US2470988A - Method of heat treatment - Google Patents

Description

Patented May 24, 1949 UNITED STATES METHOD OF JHEAT TREATMENT? nois No Drawing. Application March-15, 19416, i Serial No. 654,808 a 12 Claims. 1;;

'Tli'is invention relatesto a method of heat treatment. More particularly "it is concerned with a method of heat treating ferriticsteel or iron alloy articles while i-n contact with austenitic steel or alloy, in such a manner to remove carbon and such other elements as are hereinafter named which migrate from fer-rite to austenite.

In" thepresent method, a purification takes place which improves certain "desirable properties of the-articles so treated. More specifically, for example, by heat treating 28% chromium steel tubing 'while'in contact with iron filings, a condition is achieved wherein ierrite is in equilibrium with austenite. It has been found that the austeni-tized'iron filings absorb carbon and other elemen-ts from the 28% chromium ferritic steel rendering it-stable against embrittlement by carbide precipitation in high temperature service.

In further explanationof thebackground of this invention, certain observations on the migration,

of carbon and other light elements such as nitrogenelboronand oxygen'which are capable of solid-solution in iron alloys of ferritic character reveal a strong tendency for these elements to diffuse in a-direction-toward contacting alloys of I austenitici character. It has also been found that it 'is'immaterial whether such contact as refer-red to 'be-established by mechanical means, fusion, or by=mixture of phases of microscopic proportions, such elements as carbon will migrate out of the ferrite and into the austenite. Carbon and other light elements are known to assume inte'rstitial solid-solution in iron alloys; i. e., positions other than those'occupied at the regular space lattice points assumed by the metals atoms. While in the ferritic space'lattice, the interstitial solubility is small, interstitial solubility in the austenite space lattice onthe other hand is comparatively large. For example, in the latter connection, the solid solution of carbon in ferrite is of theorder-of-afew hundredths of a percent, while the austenities will hold carbon in solid solution in concentrations of the order of two percent. The greatercapacity of the austenite for the solid solution of carbon and the other interstitially dissolved elements named seems to render it capable of subtractingthe-same elements from interstitial solution'in'th'e 'ferritelattice where they are less commodiously "accommodated. It is as though austenite placed in contact acts as a blotter for carbonand other light elements.

. The rate of migrationof thelight elements from ferrite toaustenitabeinga solid diffusion process, is dependent. primarilyupon time .and temperature. Other factors such as stress, metal working. randsinherenti structure'iare, secondary influences upon thezraterofinnigration. 'Themost rapid migration rota-carbon; nitrogenrand .other light elementsxirlom eierritettrr austeniteseems. to occur atithez, highestftemperature at& which Lthese phases xremainfinzequilibriumr; :In llhBQCfl-SiOf iferrite ofa1ow=alloyrcontentusucha temperature will not exceed that :of whee-3&1 l:transformation. In the case of the more highlyfialloyed ,ferritessuch a limiting tempenaturewmasaibentheaiower of the two melting rpointsurepresented by .the :ferrite and .faustenite: .phase s;;. The: Jaime :necessary for austen'ite to.-substantiallyrsubtract the interstitial alloy content :of the..=-ferrite ,.may.wary from days to minutes,.dependingrnpontherlimiting temperature. at which:thestWO::phasesnemain .in equilibmum .Evidenceoi th'erability cfiaustenite; to, subtract carbon fromcferrite-zhasgibeen,shown =byv study of actual :photomicrographsyginwhich :the :fusion line of a deposit soisanuaustenitic. steelcontaining 251% of chromium, 20%;.01? nickel, upon a ferritic steel of low aalloynontentwvas, :clearly shown as freshly .depositedr: The. LSEtIIlfl-StlllClJlll'B .after a .perio'ol "of aging aatzlatemperature slightly: below the. 5A1: ipoint xrevealstzthatrzcarbon had diffused across" the ,rfusion zline zf'romithe aierr-ite .to itheaustenite, while in anothermhotomicnographa microstructure of sanausteniticstejel; containing 181% of chromum, 8%. of nickel, =and..:3% of molybdenum was shown,:, which; calthoughi-nominally an: austeniticxsteel, ::-the microsliructure .wasv characterized. by lakes =0l1ferriteyin aunatrix .austenite. After .a period.iofrexposure-itoelevated tempera- A ture, a durtherimicrographiciistudy showed that carbonhad migrated fromethe territe,giving rise toma precipitant-ester;canbiderln-the;austenite .upon cooling; In"additionakphotomicrographic-studies, it' hasibeen shownithats with more prolonged aging :the carbon hadtbeenadispersed ttoan extent .no longer occasioning wthe carbide' precipitate but the subtraction/ ot carbon from the ferrite phase wasdemonStratedby-thesofteningwhich had occurred in this phase, i

It has been discovered that this characteristic migration of hardening and em'rlor ittling impuri ties from ferrites toward austenites can be made the'basis of a-process fonthe purification offerritic alloys and for *the purpose of improving physical properties-such toughness, stability, corrosion resistancewetc: the latterconnec tion; for example-Phi!--sim13lycontacting a -ferritic material or metal article with-a-suitable encasement of austenite, later to be removed-and exposing same-fora suffi'clent time at a suitably elevated temperature, the desired removal of impurity is accomplished. It should be appreciated that such purification procedure might be employed to real advantage in the manufacture of sheets, strips, tubes, rods, vessels, containers and the like, to the enhancement of whatever properties such treatment would accomplish.

In contradistincticn to the novel heat treatment hereinabove described, it is realized of course that plain carburizing has been used in the past, but the instant invention difiers from such method in that carburizing is accomplished by diffusing carbon into the article while heated to its austenitizing range. In this invention however, the article is heated in such a range as will not cause austenitization while the article is kept in contact with austenitized material which serves to wtihdraw carbon and other elements hereinabove mentioned.

Furthermore, actual test results have shown that if ordinary carbon steel articles be heated in contact with iron-manganese alloy filings at a temperature below the A1 point for the carbon steel, but above such transformation temperature for the filings, a purified case is produced upon the carbon steel having enhanced toughness, corrosion resistance, magnetic susceptibility and other characteristics associated with carbon free and nitrogen free iron.

It is anticipated that this method of heat treatment might be applied to any combination of carbon or alloy steel remaining in its ferrite phase while a contacting ferrous or other medium is applied at a temperature efiecting its (the contacting mediums) equilibrium in austenite phase. This condition establishes the possibility of applying the principle of the heat treatment described to a large field of ferritic alloys for the accomplishment of many diverse ends.

Investigations of the strain-aging and blue metal phenomena of steel have shown that the efiect is governed to a large degree by the dissolved nitrogen content. It has been shown that the brittle shortcomings of Bessemer steels are due in a large measure to this cause. Treatment of Bessemer steel and articles made thereof by the method herein described may be used to correct such brittle tendency.

Great stress is laid by steel makers upon the importance of killing steel with aluminum, titanium and other agents to cure such various shortcomings as strain, aging and brittlement, Krupp krankheit, crystallization, poor impact strength, low temperature embrittlement, etc. The additions of such elements in the steelmaking process are effective by virtue of their affinity for nitrogen, immobilizing it against precipitation upon cleavage planes, through the formation of non-metallic inclusion complexes. By the use of the heat treatment hereinabove described, nitrogen is removed, accomplishing the same end as the steelmakers addition agents or killers without the undesirable effect of incurring the inclusions which are known to promote directional properties, instability of carbide in elevated temperature service, loss of resistance to corrosion and erosion, etc.

The application of the heat treatment herein described to the production of a case upon a ferritic steel article purified of carbon and nitrogen tends to raise the temperature of the start of the heating transformation from A1 for the eutectoid containing core to the higher A3 point in the case.

Useful applications ma be made of such a change in character. For instance, it is known that the resistance of a steel gun barrel to firing erosion could be improved if the carbon content of the steel were lowered and the transformation temperature raised. However, to lower the carbon content of the steel would reduce the strength and elasticity below the requirements of the firing pressure, but if by treating the bore by a process which would leave a thin case of ironalloy purified of carbon and nitrogen, with the body of the barrel intact in its physical properties, enhancement of firing erosion resistance and consequently an increase in the gun life would be eifected.

It is noteworthy of the heat treatment hereinabove described that it does require heating the article treated through a critical point with the dimensional changes that result therefrom. Thus it is plain that the treatment may be applied to accurately finished parts and surfaces without disturbing dimensions, as in other subcritical treatments for steel articles, as for instance, nitriding. It might be stated surrounding of the article to be treated with the contacting agent prevents oxidation of the surface of said article, which is an obvious advantage.

Another respect in which the method of heat treatment herein described may be employed to unique advantage concerns the adjusting of surface carbon contents to ideal concentrations.

After the surface of the article has been purified, it may continue to be heat treat-ed without contact with the austenitizing contact agent, until the core carbon diffuses towards the surface in desired concentration below that of the original concentration. As an example of the utility of such a treatment, the case of articles manufactured from the 11% to 14% chromium stainless steels, and irons may be cited. For certain kinds of corrosion resisting surfaces it is desirable to have a low carbon content stainless steel. Frequently the carbon content used represents a compromise between that which is low enough to permit of the best corrosion resistance and the minimum necessary to establish the requisite response to heat treatment and physical properties. By means of the present invention it becomes possible to process a stainless steel article having an adequate carbon content to establish core strength and a low enough carbon content at the surface to present high resistance to corrosion. A particular advantage in applying this type of treatment in adjusting the carbon content in the surface of a stainless steel article, accurately finished to dimensions, as for instance a valve seat or disc or both, is that the processing is carried on at a temperature below the critical transformations, thus not disturbing dimensions or shape of the article.

It might furthermore be stated that the method or principle of heat treatment herein covered has applications in the field of powder metallurgy. For instance, if it is the object to purify a ferritic iron powder of carbon, nitrogen and other like elements, in order to enhance magnetic or other properties, it may be admixed with another powder in austenite phase and the mixture heat treated at a temperature below the A1 point of the ferritic powder. Separation of the purified powder from the austenitic contacting powder may then be accomplished by magnetic separation and the purified powder may then be used either as such or for the manufacture of articles by powder metallurgy techniques.

It should of course be realized that the detailed shamans method; of heat treatment-ma-yevaryrsubstantially from the description hereinabove set forth which is only an example of the-application o'f this-inivention a'ndthereforethe scopeiof this invention 'shouldbemeasured-by-the-breadth of themiaims ."tained from the ferrite, to -.thelaustenitel the -said ferrous 1 contacti ng;,matejrial ngbeingaaiistenitic at the treating temperature whi1st';.the,said ferrous article remains. ferritic.

" 2; In a processjfof. thef.characterdescribedf the 4 steps consisting of -contacting;amarticl of. fer-- rous material with a ferrousacontactingimaterial to purify the ferrousai'ticleofcarbo1i; the contact taking place in a range of elevated temperatures below the A1 transformation of the said ferrous article and above the A1 transformation point of the said ferrous contacting material for a period of time during which a substantial rate of migration of interstitially dissolved carbon is effected from the ferrite to the austenite, the said contacting material being austenitic at the treating temperature whilst the said ferrous article remains ferritic.

3. In a process of the character described, the steps consisting of contacting an article of ferrous material with a ferrous contacting material to purify the ferrous article of nitrogen, the contact taking place in a range of elevated temperatures below the A1 transformation of the said ferrous article and above the A1 transformation point of the said ferrous contacting material for a period of time during which a substantial rate of migration of an interstitially dissolved nitrogen is attained from the ferrite to the austenite, the said ferrous contacting material being austenitic at the treating temperature whilst the said ferrous article remains ferritic.

4. In a process of the character described, the steps consisting of contacting an article of ferrous material with a ferrous contacting material to purify the ferrous article of carbon and nitrogen, the contact taking place in a range of elevated temperatures below the A1 transformation of the said ferrous article and above the A1 transformation point of the said ferrous contacting material for a period of time during which a substantial rate of migration of interstitially dissolved carbon and nitrogen are attained from the ferrite to the austenite, the said ferrous contacting material being austenitic at the treating temperature whilst the said ferrous article remains ferritic.

5. In a process of the character described, the steps consisting of contacting an article of ferrous material with an austenitic material to purify the ferrous article of interstitially dissolved elements, the contact taking place in a range of elevated temperatures at which the ferritic phase of the said article remains in equilibrium with the austenitic phase of the said contacting material for a period of time during which a submstantialnmigration rot am interstitiallyiidissolved suelemerit having :lower solubility in ferrite than: in austenite is t -attained :fromfitheasaid: articleiisconr tacted :to theisaidt contacting :material. 5 6. In a process of:the;charactersdescribect;ithe .z-estepaconsisting-70f.xcontacting an :article ofttferous z'mat'erialr with an? :austeniticmmaterial to :zzepurify ;-the :fenrous; article L of carbonntheecontact stakingrplacetin arrange oflelevated temperatures anatilWhi-Ch; thei. ferritic phase: of 2 z-the esaid article 1:; remains in;equilibriumiwithztherraustenitictphase ether saidecontacting imaterialefor. aszperiodztof ime during which. aesubstantialeratee f: migraion;- of interstitially; d-iSSOIVBdLCaI'bOH L is :attained rfromathezsaid article; contacted ltditheisaidiicom atactingmaterial.

:1 Tm-In arprccess otfitheacharacteridescribeditthe a stepsrconsisting: of:contacting11amarticlerifoiisferrous material with an austenitic materializto pifiifylcthe iferrousvanticle of nitrogen, the contact taking place in a range of elevated temperatures at which the'ferritic phase of the said ar- .,-ticle remains in equilibrium with the austenitic phase of the said contacting materialfor a, period of time during which a substantial migration of interstitially dissolved nitrogen is attained from the said article contacted to the said contacting material.

8. In a process of the character described, the steps consisting of contacting an article of ferrous material with an austenitic material to purify the ferrous articles of carbon and nitrogen, the contact taking place in a range of elevated temperatures at which the ferritic phase of the said article remains in equilibrium with the austenitic phase of the said contacting material for a period of time during which a substantial migration of interstitially dissolved carbon and nitrogen are attained from the said article contacted to the said contacting material.

9. A process of the character described, the steps consisting of contacting at the processing temperature a ferritic article with an austenitic material to purify the ferritic article to the extent of enhancing its physical and chemical properties comprising heat treatment and solid diffusion whereby migration of interstitial elements is effected from the ferritic phase of the article to the austenitic phase of the material contacted and at a range of elevated temperatures in which these two phases remain in equilibrium with each other and at which a substantial rate of migration of interstitially dissolved elements having lower solubility in ferrite than in austenite is attained.

10. A process of the character described, the steps consisting of contacting at the processing temperature a ferritic article with an austenitic material to purify the ferritic article to the extent of enhancing its physical and chemical properties comprising heat treatment and solid diffusion whereby migration of carbon is effected from the ferritic phase of the article to the austenitic phase of the material contacted and at a range of elevated temperatures in which these two phases remain in equilibrium with each other and at which a substantial rate of migration of carbon is attained.

11. A process of the character described, the steps consisting of contacting at the processing temperature a ferritic article with an austenitic material to purify the ferritic article to the extent of enhancing its physical and chemical properties comprising heat treatment and solid diffusion whereby migration of nitrogen is reffected from the ferritic phase of the article to the austenitic phase of the material contacted and at a range of elevated temperatures in which these two phases remain in equilibrium with each other and at which a substantial rate of migration of nitrogen is attained.

12. A process of the character described, the steps consisting of contacting at the processing temperature a ferritic article with an austenitic material to purify the ferritic article to the extent of enhancing its physical and chemical properties comprising heat treatment and solid diffusion whereby migration of carbon and nitrogen is effected from the ferritic phase of the article to the austenitic phase of the material contacted and at a range of elevated temperatures in which these two phases remain in equilibrium with each other and at which a substantial rate of migration of carbon and nitrogen is attained.

JEROME J KANTER.

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

UNITED STATES PATENTS Number Name Date 117,949 Wihl Aug. 8, 1871 360,732 Sawyer Apr. 5, 1887 FOREIGN PATENTS Number Country Date 22,799 Great Britain Oct. 10, 1907 187,310 Great Britain Oct. 16, 1922 419,057 Great Britain Oct. 31, 1934 OTHER REFERENCES "Steel and its Heat Treatment, vol. 1, 4th edition, 1938, John Wiley and Sons, Inc., N. Y. 0., page 101.

Practical Metallurgy, Sachs and Van Horn, 1940, American Society for Metals, Cleveland, Ohio, page 440.

Darken: Diffusion in Metal accompanied by Phase Change, published in Metals Technology, vol. 9, 1942, Technical Publication 1479, pages 1, 10-12, published by American Inst. of Mining and Metallurgical Engineers Inc., New York City.

Transactions American Institute of Mining & Metallurgical Engineers, vol. 131, Iron & Steel Div., 1938, pages 274-275.