EP0953653A1 - Method for producing forged iron-nickel-base superalloys - Google Patents

Abstract

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

Description

FIELD OF THE INVENTION
• The invention relates to the production of forged articles of iron-nickel-basesuperalloys from consolidated articles made from hot isostatically pressed prealloyednitrogen gas atomized particles.
BRIEF DESCRIPTION OF THE PRIOR ART
• It is known to use highly alloyed iron-nickel-base superalloys for the productionof forgings for use in applications requiring good mechanical properties at hightemperatures along with good corrosion resistance. These uses include themanufacture of gas turbine components and chemical processing applications.
• Presently, large ingots of alloys of this type are produced in cast and wroughtform by the use of a triple-melting operation. This melting includes vacuum inductionmelting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR). Thistriple-melting operation is both time consuming and costly, which increases significantlythe overall costs of these forgings and assemblies made therefrom, but it is necessaryto avoid elemental segregation and other problems, such as the creation of sites forcrack propagation, resulting therefrom. To minimize segregation, either the alloycontent must be reduced or excessive preliminary forging, commonly referred to asbilletizing, is necessary to promote homogenization of the material prior to forging to thedesired configuration of the article. This results in expensive furnace time and forgingpress time to heat to the required forging temperatures, both of which further add to theoverall cost of the final forged product.
• It is accordingly an object of the present invention to provide a practice forproducing forgings of this type from iron-nickel-base superalloys wherein segregation
• can be minimized and homogeneity enhanced without necessitating special melting andforging practices of the type employed in the prior art, which add considerably to theoverall cost of the final product.
SUMMARY OF THE INVENTION
• In accordance with the invention, a method is provided for producing a forgedarticle from iron-nickel-base superalloys that includes producing a melt of aniron-nickel-base superalloy. The superalloy includes alloying additions of chromium,niobium, titanium, and aluminum. The melt is gas atomized using nitrogen or argon toproduce prealloyed particles of this superalloy composition. These prealloyed particlesare hot isostatically pressed to produce a fully dense article therefrom. The fully densearticle is then forged to produce the desired forged article.
• The forged article may have preferably a grain size of ASTM No. 7 to 9.5.
• The forged article may have titanium, niobium, and titanium/niobium carbonitridecompounds at grain boundaries of the article after forging.
• The forging operation may be conducted at a temperature up to 2200°F.
• The forged article may be annealed at a temperature up to 2200°F.
• After annealing, the forged article may exhibit a grain size of ASTM No. 6 to 11.
• A preferred composition of the iron- and nickel-base superalloy is, in weightpercent, 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
• Figures 1a and b are photomicrographs of as-atomized powders of sample alloysA706 and N706 of the specific examples set forth herein;
• 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 forthin Table III, in the as-HIP condition;
• 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 at1900°F;
• Figures 4a and b are photomicrographs of cross sections of sample alloys, whichhave 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 at1900°F, respectively, as set forth in Table VII.
DESCRIPTION OF THE PREFERRED EMBODIMENTSExample 1
• 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 differentprocessing conditions is given in Table I. In the following, the argon atomized materialwill be referred to as A706, the nitrogen atomized material as N706. The designationP/M706 refers to both A706 and N706. Table II sets forth the chemical composition ofsamples A706 and N706, which have nominally the same composition, except for thenitrogen content..
• Blanks for microstructural investigations and mechanical testing weresolutionized 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 testswere conducted according to ASTM E8 and E23, respectively.

  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)

  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

• Cross-sections of as-atomized powders are shown in Figure 1. The largerpowder particles have a very fine cellular microstructure. Electron dispersive X-ray examination (EDX) showed that the cell walls, which appeared bright in backscatterelectron imaging, were rich in niobium and titanium and lean in chromium compared tothe overall composition of the alloy, while the inner part of the cell was depleted inniobium and titanium and rich in chromium. The finer powder particles appearedfeatureless. In the nitrogen atomized powders, fine black appearing precipitates ofabout 0.5 µm in size were also visible in backscatter electron imaging (Figure 1). EDXindicated that these precipitates are titanium and/or niobium rich, presumably nitrides orcarbonitrides.
• Following HIP, both alloys revealed a generally very fine microstructure typicalfor P/M processed materials (Figure 2). Particle outlining by discrete precipitates wasvisible in N706, but hardly distinguishable in A706.
• While all materials exhibited a very fine grain size, different processingconditions led to some differences in grain size as shown in Table III. Average grainsizes of the as-HIP materials ranged from 12 µm for N706, -140 mesh HIP 1950°F to19 µm for A706, -60mesh 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°Fdid for material with the same mesh size. In the as-HIP condition, the finer powderfraction of -140 mesh yielded finer grain sizes than -60 mesh fraction at the same HIPtemperature. N706 had a finer grain size than A706 with the same mesh size and HIPtemperature.
• Grain sizes increased only moderately during four hour annealing heattreatments at temperatures from 2075 to 2200°F, as shown in Table III. After fourhours at 2200°F, grain sizes varied between 17 µm for N706, -140 mesh HIP 1950°F and 28 µm for A706, -60mesh HIP 2065°F which correspond to ASTM No. 8.5 andNo. 7.1, respectively. During annealing, the tendency for finer grain size with finermesh size present in the as-HIP N706 and A706 prevailed for all conditions, while finergrain sizes resulting from lower HIP temperatures did not persist.

  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

• The as-HIP materials exhibited some degree of powder particle decoration bydiscrete precipitates, which were larger and occurred more frequently for N706 than forA706, as can be seen in Figure 2. Precipitates were also present within prior powderparticles, and more so in N706 than in A706. EDX showed that the precipitatesobserved are titanium, niobium, and niobium-titanium compounds, presumablycarbonitrides. The fine grain size resulted from grain boundary pinning by theseprecipitates during high temperature exposure, especially in N706.
• Room temperature mechanical properties of HIP, solutionized and aged P/M 706are 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 heattreating in different batches. The tensile elongations were around 20%, the reductionsof area around 30%. Charpy impact strength was 26 ft-lb forN706 HIP 2065°F in bothmesh fractions. For N706 HIP 1950°F, both mesh fractions and all A706 variants,Charpy impact strength was 20 ft-lb.

  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

• The data reported and discussed above show that the rapid cooling rate inherentto P/M processing eliminated segregation and led to very fine cellular solidificationmicrostructures in the as-atomized powders. In the consolidated materials, grainboundary pinning by the discrete carbonitride precipitates during heat treatment andduring thermo-mechanical processing resulted in very fine grain sizes. Due to thepresence of more and larger carbonitrides, N706 experienced stronger grain boundarypinning and therefore had an even finer grain size than A706. Still, these finely dispersed precipitates did not degrade the ductility or Charpy impact toughness asevident from Table IV. Similar beneficial effects have been observed in nitrogenatomized Alloy 625.
• Annealing heat treatments and quantitative microstructural investigationsindicated 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 achievingvery fine grains than was the lower HIP temperature. A significantly reduced propensityfor grain growth was also observed for HIP and forged P/M 706. This allowed higherforging temperatures during processing leading to lower forging forces, which isespecially important for large workpieces when frequently the limits of existing forgingpresses are reached. Also, the finer grain size leads to improved ultrasonicinspectability due to a reduced noise level. A decrease in grain size from ASTM No. 3to ASTM No. 8 has been found to decrease the ultrasonic noise level by factors of 3 to5 times.
Example 2
• 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 706was 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 topancakes of 1.5" height and 5.5 diameter. In the following, the argon atomized materiawill be referred to as A706 and the nitrogen atomized material will be referred to asN706. 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 intheir nitrogen content.

  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

• Blanks for microstructural investigations and mechanical testing were solutionedand subjected to a two-step aging heat treatment. Grain size was determined usingASTM E112. Tensile, Charpy impact, and fracture toughness testing was conductedusing 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 andan A-ratio of 1 (switched to load control at 5Hz after 28,800 cycles).
• The typical, but very fine pancake microstructure following the forging simulatortests is shown in Figure 3. The partly recrystallized grain structure bears noresemblance with the original as-HIP microstructure. During an annealing heattreatment, a small amount of grain growth took place and resulted in an equiaxed grainshape (Figure 4). The quantitative analysis is given in Table VI. The average grain sizeafter hot compression tests was 5 µm for N706, -60 mesh, HIP at 2065°F, deformed at1700°F, to 8 µm for A706, -140 mesh, HIP at 2065°F, deformed at 1900°F, whichcorresponds to ASTM No.11 to 12, respectively. Following an additional four hours at2000°F, the grain size increased from 13 µm to 22 µm (ASTM No. 7.5 to 9.5).

  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

• The high temperature flow stress curves obtained from the isothermalcompression tests showed that the flow stress of the P/M 706 decreased significantlywith increasing temperatures, from 38 ksi to 22.3 ksi as the temperature was raisedfrom 1700°F to 1900°F. When compared to C/W 706 material using the same testconditions, the P/M materials showed an 8-22% lower flow stress depending upontemperature and strain rate (either.05 or .5/second). There did not appear to be anydifference in the flow stress behavior for the P/M materials as atomization gas, meshsize, or HIP temperature were changed.
• The mechanical properties of HIP, forged, solution treated, and aged P/M aregiven in Tables VII and VIII. Yield strength and ultimate tensile strength (UTS) aresimilar to the as-HIP P/M 706 described in Example 1, but ductility and toughness areimproved.

  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

  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

• P/M 706 processed as described herein was fully dense and had a very finemicrostructure. The rapid cooling rate inherent to P/M processing resulted in very finecellular solidification microstructures in the as-atomized powders, while segregation waslargely absent. Also, the inclusion size was limited by the mesh size during powderscreening. The very small grain size observed here was due to grain boundary pinningduring thermo-mechanical processing by the finely dispersed carbonitride precipitates.This was most pronounced for nitrogen atomized P/M 706 with the larger number ofcarbonitrides. Similar effects have been observed in nitrogen atomized Alloy 625.Annealing studies of hot forged P/M 706 indicate higher resistance to grain growth inthis material as compared to cast and wrought (C/W) 706. P/M 706 had five to eighttimes smaller grains than C/W 706 after the same high temperature exposure (Figure5). This corresponds to ASTM No.8 for P/M material versus No. 2 for C/W material orNo. 9 for P/M material versus No. 5 for C/W material. The significantly reducedpropensity for grain growth allowed higher forging temperatures during processing andtherefore 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 fromASTM Nol. 3 to ASTM No. 8 has been found to decrease the ultrasonic noise level byfactors of three to five times.
• Low cycle fatigue (LCF) of HIP plus forged P/M 706 results were excellent (TableVIII) and exceeded those of C/W 706 by factors of three to five. The good low cyclefatigue resistance results in part from the very fine microstructure.
• All reported "mesh sizes" are U.S. Standard.
• All compositions set forth in the specification are in weight percent, unlessotherwise indicated.
• ASTM refers to the American Society for Testing Materials and related publishedtesting standards and practices.

Claims (12)

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 produceprealloyed particles of said superalloy, hot isostatically pressing said particles toproduce a fully dense article therefrom and forging said fully dense article to producesaid forged article.
2. The method of claim 1, wherein said forged article has a grain size ofASTM No. 7 to 12.
3. The method of claim 2, wherein said forged article has Ti, Nb, and Ti/Nbcarbonitride compounds at grain boundaries thereof.
4. The method of claim 3, wherein said forged article is produced byconducting said forging thereof at a temperature up to 2200°F.
5. The method of claim 4, wherein said forged article is annealed at atemperature up to 2200°F.
6. The method of claim 5, wherein after said annealing said forged articleexhibits 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 consistingessentially of 40 to 43 nickel, 15.5 to 16.5 chromium, 2.8 to 3.2 niobium, 1.5 to1.8 titanium, 0.1 to 0.3 aluminum, up to 0.1 nitrogen, up to 0.1 carbon, and balanceiron; nitrogen gas atomizing said melt to produce prealloyed particles of said superalloy;hot isostatically pressing said particles to produce a fully dense article therefrom; andforging said fully dense article to produced said forged article.
8. The method of claim 7, wherein said forged article has a grain size ofASTM No. 7 to 12.
9. The method of claim 8, wherein said forged article has Ti, Nb, and Ti/Nbcarbonitride compounds at grain boundaries thereof.
10. The method of claim 9, wherein said forged article is produced byconducting said forging thereof at a temperature up to 2200°F.
11. The method of claim 10, wherein said forged article is annealed at atemperature up to 2200°F.
12. The method of claim 11, wherein after said annealing said forged articleexhibits a grain size of ASTM No. 6 to 11.

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