US2524661A - Alloy having high elastic strengths - Google Patents

Description

Patented Oct 3, 1950 v T Oscar E. Harder and Dimon A. Roberts, Columbus, Ohio, assignors, by mesne assignments, to Elgin National Watch Company, Elgin, Ill, a corporation of Illinois N Drawing. Application May 3, 1947, Serial No. 745,716

3 Claims. (G1. 751'71) 1 I 2 V This invention pertains to an alloy which may develop a coarse grain and be too brittle, even contain cobalt, chromium, nicl el and other metafter tempering, for use as a power spring. als: a'ndwhich can provide anon-corrosive body "Even when these carbon steel springs and other of low or non-magnetic properties, capable articles have had what would be considered as the of exhibiting very high strength properties. While 5 best possible heat treatments, they are lacking it is preferred to have the major proportion of in certain desirable characteristics such as corrothe alloy constituted of cobalt, chromium and sion resistance and high-yield strength or creep nickel, molybdenum is also preferably present resistance, and they are magnetic. It is well along with carbon, iron, manganese, and berylknown in the trade that many, and probably the hum. Insignificant amounts of still other elegr b of the b eakage n power sprin s I ments may he present. result from corrosion of the steel. Even tiny rust This application is a continuatiomini-part of pots materially/reduce the section of a Spr our parent application Serial No. 567,894, filed and it is well known to engineers that C OSiOII December 12, 1944, on "Cobalt-Chromium-Nickelpits can serve as stress raisers and thus lead to Base Alloy, now abandoned, early failure by a combination of corrosion and It has been discovered. that the present alloy fatigue.

has unusual properties, and extensive tests have Fa of at Spring y breaking is Serious, bu shown its suitability for use under the exacting in e times the Spring an b placed at conditions of springs which are under maintained relatively low @0511 However, Steel Springs a 1 loading in service, such as watch mainsprings and 7 characterized by more Serious de f m the other power springs, and for other purposeslwhe e standpoint of a correct timekeeper. This defect its chemical and physical properties arefdesiriS known in e industry a king a set and able, l might properly be defined by engineers as creep- Power springs are of necessit i der. m inring or showing continued deformation in service. tained load during operation, which load may at .25 A D we Sp Wh SeTViCe, p tain an overall stress condition closely approachheated stress and release, loses its power to drive in the maxim m u l under t t t t the associated device under the selected original and above 9.11 7 present th om mb t i 3 conditions, and, as a result, the drive no longer maximum of available energy in a minimum Operates satisfactorily- U e Su'eh C rcumspace. Such springs have been. generally made "1 stances, it is impossibleoto correctly adjust and of plain carbon steel with carbon contents in the 7 ste-hderdiZe thedeviee in e i o y- T i h rrange of about 1.00 to 1.30 percent. Ira-practice, eetelistie of setting p f p w it is necessaryto heat the tape of approximately Springs results om the material avin oo low spring width and thickness to a relatively high values of h p p r as proportional l i 7 temperature and quench it, usually in oil, to deand Yield Strength l 4 velop the desired hardness. This heating may The problem i develehihg high elastic P p -1ncdify the surface composition of the steel-and ties and, at e Same time, q e r si ance generally roughens the surface in addition to p-roto eekage is fi di o and has ducing an oxide coloration. After hardening, the fronted industry for'manyyears- It a e stock t h pgjished in order to produce an) ample, become somewhat more serious as the desmooth surface. The hardened tape is too brit mand for Smaller watches has mcrea'sei If a watch mainspring is expose-d to a magnetic field, the steel becomes magnetized and the watch no longer keeps the correct time, and it may not run tie for use as a power spring and must be reheated or tempered to develop sufficient toughness for practical applications; This again produces a at all: like troubles arise with other devices where 01860101 5 9 r y magnetic fields are present and carbon steel parts In addition to the dlfilcultles ust mentioned, are present.

there aremany others connected with the heat A limited number of special 5 watch m mtreatment of Steel Sprmgs- The hardening procsprings have heretofore beenused. One such ale is highly ee i r g to thetempemtlll'e loy is of nickel-iron-chromium base with smaller and t time at the hardening temperature If amounts of molybdenum,manganese, and'berylthe temperature is too low or the time too short, 1mm, The type analysis is represented by 60% the spring will be soft and cannot be used; On nickel, 15% chromium, 15% iron, 7% molybdethe other hand, f th temp ratur is to h o num, 2% manganese, and about 0.6% beryllium. the time at temperature toolong, the steel may Suchan alloy hasimproved corrosion resistance,

because of its chromium content, as compared with the carbon steels. Tests on an alloy of this type and on commercial mainsprings made of such an alloy have ShOWn that the elastic properties, such as proportional limit and yield strength, are not adequate for watch mainsprings; in fact, the elastic properties were found to be inferior to steel mainsprings of commercial production. Tests also showed that the beryllium content is critical and must be controlled within close limits, which is difiicult in melting and otherwise processing the alloy.

Consideration has been given to the use of certain stainless steels, such as an alloy of about 18 percent chromium8 percent nickel, in the cold Worked condition. While such an alloy has considerable resistance to corrosion, it is low in mechanical properties and does not have the high proportional limit and high yield strength necessary for watch mainsprings. Its modulus of elas ticity in the cold-worked condition is also low,

and such an alloy steel cannot be hardened sufficiently by the heat treatments used on the ordinary carbon steel springs but it is necessary to rely upon cold working to develop high strength.

Consideration has also been given to alloy spring steels, such as those designated as SAE 6150, SAE 9250 and SAE 9260. While these alloy steels are highly successful in certain of the larger machine springs and where abundant space is available for them, they require a high temperature for hardening which results in surface oxidation; and such steels have not been found satisfactory for thin sections, for example .004 inch in thickness, and even thinner, as employed for watch mainsprings and in other devices which afford only a small space for the part.

Thus many deficiencies have been found in the materials which have been used or tried in the production of power springs, and a purpose of the present invention is to overcome most of these deficiencies and to result in an improved power spring or other article.

It is, therefore, one object of our invention to provide an improved power spring alloy. A more specific object of our invention is to develop a superior spring alloy for chronometric instruments, such as mainsprings for watches.

It is a further object to develop an alloy having high elastic properties, such as proportional limit,

yield strength, and modulus of elasticity.

A further object-is to develop an alloy which is highly resistant to corrosion in such applications as mainsprings in watches, etc.

Still a further object is to develop an alloy which is essentially non-magnetic.

Another object is to develop an alloy which is readily workable into spring sizes and shapes, including those used for watch mainsprings.

Yet another object is to provide an alloy which is highly resistant to creep or to taking a permanent set in service as a spring.

Another object is to provide an alloy which is capable of developing the high elastic properties desirable in springs without the use of closely- .controlled high-temperature heat treatments as required for steel springs.

Another object is to provide an alloy having the best possible combination of the desirable properties essential to watch mainsprings.

Another object is to provide a power spring manufactured from a cobalt-chromium-nickelbase alloy.

Other objects and accomplishments of the in- 4 vention will be more clearly brought out in the following specification and in the appended claims.

We have discovered that combinations of cobalt, chromium, nickel, molybdenum, carbon, and beryllium, in certain selected amounts, produce an alloy which is highly suitable for the production of improved watch mainsprings. In addition to the above-mentioned metals, the alloy preferably contains iron and manganese and may contain insignificant amounts of other elements such as silicon. The desirable features of this alloy have been confirmed by. extensive tests, including tests in watches.

Cobalt in the alloy of this invention functions to strengthen the alloy and to improve its response to the age hardening treatment. While cobalt may be used in the range of 20 to 50 percent, the preferred range is 28 to 45 percent. At around 45 to 50 percent, the alloys are inclined to be too brittle and are diificult to cold-roll; and with 28 to 20 percent, the alloys are inclined to be too low in strength for some applications.

Chromium is relied upon for corrosion resistance, and it has been found desirable to use at least 15 percent of chromium and preferably over 20 percent of chromium to develop adequate corrosion resistance. On the other hand, difficulties are encountered in melting and otherwise processing of the alloys when the chromium content is too high, but alloys containing up to 30 percent chromium have been produced and studied. The preferred range is from 20 to 26, percent of chromium. Increasing thechromium beyond 30 percent does not materially further improve the corrosion resistance of the alloy.

Nickel is preferably at 15 to 30 percent, and has been used in the range of about 5 to over 30 percent and can be used to even higher percentages. Nickel and iron are to some extent supplementary to each other, although the most satisfactory results are obtained with alloys which contain more nickel than iron. It has been found that the sum of the nickel plus iron and manganese is usable in about the range of 20 to 50 percent, and the preferred amount of nickel plus iron is in the range of 25 to 42 percent. Preferably under 15 percent of iron is present. Alloys containing iron above about 18 percent, have been found to be unsatisfactory becauseof decreased resistance to scaling when heated, and because of poor cold-rolling characteristics. 7

Beryllium is a desirable constituent of the alloy, and appears to give improvement to certain a1- loys in the strength properties on aging after a quenching treatment, more particularly on aging after a quench-anneal and cold reduction. It has been found that in an alloy of this type, relatively small amounts of beryllium are suflicient to add important properties to the alloy, and amounts as low as .01 percent have been found to add materially-to its response to hardening and strengthening by an aging treatment after quenching and cold rolling. Highly satisfactory results have been obtained by beryllium contents in the range of .02 to .05 percent. But with lowcarbon contents, there are advantages in using more beryllium, to about .09 percent. It is one of the features of this invention that the beryllium content required in the alloy is much lower than that found necessary in certain nickel-base alloys which have been used for watch mainsprings. For example, in one previously used alloy containing over 50 percent nickel, the beryllium content was of the order of 0.5 to 0.6 percent and might be as high as 1.0 percent. However, in the present alloy employing beryllium, 0.02 to 0.05 percent of beryllium is generally adequate, audit is preferable to use beryllium contents: under 0. 09 percent, because no improvement in the final mechanical properties has been found to result from the use of a higher beryllium content, and using a higher beryllium content addsto the cost of the alloy and also presents problems in hot and cold rollused with good results, and this applies to all, or

practically all, of the elements listed.

Table 1.-IZZustmtioe alloy compositions and properties Chemical Composition, Per Cent Mechanical Properties (1) m ainer Co Cr Ni Be Mo Fe Mn T. s. P. L. -Y.s. Vmkers 20 a 2....- 40.0 25.0 15.0 *.01 7.0 .15-.20 10.0 2.0 {gg% ?i 3 30.0 26.0 31.0 0.0 .15-.20 7.0 1.0 4 35.0 25.0 20.0 :03 7.0 .15-.20 11.5 1.5 {ggg f gt ff 35.0 25.0 30.0 *.03 7.0 -.20 1.51s N cairn;-

s a iii a 25.0 5.0 *.03 7.0 .l5.20 26.0 2.0 7 Too brittle for cold rolling s "was 20.0 15.5 02 7.0 *05 15.0 *189 gg gfiggi i V 40.0 20.0 15.5 02 7 0 *.11 15.0 *055 40.0 20.0 15.5 *.03 7.0 *.00 15.0 2.0 {gg tggt fi ing and in adequate solution of the excess of the beryllium compound in the heat-treating operations.

Carbon must be kept within certain composi-- i The usable range for carbon is about 0.05 to 0.30

percent, and highly satisfactory results have been obtained in alloys containing about 0.10 to 0.20 percent. Carbon functions to strengthen the alloy and also aids in its response to precipitation hardening.

Molybdenum is preferably used in the amounts of about 6 to 7 percent. When 3 percent or less of molybdenum is used, the alloy is somewhat lower in strength than when larger amounts are employed, and there is no substantial improve-' ment in the ductility: with such low-molybdenum alloys, it is advisable to emplo th upper ranges of beryllium content. The range for molybdenum is about 1 to 10 percent, for articles of high strength values.

Manganese is usually present in amounts up to about 3 percent, preferably about 0.5 to 2 percent, and it is considered as one of the additional elements which behaves like nickel in improving the alloy in its performance during hot and cold rolling.

A specific alloy containing 10 percent cobalt, 20 percent chromium, 15.5 percent nickel, 15 percent iron, 0.03 percent beryllium, 7 percent molyb denum, 2 percent man anese, and about 0.15 percent carbon, has been found excellently adapted for the requirements of watch mainsprings, and responds excellently to the'mechanical and heat treatments.

Alloy compositions, with their properties, are shown in Table 1, with this table including one 7 composition (alloy No. 7) which is outside of the range of the invention, in containing 26 percent of iron with 5 percent of nickel, and which was found to be too brittle for satisfactory cold rollwas analytically determined in all alloys because of its importance, and to show that the desired amount was present in the alloy. Chemical 40 analyses have also been made of other ingredients, and show good agreement between the intended amounts and the amounts actually present. For examplaupon limited specific analysis for the stated metal, it was found:

Intended Actual These analyses indicate that the proportions present are well Within tolerance, and that the varying percentages in Table .1 truly represent the presence of alloys of successively difiering percentage of ingredients.

In this Table 1,1 the tensile and hardness prop erties are given for each alloy under two conditions: (1) as cold-rolled which for these specimen instances represents a reduction of the order of 15:1 to 10:1, starting with hot-rolled stock ofabout 0.060 to, 0.050 inch, which was quench-annealed and then subjected to the cold rolling; and (2) as .aged, which for the specimens set out in Table 1 represents a processing of the stock asoutlined above, followed by heating for 5 hours at 900 degrees F.

Comparisons of the properties underthese two conditions show the desirable properties of the cold-rolled stock: and also the effective increase in the tensile properties, particularly the proportional limit and the yield strength, resulting from the aging treatment. The hardness is. also increased by aging, but this is largely incidental for many purposes of employment, as it.is the improvement in the elastic properties of the alloys which is so important for superior chronometric springs and other articles where the requirements of maintenance of form and strength are so critical that, for several of the alloys, the material would not be superior to steel for chronoinetric springs, without the aging treatment.

Alloy 6 with nil molybdenum is somewhat 'lower in strength than alloys of equally similar composition otherwise, but containing 6 to '7 percent of molybdenum. Alloy 6 is useful for mechanical springs, for electrical heating grids, for springs in electrical contactors where high temperatures may be encountered, etc.

Alloy 8 with only 0.05 percent carbon, is somewhat lower in tensile properties both as coldrolled and as aged than otherwise similar alloys which are higher in carbon, with alloy 1 as a comparative example. This alloy 8 has a somewhat higher ductility than alloy 1, and is somewhat more readily cold-rolled. Hence, it is useful in preparing parts and cutting instruments which must be capable of withstanding elevated temperatures and which must be formed into shapes including relatively sharp bends.

The data in Table 1 indicate that changes in proportion of the alloy composition can be made without departing from the invention, and that different compositions may be desirable for different applications, or selectively employed for similar applications when different demands for properties may predominate as to importance. It will be noted that the aged alloys all show a yield strength (0.02 percent offset) in excess of 200,000 p. s. i., an ultimate tensile strength in excess of 300,000 p. s. i., a modulus of elasticity in excess of 28,800,000 p. s. i., and a hardness (Vickers) in excess of 480, indicating that they are adapted for the critical requirements of chronometric springs.

The alloy of this invention is melted and cast by conventional methods but is preferably produced in a high-frequency induction furnace, cast into slabs or ingots which are first forged and then hot rolled to certain preferred thicknesses, after which the strip is quench annealed by heating at temperatures of 2000 to 2200 degrees F. to effect a solution of the secondary phase or phases thereby decreasing the hardness of the quench-annealed stock. This process is referred to as quench annealing as is the practice in the industry, and the alloy as quench annealed will have a hardness of the order of 250 Brinell or the equivalent. Thicknesses of 0.040 to 0.060 inch have been found satisfactory as the final hot-rolled stock, which is then quench annealed. In commercial practice, the hot reduction can be terminated at 0.200 to 0.250 inch, and then cold-rolling effected down to 0.040 to 0.060 inch, with intervening solution annealing as the hardness becomes too great for practical further reduction. After the final quench annealing, the alloy is cold rolled with at least about '70 percent reduction. It has been found feasible to cold roll the alloys of preferred composition from thicknesses of the order of 0.040 to 0.060 inch down to thicknesses required for watch mainsprings, for example, 0.0044 and 0.004 inch, and stock of anly 0.003 inch thickness has been produced. It has been found feasible to make this entire reduction without any intermediate anneal, so that reductions over 90 percent or extensions of over 1000 percent have been effected.

The cold working materially increases the strength of the alloy and refines the grain structure. However, the cold-worked alloy does not have the high strength properties, such as proportional limit, yield strength, and modulus of elasticity required for watch mainsprings. For example, in the cold-worked condition the proportional limit of one specific alloy is likely to be in the range of 130,000 to 160,000 p. s. i., the yield strength (A. S. T. M. method: offset 0.02%) in the range of 160,000 to 180,000 p. s. i. and the modulus of elasticity in the range of about 22,000,000 to 27,000,000 p. s. i. On the other hand, when the alloy has been subjected to a quench anneal, cold rolled, and then age hardened, for example, by heating 5 hours at 900 degrees F. the proportional limit will be of the order of about 200,000 to 240,000 p. s. i., the ultimate strength 340,000 to 380,000 p. s. i., and the yield strength about 250,000 to 290,000 p. s. i. The modulus of elasticity will be close to 30,000,000 p. s. i., and sometimes exceeds that value.

It 'will be, obvious from the foregoing description of the processing of the alloy of this invention that it is not necessary to harden it by quenching the final springas is the case with the steel springs, but the alloy is cold worked to develop relatively high-strength values and then subjected to an aging treatment at 500 to 1200 degrees F., which further increases the strength values, particularly the proportional limit and the yield strength; for example, an aging treatment of 5 hours at 900 degrees F. may increase the proportional limit by 90 percent or more, and the yield strength by 70 to percent. These are the properties which are considered essential for an article of high elastic properties, which is to stand up in service over long periods of time without taking a set or showing creep.

The resistance of the alloy of this invention to corrosion has been demonstrated by placing an article made of this alloy in a desiccator jar containing water so that it is subjected to a moisturesaturated atmosphere. For comparison purposes, steel springs of the type now generally used in watches were subjected to the same test. All of the steel mainsprings, representative of the existing art, failed by corrosion attack within 48 hours. On the other hand, the articles made of the alloy of this invention were subjected to the test for periods of 3 to 6 months; none of them failed under the rated load, and there was no sign of corrosion.

The ability of the alloy of this invention to resist set or creep can be determined by installing mainsprings made therefrom in watches, operating them over extended periods of time, and then removing the springs and noting if they returned to their original free-coil position. Commercial steel springs were tested under identical conditions. It is the experience of the Watch industry that in such tests, steel springs always show some set even when the service time is only a few days. In fact, watches equipped with steel ma'insprings frequently have to have their springs replaced before they leave the factory, because so much set has developed that the spring has lost torque and the watch cannot be adjusted to keep the correct time. In contrast to this behavior, mainsprings of the alloy of this invention showed no loss of torque after having been in service over test periods of 3 and 6 months. Thus, by using an alloy of high elastic properties, set or creep was eliminated; and once the watch was adjusted, it continued to be a good timekeeper.

Thus, by the propert es developed in the alloy of this invention, namely, high mechanical properties and resistance to corrosion, a superior alloy for watch mainsprings and other employments requiring high elastic properties and freedom from corrosion, has obviously been produced.

While a major objective of developing this alloy was to find a superior watch mainspring alloy, it has been found that the alloy has other useful applications. For example, it can be used where strength and long life are required at high operating temperatures, such as electrical resistor heating elements, springs for mechanical uses, instruments requiring flame sterilization, pressure conduits and vessels, etc. Electrical resistivity tests on three alloys within the composition range disclosed gave resistivities of 626, 666, and 686 ohms per circular mil foot at room temperature. Some of these resistivities are higher than those found for alloys which are now used for heating elements. For example, under the same conditions, a well known nickel chromium alloy showed a resistivity of 650 ohms percircular mil foot. Tests on the alloys as electrical resistor heating elements indicate relatively long life at temperatures up to and including 1800 degrees F.

Still other applications of an alloy of this type have been noted, one of which is for springs to operate at elevated temperatures. Since this alloy age hardens at 500 to 1200 degrees F. and continues to increase in hardness and strength on increase of time of exposure at 900 degrees F., for example, up to at least hours, it is evident that the alloy will not lose its ability to function as a spring material up to about this range of temperatures.

Still other applications of the alloy include instruments and the like requiring keen cutting edges, examples of which are razor blades and surgical instruments. Again the alloy meets the requirements for these applications by its high hardness and strength which enable it to be sharpened to a keen cutting edge and to retain that edge in service. It, furthermore, has the advantage that it is corrosion resistant even at elevated temperatures and, therefore, is not corroded and tarnished by coming in contact with atmospheres or other media which would rust steel.

Having thus described our invention, what we claim is:

1, An alloy consisting essentially of carbon up 10 to 0.30%, beryllium from 0.01 to 0.09%, manganese up to 3.0%, nickel from 5 to 30%, iron up to 18%, with the sum of the nickel, iron, and manganese being 20 to and the remainder cobalt, chromium, and molybdenum, with the cobalt from 20 to 50%, the chromium 15 to 30%,"

and the molybdenum 1 to 10%, and the sum of the chromium and molybdenum being 21 to 37%, said alloy having in solution-annealed condition a hardness below 300 (Vickers) 2. An alloy consisting essentially of carbon from 0.05 to 0.25%, beryllium from 0.01 to 0.09 manganese from 0.5 to 3.0%, with the sum of the nickel and iron being 25 to 42%, with the nickel from 15 to 30%, and the iron up to 15%, and the remainder cobalt, chromium, and molybdenum, with the cobalt from 28 to 45%, the chromium from 15 to 30%, the molybdenum from 3 to 10%, with the sum of the chromium and molybdenum being 24 to 35%, said alloy having in solution-annealed condition a hardness below 300 (Vickers).

3. An alloy consisting essentially of carbon from 0.10 to 0.20%, beryllium from 0.01 to 0.09%, manganese 2%, nickel 15.5%, iron 15%, cobalt 40%, chromium 20%, and molybdenum 7%: said alloy having in solution-annealed condition a hardness below 300 (Vickers) OSCAR E. HARDER. DIMON A. ROBERTS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,685,570 Masing et al. Sept. 25, 1928 1,698,935 Chesterfield Jan. 15, 1929 1,942,150 Rohn Jan. 2, 1934 1,945,679 Corson Feb. 6, 1934 2,072,910 Touceda Mar. 9, 1937 2,103,500 Touceda Dec. 28, 1937 2,150,255 Touceda Mar. 14,1939 2,246,078 Rohn et a1. June 17, 1941 2,288,609 Crampton et al July 7, 1942 2,370,395 Cooper Feb. 27, 1945 FOREIGN PATENTS Number Country Date 171,085 Switzerland Oct. 16, 1934,

Claims (1)

1. AN ALLOY CONSISTING ESSENTIALLY OF CARBON UP TO 0.30%, BERYILIUM FROM 0.01 TO 0.09%, MANGANESE UP TO 3.0%, NICKEL FROM 5 TO 30%, IRON UP TO 18%, WITH THE SUM OF THE NICKEL, IRON, AND MANGANESE BEING 20 TO 50%, AND THE REMAINDER COBALT, CHROMIUM, AND MOLYBDENUM, WITH THE COBALT FROM 20 TO 50%, THE CHROMIUM 15 TO 30%, AND THE MOLYBDENUM 1 TO 10%, AND THE SUM OF THE CHROMIUM AND MOLYBDENUM BEING 21 TO 37%, SAID ALLOY HAVING IN SOLUTION-ANNEALED CONDITION A HARDNESS BELOW 300 (VICKERS).