US2080368A - Process for improving the physical properties of austenitic steels - Google Patents

Description

Patented May 11, 1937 PROCESS FOR IIHPROVING THEQPHYSIOAL PROPERTIES OF AUSTENITIC STEELS Paul D. Ffield, Akron, Ohio, assignor to Goodyear-Zeppelin Corporation, Akron, Ohio, a corporation of Delaware Application June 22, 1934, Serial No. 731,938

iron alloys.

nal strains.

Austenitic steels such as chrome nickel steels and chromium or nickel manganese steels are ordinarily worked to a desired shape at the mill and quenched from'above the carbide solution temperature before shipment. According to 0 standard American mill practice this working is conducted at a high temperature which in the case of chrome nickel steels may be 1800 F. or Regardless of the composition of the steel it is customary to conduct this working at a temperature above the recrystallization temperature, that is at a temperature at which the various components of the alloy are present in solution. For certain purposes the alloy may be worked cold and then heated above the recrystallization and carbide re-solution temperature before quenching. It is customary to quench the alloy, after the working operation in which it is formed into ingots or bars or other standard forms, from a temperature above the recrystallization temperature to normal temperature, and the various elements are present in the finished steel as a solid solution which is super-saturated with respect to certain compounds. Austenitic steels and particularly 18-8 chrome-nickel steels are sometimes worked cold after quenching to By this treatment alone it is possible to obtain a chromium nickel steel with a tensile strength of about 200,000 lbs/sq. in., but in general such steels are not cold worked above a tensile strength of 165,000

above.

improve their properties.

lbs/sq. in.

According to this invention there is included in the alloy a stabilizer such as titanium or columbium which when the alloy is subjected to the usual treatment above outlined will be present in a super-saturated state in the solid solution obtained and, when subjected to the further treatment described herein, will become precipitated probably in combination with carbon or other element present in the alloy. However, the precipitation eflected by the process of this invention differs from that which takes place when an alloy containing no columbium is heated. The precipitate does not rob the chromium at the grain boundaries but is uniformly distributed throughout the alloy. To bring about this precipitation the alloy, after the usual quenching (above described) is strain-hardened by being subjected to a cold-working at a temperature below the recrystallization temperature, and this strain-hardened product is then subjected to a precipitation heat treatment which is also below the recrystallization temperature. In the strain hardening treatment the internal slip which takes place in the metal forms nuclei for the subsequent precipitation by heat treatment of the super-saturated constituent. An initial precipitation of this super-saturated constituent probably occurs in the strain-hardening opera- 5 Claims.

about the optimum properties in the alloy.

Heretofore titanium has been included as a true alloying element in heat and corrosion resistant austenitic steels of high alloy content, but it has not been found possible to retain in such alloy steels the resistance to corrosion which the same alloy steels without the titanium possess, and at the same time to impart suitable physical properties to such steels. For a number of uses it has heretofore been unnecessary to improve upon the physical properties of these corrosion-resisting alloy steels of the prior art. However, for other purposes these same alloy steels because of their poor physical properties or, if their physical properties have been sufficiently improved, because of their decreased resistance to corrosion, have not been regarded as practicable.

That these titanium-containing steels have not in the past been capable of being subjected to the known age-hardening processes and still retain their corrosion-resisting properties is one of the reasons why they have not found more use in industry. As disclosed in papers by W. Kroll (Wissenschaftliche Veroeffentlichungen des Siemens-Ko nzerns, v.'8, 1929, pp. 220-35; Metallwirtschaft, v. 9, 1930, pp. 1043-45) and by R. -Wasmuht (Arch. 1'. Eisenh., v. 5, 'pp. 45-56 and 261-66), titanium may be introduced into such alloy steels to give the useful age hardening effects, but as pointed out by Bennek and Schafmeister, (Archiv F. Eis'enhiittenwesen, v. 5,=.pp. 615-20, June 1932) in every case it has been necessary to employ so much titaniumthat the corro sion-resistant properties have been lost, thus rention, but heat treatment is necessary to bring dering the steels no longer resistant to corrosion, and if a full age-hardening treatment is given, much too coarse and brittle.

The benefits derived from the heat treatment following strain-hardening will vary with the amount of strain-hardening to which the alloy is subjected. For. instance, a steel analyzing 18.0% of chromium, 8.0% nickel, 0.05% carbon and 0.40% titanium when strain-hardened, from a tensile strength of 90,000 lbs/sq. in., its original annealed state, to a tensile strength of 140,000 lbs/sq. in., was found to increase on subsequent heat treatment to atensile of 200,000 lbs./sq. in. The same steel when strain-hardened to a tensile of 165,000 lbs/sq. in. is strengthened to a tensile of 240,000 lbs/sq. in. by the heat treatment herein described. Such treatment of titanium-containing steels is covered in my copending application Serial No. 685,069, filed August 14, 1933.

As a general rule for a steel containing a small amount of titanium, columbium, etc., the preferred temperature of the precipitation heat the properties of a steel to which columbium has been added. The titanium containing steel of Fig. 1 had the following analysis: chromium 18%, nickel 8%, carbon 0.05%, titaniiun 0.40% and other ingredients as above. This steel, having a maximum tensile strength inits original annealed state of 90,0001bs./sq. in. was strain-hardened to an approximately 40% reduction in draft, its

tensile strength being 165,000 lbsi/sq. in. It was then subjected to a temperature of approximately v 920 F. for a period of an hour, after which it treatment step will be found to be between 600 "\was quenched in water,

and 1200 F., the best results having been obtained with temperatures in the neighborhood of 900 F. While heating the quenched steel to a temperature of between 600 and 1200. F. and at once cooling it gives an improvement. in the physical properties, an even greater improvement is obtained by retaining it in this temperature range for a period of approximately one hour. The time may be extended, but generally it will be found that there is no further increase in physical properties even though the steel is maintained at the desired elevated temperature for a period of as long as 24 hours. Heat treatment for a period of one-half hour gives greatly improved properties, but apparently treatment for one hour gives the optimum improvement.

Although any austenitic steel with the inclusion of titanium or columbium or the like, may

'be improved in the practice of the invention the austenitic chrome-nickel steels such, for example, as those having chromium and nickel contents of approximately 18% and 8% respectively, and those having chromium and nickel contents of approximately 25% and 12% respectively have been found particularly valuable. The steels preferred in the practice of the invention should in general contain not more than approximately 0.20% of carbon and suflicient columbium or the like to bring about the desired properties and precipitation. For titanium this is usually found to be in an amount of approximately five times that of the amount of carbon employed and includes the range of from .2% to 1.0% titanium. For columbium the range is higher and may be as high as 3%. Where metals other than titanium or columbium are employed the amount will vary with'the molecular weight. Even using less than this amount, that is using less than five times as much titanium as carbon, a marked increase in the properties of a titanium-containing steel may be obtained by cold-working with subsequent precipitation heat treatment. It has been found that with more than 1% titanium the corrosion resistance of the steel is lessened somewhat. Columbium does not affect the corrosion resistance in the same way as titanium and as much as 3% has been found not to materially impair the corrosion properties of the steel. The other ingredients usually found in steels of this type, e. g. sulfur, phosphorus, manganese and silicon may be varied at will. Thus the amounts of It is to be understood that the step of quenching in a liquid is not necessary, but that the steel may be cooled oif slowly in the air with equally successful results.

, After this treatment it was found that the modulus of elasticity had recovered from 23,-

lbs/sq. in. as strain-hardened to 240,000 lbs./sq.

in. an increase of approximately The yield point (0.1%) was also raised from;145,000 lbs./sq. in. to 200,000 lbs/sq. in. an increase of approximately 38%. noted, a Rockwell hardness of A 69.5 to "A 70 (60kg. load with Brale diamond penetrator) being increased to A 75. The elongation was found to. have decreased slightly, but not 'to an excessive extent. Moreover, in addition to all these improved properties, the steel retained to a marked degree the characteristic resistance to corrosion of the austenitic chrome-nickel steels. It was appreciably magnetic.

The analysis of the steel shown in Fig.2 is as follows: chromium 18.39%, nickel 8.97%, silicon 0.68%, manganese 0.63%, columbium 1.16%, carbon .095%, and small amounts of sulfur and phosphorus. This steel was strain-hardened to a tensile strength of 177,000 lbs/sq. in. It was then subjected to a temperature of approximately 920 F. for a period of 1 hour after which it was allowed to cool. After this treatment it was found that the modulus of elasticity had recovered from 24,000,000 approximately to 27,000,000 approximately an increase of about 12%. The tensile strength was raised from 177,000 lbs/sq. in. as strain-hardened to244,000 lbs/sq. in. an increase of approximately 35%. The yield point (0.1%) was also. raised from 130,000 lbs/sq. in. to 225,000 lbs. /sq. in. an increase of approximately 70%. Thissteel was also decidedly magnetic.

0n referring to the drawing it will be observed that the optimum increase in properties in the titanium-containing steel-was obtained at a temperature of about 900 F. and with the columbium-containing steel at possibly somewhat lower temperature. Comparing these with the properties of the steel which contains neither titanium nor columbium as shown in both the graphs, it will be seen that only a slight increase in tensile and yield is obtained with such a steel on heat- An increase in hardness was also.

ing up to 1000 F. The marked improvement in physical properties is readily seen by referring to these graphs.

It is to be understood that the invention is not limited to the two steps of strain-hardening and heat-treating, but extends to numerous variations of these steps. Thus, one alternative procedure is to strain-harden the steel to a lesser degree, heat-treat it and then repeat the strainhardening step. Also it may be desirable, in some instances, to include a large number of steps of strain-hardening followed by heat treating. It will be understood that any of the known methods of strain-hardening may be employed in the practice of the invention. It will further be understood that the precipitation heat treatment may be followed by any method of cooling.

In the practical application of this invention, the steel after the usual quenching step at the mill, may be cold-rolled to sheet form to a comparatively low tensile strength to maintain considerable ductility. In the production of girders for airships or other structural shapes for any use, it is then formed to the desired shape (while in this ductile condition) and subsequently heat treated to higher tensile properties. By this process it is possible to form intricate shapes of high properties which could not be made from steel of initially similar properties because the ductility would be too low. Briefly, the forming is carried out while the steels are ductile and the steel is later brought up. to strength by heat treatment.

It is readily apparent that the products of this invention have greatly improved properties, makingpossible the employment of corrosion-resisting steels in many new ways. Thus, the alloy steels of this invention by virtue of their resistance to corrosion and their great strength may be used in place of the present alloys used in the aircraft industry, both heavier-than and lighter-than-air. The alloy steels of this invention are so much stronger than duralumin, for instance, that by employing a suflicient amount to give the same tensile strength as given by duralumin, an even smaller weight may be employed even though the specific gravity of the steel is higher than that of duralumin. Being in addition resistant to corrosion, the steels of this invention will thus find use wherever the two qualities of great strength and corrosion-resistance are required.

Apparently the mechanism of stabilization is to introduce an element which has a greater affinity for the carbon present than the chromium has. It is generally believed that the steel on being heated to the embrittling temperatures will precipitate (when titanium is the stabilizer) titanium carbides instead of chrome carbides. Various investigators have reported either complete or partial stabilization by alloying with one or more of the following elements: beryllium, copper, cobalt, molybdenum, silicon, tantalum, tungsten, and vanadium. The invention includes the use of one or more of these or other elements Although only preferred forms of the invention have been illustrated and described in detail, it will be apparent to those skilled in the art that the invention is not so limited but that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims. It is intended that the patent shall cover, by suitable expression in the appended claims, whatever features of patentable novelty reside in the invention herein disclosed. a

This application is in part a continuation of my copending application, Serial No. 685,069, filed August 14, 1933.

. I claim:

1. The process of improving the physical properties of a chromium-nickel austenitic steel containing a small amount of columbium which steel has been quenched from a temperature above the \solution temperature to a normal temperature, which comprises the steps of cold-working and then re-heating below recrystallization temperature but at such a temperature as to obtain a critical dispersion of carbide therein.

2. The process of improving the physical properties of a chromium-nickel austenitic steel containing a small amount of columbium which has been quenched from a temperature above the solution temperature to a normal temperature, which comprises the steps of cold-working and then heating to a temperature between 600 and.

1200 degrees F. to obtain a critical dispersion of of cold-working and then re-heating below re- -crystallization temperature but at such a tem perature as to obtain a critical dispersion of' carbide therein.

5. The process of improving a steel having a chemical analysis of approximately 18% chromium, 8% nickel, less than .070% carbon, remainder iron with normal additions of elements such as manganese, silicon, sulfur and iron, which comprises adding not more than 3% of columbium thereto, and then after quenching from a temperature above the solution temperature, improving the tensile strength of said steel by cold-working and then artificially ageing it by re-heating to a temperature of not more than 1200 F. but at such a temperature as to obtain a critical dispersion of carbide therein. 2

PAUL D. FFIELD.

CERTIFICATE OF CORRECTION.

Patent No. 2,080,368. May 11, 1957.

PAUL D. FFIELD.

It is hereby certified that error appears, in the printed specification of the above numbered patent requiring correction as follows: Page 1', first column, line 48, strike out the words "titanium or"; and that the said Letters Patent should be read with this correction therein that the same may conformv to the record .of the case in the Patent Office.

Signed and sealed this 20th day of July. A. D. 1937.

Henry Van Arsdale (Seal) 7 Acting Commissioner of Patents.

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