US3794532A - Spherodization of grain boundary precipitates - Google Patents

Spherodization of grain boundary precipitates Download PDF

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Publication number
US3794532A
US3794532A US00234483A US3794532DA US3794532A US 3794532 A US3794532 A US 3794532A US 00234483 A US00234483 A US 00234483A US 3794532D A US3794532D A US 3794532DA US 3794532 A US3794532 A US 3794532A
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alloy
spheroidization
grain boundary
alloys
temperature
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R Jones
E Parker
V Zackay
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US Atomic Energy Commission (AEC)
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US Atomic Energy Commission (AEC)
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • C21D1/32—Soft annealing, e.g. spheroidising
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium

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  • This invention relates to the heat treatment of steels, particularly to the spheroidization of the grain boundary precipitates in Fe-Ta binary systems, and more particularly to the spheroidization of the grain boundary precipitates in 'FeTa-Cr alloys.
  • the yield strength has been related to dislocation morphology, alloy content, grain size, internal stresses, particle morphology and many other variables.
  • the work hardening of single phase alloys has been related to grain size, dislocation density and stacking fault energy.
  • nucleation and growth (2) internal oxidation, and (3) powder metallurgical techniques.
  • a complex grain boundary structure may result from the nucleation and growth process in polycrystalline materials.
  • This boundary region may have a heavy grain boundary network of the second phase with a precipitate free zone adjacent to the network.
  • a grain boundary structure such as this would alter the yield and flow behavior of an alloy when compared to an alloy without this grain boundary structure.
  • the present invention overcomes the prior problems of the above-mentioned grain boundary structure by providing a method which involves an allotropic phase change in binary systems after the aging treatment, which refined the grain structure and spheroidized the grain boundary network.
  • the final structure produced by the invention is a random dispersion of particles in a soft polycrystalline matrix with the grain boundary network spheroidized and no longer positioned at a grain boundary. It has been found that this spheroidization can be carried out in binary systems, such as Fe-Ta, and ternary systems, such as 3,794,532 Patented Feb. 26, 1974 Fe-Ta-Cr. Both the ductility and strength are increased by the spheroidization treatment.
  • a further object of the invention is to provide a process which grain boundary precipitates are fully spheroidized by cycling through the u-v phase transformation.
  • Another object of the invention is to provide a method which produces a material having enhanced oxidation corrosion resistance, utilizes lower heat treating temperature, and minimize embrittlement of grain boundaries.
  • Another object of the invention is the producing of an alloy, either binary or ternary, which has spheroidized grain boundary network having a random dispersion of particles.
  • Another object of the invention is to provide an Fe- Ta-Cr alloy wherein the grain boundary precipitates are fully spheroidized.
  • FIG. 1 is a flow chart illustrating the heat treating cycle which includes the novel spheroidization technique.
  • FIG. 2 is an equilibrium phase diagram for the Fe-Ta binary system.
  • FIG. 3 illustrates the grain boundary network without the spheroidization process.
  • FIG. 4 illustrates the effect of the spheroidization process on the grain boundary of FIG. 3.
  • FIG. 5 is a graph showing temperature vs. yield stress of various materials including the novel Fe-Ta-Cr alloy.
  • the alloys were cast from 99.95% purity electrolytic iron and 99.9% initial purity tantalum.
  • the composition preparation of the alloy does not constitute part of this invention and is not described in detail, but is set forth in coinventor Jones Ph.D. Thesis dated October 10, 1971, LBL-l79, entitled Prediction of the Stress-Strain Behavior of Polycrystalline Ot-II'OII Containing Hard Spherical Particles.
  • the heat treating cycle utilized is schematically shown in FIG. 1.
  • the second phase was formed, after solution treating at 1400 (3., shown as the first leg of FIG. 1, and quenching to room temperature, by the reaction: a(supersat.) x+F6 Ta at 700 C.
  • the Fe Ta particles which formed during this reaction where plate shaped and there was a heavy network of the Laves phase at the grain boundaries, as shown in FIG. 3.
  • the alloy was then age treated as indicated by the second leg of FIG. 1.
  • the treatment provided by the inventive technique was accomplished by heating the alloy to 1100 C., as shown in FIG. 1 at the third leg of the flow chart, at which the stable structure is 'y+Fe Ta.
  • the eifect of this novel treatment was to spheroidize the matrix and grain boundary particles, and refine the grain size as shown in FIG. 4.
  • Specimens for u-v phase transformation temperature determination by the metallographic method were held at 860 C. for 12 hours, and then heated up to 890 C. and 950 C., respectively, at a heating rate of 1 C./minute, followed by quenching into salt water.
  • Tensile specimens were quenched from 1320 C. (similar to the first leg of FIG. 1) after one hour solution treatment into 45 C. water.
  • the dilatometer analysis showed that the 'y-6 phase transformation temperature had been lowered about 60 C. and the a-q phase transformation temperature had been lowered about 100 C. by the addition of chromium to Fe-Ta binary alloys.
  • the results of the metallographic examination agreed reasonably with the dilatometer analysis.
  • chromium up to 7 atomic percent has the efiect of lowering both the rut-*y and -6 transformation temperatures. Further increase in chromium content will lower the -6 transformation temperature but will raise the oz'y transformation temperature.
  • spheroidization in the binary alloy was accomplished by cycling through the oz'y allotrophic phase transformation.
  • some grain boundary films were already broken up and some particles were rather spherical before the spheroidization treatment.
  • the degree of spheroidization was not suflicient to eliminate the brittleness of the lower chromium alloys; thus, it was necessary to use the oc'y transformation treatment for these materials.
  • the spheroidization temperature in the 'y+Fe Ta region affects the hardness of the Fe-lTa-SCr alloy, wherein the peak aged hardness was 169, and the spheroidization treatments above 975 C. produced an added increment of hardness.
  • the strength increased with increasing chromium content and the ductility decreased with increasing chromium content.
  • FIG. 5 illustrates the yield stress of various alloys including the above described Fe-Ta-Cr alloy.
  • the ternary alloy produces, with the exception of Fe-l at percent Ta Cold Worked Steel, higher yield stress at temperatures below 600 C. and higher above 600 C. except for Hastelloy X, thus further illustrating the advantages of the novel Fe-Ta-Cr alloy.
  • the 11- phase transformation temperature was lowered about C. by adding not greater than 7 at percent chromium, and the -6 transformation temperature was lowered about 60 0., relative to the binary phase diagram.
  • the ductility can be increased and a fine grain size can be obtained.
  • the present invention provides a spheroidization method and ternary composition which greatly advances the state of art.
  • steps of cycling the alloy through the alpha-gamma phase transformation includes the steps of heating the age treated alloy to the gamma+Fe Ta phase field, and cooling the thus heated alloy.
  • cycling steps are carried out by heating the alloy to a temperature of about 950 C. to 1100 C., maintaining the alloy at such temperature for a time period of no greater than 10 minutes, furnace cooling the alloy to a temperature of about 750 C., and air cooling the alloy to ambient temperature.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Manufacture And Refinement Of Metals (AREA)
US00234483A 1972-03-14 1972-03-14 Spherodization of grain boundary precipitates Expired - Lifetime US3794532A (en)

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US (1) US3794532A (sv)
JP (1) JPS48102719A (sv)
DE (1) DE2312673A1 (sv)
FR (1) FR2176015B1 (sv)
GB (1) GB1383868A (sv)
SE (1) SE386912B (sv)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4087287A (en) * 1977-04-15 1978-05-02 The United States Of America As Represented By The Secretary Of The Interior Method for providing ferritic-iron-based alloys

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4087287A (en) * 1977-04-15 1978-05-02 The United States Of America As Represented By The Secretary Of The Interior Method for providing ferritic-iron-based alloys

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Publication number Publication date
FR2176015A1 (sv) 1973-10-26
FR2176015B1 (sv) 1976-06-11
JPS48102719A (sv) 1973-12-24
SE386912B (sv) 1976-08-23
DE2312673A1 (de) 1973-09-27
GB1383868A (en) 1974-02-12

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