US2320260A - Columbium-bearing austenitic steel and articles produced therefrom - Google Patents

Description

Patented May 25, 1943' COLUMBIUM-BEARING AUSTENITIC STEEL AND ARTICLES PRODUCED THEREFROM Vere B. Browne, Brackenridge, Pa., assignor to Allegheny Ludlum Steel Corporation, a corporation of Pennsylvania No Drawing. Application November 22, 1940, Serial No. 366,556

6 Claims.

m present invention relates to columbiumcontaining austenitic chromium nickel steels and alloys from which various articles of manufacture can be produced by hot working operations as well as to the hot worked articles so produced. The present application is in part a continuation of my copending application Serial No. 17,154, filed April 18, 1935.

As I have outlined in the application referred to, it is well known that columbium-bearing chromium nickel austenitic steels of the familiar 18-8 type cannot be commercially and economically hot rolled, forged or otherwise hot worked to produce fabricated articles therefrom due to the fact that such materials, when subjected to hot working operations such as those mentioned, crack, tear and otherwise suffer physical deterioration. Within the term steels of the 18-8 type I include the usual or conventional steels which are made up of approximately 18% chromium, approximately 8% nickel, low carbon and the balance chiefly iron, as well as those differing somewhat from the foregoing analysis. Columbium-bearing steels of this character contain a relatively small amount of columbium. It is likewise well known to those versed in this field that the same steels without a columbium content are not subject to the defect hereinabove noted.

Such steels containing columbium, however, have other desirable properties but have been commercially unsatisfactory in that the percentage of defective material or imperfect articles produced therefrom was so large that the cost became prohibitive and the operation became non-commercial in character. due either to lack of control over the steel-making operations or to adventitious alloying operations, some of the 18-8 type steels and alloys have somewhat departed from the so-called 18-8 proportions. Insofar as I am aware, however, no one has heretofore conceived of a new relationship between the chromium and nickel contents of such columbium-bearing steels and alloys, nor deliberately produced any such steels or alloys wherein the above noted defects and disadvantages were consistently and reliably eliminated. If in sporadic instances a material was produced which could be hot worked without cracking or tearing, such could not be reproduced and the reason therefor was'not known.

Such cases served only to emphasize the existence of the problem.

The reason for the cracking. tearing or rupturing of columbium-bearing austenitic chromium nickel steels or alloys of the 18-8 type In some cases,

resides in the formation of a phase or constituent denominated as delta ferrite. Delta ferrite is a fourth allotropic modification of iron which is formed at temperatures in the neighborhood of 2200 F. in the columbium-bearing steels and alloys already referred to but which occurred only at considerably higher temperatures in the columbium-free steels and alloys of the same type. Therefore, the addition of columbium causes a lowering of the temperature at which delta ferrite forms and it is an unfortunate coincidence from the standpoint of fabrication that columbium reduces the temperature of delta ferrite formation to precisely the temperatures at which the fabrication is carried out. This explains why columbium-bearing austenitic chromium nickel steels cannot be subjected to hot working operations successfully and regularly.

One of the objects of the present invention -is to produce columbium-bearing austenitic chromium nickel steels and alloys which can be hot worked at the usual hot working temperatures without cracking, tearing or rupturing as a regular commercial procedure reproducible at will.

Another object of the invention resides in rationally readjusting the composition of the columbium-bearing 18-8 type of steel or alloy above referred to so as to substantially or entirely eliminate the formation of delta ferrite therein at the temperature of the hot working or hot forming operation.

A further object of my invention resides in increasing the nickel content of columbiumbearing austenitic chromium nickel steels and alloys relatively to the nickel content of. the 18-8 type materials hereinabove discussed and in so mutually correlating and modifying ,-the chromium and nickel contents of such 18-8 type ma terials as will entirely eliminate the uncommercial features heretofore encountered in attempting to hot work the same.

An additional object of my invention resides in producing a columbium-bearing austenitic chromium nickel steel and hot worked articles composed thereof which contain, addition to iron and the usual contaminants found in materials of this character, approximately 16% to 20% chromium, approximately-8%'qto 14% nickel, carbon up to but not substantially in excess of 0.2% and columbium to the extent of about 6-10 times the amount of carbon but pref erably not substantially in excess of about 2%.

A still further and more specific object of my invention resides in producing columbium-bean ing austenitic chromium nickel steels and alloys and hot worked articles composed thereof which consist of approximately 18% to 18 chromium, approximately 10 to Il 5% nickel,

tenitic chromium nickel steel or alloy in which the chromium may range from approximately 16% to approximately 20%, the nickel may range from approximately 8% to approximately 14%. the carbon may range from as little as about .03% up to about 0.2% but is preferably not over approximately 0.10% to 0.14% maximum, and columbium from about 6 to about 10 times the carbon content and preferably not substantially over about 2%. Within the ranges just above set forth my invention further comprises an apportioning of the chromium and nickel contents in particular in such manner that the formation of delta ferrite at conventional working temperatures is substantially or entirely'eliminated. I have found that this condition can consistently be accomplished by limiting the chromiumnickel relationship to a maximum ratio not ex ceeding 2:1. Such materials can readily be subjected to the usual hot rolling, forging or other hot working operations without the formation of harmful percentages of delta ferrite and,therefore, without cracking, tearing or rupturing during such hot working operations. The commercial and economic advantages involved in the ability to produce hot fabricated articles even of relatively complex cross section or configuration by conventional procedures are evident to those familiar with this subject.

As illustrative of the features and principles of the present invention, the following example is referred to as constituting a composition responding fully to the present invention and providing the benefits and advantages herein set forth and claimed. Such a composition consists of approximately 18% chromium, approximately 1l% nickel, .07% to .08% carbon and about .I% to .8% columbium, the balance being substantially all gamma iron except for the usual impurities including phosphorus and sulphur in common amounts.

Preferably, I produce a composition of the above character which has an analysis such as that above set forth but wherein variations above and belowthe stated percentages are permissible without detracting in any appreciable manner from the value and properties hereof. Compositions like that stated but containing from about 18% to about l8/2% chromium and from about l0 to about ii /2% nickel have been found by me to be especially desirable steels or alloys. Depending upon the intended use of the material and the precise temperature of the hot working or hot fabricating operation, the chromium and nickel contents may still further vary from those just above set forth while still retaining substantially or entirely the benefits and advantages of this invention. I therefore deem that chromium percentages ranging from about 16% to about 20% and nickel percentages ranging from about 8% to about 14%, with the columbium, carbon and other contents already mentioned, form a definite and intended part of this invention. In no case, however, do I claim percentages of chromium and nickel even within the limits above defined which exceed a chromium-nickel 'ratio more than 2:1; my invention does, however, include compositions within the ranges of constituents set forth wherein the nickel exceeds one-half the chromium content. For example, the 18% chromium, 11% nickel alloy steelabove set forth falls precisely within the invention here defined.

It is clear, moreover, that within the wider ranges of percentages included herein my invention involves many possible compositions in which the chromium-nickel ratio is not greater than 2:1. Thus, I do not claim 20% chromium and 8% nickel as a part of my present invention, but where a chromium content of 20% is employed the nickel content must be a minimum of 10% and may exceed 10%. 16% chromium and 14% nickel do, however, form a part of the present invention as do 16% chromium and 8% nickel since such fully respond to the principles herein outlined and will produce materials which can be hot worked and hot fabricated without the formation of harmful percentages of delta ferrite. Between the end points referred to many other specific compositions can be employed while still remaining within-the metes and bounds of the invention. It will be particularly noted in connection with the foregoing that compositions containing 18% chromium and 8% nickel or chromium-nickel contents in the same proportions as they :occur in the 18-8 materials are definitely excluded and form no part of my invention.

It will be understood from the foregoing that heretofore it has been impossible to predict, from the individual percentages of chromium and nickel included in an alloy, whether or not that alloy would or would not form delta ferrite at conventional hot working temperatures. While the percentages of both the chromium and the nickel must be considered, my invention makes it possible to produce austenitic chromium nickel columbium-bearing steel which is substantially entirely free from delta ferrite even after sustained hot working at the customary hot working temperatures. It will also be evident therefore that for each particular chromium content thereis a definite minimum percentage of nickel and my invention discloses that that percentage must be at least one-half the chromium content. Inasmuch as the proportion of columbium employed is usually relatively small and most commonly is in the neighborhood of from about 0.5% to 1%, the effect of that amount of columbium on the temperature of delta ferrite formation may be considered as constant for all practical purposes. The percentage of nickel for any given chromium content therefore should be such that no appreciable amount of the delta constituent will form when the material is heated to approximately 21002300 F.

It will further be appreciated from the foregoing that I have not only discovered why columbium-bearing austenitic chromium nickel steels could not be commercially and economically produced by conventional hot working and hot fabricating operations but I have also discovered how to circumvent those failures. As already pointed out, this includes th increase of the nickel content of such steels and alloys to an extent adequate to constitut the nickel not less than onehalf the chromium content. Stated in another way, the present invention is predicated on the discovery that a definite relationship must be maintained betweenthe chromium and nickel constituents of a columbium-bearing austenitic chromium nickel steel in order to prevent the formation of delta ferrite in detrimental amounts under normal hot working temperatures. As hereinbefore pointed out, this relationship between the chromium and nickel is such that the nickel constituent of the alloy must always be at least one-half that of .the chromium constituent. Where this is true, delta ferrite, if formed during ordinary hot working procedures, will only be formed in amounts which are not detrimental to the physical characteristics of the alloy.

Hot fabricated and hot worked products of many different kinds and shapes fall within the purview of this invention, which is, however, es-

.pecially characterized by the ease of production of complex shapes such as those havin complicated cross-sectional configurations, re-entrant curves and angles and other products which are difficult to produce from any material. The simpler and more common commercial shapes and forms are, however, a definite part of this invention-andwithin such'I specifically include .by way of-example sheets, strips, plates, bars, -rods, wires, tubes. pipes and the like. My inven- 'tion' includes not only hot'rolled products, i. e.. those products produced on or by hot mills, but also hot forged products.- Within this latter term I include theusua'lvarieties of hot forgings such as are well knownjto-those familiar with this subject. Products produced by other or miscellaneous hot working. hot forming or hot fabricating operations are likewise deemed to be a part hereof. These articles find innumerable uses and I refer merely by way of example to uses in chemical and metallurgical equipment, use in the process industries and generally in those commercial operations which require materials or products having the properties and characteristics of those herein defined.

Procedurally, the results of this invention can b readily achieved by adjusting the composition of the steel or alloy during the manufacture or production thereof and while the same is preferably still in molten form. Before the usual pouring. forming. rolling. forging, casting or other operation the chromium content of the melt is so adjusted as to bring the percentage thereof within the limits of about 16% to 20%, the nickel content is adjusted so that it falls within the range of about 8% to 14%, and the chromium and nickel percentages in the melt are so correlated to each other that the nickel is not less than one half the chromium, that is, the chromium-nickel ratio is a maximum of 2:1. The carbon and columbium contents are appropriately adjusted within the ranges set forth and in accordance with th principles laid down in this and my aforesaid copending application. The manipulations involved are understood by those skilled in this art.

The foregoing is intended as illustrative rather than limitative and within the appended claims I may make such other and further modifications, substitutions. additions and omissions as will be appreciated by those versed herein. While I have described the invention with particular reference to the production of flat products, it is clear that I am not limited in this respect to products of any particular type or configuration.

Relatively complex products are readily produced by means of the present invention in contr'adi'stinction to previous experience. While I' have moreover-described my invention with particular reference to th use of the element columbium as the alloying agent. I wish it to be understood that stabilizing alloying agents other than'columbium may be employed as weil'asany other suitable element classified with columbium in Mendeleje'ff's periodic arrangement of th'e'ielements. While I have also described my invention in more particular connection with products which are produced by a hot working or forming operation at a temperature of about 2200 F., it is to be understood that I am not limited to products made or treated at or approximately at this temperature as I may apply the invention to articles and products produced within the usual hot working ranges of the particular material involved and especially within the ranges of about 2100-2300 F. The invention comprises the new steels and alloys described and claimed-as well as the products produced therefrom.

Having thus described my invention, what I claim as new and' desire to secure by Letters.

Patent is:

1. Austenitic steel which is composed of about 16-20% chromium, about 844% nickel, below about 0.2% and a small percentage of columbium ranging from about 6-10 times the carbon contact, and characterized by having a chromium-nickel ratio not over 2:1 and the ability to be subjected to conventional hot working operations at the usual temperatures thereof without the formation of suflicient delta ferrite to cause tearing, cracking and rupturing, the balance thereof being substantially all iron except for the usual impurities in common amounts.

2. Austenitic steel which is composed of about 16-20% chromium, about 844% nickel, carbon up to about 0.14%. columbium about '6-10 times the carbon content and the balance substantially all iron except for the usual impurities in common amounts, the chromium and nickel contents of the austenite within the ranges recited having a maximum limiting-ratio of 2:1 and the steel being characterized by the ab lity to be subjected to not working operations at temperatures ranging from about 2100-2300 F. without the formation of harmful percentages of delta ferrite.

3. Hot worked articles of manufacture which are characterized by substantial freedom from cracks, tears and ruptures although containing a substance normally rendering them susceptible to such, which are composed of austenitic steel having substantially the chromium. nickel. carbon and columbium contents set forth by claim 1 and which remain in the austenitic state under conditions which normally produce ferrite development.

4. Hot worked articles of manufacture which are characterized by substantial freedom from cracks. te "rs and ruptures although containing columbiu which renders them susceptible to such. and! which are composed of austenitic steel having substantially the chromium, nickel, carbon and columbium contents and relationships set forth by claim 2.

5. An austenitic columbium-bearing low-carbon chromium nickel alloy steel which retains its austenitic character under conditions which normally produce ferrite precipitation such as those encountered in conventional hot rolling and hot forging procedures, said austenitic alloy steel having a chromium content ranging from about 16-20%, a nickel content ranging from about 13-14%, a chromium nickel ratio which is not in excess of 2:1; low carbon 0! the order of .07-.08% and columbium to the extent of about 6-10 times the carbon content.

6. A chromium nickel coiumbium-bearing austenitic steel composed of about 16-20% chromlum, about 8-i4% nickel, carbon not to exceed c approximately 0.14%, oolumbium not less than about .5% and the balance substantially all iron and characterized-by the fact that the nickel content is at least half the chromium content and that the alloy is substantially entirely free from delta ferrite after a sustained hot working at approximately 2200 F.

4 I VERE B. BROWNE.

I CERTIFICATE OF CORRECTIOIi.

Patent No. 2,320 ,260. m 1%5 v w VERE B. BROWNE.

It is hereby certified that error appears in the printed specification of the above nu mbered patent requiring correction as follows; Page 5, sec-- 0nd column, line 28, before the word "below" insert -car bon--; line 51,

for "Contact" read "content"; and that the said Letters Pa tentfsh'ould be read with this correction therein that the same may conform to'the rec 5 V 7 0rd of' the case in the Patent Office. I I

Signed and sealed this 6th day of July, A. D. 191;

Henry Van Arsdale, (eal) Acting Commissioner of Patents.