention may be gained by reference to the drawings, in which:

FIGURE 1 is a fragmentary view in perspective of a generator rotor showing elements made from the alloy of this invention, and

FIG. 2 is a fragmentary view in section taken along the line 11-11 of FIG. 1; and

PEG. 3 is a fragmentary view in section taken along the line llllll of FIG. 1.

This invention is directed to substantially non-magnetic iron-base alloys capable of both work hardening and precipitation hardening which contain a minimum amount of carbon and nickel. More particularly, the alloy comprises, byweight, from 9% 'to 30% manganese, from 0 to 9% nickel, the total of nickel and manganese lying in the range from 16% to 30%, from 1.5% to 3.5%" titanium, from 0 to 12% chromium, a maximum of 08% carbon, and the balance iron, with small amounts of additives, and incidental impurities. Other additives which may sometimes be present in relatively small amounts are molybdenum, aluminum and silicon. Among the impurities which may be present are phosphorus, sulfur, oxygen and nitrogen.

An alloy of the type described above which has been found to be particularly good comprises, byweight, 25% to 26% manganese, from 1.8% to 3.5% titanium, a maxi mum of .03% carbon, and the balance essentially iron with the usual small amounts of impurities therein.

3 As the term nonmagnetic is employed in this descrip tion, it is intended to include alloys exhibiting no magnetic characteristics, and also those loys which are only slightly magnetic. Some alloys of this invention, under which strengths would be desirable in many instances. This strength level is diificult to attain with consistency, and is particularly diilicult to reach in alloy No. 1. It should be understood that while the average strength may Manufacturers of these alloys are very reluctant to guarantee ultimate tensile strengths of over 125,000 p.s.i.,

certain conditions, exhibit very slight magnetic properties. 5 be high, a substantial proportion of alloy members will Referring to FIGS. 1 and 2 of the drawings, there is be considerably less than this average. 2 shown a two pole rotor having a rotor body 11 with The compositions of the alloys of this invention are bearing shafts 12 at opposite ends of the rotor body. set forth, in weight percent, in Table III.

The rotor body 11 has smooth pole faces 13 at opposite Table III sides of the body, only one of which is visible in FIG. 1. 10

Between the pole faces 13 the rotor body has a series of P h deep longitudinal slots 14 which are machined parallel to Alloy M1 or N1 M0 Ba] 0t m the axis of the rotor body. The slots as shown, extend 0 2 2 substantially radially into the body, though they may also 321 III: 219

be parallel to each other. A cross-sectional view of the 15 slots 14 may be seen in FIG. 2 where it will be observed 1 1 that in the opposing side walls of each of the slots 14 are 3-;

a pair of opposed longitudinal grooves 17. In each of 1110 6:8 33

the slots 14 is positioned a coil winding 15 with the outermost surface of the coil winding fiush with the innermost wall of the opposed grooves 17. All the coil windings in the rotor taken together constitute the field coil of the is; 1 g 21: 5: 2 ?i fig g g g gs g i 122 33 222 33% generator.

hs lmmedlately below. A plurahty of wedges 18 having T-shaped cross sections the paragrap (as seen in FIG. 2) are slidably mounted in each of the 20 i "gg g fs g gfig i gg zi g ge g i' 523 32 slots 14. The wedges 18 extend into the grooves 17 and 6 72 h um t and 24 at C are arranged in abutting relation with the coil windings A110 d 8 haat trez'lted M1190. C uenched and with each other, and thus both the coil windings and 1d fled b 3 t C 3 ho S the wedges are restrained against radial movement. gi g on g go C ag ur However, the ends of the coil windings extend beyond a A110 v g s 1 g g treat d C chad the slots and require restraining means to hold them in i y M t 5 5 C d 93% 300 place. This is accomplished by the use of retaining rings. 3 C a u 0 $25 3 a retammg ring {elated coll Wmdmg Alloy P was solution heat treated at 1050 C., quenched re 1s set fonth. In FIG. 3 it 1s seen that the end 01d 11 d b t 507 t C d d 300 windings 21 of the coil windings 15, closely surround the 1 2 3 a W a a an age Fowl. 11 whlch has F rfaduced end portion 11 adjacla'nt Alloy 6 was solution heat treated at 1050 C quenched l- The wmdmgs are t have in oil cold rolled about 50% at 700 c and a ed 300 condu1ts 22 thereln for the c1rculat1on of hydrogen gas hoursat c g or other. Smtable i A retammg. ring 23 is i a 40 Alloy 5 was solution heat treated at 1050 C quenched close fitt1ng, surround1ng engagement w1th the end w1ndin oil cold rolled abzmt 50% at C d 3 ed 300 ings preventmg rad1al or axial displacement of the windboursat C g ings. The retaining ring 23 may be secured to the rotor All o 12 was solut1on heat treated at 1120 C., at its po1nts of contact therewith in any convent1onal quenchxd in on, cold rolled about 50% at C and mammaged 300 hours at 550 c.

The composmons of two prior art alloys which are Alloy 9 was Solution heat treated at 11700 C presenfly used as genemtor rings and wedges are set quenched in oil, cold rolled about at 700 C. and forth Table I: T bl I aged hours at 550 C.

a 9 Alloy 1267 was solution heat treated at 1140 C., [Composition wt. percent] 50 quenched in oil, cold rolled about 50% at 700 C., and

aged 120 hours at 550 C. Alloy N1 Mn Cr Mo 0 Al st Ti Fe Alloy U was solution heat treated at 1160 C., quenched in oil, cold rolled about 50% at 700 C., and

g 9 g 2 0.4 5 aged 120 hours at 550 C.

"""""""""""""" 55 T "5 Ime hanical properties developed by the alloys of The mechamoal propertles character1st1c of these alloys, Table 6 Vor and heat trcatm nt are hsted m together with present and proposed commercial n1echanical property standards for alloys of this type are set forth a le IV for purposes of comparison in Table II. 6

O 0.02 Per- 02? 'r Table II Alloy scanty gang lsltltimattltla ga ing i i r l iii fi tff 1 I! [lvteehanicalpropernesl sgeiiggh sgesng gh (52 .5) iglil 00$? (P i c e nt) .l. A I A g 0.2 i r izi ht Ul ii rii at e Elongation Reduction 111 300 00 Alloy Yield Tensile in 2" of Area 1221000 133350 288 Sgageggggh Sg cggg er t) (Percent) 103,580 1151000 1401000 2331800 2326 44:1

112' an la l iii 1 25,5 0 20.4 00.4 1 120x10 135x10 25 33 120-139x10 15e-105 10 26-31 42-40 Standard: 1303550 Present 115x10 125x10 15 20 I Proposed. 120x10 x10 20 40 The schedules for working and heat treatment listed above will yield satisfactory products, but are by no means the only acceptable procedures. For example, the

5. precipitation times may be reduced by increasing the aging temperature somewhat. Increasing the aging temperature, however, does have the disadvantage that the ductility will usually be decreased. The precipitation times can also be shortened by increasing the amount of cold work. 7

It will be noted that alloys Q and O which do not exhibit magnetic properties and contain neither nickel nor chromium, more than satisfy both the present and proposed commercial standards for alloys for this application. Seven other alloys, which contain moderate amounts of chromium and nickel, also satisfy the en gineering requirements for the proposed application. Al oy V, however, containing very little nickel and over 20% manganese, falls just short of the desired strength level. The strength of thisalloy may be raised to the desired level by lowering the aging temperature or increasing the aging time. The character of these alloys is such that, with proper heat treating and working, uniform and consistent mechanical properties are readily obtained. With these alloys, as with the alloys of Table I, the required combination of strength and ductility cannot be reached if, after hot working such as forging, the conventional precipitation hardening procedure of solution heat treatment, rapid cooling, and aging is used. Cold working to effect a reduction of at least 30% before or during aging must be applied to obtain the required combination of mechanical properties.

In alloys Q and O, manganese acts alone to establish the austenitic structure. The alloys other than Q and all have substituted therein for a part of the manganese, varying amounts of nickel, up to about 9% maximum, to supplement the action of manganese in promoting the formation of austenite. In these latter alloys it is the total of manganese and nickel which imparts the austenitic structure. The total of manganese and nickel may range between 16% to 30%, but the preferred range is to In this preferred range, manganese is present in amounts from 11% to 25%, by weight. The high proportion of manganese in the alloys of this invention serves also to improve impact strength at low temperatures.

The titanium in the alloy, which lies in the range 1.5% to 3.5%, and preferably in the range 2.25% to 3.25%, is present to promote precipitation hardening by the formation of compounds with iron and perhaps other elements in the alloy.

Although chromium is not essential to the alloys of this invention, up to 12% chromium may be added where it is desired to improve the corrosion resistance of the alloys.

In the alloys of this invention carbon is purposely maintained at a low level in order to avoid the shortcomings of the prior art alloys. For the purposes of this invention, carbon should be held to a maximum of .08%, by weight.

Application of the alloys Q and O of the present invention to the field of turbogenerator retaining rings will now be discussed. In order to appreciate the advantages of this alloy, the procedure followed with the alloys of Table I will first be considered. In the alloys of Table I, with carbides as the precipitate, the ingot is first solution heat treated to take the carbon into solution. After the solution heat treatment the ingot is mandrel forged in several steps, and then, in a final working step, it is cold expanded over a tapered mandrel at a relatively low temperature for a relatively long time. This particular wor :ing step results in a very sharp increase of the hardness with increasing deformation because of the simultaneous contributions of cold working and precipitation. The sharp increase of the hardness and thus of the yield strength require high capacity presses to carry out the working, and even though such presses are available, the reduction of area cannot proceed beyond a relatively low reduction because of the rapid increase of the yield strength and the simultaneous decrease of ductility. It is obvious that Where only such low reductions are attainable, relatively great differences will result in the amount of deformation over the cross section of the Worked member, especially if certain areas precipitation harden before others. The non-uniformity of working and aging over the cross section will result in pronounced nonuniformity of the mechanical properties over the cross section of the ring.

In order to show that the precipitation of chromium carbide or other carbides takes place during cold working, the following experiment was conducted: A rod of each of the alloys 1 and 2, about 0.5 inch in diameter was solution heat treated at 1050 C., quenched in water, rolled at 625 C. to 40%, 50% and 60% reduction, respectively, and then the specimens were aged at 525 C. The measured diamond point hardness values (30 kg.) were:

Table V [Hardness (D.P.I-I.)]

As Rolled and As Rolled Aged, 25 Hrs. at Alloy As 525 C. I

Quenched Table VI Treatment: Hardness (D.P.H.) As quenched after solution heat treatment- 150 40% rolled at 630 C 230250 50% rolled at 630 C 250-260, 60% rolled at 630 C 260-270 Aged from 24- hrs. to 300 hrs. at 525 C.

to 550 C 350-370 It should be understood that even higher hardness values can be attained than are indicated in the above table by longer aging or aging at higher temperatures, but the ductility would drop below acceptable values for the contemplated application if this were to be done.

A comparison of Tables V and VI for the alloys shows striking diiferences which are important for this invention: (1) the hardnesses after cold rolling at 630 C. are relatively low with the new alloys; (2) the reduction of area to reach a given hardness level is obviously much higher in the case of the new alloys than was the case with the old alloys. The higher reduction of area possible with the new alloys, results in a much more uniformly worked material, and consequently, a member which will possess uniform mechanical properties throughout the body thereof.

In the production of retaining rings, the properties of the new alloys suggest the substitution of the economical tire rolling method which uses relatively low capacity, inexpensive equipment, and requires relatively short times to roll the ring as compared to the high capacity presses essential with present alloys. Since with these alloys the retaining rings can be formed while in a relatively soft condition the cost of processing is materially reduced. Thus, rough machining and drilling can be performed while the retaining ring is in the unhardened condition,

and there is consequently less danger of crack formation at stress raising points. It should be noted that the uniform structural conditions achieved in the working of these alloys carries over into the aging step in a manner such that the precipitation hardening occurs in a very uniform manner.

While alloys Q and O are particularly suitable for use as rings and wedges in generators, the other alloys of Table III are well suited for this same purpose. However, due to the additional alloying elements in these latter alloys, they are somewhat more expensive and, therefore, might better serve where their improved properties such as high yield strength and ductility are required. Some such uses are in underwater detection apparatus, mine sweeping devices, and in submarines.

v It will be understood by those skilled in the art that although the present invention has been described in connection with preferred embodiments, modifications and variations may be employed without departing from the underlying spirit and scope of the invention. It is intended to claim all such variations and modifications.

I claim as my invention:

1. An austenitic iron-base alloy consisting essentially of, by weight, from 16% to 30% manganese, from 2.25% to 3.25% titanium, carbon in an amount not exceeding 08%, and the balance iron except for incidental impurities and additives.

2. A nonmagnetic precipitation hardenable iron-base alloy composed of as its essential constituents, by weight, from about 25% to 26% manganese, from 1.8% to 3.5% titanium, a maximum of .08% carbon, and the balance iron with small amounts of incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS 1,574,782 Becker Mar. 2, 1926 2,048,167 Filling July 21, 1936 2,266,481 Talbot Dec. 16, 1941 2,378,993 Franks June 26, 1945

Claims (1)

1. AN AUSTENITIC IRON-BASE ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, FROM 16% TO 30% MANGANESE, FROM 2.25% TO 3.25% TITANIUM, CARBON IN AN AMOUNT NOT EXCEEDING 08%, AND THE BALANCE IRON EXCEPT FOR INCIDENTAL IMPURITIES AND ADDITIVES.

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