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Thermal stability of the superalloys Inconel 625 and Nimonic 86
Journal of Nuctear Materials 101 (1981) 243-250 North-Holland Publishing Company
243
THERMAL STABILITY OF THE SUPERALLOYS INCONEL 625 AND NIMONIC 86 H.K. KOHL
and K. PENG
*
Swiss Federal Institute fur Reactor Research, Wiirenlingen, Switzerlund Received I June 1981; in revised form 2 July 198I The thermal stability of Inconel 625 and Nimonic 86, as received, cold worked (IO, 20, and 40%). and solution treated, was investigated in the temperature range 500-900°C. The annealing times varied from 0.3 (0.03) to 100 days. Precipitation hardening and recovery (recrystallisation) takes place in cold worked material, beginning after shorter times in cold worked material than in as received material. The temperature interval for precipitation hardening is extended in Nimonic 86, due to cold working, from about 500-600°C to about 450-7OO’C. It is possible to suppress or retard the precipitation hardening in solution treated Inconel625 and Nimonic 86 by fast cooling after solution annealing. Hardness was measured at room temperature with five different loads, so that the parameters k and n from Meyer’s_law, and the Brine11hardness number (for F+ 0’ = 30) could be determined. The lattice tontraction of Inconel625 due to ageing was investigated with X-ray measurements. The change of intensities of the diffractometer traces due to recovery was also determined.
1. Introduction Superalloys are used and contemplated in nuclear engineering in HTRs and fusion reactors, e.g. for hot gas pipes, heat exchangers, control rods or the vacuum vessel, respectively. It has been known for about ten years that Inconel625 shows precipitation hardening at higher temperature after annealing over periods of months as a result of developing phases, which are precipitated in the temperature range of -(700 “r 15O)‘C [l-4]. Generally, the first precipitates, found in the solution treated alloy, are carbides, and different carbide phases are generated, until the origination of intermetallic compounds is favoured. These also follow a certain sequence, and are mainly responsible for changes in hardness. Nimonic 86 is the same type of alloy as Inconel625, but it is not as complex. Both are solid solution strengthened alloys. Inconel 625 was considered as unsuitable for load-bearing applications in elevated temperature regions of the HTGR (51 due to precipitation hardening. In the present investigation, the precipitation (age) hardening of Inconel625 was examined once again and the influences of cold working and cooling time after the solution treatment on the precipitation hardening were investigated. * K. Peng is now retired. 0022-3 115/8 1/OOOO-0000/$02.75
0 198 1 North-Holland
The examination of a high temperature alloy at room temperature is no contradiction, because a high temperature alloy must, for certain applications, have adequate mechanical properties at room temperature as well. The loss in ductility at room temperature together with the precipitation hardening is of importance for the superalloys under consideration. Room temperature embrittlement and reduction of creep strength at higher temperatures is not a consequence of the precipitates, their morphology and distribution, but rather a consequence of the alloy dilution adjacent to the precipitates [6]. Accordingly, a maximum in ductility for superalloys was measured using hot tensile tests after ageing in the temperature range where precipitations were found [7].
2. Experimental prixdures 2. I. Materials The manufacturer, the as received status, the cast analysis, and mechanical test results of the investigated materials are presented in table 1. Bars of cross section 30 X 20 mm2 or 20 X 20 mm* were cut off the plates in the rolling direction. Round bat> were then fabricated with 19 mm diameter. Part of these bars were cold drawn to diameters of 18, 17, and 15 mm, respectively, which represented
H.K. Kohl. K. Peng / Thermal stuhrhty of Inconel625 und Nlmonic X6
244 Table I Cast analysis
(wt%) of the investigated
materials
and mechanical
test results Nimonic
Inconel625
86
Manufacturer
Stellite Div., Cabot Corp. Indiana,
USA
Henry Wiggin and Co. Ltd., Hereford,
As received
plate, 30 mm, hot rolled, annealed
and pickled
plate, 20 mm, hot rolled, annealed (3 min 115O”C, water quenched) and pickled
C Si Mn Cf Ni Fe MO co Al Ti Nb+Ta P S CU
0.070 0.33 0.29 20.84 bal. 3.82 8.89 0.25 0.32 0.30 3.60 0.009 <0.002
Mg Zl Ce Pb B Bi Ag 0.2 yield stress, MPa Tensile strength, MPa Hardness, HV Elongation, % Reduction of area, %
503 889 46.0 50.0
degrees of deformation of about 10,20, and 40% (degree of deformation (W) = (F, - F,). 100/F,, where F. and F, represent the cross sections before and after cold working). Discs of 5 mm thickness were separated from the rods using electro-erosion. Each disc face was fine ground and polished. The specimens were enclosed in evacuated quartz ampoules for the heat treatments.
England
0.032 0.28 0.06 24.85 bal. 1.46 10.92 0.38 0.05 0.02
0.005 0.08 0.019 to.010 0.035 0.7 ppm < 20 ppm CO.1 ppm to.1 ppm 349 750 221/232 58.4
1200°C and Nimonic 86 at 1150°C. The specimens were brought to solution temperature in 1 h then held 1 h at solution temperature, and then afterwards cooled to room temperature at different times (4 X 10 -3 to > IO3 h). The heating and cooling sequences were linear because an electronically controlled device was used. 2.3. Experiments
2.2. Thermal treatments Ageing was done at temperatures of 500, 550, 600, 750, 800, and 900°C. The temperature variations were less than f 1%. The ageing times were 0.3, 1, 3, 10, 30, 60, and 100 days. At 800 and 900°C shorter times were also used, i.e. 0.03 and 0.1 days. Solution treated specimens were aged in addition to the specimens in the as received status and the cold worked specimens. Inconel625 was solution annealed at
2.3.1. Hardness testing The disc specimens were hardness tested on the polished face, i.e. the test direction was the same as the hot rolling direction of the original plates and also the drawing direction of the rods during the cold working sequences. Higher hardness values were measured at the rolling surface of the plate of Nimonic 86. This was not observed for Inconel625. The hardness test was performed using five different
H. K. Kohl, K. Peng / Thermal stability of fnconel62S and Nimonic 86
loads from 490 to 2450N. This allowed the determination of the parameters for Meyer’s law [8,9] F= k(d/D)“,
(1)
where F represents the load in N; k, the load which would be necessary to press a sphere down to its equator into a material (if (d/D) = 1, then F= k); d is the diameter of the indentation; and D is the diameter of the spherical indenter (= 2.5 mm). The exponent n is related to the strain hardening exponent m of the Ludwik equation, u = kl&” [lo,1 I], according to [ 121 n-m++.
(2)
Different loads must give geometrically dissimilar indentations (this is only the case with a spherical indenter), so that n-values greater than 2 are obtained. In fig. 1 the load F is plotted logarithmically against (d/D) (which is also plotted on a logarithmic scale) for Inconel 625 for different treatments, without and after ageing. The value of k can be obtained from the intersection point of the Meyer-line with (d/D) = 1, and n
245
represents the slope of the straight line; n depends somewhat on the load interval used for hardness testing, even at high correlations (r > 0.99). For all measured points, 0.2 < (d/D) C 0.7 should be held; therefore the smaller loads had to be increased at higher hardness values. The Brine11 hardness number (BHN) was evaluated for F(N) X 0. 102/D2 = 30. The value of n represents a quantity (related to the true uniform strain) which characterizes ductility. However, necking may be of greater importance in some cases. Hardness is usually correlated with tensile strength or yield stress [ 131, respectively. 2.3.2. X-ray testing The X-ray investigations were undertaken with a diffractometer. The interplanar spacings and lattice parameters of the alloys were determined and the hardness and lattice parameter were correlated. The lattice contraction of Inconel 625 caused by ageing was measured and the intensities of the diffractometer traces after cold working and their changes after ageing were also measured. 2.3.3. MetalIagrap~y The met~lograp~c investigation of Inconel625 and Nimonic 86, as received, solution annealed, cold drawn, and aged was undertaken light-optically, whereas Incone1 625, as received, was also investigated electronoptically and with the microprobe-analyser.
3. Results 3.1. Hardness and lattice parameter
A:
as
received
El: lO%C.W.
2
c
I
: 2owoc.w.
D:40%c.w.
a
aged,
b
100
c
600%
‘%e correlation of hardness with the lattice parameters of very pure and pure nickel, Nionic 86, and Inconel 625 is shown in fig. 2. Hardness increases with increasing lattice parameter (determined from (3 11) with CoK,). This correlation is only valid for solution treated material, where solution strengthening determines the hardness. The lattice shrinks with ageing and the hardness is then dependent upon the precipitation hardening.
days,
d:
Fig. I. Meyer’s law for differently treated Inconel 625.
3.2. Lattice parameter traces after ageing
and intensities
of diffractometer
The changes in the lattice parameters of Inconel625 after ageing at 750°C is shown in table 2. Ageing results in a lattice contraction and this process is favoured by cold working. The lattice shrinkage is probably a conse-
II. K. Kohl, K. Peng / Thermul srahi1it.vof Itlconel 625 crnd Nimonic 86
246
I
I
3.500 50
150
100 BHN.
Fig. 2. Lattice
texture and lattice distortion are reduced, due to recovery, as can be seen in table 3. Micrographs of Inconel 625 are presented in fig. 3. Inconel 625, as received, is shown in fig. 3(a) and in fig. 3(b) the micrograph of 20% cold worked and aged material is shown. In fig. 3(a) coarser primary carbides are present as well as finer grain boundary carbides. The coarser carbides are of the type MC and include niobium and titanium (from the microprobe analysis). The grain boundary carbides are of the type M,,C, and M,C; they contain molybdenum in addition to niobium and silicon. Primary carbides, grain boundaries, and the needle-shaped inter-metallic compound Ni,(Nb,Mo) are visible in fig. 3(b). No analogous precipitations were detected in Nimonic 86 after ageing by light-microscopy. After solution annealing and fast quenching, carbide precipitation was suppressed and twinning was in evidence.
parameter
1130
and
or
200
250
2 S/187.5
hardness
of nickel
and
nickel
alloys.
3.3. Hardness
quence of the generation of precipitates whereby atoms in solid solution form new phases and leave the lattice. It was attempted to characterize the state of the material after ageing from the size and shape of the diffractometer traces. The height of the traces is mainly influenced by the texture of the material and the width through lattice distortion. The product of the half-value of the width times the height of the trace was taken as its intensity. The intensities of the (11 I)-traces of Incone1 625, aged at 750°C are presented in table 3. The trace intensity increases with cold working because of texture formation and lattice distortion. During ageing,
Table 2 Lattice parameters
(A) of Inconel
Deformation (%)
Deformati
0 IO 20 40
of Inconcl
By plotting hardness against temperature, with ageing time as a parameter, diagrams showing the range of precipitation hardening were obtained. Cold worked material has a higher initial hardness and, during ageing, recovery takes place. The temperature ranges for precipitation hardening and recovery can more or less overlap. Hardness is plotted against temperature for Inconel 625, as received, and 40% cold worked in fig. 4. In the as received material only precipitation hardening takes
and cold worked)
after annealing
at 750°C
(determined
from (3
I 1) with CoK,)
Annealing time (days)
-?I IO 20 $0
Table 3 I I I-intensities
625 (as received
after ageing
0
0.3
I
3
IO
30
3.601, 3.602 ., 3.602, 3.602,
3.602, 3.600, 3.599, 3.591,
3.601, 3.598, 3.598, 3.597,
3.599, 3.600, 3.597, 3.596,
3.599, 3.598, 3.596, 3.593,
3.587, 3.596, 3.593, 3.5908
625 (as received and cold worked) Annealing
after annealing
at 750°C (CoK,)
time (days)
(So) 0
0.3
1
100 133 204 3x3
x5 I40 194 37x
YX 142 203 314
3
IO
30
xx
x3
X3
I30
I29
I I4
IO4 371
I YO 355
17.5 ZXX
H. K. Kohl, K. Peng / Thermal siuhility of Inconel6.25 and Nimonic 86
Fig. 3. (a) Inconel 625, as received (electron back scattering). etched with IO g oxalic acid in 100 ml water, 3 V/20 s).
(b) Inconel
place. In the cold worked material two processes, recovery and ageing, occur together depending on temperature. The results of Nimonic 86, as received, and 20% cold worked are shown in fig. 5. The range of precipitation hardening in the unformed material is rather small, but is enlarged in cold worked material after 100 days. Age
*
625, 20% cold worked,
s ; 4 300. z as
received
250.
Temperature Fig. 4. Hardness
of Inconel
625, as received,
(%I
and 40% cold worked,
(electrolytically
hardening in cold worked material begins after shorter times than in material without cold working. In comparison with Inconel625, the two processes, age hardening and recovery, in Nimonic 86 overlap to a lesser extent, since the temperature range for precipitation hardening of Nimonic 86 lies at lower temperatures, i.e. between 500 and 600°C.
350.
s
30 days at 750°C
after ageing.
248
H.K. Kohl. K. Peng / Thermul stability of Inconel 625 and Ntmmic
Nimonic
86
86
350
Temperature Fig. 5. Hardness
At short dominant.
Inconel625
of Nimonic
annealing The
86, as received.
times (<
recovery
and 20% cold worked,
10 h) recovery
is more
of 20% cold
worked 86 at various temperatures are
kinetics
and Nimonic
(“cl _ after ageing.
presented in fig. 6. The fraction of hardness increment which remains after recovery is represented by (1 - R): R=
D
c : D ; Jz 1_
BHN_
- BHN, ’
where BHNCw is the Brine11 hardness number after deformation, BHN, is the,recovered hardness and BHN, is the hardness of the as received material. The recovery process in Inconel 625 is seen to be faster than that in Nimonic 86. The exponents n from eqs. (1) and (2) are plotted against temperature in fig. 7. For every ageing temperature, a mean value with standard deviation is given for annealing times from 0.3 to 100 days. There are differences between n-values due to cold working. Generally, n increases slightly at ageing temperature, but it is almost independent of any hardness increase.
.4
lnconel 625 20%cold worked
.a
BHNCw - BHN,
3.4. Hardness as a function and ageing treatment 2
3
4
5 Tim*
6
7
8
9
10
I h)
Fig. 6. Hardness recovery kinetics in 20% cold worked 625 and Nimonic 86 at various temperatures.
Inconel
of cooling time after solution
The superalloys Inconel 625 and Nimonic 86, after the usual solution treatment, represent supersaturated solid solutions and are therefore not in thermodynamic equilibrium. The hardness is plotted against the cooling
H.K. Kohl, K. Peng / Therm& srubiliility of Inconelb25
and Nimonic 86
249
time after solution and ageing treatment in fig. 8. The approach to eq~~b~~ begins in Inconel 625 and Nimonic 86 at cooling times of about 500 to 1000 h, which is indicated by the increase of hardness due to the formation of the corresponding phases. No difference in hardness was measured after the solution treatment over a wide range of cooling times (m I5 s to= 1O’h). A dependence of hardness upon cooling time was found after the ageing treatment (Inconel 625 : 6OO*C, 10 and 42 days; Nimonic 86: SOT, 10 and 42 days). At short cooling times age hardening is retarded or suppressed in both alloys.
4. Discussion and 42om&sions
Fig. 7. Exponent temperature.
Coaling
n From eqs. (I) and (2) as a function of
time
from
1200%
io room
tomparat~ga
(h)
loo
Fig. 8. Hardness of Iaconel625 and Nimonic 86 as a function of cooling time after solution and ageing treatment.
lnconel625 and Nimonic 86 show similar behaviour after solution and ageing treatment. Age hardening was found between about 550 and 850°C for Inconel 625 and between 500 and 6OO’C for Nimonic 86; these temperature ranges were somewhat more expanded with cold working, to about 450-700°C in the case of Nimonic 86. Age hardening begins in cold worked material after a shorter time, i.e. cold working enhances the ageing process. It can be supposed that nucleation in cold worked material is favoured, and that this is responsible for faster formation of the precipitates (or even perhaps zones). It is known that the cooling rate after solution annealing has no essential influence on mechanical properties. This paper shows there does however exist an influence on the subsequent age hardening for Inconel 625 and Nimonic 86. Precipitation hardening occurs as a consequence of the approach to a more equably condition. Precipitation hardening therefore cannot entirely be suppressed. A fast quench retards the ageing process; in Nimonic 86 the effect is somewhat stronger than in Inconel625. Fast quenched material contains a higher excess vacancy ~on~n~ation than slower cooled material [ 141. The supposition in this paper is that during reheating for ageing and at ageing temperature the entities responsible for nucleation of hardening phases react with the excess vacancies and that nucleation and growth of h~de~ng phases are therefore retarded or suppressed. The cooling rate effect after the solution treatment represents a possibility to influence the age hardening of Inconel 625 and Nimonic 86. Cold working and fast quenching have the opposite effects.
H. K. Kohl, K. Peng / Thermal stahilily
250
Acknowledgement The authors wish to express their thanks to Dr. P. Tipping for discussions and checking the English text, and Mr. E. Guyer for the computer evaluations of the measurements.
References 111 F. GarzaroBi, VI 131 141
151
A. Gerscha and K.P. Francke, Z. Metallk. 60 (I 969) 643. H. Bohm, K. Ehrhch and K.-H. Kramer, Metal1 24 (1970) 139. E. Schnabel, H.-J. Schtiler und P. Schwab, Prakt. Metallogr. 8 (1971) 521. H.J. Blumenkamp, H. Hoven, K. Koizlik and H. Nickel, Optimierung der Phasentrennung mittels Kontrastierkammer fur hochwarmfeste metallische Legierungen, Jul.1654 (April 1980) p. 51. O.F. Kimball, W.R. Pieren and D.I. Roberts, Effects of Elevated-Temperature Ageing on the Mechanical Proper-
of Inconel625und Nimonic X6 ties and Ductility of Ni-CrMO-Cb Alloy 625, GulfGA-A 12683 (Oct. 1973). (Karlsruhe, 1978) [61 W. Dienst, Hochtemperaturwerkstoffe p. 58. Ductility and Toughness Considerations 171 J.P. Hammond, in Elevated Temperature Service, Winter Annual Meeting of the ASME, San Francisco (1978) p. 63. Bl R.F. Bunshah, ed., Techniques of Metals Research, Measurements of Mechanical Properties, Vol. 5, Part 2 (New York, 1971) p. 165. [91 J.H. Westbrook and H. Conrad, The Science of Hardness Testing and its Research Applications (Am. Sot. Metals Ohio, 1973) p. 102. [lOI W. Dahl and H. Rees, Die Spannungs-Dehnungs-Kurve von Stahl (Dusseldorf, 1976) p. 89. K.H. Schriider, Z. Werkstofftech. I I (1980) 73. G.E. Dieter, Jr., Mechanical Metallurgy (New York, 1961) p. 285. J.R. Cahoon, W.H. Broughton and A.R. Kutzak, Met. Trans. 2 (1971) 1979. R.E. Smallman, Modem Physical Metallurgy (London, 1976) p. 425.
243
THERMAL STABILITY OF THE SUPERALLOYS INCONEL 625 AND NIMONIC 86 H.K. KOHL
and K. PENG
*
Swiss Federal Institute fur Reactor Research, Wiirenlingen, Switzerlund Received I June 1981; in revised form 2 July 198I The thermal stability of Inconel 625 and Nimonic 86, as received, cold worked (IO, 20, and 40%). and solution treated, was investigated in the temperature range 500-900°C. The annealing times varied from 0.3 (0.03) to 100 days. Precipitation hardening and recovery (recrystallisation) takes place in cold worked material, beginning after shorter times in cold worked material than in as received material. The temperature interval for precipitation hardening is extended in Nimonic 86, due to cold working, from about 500-600°C to about 450-7OO’C. It is possible to suppress or retard the precipitation hardening in solution treated Inconel625 and Nimonic 86 by fast cooling after solution annealing. Hardness was measured at room temperature with five different loads, so that the parameters k and n from Meyer’s_law, and the Brine11hardness number (for F+ 0’ = 30) could be determined. The lattice tontraction of Inconel625 due to ageing was investigated with X-ray measurements. The change of intensities of the diffractometer traces due to recovery was also determined.
1. Introduction Superalloys are used and contemplated in nuclear engineering in HTRs and fusion reactors, e.g. for hot gas pipes, heat exchangers, control rods or the vacuum vessel, respectively. It has been known for about ten years that Inconel625 shows precipitation hardening at higher temperature after annealing over periods of months as a result of developing phases, which are precipitated in the temperature range of -(700 “r 15O)‘C [l-4]. Generally, the first precipitates, found in the solution treated alloy, are carbides, and different carbide phases are generated, until the origination of intermetallic compounds is favoured. These also follow a certain sequence, and are mainly responsible for changes in hardness. Nimonic 86 is the same type of alloy as Inconel625, but it is not as complex. Both are solid solution strengthened alloys. Inconel 625 was considered as unsuitable for load-bearing applications in elevated temperature regions of the HTGR (51 due to precipitation hardening. In the present investigation, the precipitation (age) hardening of Inconel625 was examined once again and the influences of cold working and cooling time after the solution treatment on the precipitation hardening were investigated. * K. Peng is now retired. 0022-3 115/8 1/OOOO-0000/$02.75
0 198 1 North-Holland
The examination of a high temperature alloy at room temperature is no contradiction, because a high temperature alloy must, for certain applications, have adequate mechanical properties at room temperature as well. The loss in ductility at room temperature together with the precipitation hardening is of importance for the superalloys under consideration. Room temperature embrittlement and reduction of creep strength at higher temperatures is not a consequence of the precipitates, their morphology and distribution, but rather a consequence of the alloy dilution adjacent to the precipitates [6]. Accordingly, a maximum in ductility for superalloys was measured using hot tensile tests after ageing in the temperature range where precipitations were found [7].
2. Experimental prixdures 2. I. Materials The manufacturer, the as received status, the cast analysis, and mechanical test results of the investigated materials are presented in table 1. Bars of cross section 30 X 20 mm2 or 20 X 20 mm* were cut off the plates in the rolling direction. Round bat> were then fabricated with 19 mm diameter. Part of these bars were cold drawn to diameters of 18, 17, and 15 mm, respectively, which represented
H.K. Kohl. K. Peng / Thermal stuhrhty of Inconel625 und Nlmonic X6
244 Table I Cast analysis
(wt%) of the investigated
materials
and mechanical
test results Nimonic
Inconel625
86
Manufacturer
Stellite Div., Cabot Corp. Indiana,
USA
Henry Wiggin and Co. Ltd., Hereford,
As received
plate, 30 mm, hot rolled, annealed
and pickled
plate, 20 mm, hot rolled, annealed (3 min 115O”C, water quenched) and pickled
C Si Mn Cf Ni Fe MO co Al Ti Nb+Ta P S CU
0.070 0.33 0.29 20.84 bal. 3.82 8.89 0.25 0.32 0.30 3.60 0.009 <0.002
Mg Zl Ce Pb B Bi Ag 0.2 yield stress, MPa Tensile strength, MPa Hardness, HV Elongation, % Reduction of area, %
503 889 46.0 50.0
degrees of deformation of about 10,20, and 40% (degree of deformation (W) = (F, - F,). 100/F,, where F. and F, represent the cross sections before and after cold working). Discs of 5 mm thickness were separated from the rods using electro-erosion. Each disc face was fine ground and polished. The specimens were enclosed in evacuated quartz ampoules for the heat treatments.
England
0.032 0.28 0.06 24.85 bal. 1.46 10.92 0.38 0.05 0.02
0.005 0.08 0.019 to.010 0.035 0.7 ppm < 20 ppm CO.1 ppm to.1 ppm 349 750 221/232 58.4
1200°C and Nimonic 86 at 1150°C. The specimens were brought to solution temperature in 1 h then held 1 h at solution temperature, and then afterwards cooled to room temperature at different times (4 X 10 -3 to > IO3 h). The heating and cooling sequences were linear because an electronically controlled device was used. 2.3. Experiments
2.2. Thermal treatments Ageing was done at temperatures of 500, 550, 600, 750, 800, and 900°C. The temperature variations were less than f 1%. The ageing times were 0.3, 1, 3, 10, 30, 60, and 100 days. At 800 and 900°C shorter times were also used, i.e. 0.03 and 0.1 days. Solution treated specimens were aged in addition to the specimens in the as received status and the cold worked specimens. Inconel625 was solution annealed at
2.3.1. Hardness testing The disc specimens were hardness tested on the polished face, i.e. the test direction was the same as the hot rolling direction of the original plates and also the drawing direction of the rods during the cold working sequences. Higher hardness values were measured at the rolling surface of the plate of Nimonic 86. This was not observed for Inconel625. The hardness test was performed using five different
H. K. Kohl, K. Peng / Thermal stability of fnconel62S and Nimonic 86
loads from 490 to 2450N. This allowed the determination of the parameters for Meyer’s law [8,9] F= k(d/D)“,
(1)
where F represents the load in N; k, the load which would be necessary to press a sphere down to its equator into a material (if (d/D) = 1, then F= k); d is the diameter of the indentation; and D is the diameter of the spherical indenter (= 2.5 mm). The exponent n is related to the strain hardening exponent m of the Ludwik equation, u = kl&” [lo,1 I], according to [ 121 n-m++.
(2)
Different loads must give geometrically dissimilar indentations (this is only the case with a spherical indenter), so that n-values greater than 2 are obtained. In fig. 1 the load F is plotted logarithmically against (d/D) (which is also plotted on a logarithmic scale) for Inconel 625 for different treatments, without and after ageing. The value of k can be obtained from the intersection point of the Meyer-line with (d/D) = 1, and n
245
represents the slope of the straight line; n depends somewhat on the load interval used for hardness testing, even at high correlations (r > 0.99). For all measured points, 0.2 < (d/D) C 0.7 should be held; therefore the smaller loads had to be increased at higher hardness values. The Brine11 hardness number (BHN) was evaluated for F(N) X 0. 102/D2 = 30. The value of n represents a quantity (related to the true uniform strain) which characterizes ductility. However, necking may be of greater importance in some cases. Hardness is usually correlated with tensile strength or yield stress [ 131, respectively. 2.3.2. X-ray testing The X-ray investigations were undertaken with a diffractometer. The interplanar spacings and lattice parameters of the alloys were determined and the hardness and lattice parameter were correlated. The lattice contraction of Inconel 625 caused by ageing was measured and the intensities of the diffractometer traces after cold working and their changes after ageing were also measured. 2.3.3. MetalIagrap~y The met~lograp~c investigation of Inconel625 and Nimonic 86, as received, solution annealed, cold drawn, and aged was undertaken light-optically, whereas Incone1 625, as received, was also investigated electronoptically and with the microprobe-analyser.
3. Results 3.1. Hardness and lattice parameter
A:
as
received
El: lO%C.W.
2
c
I
: 2owoc.w.
D:40%c.w.
a
aged,
b
100
c
600%
‘%e correlation of hardness with the lattice parameters of very pure and pure nickel, Nionic 86, and Inconel 625 is shown in fig. 2. Hardness increases with increasing lattice parameter (determined from (3 11) with CoK,). This correlation is only valid for solution treated material, where solution strengthening determines the hardness. The lattice shrinks with ageing and the hardness is then dependent upon the precipitation hardening.
days,
d:
Fig. I. Meyer’s law for differently treated Inconel 625.
3.2. Lattice parameter traces after ageing
and intensities
of diffractometer
The changes in the lattice parameters of Inconel625 after ageing at 750°C is shown in table 2. Ageing results in a lattice contraction and this process is favoured by cold working. The lattice shrinkage is probably a conse-
II. K. Kohl, K. Peng / Thermul srahi1it.vof Itlconel 625 crnd Nimonic 86
246
I
I
3.500 50
150
100 BHN.
Fig. 2. Lattice
texture and lattice distortion are reduced, due to recovery, as can be seen in table 3. Micrographs of Inconel 625 are presented in fig. 3. Inconel 625, as received, is shown in fig. 3(a) and in fig. 3(b) the micrograph of 20% cold worked and aged material is shown. In fig. 3(a) coarser primary carbides are present as well as finer grain boundary carbides. The coarser carbides are of the type MC and include niobium and titanium (from the microprobe analysis). The grain boundary carbides are of the type M,,C, and M,C; they contain molybdenum in addition to niobium and silicon. Primary carbides, grain boundaries, and the needle-shaped inter-metallic compound Ni,(Nb,Mo) are visible in fig. 3(b). No analogous precipitations were detected in Nimonic 86 after ageing by light-microscopy. After solution annealing and fast quenching, carbide precipitation was suppressed and twinning was in evidence.
parameter
1130
and
or
200
250
2 S/187.5
hardness
of nickel
and
nickel
alloys.
3.3. Hardness
quence of the generation of precipitates whereby atoms in solid solution form new phases and leave the lattice. It was attempted to characterize the state of the material after ageing from the size and shape of the diffractometer traces. The height of the traces is mainly influenced by the texture of the material and the width through lattice distortion. The product of the half-value of the width times the height of the trace was taken as its intensity. The intensities of the (11 I)-traces of Incone1 625, aged at 750°C are presented in table 3. The trace intensity increases with cold working because of texture formation and lattice distortion. During ageing,
Table 2 Lattice parameters
(A) of Inconel
Deformation (%)
Deformati
0 IO 20 40
of Inconcl
By plotting hardness against temperature, with ageing time as a parameter, diagrams showing the range of precipitation hardening were obtained. Cold worked material has a higher initial hardness and, during ageing, recovery takes place. The temperature ranges for precipitation hardening and recovery can more or less overlap. Hardness is plotted against temperature for Inconel 625, as received, and 40% cold worked in fig. 4. In the as received material only precipitation hardening takes
and cold worked)
after annealing
at 750°C
(determined
from (3
I 1) with CoK,)
Annealing time (days)
-?I IO 20 $0
Table 3 I I I-intensities
625 (as received
after ageing
0
0.3
I
3
IO
30
3.601, 3.602 ., 3.602, 3.602,
3.602, 3.600, 3.599, 3.591,
3.601, 3.598, 3.598, 3.597,
3.599, 3.600, 3.597, 3.596,
3.599, 3.598, 3.596, 3.593,
3.587, 3.596, 3.593, 3.5908
625 (as received and cold worked) Annealing
after annealing
at 750°C (CoK,)
time (days)
(So) 0
0.3
1
100 133 204 3x3
x5 I40 194 37x
YX 142 203 314
3
IO
30
xx
x3
X3
I30
I29
I I4
IO4 371
I YO 355
17.5 ZXX
H. K. Kohl, K. Peng / Thermal siuhility of Inconel6.25 and Nimonic 86
Fig. 3. (a) Inconel 625, as received (electron back scattering). etched with IO g oxalic acid in 100 ml water, 3 V/20 s).
(b) Inconel
place. In the cold worked material two processes, recovery and ageing, occur together depending on temperature. The results of Nimonic 86, as received, and 20% cold worked are shown in fig. 5. The range of precipitation hardening in the unformed material is rather small, but is enlarged in cold worked material after 100 days. Age
*
625, 20% cold worked,
s ; 4 300. z as
received
250.
Temperature Fig. 4. Hardness
of Inconel
625, as received,
(%I
and 40% cold worked,
(electrolytically
hardening in cold worked material begins after shorter times than in material without cold working. In comparison with Inconel625, the two processes, age hardening and recovery, in Nimonic 86 overlap to a lesser extent, since the temperature range for precipitation hardening of Nimonic 86 lies at lower temperatures, i.e. between 500 and 600°C.
350.
s
30 days at 750°C
after ageing.
248
H.K. Kohl. K. Peng / Thermul stability of Inconel 625 and Ntmmic
Nimonic
86
86
350
Temperature Fig. 5. Hardness
At short dominant.
Inconel625
of Nimonic
annealing The
86, as received.
times (<
recovery
and 20% cold worked,
10 h) recovery
is more
of 20% cold
worked 86 at various temperatures are
kinetics
and Nimonic
(“cl _ after ageing.
presented in fig. 6. The fraction of hardness increment which remains after recovery is represented by (1 - R): R=
D
c : D ; Jz 1_
BHN_
- BHN, ’
where BHNCw is the Brine11 hardness number after deformation, BHN, is the,recovered hardness and BHN, is the hardness of the as received material. The recovery process in Inconel 625 is seen to be faster than that in Nimonic 86. The exponents n from eqs. (1) and (2) are plotted against temperature in fig. 7. For every ageing temperature, a mean value with standard deviation is given for annealing times from 0.3 to 100 days. There are differences between n-values due to cold working. Generally, n increases slightly at ageing temperature, but it is almost independent of any hardness increase.
.4
lnconel 625 20%cold worked
.a
BHNCw - BHN,
3.4. Hardness as a function and ageing treatment 2
3
4
5 Tim*
6
7
8
9
10
I h)
Fig. 6. Hardness recovery kinetics in 20% cold worked 625 and Nimonic 86 at various temperatures.
Inconel
of cooling time after solution
The superalloys Inconel 625 and Nimonic 86, after the usual solution treatment, represent supersaturated solid solutions and are therefore not in thermodynamic equilibrium. The hardness is plotted against the cooling
H.K. Kohl, K. Peng / Therm& srubiliility of Inconelb25
and Nimonic 86
249
time after solution and ageing treatment in fig. 8. The approach to eq~~b~~ begins in Inconel 625 and Nimonic 86 at cooling times of about 500 to 1000 h, which is indicated by the increase of hardness due to the formation of the corresponding phases. No difference in hardness was measured after the solution treatment over a wide range of cooling times (m I5 s to= 1O’h). A dependence of hardness upon cooling time was found after the ageing treatment (Inconel 625 : 6OO*C, 10 and 42 days; Nimonic 86: SOT, 10 and 42 days). At short cooling times age hardening is retarded or suppressed in both alloys.
4. Discussion and 42om&sions
Fig. 7. Exponent temperature.
Coaling
n From eqs. (I) and (2) as a function of
time
from
1200%
io room
tomparat~ga
(h)
loo
Fig. 8. Hardness of Iaconel625 and Nimonic 86 as a function of cooling time after solution and ageing treatment.
lnconel625 and Nimonic 86 show similar behaviour after solution and ageing treatment. Age hardening was found between about 550 and 850°C for Inconel 625 and between 500 and 6OO’C for Nimonic 86; these temperature ranges were somewhat more expanded with cold working, to about 450-700°C in the case of Nimonic 86. Age hardening begins in cold worked material after a shorter time, i.e. cold working enhances the ageing process. It can be supposed that nucleation in cold worked material is favoured, and that this is responsible for faster formation of the precipitates (or even perhaps zones). It is known that the cooling rate after solution annealing has no essential influence on mechanical properties. This paper shows there does however exist an influence on the subsequent age hardening for Inconel 625 and Nimonic 86. Precipitation hardening occurs as a consequence of the approach to a more equably condition. Precipitation hardening therefore cannot entirely be suppressed. A fast quench retards the ageing process; in Nimonic 86 the effect is somewhat stronger than in Inconel625. Fast quenched material contains a higher excess vacancy ~on~n~ation than slower cooled material [ 141. The supposition in this paper is that during reheating for ageing and at ageing temperature the entities responsible for nucleation of hardening phases react with the excess vacancies and that nucleation and growth of h~de~ng phases are therefore retarded or suppressed. The cooling rate effect after the solution treatment represents a possibility to influence the age hardening of Inconel 625 and Nimonic 86. Cold working and fast quenching have the opposite effects.
H. K. Kohl, K. Peng / Thermal stahilily
250
Acknowledgement The authors wish to express their thanks to Dr. P. Tipping for discussions and checking the English text, and Mr. E. Guyer for the computer evaluations of the measurements.
References 111 F. GarzaroBi, VI 131 141
151
A. Gerscha and K.P. Francke, Z. Metallk. 60 (I 969) 643. H. Bohm, K. Ehrhch and K.-H. Kramer, Metal1 24 (1970) 139. E. Schnabel, H.-J. Schtiler und P. Schwab, Prakt. Metallogr. 8 (1971) 521. H.J. Blumenkamp, H. Hoven, K. Koizlik and H. Nickel, Optimierung der Phasentrennung mittels Kontrastierkammer fur hochwarmfeste metallische Legierungen, Jul.1654 (April 1980) p. 51. O.F. Kimball, W.R. Pieren and D.I. Roberts, Effects of Elevated-Temperature Ageing on the Mechanical Proper-
of Inconel625und Nimonic X6 ties and Ductility of Ni-CrMO-Cb Alloy 625, GulfGA-A 12683 (Oct. 1973). (Karlsruhe, 1978) [61 W. Dienst, Hochtemperaturwerkstoffe p. 58. Ductility and Toughness Considerations 171 J.P. Hammond, in Elevated Temperature Service, Winter Annual Meeting of the ASME, San Francisco (1978) p. 63. Bl R.F. Bunshah, ed., Techniques of Metals Research, Measurements of Mechanical Properties, Vol. 5, Part 2 (New York, 1971) p. 165. [91 J.H. Westbrook and H. Conrad, The Science of Hardness Testing and its Research Applications (Am. Sot. Metals Ohio, 1973) p. 102. [lOI W. Dahl and H. Rees, Die Spannungs-Dehnungs-Kurve von Stahl (Dusseldorf, 1976) p. 89. K.H. Schriider, Z. Werkstofftech. I I (1980) 73. G.E. Dieter, Jr., Mechanical Metallurgy (New York, 1961) p. 285. J.R. Cahoon, W.H. Broughton and A.R. Kutzak, Met. Trans. 2 (1971) 1979. R.E. Smallman, Modem Physical Metallurgy (London, 1976) p. 425.