US2009713A - Free machining ferrous alloy - Google Patents

Description

I Patented July 30, 1935 K UNITED *STATES FREE MACHINING FERROUS ALLOY Frank R. Palmer, Reading, Pa., assignor to The Carpenter Steel Company, Reading, Pa., a corporation of New Jersey No Drawing. Application January 14, 1932,

Serial No. 586,697

10 Claims.

My invention relates to ferrous alloys such as are used in the manufacture of machined articles or parts; my essential purpose being to impart relatively free machining quality to such alloys 5 by the addition thereto of metalloid elements effective in securingsuch relatively free machining quality; as" fully set forth in the following specification with clear definition of the invention in the subjoined claims.

In Patent No. 1,835,960 issued to me December 8, 1931, I have fully set forth the invention involved in deliberatelyusing a substantial content of the element sulphur, to correct particularly the poor-machining frictional quality of corrosion resisting chromium steels. I have determined by extensive experimentation that the metalloids selenium and tellurium, which group with sulphur in the periodic table of elements,

' may be employed for such purpose similarly to 20 sulphur but with-avoidance of collateral objectionable effects and greatly improved results.

' My work also has been directed to a wide variety of ferrous alloys such as are now in extensive commercial use; including those straight carbon and alloy steels covered by the specifications of the American Society of Automotive Engineers, Inc., and published in the S. A. E. Handbook dafed 1930 to which reference is made. I have also experimented with many special alloy steels not comprehended by S.'A. E. specifications such as ferritic high chromium corrosion resisting steels, austenitic alloy steels containing high percentages of manganese, chromium and nickel, etc. a

In connection with my invention, I have found that the element carbon, although very important in buildingup the base composition of the alloy, plays no essential part in the application of my invention-as I have proven by experimenting with alloys containing from the lowest carbon practicable in commercial manufacture up to the highest carbon commonly used in tool steel analyses. The element, carbon, is therefore ignored in the following specification and claims with the understanding that it may be used in compounding the base analysis in accordance with the practice well known to the art. Broadly speaking, the ferrous alloys to which my invention relates may be divided into four general classes having fairly well understoodbut not definitely definable boundaries. The first class comprises the non-alloy or so-called straight carbon steels, containing approximately 98% or more of iron, and typified in the S. A. E. specifications under the 1000 series. This class would also include commercially pure ingot iron. In the steels of this class, the carbon is adjusted to suit requirements, manganese is present in various amounts up to 1.55%, phosphorous and sulphur (with exceptions) occur as unavoidable impuritiea'and possibly silicon up to about 50%, without departing from the conception of a straight carbon steel. In the S. A. E. 1000 series are found such steels as X4315 containing low carbon, 1.25/1.55% manganese, .05% phosphorus, and .080/.130% sulphur. Nevertheless, such modifications are commercially regarded as non-alloy steels and for the purpose of this invention are so considered. A second class can be formed to comprehend the so-called structural alloy steels, commonly employed for heat treated machine parts and in which the iron base may approximate 90%, to 98%as embraced for example, by S. A. E. series 2000, 3000, 4000, 5000, 6000 and 9000. Thus it appears that these so-called structural alloy steels are distinguished from the straight carbon steels by the presence of one or more of the group of metallic alloying elements comprising manganese, chromium, nickel, vanadium, and molybdenum. Obviously these structural alloy steels can be compounded outside of the specific limits set by the S. A. El specification, but such steels, some of which contain other alloys such as tungsten would be readily classified by those familiar with the art as belonging to the structural alloy group which I have defined. The third class would comprehend the higher carbon and harder -materials-usually employed for the manufacture of tools and which ordinarily can be hardened harder than about on the C scale of the Rockwell hardness testing. This would embrace such steels as S. A. E. 1095, 52100, 6195, the 7000 series, and a wide ,variety of other hardenable alloy steels readily recognized by the trade as tool" steels. The alloy content of the "tool steel class varies widely from less than 1% up to about- 30% as in the cobalt high-speed steels; the'balance being iron. The fourth class is really a remainder containing "special alloy steels not embraced by the three classes already mentioned;

including high chromium ferrous alloys of the corrosion resisting type; austenitic alloys of the high manganese, high nickel or nickel-chromium type; high silicon alloys used for electrical purposes; tungsten or cobalt magnet steels, etc.; in which the percentage of iron may run as low as 50% of the total, the balance comprising alloying elements. In the prosecution of my invention I have experimented with a variety of ferrous alloys in each of the above four broad classes giving particular attention to the ferritic highchromium, corrosion-resisting alloys and the austenitic ferrous alloys; 'all' as hereinafter described.

Since this invention has for its object the production of ferrous alloys of relatively freemachining properties, some apparent facts regarding the machinability of ferrous alloys in general should be considered. Diflicult machining of ferrous alloys arises from a number of independent causes but these causes are frequently combined in a given alloy. Some alloys are diflicult to machine because of their inherent hardness, as for example high speed tool steel,

more diflicult to cut. Somewhat related, butstill different from the last mentioned cause for difficult machining, is. the inherent toughness of the alloy being cut. Two steels of the same hardness and the same ultimate tensile strength may machine very differently due to the fact that one is comparatively brittle while the other is comparatively tough; the tough steel being the more difficult because it requires more work to tear off the chips. This same difliculty of toughness seriously interferes with machining some ferrous alloys after they have been annealed too soft, when they exhibit a stringy tendency with likelihood of tearing, and the chips are most diflicult to clear. from the tools. In such cases, it is well known that machinability can be im-. proved by not annealing them too soft, in. which case they will machine better even though they are harder and have a higher tensile strength. Still another source of variable machining quality is to be found in the structural composition of the alloy. This is well illustrated in the case of straight carbon tool steel which may be annealed in such a manner as to produce a predominanceof lamellar pearlite on the onehand or a predominance of spheroidized cementite on the other. Although the analysis and hardness may be the same in each case, a great difference will be found in machinability, usually favoring the spheroidized steel. Difficult machining properties are introduced in some steels-'- notably the high chromium, corrosion resisting steels-by reason of high frictional properties which tend to cause the chips to adhere to the cutting edge-of the tool and produce what is commonly known as a bug. This seriously interferes with the speed of cutting and the smoothness of the cut and is generally recognized as acause independent of all other machining difficulties.

For many years, it has been generally known that a comparatively high sulphur content will render a steel more free-machining. Of course,

. limit, the S. A. E. specifications hardly tolerating it in percentages over .05 and it being usually required to be much lower; an exception being found however in so-called screw stock where sulphur is permissibly used in percentages up to .15 in order to obtain. specially desired free-machining quality.

While a relatively large sulphur content may be deliberately and effectively employed for securingmachinability in screw stock and as set forth in my Patent No. 1,835,960 above referred to, I have found that the collateral objectionable defects incident to use of this element practically limit its satisfactory commercial employment; the fact appearing that sulphur combines with a metal present in the alloy to form a sulphide which occurs as a slag-like inclusion hardly soluble in the ferrous matrix. While these slaglike sulphides occur usually in more or less globular form in the cast ingot, in forged or rolled products they are drawn out into long stringers which impair the quality of the metal and particularly depreciate its transverse ;ductility. This apparent fact supports the great prejudice against a sulphur content in commercial alloys, so as to interfere in any case with its generaluse in the manufacture of free-machining alloy steels, notwithstanding the great desire on the part of industry to secure metals which can be freely, rapidly and economically cut.

I have discovered that metalloid of the group selenium-tellurium can be advantageously used in a great number of ferrous alloys, to secure free-machining properties comparable to those which could be secured by the use of sulphur but with scarcely any of the disadvantages of sulphur. I have also discovered that certain very tough ferrous alloys, such as those of the austenitic group, can be further improved and made more free-machining by the addition of an embrittling element such as phosphorus or arsenic in conjunction with the selenium or tellurium addition. My experiments show that the invention operates satisfactorily on castings as well as on rolled or forged alloys.

Selenium and tellurium, in all steels that I have investigated, differ primarily from sulphur in the fact that they are largely soluble in the ferrous matrix, onlya small residuum being present in the form of slag-like inclusions. It is quite evielements and, furthermore, in many cases they seem more potent in their benefits than sulphur, since a smaller total percentage of them is necessary in order to produce satisfactory machining qualities.

In that non-alloy or straight carbon steel group previously referred to, I have variously added selenium 'and tellurium and find that it can be effectively used to replace the sulphur of common screw stock with only a. small percentage of resulting slag-like inclusions. Such steels are freely machinable but are much stronger, tougher and more commercially usable than the high sulphur product known as screw stock. I have further experimented with selenium and tellurium in steels containing manganese, similar to S. A. E. X 1315, and also with lower manganese, such as S. A. E. 1112, but find that I always get freemachining qualitywith a minimum of slag-like inclusion and no evidence of red-shortness, which would indicate that abnormal manganese is in no way necessary to compensate for the presence of selenium and tellurium.

. through my invention, are now made available to In the group previously referred to as structural alloy steels, I have experimented with' nickel steels, nickel chromium steels, nickel molybdenum, chrome molybdenum, straight chromium, chrome vanadium, silico-manganese steels, and others and find, regardless of thebase alloy composition of the material, that selenium or tellurium, or the two metalloids jointly,

produce relatively free-machining qualities with.

a minimum of slag like inclusions and with verymuch less detriment in any case, to the high quality of the alloy, than, wouldbe incurred by using an equivalent percentage of sulphur for such purpose.

For example, I have taken the base analysis represented by S. A. E 3250 and have added selenium and/or tellurium in percentages ranging from about .05% to about 2%. I. have com pared these selenium-tellurium alloys with a similar composition without seleniumor tellurium and find that the tensile strength, the response to heat treatment, the ductility and the impact resistance are not aiIected to any objectionable extent. It is indicated by experiments on this and other base compositions falling within the structural alloy group that these steels,

machining qualities inevitably follow the addition of appropriate quantities of selenium o'r tel-i lurium, or both. Apparently the use of these metalloidsdoes not seriously interfere with the hardening properties of these tool steels nor. wit their utility in the formof treated tools. I

I have also investigated the effect of selenium, tellurium, phosphorus and arsenic on various ferrous alloys within the special alloy class.

As is well known, some commercial alloys within this class are very hard or very brittleor both-and will scarcely lend themselves under any circumstances to free-machining. For

example I have studied the high silicon steels used for electrical'purposes and find that an ironsilicon alloy containing approximately 6% silicon is very brittle ,(having negligible elongation in a tensile test) and hard (over 250 Brinell hardness). When attempting to cut a thread on such alloyswith a threading die, the alloy simply crumbles under the tool and is destroyed. It is therefore obvious that in this special alloy classmy selenium-telluriuminvention can only be applied to ,base alloy compositions that have commercial machining possibilities, that is practically to alloys capable of being annealed to softer than 15000 pounds tensile strength per square inch.

I have studiedfin considerable detail the eifect' of selenium and tellurium on that class of ferritic high chromium, corrosion resisting steels and irons containing chromium from 4% to 60% and find that seleniumor tellurium, or both used Jointly, are even more effective than sulphur as described in my Patent No. 1,835,960 in producing free-machining qualities. The seleniumtellurium addition appears to act equally as well as'sulphur in reducing the high frictional quality containing from about 15%: to 50% nickel, and I the high chrome-nickel alloys containing a total chromeenickel content from about 20% to 50%, the principal part of the balance in all cases being iron. In this group of alloys I find that the useof selenium and/or tellurium in quantity from .03% to 2.00% effects a noticeable improvement in machinability but because of the super tough nature of these alloys they cannot be cut as rapidly as is commercially desired. Austenitic ferrous alloys, when subjected to a tensile test commonly show an elongation in 2" or 40% or more .with corresponding high impact tough ness. .1 have found that this excessive toughness can well be. reduced by adding in addition to the selenium or tellurium, an embrittling agent such as phosphorus thereby reducing the elongation in 2" by 10% or even 20%, greatly improving the free cutting qualities of the alloy. and yetnot reducing theeffective toughness of the alloy sufiiciently to interfere with its application for bolts, nuts, pump shafts, valve parts etc.. where free machining qualities are highly desirable; Also these austenitic ferrous alloys retain their characteristically lower magnetic qualit es when treated with selenium or tellurium and phosphorus as described. 'The amount of embrittling agent to be used may vary from .05% to 50% depending on the degree of brittleness desired, about .12% to .18% being sufiicient in conjunction with selenium or tellurium to reduce the elongation by about 10% and is ample for most purposes. The use of an embrittling agent alone. without selenium or tellurium, has very little beneficial efiect on the machinability of an austenitic alloy and my invention contemplates that the embrittling agent shall always be acthese explanations seem to adequately explain the behavior of selenium and tellurium, since there is a minimum of slag-like inclusion, very little discontinuity in the metallic matrix and very little evidence of decreased toughness as a result of that portion of the selenium and tel lurium which dissolved in the ferrous matrix.

I find apparent benefit in using the selenium and tellurium elements jointly, inasmuch as each element is largely soluble but partly insoluble in the ferrous matrix. It would appear that greater quantities of these metalloids can be used jointly to produce a smaller quantity of slag-like inclusions than results if either of the metalloids is used singly in the same amount. I have also observed that sulphur may be supplemented by the use of selenium or tellurium, or both, in or der to produce better machining qualities, but. ordinarily I prefer to keep the sulphur low be-- cause of the insolubility of its metal-sulphides.

I find noparticular advantage or disadvantage to the use of so-called neutralizers such. as manganese, zirconium, etc.; in connection .with my selenium-tellurium addition. These neutralizers do not appear necessary in order to insure hotusing the selenium or tellurium therewith.

Selenium and tellurium are commercially available in the elemental form and Ihave seen them most generally in the formof a powder. This powder does not lend itself readily to making additions to the molten alloy and I therefore find it advantageous to first produce an alloy of iron and selenium or iron and tellurium and then add this alloy to my molten bath. Both selenium and tellurium, when mixed with finely divided iron and heated to a black-red temperature, will combine with evolution of heat to form a product which is suitable for steel making addition's of iron and 50% of the metalloid appearing to make a satisfactory combination-although it is entirely likely that their admixture in some atomic proportions would produce a more stab compound.

In making my free-machining alloy, I melt in the open hearth, electric furnace, crucible furnace, or by other means, the base ferrous composition and then add my iron-tellurium or iron-selenium alloy at the end of the melt in a manner which seems best calculated to give maximum yield of the added metalloid. For example, in the high frequency, electric induction furnace, I add my iron-selenium or iron-tellurium alloy a few minutes before the heat is ready for, pouring and it isvery rapidly assimilated by the bath. I find that small-additions up to about 110% incur scarcely any loss of metalloid and even with additions up to 20%, the percentage lost is small. As the percentage of metalloid .added increases, the amount lost through volatilization also increases so that to get percentages as high as 2%, it is presently indicated that at least twice this amount mustbe added. This loss, however, can doubtless be further reduced by more expert manipulation and the employment of-means which are commonly known to the steel making art. In the case of open hearth or arc electric furnace steel, the addition 'can well be made to the ladle, throwing the iron-selenium or iron-tellurium compound into the flowing stream.

'I have observed that the addition of tellurium to a molten steel bath has rather dangerous toxic effects; such molten tellurium alloys giving off voluminous fumes very disagreeable and dangerous to the workmen. This difficulty has limited my work on tellurium steels to a point only sufficient to establish the facts herein stated that either selenium or tellurium or both Jointly may be employed for the purpose of my invention. Their absolute quantitative equivalence is not to be understood, and in the case of some "special alloy steels I have occasionally observed that tellurium seems more effective than selenium in producing free machining qualities and thereforemay be employed in somewhat smaller percentages for my.purposes apart from possible different effects.

I have thus investigated a wide range of ferrous alloys with regard to the effect of adding metalloid of the group selenium-tellurium, in order to produce free-machining qualities, and find that percentages of these metalloids, even as small as 03% are reflected in noticeably improved machining qualities; these qualities be ing further improved by additions within the range of 20% to 30% where the results are satisfactory for most commercial purposes. I have added further quantities of these metalloids up to approximately 2% and find that, while these higher percentages yield improved machining alloy to another and from one application to another. Also the use of an embrittling agent like phosphorus must be done with discrimination and proper regard to the purpose for which the alloy is to be used.

Essentially, my invention consists in adding to a ferrous alloy, between .03% and 2% of metalloid of the group selenium-tellurium, as required to desiredly improve its free-machining qualities with further addition of an embrittling element when and if desired. It will be understood that the iron element of the alloys in all cases exceeds fifty per cent as stated, and may exceed ninety eight per cent as in straight carbon steels and commercially pure ingot iron; and that various substantial percentages of different me- In the subjoined claims the term balancei substantially iron contemplates that the balance of the composition is largely iron but may contain percentages of non-ferrous alloying elements of such nature and in such quantity as to not alter the basic nature of the alloy for purposes'of my invention. For example, in my special alloy group there occurs a steel having the following nominal composition:

o .10% or. 18.00% Ni. 8.00%

Many modifications of this basic composition are well known to the art containing added percentages of other alloying elements:

C .10% Cr 18.00% Ni 8.00% Ti 1/2.0% .10% C1 18.00% Ni 8.00% Cu 2.45% .10% Cr 18.00% Ni 8.00% W 3.80% .10% Cr 18.00% Ni 8.00% Me 3.42% .10% C1 18.00% Ni 8.00% Si 4.59% (I .10% Cr 18.00% Ni 8.00% Al 2.00%

What I claim is: 1. A ferrous alloy for the manufacture of machined articles containing: .03 to 2.00% metalloid of the group selenium-tellurium and .05% to 50% C C C C of an embrittling agent of the group phosphorusmium between 4% and 45% and nickel between and 46% with a total percentage of said elements between and 50%, metalloid of the group selenium-tellurium .03% to 2.0% phosphorus .05% to 50%, the balance being substantially iron and the alloy being characterized by relatively free machining properties.

3. The combination of from .03% to 2% metalloid of the group selenium-tellurium with an inherently machinable ferrous base, the latter comprising allowing elements and from 50% to 98% of iron and being capable of annealing softer than 250 Brinell hardness. and the combination of said metalloid with said base imparting materially increased machinability to the resulting alloy.

4. The combination of from .03% to 2% metalloid of the group selenium-tellurium with an inherently machinable ferrous base, the latter comprising alloying elements and from 50% to 98% of iron and being capable of annealing softer than 150,000 pounds per square inch ultimate tensile strength with more than 3% elongation stantially iron; the resulting alloy being characterized by relatively free machining properties.

6. An austenitic alloy characterized by relatively free-machining properties. containing 5% to 50% of metallic alloy of the group manganesenickel-chromium, .03% to 2% of metalloid of the group selenium-tellurium, .05% to .50% of phosphorus, and the balance substantially iron.

7. The combination of from .03% to 2% metalloid of the group selenium-tellurium and .05% to .50% phosphorous with an inherently machinable ferrous base, the latter comprising alloying elements and from 50% to 98% of iron and being capable of annealing softer than 250 Brinell hardness; the combination of said phosphorous and metalloid with said base imparting materially increased machinability to the resulting alloy.

8. The combination of from .03% to 2% metalloid of the group selenium-tellurium and .05%

to .50% phosphorous with an inherently machinable ferrous base, the latter comprising alloying elements and from 50% to 98% of iron and being capable of annealing softer than 150,000 pounds per square inch ultimate tensile strength with more than 3% elongation in two inches and the combination of said phosphorous and metalloid with said base imparting materially increased machinability to the resulting alloy.

9. An alloy containing essentially chromium between 4% and 45% and nickel between 5% and 46% with a total percentage of said elements between 10% and 50%, metalloid of the group selenium-tellurium .03% to 2%, the balance being substantially iron and the alloy being characterized by relatively free machining properties.

10. An austenitic alloy steel containing essentially chromium between 4% and 35% and nickel between 5% and 46% with a total percentage of said elements between 10% and 50%, .03% to .2% tellurium, the balance of the steel being substantially all iron and the alloy steel being characterlzed by relatively free machining properties.

FRANK R.

CERTIFICATE OF CORRECTION.

Patent No. 2,009,713. July 30, 1935.

FRANK R. PALMER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3, first column, line 65, for "15000" read 150000; and second column, line 21, for "or" first occurrence, read of; page 4, second column, after line 70, insert the following paragraph: Such modified types are still in need of improved machinability and these added percentages of copper, .tungsten, molybdenum, etc., while substantial, do not obscure the basic nature of the austenitic chrome-nickel analysis. or interfere with the applicability of my invention. Therefore, such extra alloys are comprehended in my term "balance substantially iron".; page 5, first column, line 7, claim 2, after "2.0%" insert a comma; and line 14, claim 3, for "allowing" read alloying; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 10th day of September, A. D. 1935.

Lea-l ie Frazer (Seal) Acting Commissioner of Patents.