Extra Low Carbon Alloy 718 - TMS

EXTRA LOW CARBON ALLOY 718

J. M. Moyer

Technology Department Teledyne Allvac

Monroe, North Carolina 28110 USA

Summary

The effects of carbon content, magnesium add itions, thermomechan processing, and heat treatment on the mechanical properties of Allvac

ical 718

have been investigated, both in the laboratory and with production heats. The results show that lowering the carbon content below 0.010% produces mechanical properties similar to those measured in heats containing more typical carbon contents in the range of 0.030 to 0.040%. Contrary to data published previously, stress-rupture properties are not degraded by carbon contents below 0.010% if the alloy contains a suitable addition of magnesium. Lowering the carbon below approximately 0.010% also reduces significantly both the total number of carbide particles and their tendency to be present as stringers. As a consequence of their improved carbide distributions, extra low carbon versions of Allvac 718 are expected to display improved low cycle fatigue performances compared to conventional 718.

443

Introduction

Superalloys selected for rotating parts in jet engines are being subjected to increasingly severe operating conditions as applied stresses continue to rise. As a result, producers of engines are becoming more concerned about premature failures caused by low cycle fatigue (LCF) nucleating at oxide, nitride, and carbide particles. In response to demands for the cleanest possible alloys, Teledyne Allvac has been exploring many avenues in order to reduce both the frequency and size of the particles normally found in superalloys. Some of these approaches include: careful selection of all refractories that come in contact with the liquid alloy, use of high purity raw materials, liquid metal filtration using porous refractory filters, multiple remelting, electroslag remelting, and electron beam remelting. Improving LCF life by removing or preventing the formation of oxides has recently been the subject of a number of papers appearing in the literature as well (l-4). Once effective solutions to the problem of oxides have been identified and successfully implemented, the problem of reducing nitrides and carbides can be expected to receive increasing attention.

Published literature from some years ago (5,6) and conventional wisdom have suggested the need for a minimum carbon content in superalloys to achieve an adequate balance of mechanical properties. Stroup and Pugliese (5,6) showed, for a variety of superalloys, that although strength at room temperature increased, the stress-rupture life and stress-rupture elongation both dropped appreciably, and the tendency for notch sensitivity increased dramatically as the carbon content was lowered. For the 718 alloy used in that investigation, the degredation of stress-rupture properties began as the carbon was lowered below 0.057%. These results suggested that it may not be possible to lower the carbon content of superalloys without having an adverse effect on stress-rupture properties. The work described in this paper was conducted to re-examine the effect of lowered carbon contents on the mechanical properties of Allvac 718 in light of the expected improvements in LCF life brought about by a reduction in the amount of and an improvement in the distribution of carbide particles.

Procedure

First Series of Laboratory Heats

A series of five heats of Allvac 718 was designed with carbon contents ranging from the high side of normal commercial practice (0.060%) to as low as possible (0.006%). One hundred fifty pound heats were vacuum induction melted and cast into electrodes four inches in diameter, vacuum arc remelted to six inch diameter ingots, homogenized, and hot rolled to two inch round cornered square bars. Chemical analyses of samples cut from the two inch bars are listed in Table I. Longitudinal metallographic samples were also cut from the bars. Samples for mechanical testing were produced by forging two inch cubes into "pancakes" 518 inch thick. Upset forging (hammer motion parallel to previous rolling direction) and transverse forging (hammer motion perpendicular to previous rolling direction) were employed. Blanks for mechanical test specimens were cut from all upset forged and transverse forged pancakes so that the longitudinal axis of each test specimen was perpendicular to the previous rolling direction. The pancakes were also forged using two different types of thermomechanical processing, the type of processing being correlated with the heat treatment given subsequently to samples used for the mechanical tests. Samples to be solution treated plus aged (STA) were cut from pancakes that had been forged from 1900 to 1720'F. They were then solution treated at 1750°F for one hour and cooled in air, followed by an aging treatment at 1325'F for eight hours

444

Table I. Chemical Analyses of Experimental Allvac 718 Heats (Wt. X)

Element

C Ni Fe CT Cb MO Ti Al

B o* N"

Ca * Mg *

*PP*

First Series -

5961 5962 5963 5964 5965 -- __ ~ ~ -

0..006 0.012 0.023 52.5 52.9 53.0 18.6 18.8 18.8 18.3 18.5 18.8

4 . 9 4.9 4.9 2 .8 2.9 2.9 0.92 0.92 0.96 IO.49 0.52 0.53

0.002 0.001 0.001 0.002 0.002 0.003 0.002 0.001 0.001

12 32 8 33 27 37 21 15 24 13 14 16

0.040 53.2 18.8 18.7

4.9 2.9 0.94 0.58

0.001 0.003 0.001

24 18 17

0.060 53.2 18.8 18.7

5.0 2.9 0.95 0.59

0.001 0.003 0.001

10 33

6 19

Second Series

A077 A079 A078 A064 ___---

0.006 0.032 0.003 0.038 -- 52.9 52.9 52.7

18.8 18.8 18.8 18.7 18.3 18.5 18.6 18.3

4.8 4.9 5.0 5.0 2.8 2.9 2.9 2.9 0.93 0.95 0.94 0.95 0.55 0.58 0.52 0.55

0.003 0.002 0.001 0.001 0.002 0.003 0.002 0.002 0.001 0.001 0.001 0.001

-- -- -- --

12 7 12 14

15 L!L.-3 Composite Composite Bars 3&4 Bars l&2

and cooling to 1150°F for an additional eight hours. Samples that were to be directly aged (DA:) were cut from pancakes that had been forged between 1850 and 1725°F to an intermediate thickness of one inch, reheated to 1800"F, and re- forged to a thickness of 518 inch between 1800 and 1440°F. They were then aged at 1325 and 1150"1?, the same as were the STA samples. Fully heat treated sample blanks were machined to 0.252 inch diameter specimens for tensile and stress-rupture testing. In addition, blanks were also submitted to an outside lab for analysis of the phases present.

Second Series of Laboratorv Heats

In a second ,phase of the program, four additional 150 pound heats of Allvac 718 were produced and rolled to two inch round cornered square bars the same way as those in the first series had been. The chemical compositions of the second series of heats are listed in Table I. This time two heats contained essentially normal amounts of carbon, but the other two contained extra low levels. One normal carbon heat and one reduced carbon heat contained deliberate additions of Pig, while the other two heats did not. Pairs of 12 inch lengths of bar were welded together to produce composite bars 24 inches long. Pour composite bars were assembled as described below:

1. Normal carbon with ?&/Reduced carbon with Kg--STA processing 2. Normal carbon with Mg/Reduced carbon with Mg--DA processing 3. Normal carbon without Mg/P.educed carbon without Mg--STA processing 4. Normal carbon without Mg/Reduced carbon without ?&--DA processing

Utilizing this approach, two alloys differing only with respect to their carbon contents were rolled simultaneously to bars approximately 314 inch in diameter, with both alloys receiving identical thermomechanical processing. The two composite bars that received the STA processing were rolled to 314 inch bar betweel 2025 and 1745"F, cut into convenient lengths, and solution treated and aged as described previously. The two composite bars given the DA processing were rolled between 18#50 and 1560"F, cut to appropriate lengths, and also aged as described previously. After heat treatment, tensile, stress-rupture, and low cycle fatigue tests were conducted.

445

Scale-up to Commercial Product

As a direct result of the promising data obtained from the laboratory program on 150 pound heats, a 15,000 pound, commercial, triple melted (VIM/ESR/ VAR) heat containing 0.010% carbon was made. The 20 inch diameter ingot was converted using standard practices to 10 inch diameter billet, from which cubes two inches on a side were cut and upset forged to 5/8 inch thick pancakes. The same STA and DA processing procedures described for the first series of laboratory heats were employed. Metallographic samples were cut directly from the billet and examined. These same procedures were also used to evaluate identically produced heats containing the normal amount of carbon to provide a basis for comparison.

Results and Discussion

Tensile Properties at Room Temperature

The effects of carbon content on the room temperature tensile properties of Allvac 718 are illustrated in Figures 1 and 2 for upset forged pancakes and in Tables II and III for 314 inch diameter bars. From Figure 1 it is apparent that the strengths of both STA and DA pancakes increase as the carbon level decreases. This observation is in excellent agreement with the data of Stroup and Pugliese (5,6), who argued that as the carbon content increases, more Cb and Ti are tied up as carbides instead of being available for precipitation strengthening. Because of the cold work being retained by the DA material, it comes as no surprise that the DA pancakes exhibit higher strengths than the STA material.

Figure 2 shows there is little effect of carbon on ductility measured at room temperature in either the STA or the DA condition. Eowever, any trend there may be indicates that higher ductilities correspond to lower carbon conten.ts.

Tables II and III cover a more restricted range of carbon than do Figures 1 and 2. Probably because of differences in processing (rolling vs forging), the strengths of the bars tend to be lower and the ductilities tend to be higher for the rolled bars than for the corresponding forged pancakes. Otherwise, however, the same general trends noted for the pancakes are evident with just one exception--Table II shows a slight decrease in strength as the carbon content is lowered in the Kg-containing alloys. Interestingly, Table II also shows there to be lower ductilities in the Mg-bearing heats than in the heats without the Mg addition. Both unexpected results would essentially vanish if the 0.038% carbon heat with Mg would have a slightly lower strength and correspondingly higher ductility, more in line with the balance of properties displayed by the other three heats in Table II. Why this minor discrepancy occurred is unknown.

Tensile Properties at 1200°F I_

Elevated temperature tensile properties are included in Figures 3 and 4, and in Tables IV and V. As was the case for the room temperature tensile test results, decreasing the carbon produces either a beneficial effect on properties or no significant effect at all. This time, however, both Tables IV and V show there to be a significant improvement in ductility (ROA) brought about by the addition of Mg. The strengths do not appear to be affected significantly by Mg.

446

200 200 .- .- In In

s 190 z 190

Solution Treat and Age Solution Treat and Age

I 1

Solution Treat and Age

Q 0

h ~ PA Elongation*

I

f\

Direct Age

./SW s

a

+A------ A Elongation A

I I I 4 I 1

0.02 0.04 0.06 0 0.02 0.04 0.06

Carbon (%) Carbon (%I

Figure 1 - Room temperature strengths Figure 2 - Room temperature ductilities of samples cut from upset forged pan- of samples cut from upset forged pancakes. cakes.

Table II. Room Temperature Tensile Properties--STA Condition -

Composite Bar With Mg Composite Bar Without Mg

CdrbCln UTS YS Elong ROA Carbon UTS Elong ROA (Wt 1:) (ksi) (ksi) (X) (%) (Wt %) (ksi) (ki:) (%) __- __ ~ __ - - - -CL-

0.003 203 166 23 46 0.006 202 164 24 48 204 169 23 45 200 164 26 49

0.038 208 172 21 37 0.032 200 162 24 47 210 173 21 38 202 162 24 48

Table III. Room Temperature Tensile Properties--DA Condition


Carbon UTS YS Elong ROA Carbon UTS YS Elong ROA (Wt Z) (ksi) (ksi) (%) (%) (Wt Z) (ksi) (ksi) pp--o (a ___ __ ~ ____ -

0.003 218 192 24 59 0.006 216 189 23 57 218 194 24 60 217 190 23 57

0.038 213 187 22 47 0.032 209 177 22 51 215 190 22 49 209 175 23 51

Stress-Ruoture Properties p.-L-____----

Stress-rupture properties of Allvac 718 measured on upset forged and trans-

verse Iorged pancakes are presented in Figures 5, 6, and 7 (not all reductions of area were reported). Although the scatter is large, there does not appear to be any trend related to carbon content observed for rupture life or ductility, regardless of the forging practice. This is in sharp contrast to the results of Stroup nnd Pugliese, who reported 3 dramatic drop in rupture life (55.6 to 26.9 hrs) and rupture elongation (14.6 to 2.4X), as well as an increase in notch brittleness for STA 718 tested at 12OO'F and 100 ksi when the carbon content was

lowered from 0.057 to 0.008%.

447

Solution Treat and Age

A Yield A A A A

2

;ii

g 190- “@q; Direct Age

5 z l

cn 160-

5’ Direct Age

160 - .

20

t A-4-4 A Elongation

4

I I I I I I I 1 I I I I I 0 0.02 0.04 0.06 0 0.b2 0.04 0.06

Carbon (36) Carbon (%1

Figure 3 - Elevated temperature strengths of samples cut from upset forged pancakes.

Figure 4 - Elevated temperature ductilities of samples cut from upset forged pancakes.

Table IV. 1200°F Tensile Properties--STA Condition


Carbon UTS (Wt %) (ksi) (kzs)

Elong ROA Carbon UTS YS Elong ROA (8 (Wt %) (ksi) (ksi) 63 m --- (%)- -_____--

0.003 165 139 26 66 0.006 167 142 24 51 167 143 26 64 165 136 23 47

0.038 169 l43 23 58 0.032 166 140 21 34 170 l44 24 58 163 133 23 45

Table V. 1200°F Tensile Properties--DA Condition


Carbon UTS YS Elong ROA Carbon UTS YS Elong ROA (Wt %) (ksi) (ksi) (%) (Wt X) (ksi) (ksi) -(x)- (%I -- -(%)- __-

0.003 182 160 24 67 0.006 180 158 13 17 177 154 22 58 176 153 12 17

0.038 181 157 23 55 0.032 176 152 17 29 180 154 24 61 175 149 18 32

The reason for the difference in results between the present experiments and those of Stroup and Pugliese may be partly explained by the data for composite bars* shown in Tables VI and VII. While supporting the results from the pancakes that there is no clear effect of carbon content on rupture life or ductility (in Mg-containing alloys), these data clearly show a major improvement in ductility (both elongation and reduction of area) due to the presence of a

*The composite bar portion of the investigation was initiated to reduce the scatter in stress-rupture data measured on pancakes. The bars had more uniform grain sizes (ASTM 8-10) than the forged pancakes did (ASTM 6-10).

448

100 ksi Solution Treat and Age

me 110 ksi Direct Age

0 0.02 0.04 0.06 Carbon (%)

r'igure 5 - Stress-rupture lives of Figure 6 - Stress-rupture elongations samples cut from forged pancakes. of samples cut from forged pancakes.

0 0.02 0.04 0.06

Carbon (%I

c I Broke at or wltc” one Diameter 01 Punch

2 -40 c

0 0 Lb. 0 0 g30 e/-, 0 ,---C-l--- -0

P-+-r-7

(0) . 0 0 5 20

z (0)

.- o--o Upset Forged

ZlO *--a Transverse Forged

: t ii - I 100 ksi Solution Treat and Age

I I I I I I I

110 ksi ’ ’ ‘Direct Age

A

0.02 0.04 0.06

Carbon (Xl

small addition of Mg. The only case where an extra low carbon content clearly causes a deterioration in stress-rupture properties is in Table VII for the CSY bar in the DA condition in the composite bar not containing the addition of Mg.

One reason why the stress-rupture ductilities reported by Stroup and Pugliese at 0.008% carbon are so low may be that they performed their work prior to 1967, when additions of magnesium to alloy 718 were not commonplace. As they did not list a magnesium analysis in their papers, it appears likely that their alloys did not contain the critical addition of magnesium that would have improved their stress-rupture ductilities. Other factors that may have contribu- ted to their low properties at 0.008% carbon include: less pure raw mat-

Figure 7 - Stress-ruiJture reductions of erials and/or less sophisticated area in samples cut from forged pancakes. processing in use nearly 29 years

ago, slight differences in chemical

449

Table VI. Stress-Rupture Properties at 1200OF and 100 ksi--STA Condition


Carbon Bar* Life Elong ROA Carbon Bar* Life Elong ROA (Wt %) Type (hrs) (X) (%) (Wt X) Type (hrs) L-0 (Xl - - - - - - - - ~ ~

0.003 s 100 25 57 0.006 S 109 8 11 S 110 33 60 S 113 7 14

CSN ** ** ** CSN 113 15 CSN ** ** ** CSN (158) (9) (E)

0.038 S 58 28 57 0.032 S 114 12 18 S a4 34 59 S 132 12 19

CSN 108 30 57 CSN 155 14 19 CSN 115 30 57 CSN 176 12 17

*S = Smooth. CSN = Combination smooth + notched, but failed in smooth section.

**Material supply exhausted before tests were run. ( ) Sample broke within one diameter of punch.

Table VII. Stress-Rupture Properties at 1200°F and 110 ksi--DA Condition


Carbon Bar* Life Elong ROA Carbon Bar* Life Elong ROA (Wt X) Type (hrs) m (X) wt %) Type ------0 0-j (X) --~--

0.003 s 54 12 32 0.006 S 33 7 a S 37 13 22 S 22 5 10

CSN 58 23 56 CSN 11 Notchbreak CSN 51 13 26 CSN 29 Notchbreak

0.038 S 48 22 36 0.032 S 32 9 14 S 42 30 54 S 28 8 9

CSN 26 22 40 CSN 32 9 16 CSN 41 12 20 CSN 47 13 34

*S = Smooth. CSN = Combination smooth + notched, but failed in smooth section unless indicated otherwise.

composition (the 718 used in the present study contained lower levels of Cr, MO , Cb, B, and usually Ti than that used previously), different solution treatment temperatures (1750°F for this study vs 1800'F for the previous work), and very different thermomechanical processing.

A major conclusion that can be drawn from the present work is that Allvac 718 with a reduced level of carbon need not have reduced stress-rupture properties if the alloy contains Mg.

Structure and Phase Analvsis

Analysis of precipitates in forged pancakes from this investigation performed by Radavich and Kolodziej (7) agrees very well with that done by Stroup and Pugliese (5,6). Both studies showed that the amount of M13Cb increases and CbC decreases as the carbon level of alloy 718 is lowered. However, Stroup and Pugliese attributed the poor stress-rupture properties measured in their 0.008% carbon 718 alloy to the presence of a semi-continuous, brittle network of plate- lets along the grain boundaries that became more pronounced as the carbon level dropped (and the amounts of Ni3Cb and Ni3Ti simultaneously increased). A similar network of iTi3Cb has also been observed in the Mg-containing alloys used in the present investigation, the network becoming more pronounced as the carbon level dropped and the total amount of Ni3Cb increased (7). Consequently, the

450

semi-continuous network, per se, does not seem to account for the poor stress- rupture properties observed earlier at 0.008% carbon.

As mentioned previously, the grain sizes produced in the various samples depended largely on the type of processing employed. Grain sizes in the forged pancakes exhibited the widest variation, from ASTM 6 to 10. Rolled composite bars were produced with both the normal carbon and reduced carbon ends exhibit- ing the same ASTM 8-10 grain size. The only noticeable difference in grain size observed between normal and reduced carbon versions of Allvac 718 resulted from annealing at temperatures above approximately 1850°F. Figure 8 demon- strates that the higher the annealing temperature, the larger the difference in

grain size between reduced carbon (0.003%) and normal carbon (0.03SZ) versions of Allvac 718. Another difference between normal carbon and reduced carbon Allvac 718 is the distribution of carbides. Figure 9 compares the carbide mor- phologies found in two inch square bars rolled from 150 pound heats (photos A-D) containing four dif- erent levels of carbon. Also included in Figure 9 are two photos, taken at a lower magnification, of identically processed commercial heats of Allvac 718. Comparing a11 six photos, one comes to the conclusion that at a carbon level very close to O.OlO%, not only are there relatively few total carbide particles, but their tendency to be present as stringers also disappears.

” 1800 1900 2000 2100 2200

Annealing Temperature (FI

Figure 8 - Effect of annealing for two hours on the grain size of Allvac 718.

Scale-up to Commercial Product I.--

Table VIII lists mechanical properties 718 compared to three identically processed

for a reduced carbon heat of Allvac heats with conventional carbon con-

tents. Eo changes in processing (melting, homogenizing, working, etc.) were made to accommodate the lower carbon content of heat W172-1. Every single

t, W172-1, is at leas t as good as mechanical property for the reduced carbon hea that measured for the normal carbon heats.

Low Cycle Fatigue Tests

Low cycle fatigue tests have been run on both laboratory and production heats, so far without any effect of carboil being revealed. There are several factors to account for the apparent lack of a trend: relatively few tests being conducted to date, tremendous scatter in LCF life data among supposedly- identical samples, and low frequency of failures nucleated at carbides in both normal and reduced carbon samples. Although the improvement in carbide distribution brought about by reducing the carbon content (Figure 9) can be expected to increase low cycle fatigue life in uniformly fine grained material essentially free of large oxide and nitride inclusions that would also serve as LCF initiation sites, this increase has not yet been demonstrated to the knowledge of the author.

451

Figure 9 - Longitudinal carbide distributions in six heats of Allvac 718. Photos A through D are for 150 pound experimental heats J961, 5962, 5964, and 5965, respectively. Photos E and F are for 20 inch ingots sampled at 10 inch diameter billet, heats W172-1 and E538-6, respectively. Note the difference in magnification.

452

Table VIII. Mechanical Properties of Commercial Heats of Allvac 718

Heat Condition

W172-1 STA

E538-1 STA

E542-4 STA

E542-1 STA

W172-1 DA 0.010

E538-1 DA 0.031

E542-4 DA 0.042

E542-1 DA 0.044

7, c

0.010

0.031

0.042

0.044

Tensile

Room Temperature 1200'F

UTS YS Elong ROA UTS YS Elong ROA (ksi) (ksi) (72 o-- (ksi) (ksi) ~ ___ - (x)- (Xl

214 183 18 30 180 155 23 63

216 179 15 22 181 158 17 31

209 176 16 19 176 155 18 35

215 183 16 24 178 160 15 32

229 212 14 32 194 174 18 53

228 212 14 29 194 177 14 32

221 203 14 27 189 171 15 31

229 212 14 29 191 176 12 32

* Smooth bar

Conclusions

*Stress-Rupture

1200"F/120 ksi

Life Elong (hrs) (%I __ -

43 24

48 13

35 20

32 21

167 19

111 12

198 17

211 18

1.

2.

3.

4.

Reducing the (carbon content of alloy 718 to or below 0.010% has no significant deleterious effect on any tensile properties at room temperature or at 1200°F. In most cases, tensile properties appear to be improved by a reduction in carbon content.

Results reported seventeen years ago in the literature showing an adverse effect of reduced carbon content on stress-rupture properties of alloy 718 do not apply to Allvac 718 produced with current technology.

A small addition of rJlg improves stress-rupture ductility markedly. Excel- lent stress-rupture properties can be maintained at very low levels of carbon (at least as low as 0.0031) if the alloy also contains a critical addition of Mg.

A carbon content below approximately 0.010% not only reduces the total amount of carbide compared to that found in conventional 718, but also eliminates the tendency for carbides to be present as stringers. As a consequence, extra low carbon 718 has the potential for enhanced LCF life compared to conventional 718.

Acknowledgements

The author would like to thank E. P. Baugham for supplying the data and photomicrographs for the commercial heats, R. K. Chien for suggestions regarding the form and content of this paper, and R. L. Kennedy for suggesting the pro- ject, guidance, and detailed critiques of the manuscripts.

453

References

1. W. H. Sutton, "Measurement and Control of Nonmetallic Particles in VIM- Melted Superalloys," paper presented at 7th ICVM Meeting, Tokyo, Japan, Nov. 1982.

2. E. E. Erown et al, "The Influence of VIM Crucible Composition, Vacuum Arc Remelting, and Electroslag Remelting on the Non-Metallic Inclusion Content of MERL 76," paper presented at Fourth International Symposium on Super- alloys (Superalloys 1980), Champion, Pa., Sept. 1980.

3. G. A. Vaughn and G. H. Geiger, "Melt-Crucible Interactions and Inclusion Formation in Vacuum Melted Ni-Al-Ti Alloys," paper presented at Sixth International Vacuum Metallurgy Conference on Special Melting, San Diego, Calif., April 1979.

4. J. K. Tien and E. A. Schwarzkopf, "Assessing the Needs for EB Refining of Superalloys," paper presented at Electron Beam Melting and Refining State of the Art 1983, Reno, Nev., Nov. 1983.

5. J. P. Stroup and L. A. Pugliese, "The Influence of Very Low Carbon Contents on the Properties and Structures of Nickel and Nickel-Iron Base Super- alloys," paper presented at 96th AIME Annual Meeting, Los Angeles, California, Feb. 1967.

6. J. F. Stroup and L. A. Pugliese, "How Low-Carbon Contents Affect Super- alloys," Metal Progress, 99 (2), 1968, pp 96-100.

7. J. F. Radavich and K. B. Kolodziej, "Effect of Carbon on Behavior of Alloy 718," paper presented at Eighth Annual Purdue University Student-Industry High Temperature Materials Seminar, West Lafayette, Indiana, Dec. 1982.

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Documents

Extra Low Carbon Alloy 718 - TMS