(19)
(11) EP 0 953 653 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication:
03.11.1999 Bulletin 1999/44

(21) Application number: 99810326.1

(22) Date of filing: 20.04.1999
(51) International Patent Classification (IPC)6C22C 33/02, C22C 1/04
(84) Designated Contracting States:
AT BE CH CY DE DK ES FI FR GB GR IE IT LI LU MC NL PT SE
Designated Extension States:
AL LT LV MK RO SI

(30) Priority: 20.04.1998 US 62585

(71) Applicant: CRUCIBLE MATERIALS CORPORATION
Syracuse, New York 13201-0977 (US)

(72) Inventors:
  • Conway, John J.
    Pittsburgh, PA 15243 (US)
  • Rizzo, Frank J.
    McMurray, PA 15317 (US)
  • Habel, Ulrike
    Pittsburgh, PA 15208 (US)

(74) Representative: Hug Interlizenz AG 
Nordstrasse 31
8035 Zürich
8035 Zürich (CH)

   


(54) Method for producing forged iron-nickel-base superalloys


(57) Two P/M versions of the Superalloy 706 have been produced by argon and nitrogen atomization. Both have a very fine and homogeneous microstructure with grain sizes of ASTM No. 6 to 12 depending on thermo-mechanical treatment. The compacted P/M 706 contains discrete titanium-, niobium- and titanium-niobium-carbonitride compounds which pin grain boundaries during thermo-mechanical treatment. Nitrogen-atomized P/M 706 contains more frequent and larger carbonitrides that argon-atomized P/M 706. P/M 706 shows very moderate grain growth even after annealing at temperatures up to 2200°F due to grain boundary pinning by the carbonitride precipitates. Due to its higher content of precipitates, nitrogen-atomized P/M 706 has a finer grain size than argon-atomized P/M 706.




Description

FIELD OF THE INVENTION



[0001] The invention relates to the production of forged articles of iron-nickel-base superalloys from consolidated articles made from hot isostatically pressed prealloyed nitrogen gas atomized particles.

BRIEF DESCRIPTION OF THE PRIOR ART



[0002] It is known to use highly alloyed iron-nickel-base superalloys for the production of forgings for use in applications requiring good mechanical properties at high temperatures along with good corrosion resistance. These uses include the manufacture of gas turbine components and chemical processing applications.

[0003] Presently, large ingots of alloys of this type are produced in cast and wrought form by the use of a triple-melting operation. This melting includes vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). This triple-melting operation is both time consuming and costly, which increases significantly the overall costs of these forgings and assemblies made therefrom, but it is necessary to avoid elemental segregation and other problems, such as the creation of sites for crack propagation, resulting therefrom. To minimize segregation, either the alloy content must be reduced or excessive preliminary forging, commonly referred to as billetizing, is necessary to promote homogenization of the material prior to forging to the desired configuration of the article. This results in expensive furnace time and forging press time to heat to the required forging temperatures, both of which further add to the overall cost of the final forged product.

[0004] It is accordingly an object of the present invention to provide a practice for producing forgings of this type from iron-nickel-base superalloys wherein segregation

[0005] can be minimized and homogeneity enhanced without necessitating special melting and forging practices of the type employed in the prior art, which add considerably to the overall cost of the final product.

SUMMARY OF THE INVENTION



[0006] In accordance with the invention, a method is provided for producing a forged article from iron-nickel-base superalloys that includes producing a melt of an iron-nickel-base superalloy. The superalloy includes alloying additions of chromium, niobium, titanium, and aluminum. The melt is gas atomized using nitrogen or argon to produce prealloyed particles of this superalloy composition. These prealloyed particles are hot isostatically pressed to produce a fully dense article therefrom. The fully dense article is then forged to produce the desired forged article.

[0007] The forged article may have preferably a grain size of ASTM No. 7 to 9.5.

[0008] The forged article may have titanium, niobium, and titanium/niobium carbonitride compounds at grain boundaries of the article after forging.

[0009] The forging operation may be conducted at a temperature up to 2200°F.

[0010] The forged article may be annealed at a temperature up to 2200°F.

[0011] After annealing, the forged article may exhibit a grain size of ASTM No. 6 to 11.

[0012] A preferred composition of the iron- and nickel-base superalloy is, in weight percent, 40 to 43 nickel, 15.5 to 16.5 chromium, 2.8 to 3.2 niobium, 1.5 to 1.8 titanium, 0.1 to 0.3 aluminum, up to 0.1 nitrogen, up to 0.1 carbon, and balance iron.

BRIEF DESCRIPTION OF THE DRAWINGS



[0013] Figures 1a and b are photomicrographs of as-atomized powders of sample alloys A706 and N706 of the specific examples set forth herein;

[0014] Figures 2a and b are photomicrographs of cross sections of sample alloys A706, -140 mesh, HIP at 2065°F, and N706, -60 mesh, HIP at 1950°F, respectively, set forth in Table III, in the as-HIP condition;

[0015] Figures 3a and b are photomicrographs of cross sections of A706, -140 mesh, HIP at 2065°F, deformed at 1900°F; and N706, -60 mesh, HIP at 1065°F, deformed at 1900°F;

[0016] Figures 4a and b are photomicrographs of cross sections of sample alloys, which have been compressed and then annealed for four hours at 2000°F A706, -140 mesh, HIP at 2065°F, deformed at 1900°F and N706, 60 mesh, HIP at 2065°F, deformed at 1900°F, respectively, as set forth in Table VII.

DESCRIPTION OF THE PREFERRED EMBODIMENTS


Example 1



[0017] Nitrogen and argon atomized powders of Alloy 706 were screened to -60 mesh (<250 µm) and -140 mesh (<106 µm) size and subsequently hot isostatically pressed (HIP) at two different temperatures, 1950°F and 2065°F. A summary of the different processing conditions is given in Table I. In the following, the argon atomized material will be referred to as A706, the nitrogen atomized material as N706. The designation P/M706 refers to both A706 and N706. Table II sets forth the chemical composition of samples A706 and N706, which have nominally the same composition, except for the nitrogen content..

[0018] Blanks for microstructural investigations and mechanical testing were solutionized and subjected to a two-step aging heat treatment. For a grain size study, additional as-HIP blanks were annealed for four hours at 2075°F, 2160°F, or 2200°F. Grain sizes were measured according to ASTM E112. Tensile and Charpy impact tests were conducted according to ASTM E8 and E23, respectively.
Table I.
Summary of Processing Conditions
Grade A706 A706 A706 A706 N706 N706 N706 N706
Atomization Gas Ar Ar Ar Ar N N N N
Mesh Size -60 -60 -140 -140 -60 -60 -140 -140
HIP Temperature °F (°C) 2065 (1130) 1950 (1065) 2065 (1130) 1950 (1065) 2065 (1130) 1950 (1065) 2065 (1130 1950 (1065)
Table II.
Chemical Composition (Wt%)
Heat Heat Ni Cr Nb Ti Al Si P S N C O Fe
A706 40.93 15.75 3.11 1.68 0.18 <0.01 ≤.002 .001 .001 .004 .0109 38.34
N706 40 82 16.01 3.02 1.65 0.20 <0.01 <0.02 .001 .039 .003 .0072 38.25


[0019] Cross-sections of as-atomized powders are shown in Figure 1. The larger powder particles have a very fine cellular microstructure. Electron dispersive X-ray examination (EDX) showed that the cell walls, which appeared bright in backscatter electron imaging, were rich in niobium and titanium and lean in chromium compared to the overall composition of the alloy, while the inner part of the cell was depleted in niobium and titanium and rich in chromium. The finer powder particles appeared featureless. In the nitrogen atomized powders, fine black appearing precipitates of about 0.5 µm in size were also visible in backscatter electron imaging (Figure 1). EDX indicated that these precipitates are titanium and/or niobium rich, presumably nitrides or carbonitrides.

[0020] Following HIP, both alloys revealed a generally very fine microstructure typical for P/M processed materials (Figure 2). Particle outlining by discrete precipitates was visible in N706, but hardly distinguishable in A706.

[0021] While all materials exhibited a very fine grain size, different processing conditions led to some differences in grain size as shown in Table III. Average grain sizes of the as-HIP materials ranged from 12 µm for N706, -140 mesh HIP 1950°F to 19 µm for A706, -60 mesh HIP 2065°F, which correspond to ASTM No. 9.5 and No. 8.1, respectively. The lower HIP-temperature, 1950°F resulted in finer grains than 2065°F did for material with the same mesh size. In the as-HIP condition, the finer powder fraction of -140 mesh yielded finer grain sizes than -60 mesh fraction at the same HIP temperature. N706 had a finer grain size than A706 with the same mesh size and HIP temperature.

[0022] Grain sizes increased only moderately during four hour annealing heat treatments at temperatures from 2075 to 2200°F, as shown in Table III. After four hours at 2200°F, grain sizes varied between 17 µm for N706, -140 mesh HIP 1950°F and 28 µm for A706, -60 mesh HIP 2065°F which correspond to ASTM No. 8.5 and No. 7.1, respectively. During annealing, the tendency for finer grain size with finer mesh size present in the as-HIP N706 and A706 prevailed for all conditions, while finer grain sizes resulting from lower HIP temperatures did not persist.
Table III.
Grain Sizes (µm) As-HIP and Annealed at Different Temperatures
Grade Mesh Size HIP Temp (°F/°C) As-HIP HIP + 4 hrs. 2075°F/1135°C HIP + 4 hrs. 2160°F/1180°C HIP + 4 hrs. 2200°F/1205°C
N706 -60 2065/1130 15 15 16 20.5
N706 -60 1950/1065 13 17 16.5 19.5
N706 -140 2065/1130 14.5 13.5 16 17.5
N706 -140 1950/1065 12 12.5 15.5 17
A706 -60 2065/1130 18.5 22 25 28
A706 -60 1950/1065 17 22 25 27.5
A706 -140 2065/1130 17 17 18 20
A706 -140 1950/1065 12 16 18 20


[0023] The as-HIP materials exhibited some degree of powder particle decoration by discrete precipitates, which were larger and occurred more frequently for N706 than for A706, as can be seen in Figure 2. Precipitates were also present within prior powder particles, and more so in N706 than in A706. EDX showed that the precipitates observed are titanium, niobium, and niobium-titanium compounds, presumably carbonitrides. The fine grain size resulted from grain boundary pinning by these precipitates during high temperature exposure, especially in N706.

[0024] Room temperature mechanical properties of HIP, solutionized and aged P/M 706 are shown in Table IV. The 0.2% yield strength varied between 149 and 165 ksi, while the UTS varied between 192 and 199 ksi. This variation in strength is typical for heat treating in different batches. The tensile elongations were around 20%, the reductions of area around 30%. Charpy impact strength was 26 ft-lb for N706 HIP 2065°F in both mesh fractions. For N706 HIP 1950°F, both mesh fractions and all A706 variants, Charpy impact strength was 20 ft-lb.
Table IV.
Room Temperature Mechanical Properties of P/M 706, HIP, Solution Treated and Aged
Grade Mesh Size HIP Temp (°F/°C) YS (ksi/MPA) UTS (ksi/MPA) Tens. EI. (%) RA (%) Impact Energy (ft-lb/J)
N706 -60 2065/1130 149/1025 192/1325 23 30 26/35
N706 -60 1950/1065 150/1035 192/1325 19 25 20/27
N706 -140 2065/1130 162/1115 197/1355 22 38 26/35
N706 -140 1950/1065 163/1125 198/1365 20 33 20/27
A706 -60 2065/1130 165/1130 196/1350 21 34 19/26
A706 -60 1950/1065 165/1135 199/1370 17 23 22/30
A706 -140 2065/1130 150/1035 193/1330 20 27 19/26
A706 -140 1950/1065 151/1040 195/1345 21 29 20/27


[0025] The data reported and discussed above show that the rapid cooling rate inherent to P/M processing eliminated segregation and led to very fine cellular solidification microstructures in the as-atomized powders. In the consolidated materials, grain boundary pinning by the discrete carbonitride precipitates during heat treatment and during thermo-mechanical processing resulted in very fine grain sizes. Due to the presence of more and larger carbonitrides, N706 experienced stronger grain boundary pinning and therefore had an even finer grain size than A706. Still, these finely dispersed precipitates did not degrade the ductility or Charpy impact toughness as evident from Table IV. Similar beneficial effects have been observed in nitrogen atomized Alloy 625.

[0026] Annealing heat treatments and quantitative microstructural investigations indicated high resistance to grain growth for P/M 706 at temperatures up to 2200°F. Finer mesh size and nitrogen atomization were found to be more efficient for achieving very fine grains than was the lower HIP temperature. A significantly reduced propensity for grain growth was also observed for HIP and forged P/M 706. This allowed higher forging temperatures during processing leading to lower forging forces, which is especially important for large workpieces when frequently the limits of existing forging presses are reached. Also, the finer grain size leads to improved ultrasonic inspectability due to a reduced noise level. A decrease in grain size from ASTM No. 3 to ASTM No. 8 has been found to decrease the ultrasonic noise level by factors of 3 to 5 times.

Example 2



[0027] P/M 706 powders were produced by both nitrogen and argon gas atomization. Nitrogen atomized 706 was screened to -60 mesh size (250 µm), argon atomized 706 was screened to -140 mesh size (106 µm). Both variants were hot isostatically pressed (HIP) at two different temperatures, 1950°F and 2065°F and subsequently forged to pancakes of 1.5" height and 5.5 diameter. In the following, the argon atomized materia will be referred to as A706 and the nitrogen atomized material will be referred to as N706. The designation P/M706 refers to both N706 and A706. The chemical compositions of both versions are given in Table V. N706 and A706 differ mainly in their nitrogen content.
Table V.
Chemical Composition (Wt%)
Heat Ni Cr Nb Ti AI Si P S N C O Fe
A706 40.93 15.75 3.11 1.68 0.18 <0.01 ≤.002 .001 .001 .004 .0109 38.34
N706 40.82 16.01 3.02 1.65 0.20 <0.01 ≤0.02 .001 .039 .003 .0072 38.25


[0028] Blanks for microstructural investigations and mechanical testing were solutioned and subjected to a two-step aging heat treatment. Grain size was determined using ASTM E112. Tensile, Charpy impact, and fracture toughness testing was conducted using ASTM E8, E23, and E813, respectively. LCF tests were conducted at 0.7% plastic strain using a triangular waveform with a frequency of 20 cycles per minute and an A-ratio of 1 (switched to load control at 5Hz after 28,800 cycles).

[0029] The typical, but very fine pancake microstructure following the forging simulator tests is shown in Figure 3. The partly recrystallized grain structure bears no resemblance with the original as-HIP microstructure. During an annealing heat treatment, a small amount of grain growth took place and resulted in an equiaxed grain shape (Figure 4). The quantitative analysis is given in Table VI. The average grain size after hot compression tests was 5 µm for N706, -60 mesh, HIP at 2065°F, deformed at 1700°F, to 8 µm for A706, -140 mesh, HIP at 2065°F, deformed at 1900°F, which corresponds to ASTM No.11 to 12, respectively. Following an additional four hours at 2000°F, the grain size increased from 13 µm to 22 µm (ASTM No. 7.5 to 9.5).
Table VI.
Grain Size of P/M 706 (pm) As-Deformed and Heat Treated* All Materials HIP at 2065°F
Grade Mesh Size Deformation Temp. (°F) As Deformed 8 hrs. 1800°F Deformed plus 4 hrs. 2000°F
N706 -60 1700 5.4 ± 0.5 13.5 ± 1 20 ± 0.5
N706 -60 1800 5.8 ± 0.9 11.5 ± 1 17 ± 4
N706 -60 1900 8.0 ± 0.7 12 ± 1 13 ± 0.5
A706 -140 1700 7.6 ± 1.0 15.5 ± 1 18 ± 0.6
A706 -140 1800 6.6 ± 0.4 13.5 ± 1 22 ± 1
A706 -140 1900 7.0 + 0.7 15 ± 1 20 ± 2


[0030] The high temperature flow stress curves obtained from the isothermal compression tests showed that the flow stress of the P/M 706 decreased significantly with increasing temperatures, from 38 ksi to 22.3 ksi as the temperature was raised from 1700°F to 1900°F. When compared to C/W 706 material using the same test conditions, the P/M materials showed an 8-22% lower flow stress depending upon temperature and strain rate (either.05 or .5/second). There did not appear to be any difference in the flow stress behavior for the P/M materials as atomization gas, mesh size, or HIP temperature were changed.

[0031] The mechanical properties of HIP, forged, solution treated, and aged P/M are given in Tables VII and VIII. Yield strength and ultimate tensile strength (UTS) are similar to the as-HIP P/M 706 described in Example 1, but ductility and toughness are improved.
Table VII.
Room Temperature Mechanical Properties of P/M 706, HIP, Forged, Solution Treated, and Aged
Grade Mesh Size HIP Temp. (°F) 0.2% YS ksi UTS ksi Ten. El. (%) RA (%) Impact Energy ft-lb
N706 -60 2065 149 192 24 48 31
Table VIII.
Room Temperature Fracture Toughness and Cycles to Failure (LCF) at 0.7% Strain at 750 and 900°F of P/M 706, HIP, Forged, Solution Treated, and Aged
Alloy Mesh Size HIP Temperature (°F) Cycles to Failure at 750°F Cycles to Failure at 900°F
N706 -60 2065 15,719 28,121


[0032] P/M 706 processed as described herein was fully dense and had a very fine microstructure. The rapid cooling rate inherent to P/M processing resulted in very fine cellular solidification microstructures in the as-atomized powders, while segregation was largely absent. Also, the inclusion size was limited by the mesh size during powder screening. The very small grain size observed here was due to grain boundary pinning during thermo-mechanical processing by the finely dispersed carbonitride precipitates. This was most pronounced for nitrogen atomized P/M 706 with the larger number of carbonitrides. Similar effects have been observed in nitrogen atomized Alloy 625. Annealing studies of hot forged P/M 706 indicate higher resistance to grain growth in this material as compared to cast and wrought (C/W) 706. P/M 706 had five to eight times smaller grains than C/W 706 after the same high temperature exposure (Figure 5). This corresponds to ASTM No.8 for P/M material versus No. 2 for C/W material or No. 9 for P/M material versus No. 5 for C/W material. The significantly reduced propensity for grain growth allowed higher forging temperatures during processing and therefore required lower forging forces. Also, the finer grain size leads to improved ultrasonic inspectability due to a reduced noise level. A decrease in grain size from ASTM Nol. 3 to ASTM No. 8 has been found to decrease the ultrasonic noise level by factors of three to five times.

[0033] Low cycle fatigue (LCF) of HIP plus forged P/M 706 results were excellent (Table VIII) and exceeded those of C/W 706 by factors of three to five. The good low cycle fatigue resistance results in part from the very fine microstructure.

[0034] All reported "mesh sizes" are U.S. Standard.

[0035] All compositions set forth in the specification are in weight percent, unless otherwise indicated.

[0036] ASTM refers to the American Society for Testing Materials and related published testing standards and practices.


Claims

1. A method for producing a forged article from iron-nickel-base superalloys, comprising producing a melt of an iron-nickel-base superalloy including chromium, niobium, titanium, and aluminum, nitrogen gas atomizing said melt to produce prealloyed particles of said superalloy, hot isostatically pressing said particles to produce a fully dense article therefrom and forging said fully dense article to produce said forged article.
 
2. The method of claim 1, wherein said forged article has a grain size of ASTM No. 7 to 12.
 
3. The method of claim 2, wherein said forged article has Ti, Nb, and Ti/Nb carbonitride compounds at grain boundaries thereof.
 
4. The method of claim 3, wherein said forged article is produced by conducting said forging thereof at a temperature up to 2200°F.
 
5. The method of claim 4, wherein said forged article is annealed at a temperature up to 2200°F.
 
6. The method of claim 5, wherein after said annealing said forged article exhibits a grain size of ASTM No. 6 to 11.
 
7. A method for producing a forged article from iron-nickel-base superalloys, comprising producing a melt of an iron- and nickel-base superalloy consisting essentially of 40 to 43 nickel, 15.5 to 16.5 chromium, 2.8 to 3.2 niobium, 1.5 to 1.8 titanium, 0.1 to 0.3 aluminum, up to 0.1 nitrogen, up to 0.1 carbon, and balance iron; nitrogen gas atomizing said melt to produce prealloyed particles of said superalloy; hot isostatically pressing said particles to produce a fully dense article therefrom; and forging said fully dense article to produced said forged article.
 
8. The method of claim 7, wherein said forged article has a grain size of ASTM No. 7 to 12.
 
9. The method of claim 8, wherein said forged article has Ti, Nb, and Ti/Nb carbonitride compounds at grain boundaries thereof.
 
10. The method of claim 9, wherein said forged article is produced by conducting said forging thereof at a temperature up to 2200°F.
 
11. The method of claim 10, wherein said forged article is annealed at a temperature up to 2200°F.
 
12. The method of claim 11, wherein after said annealing said forged article exhibits a grain size of ASTM No. 6 to 11.
 




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