EFFECTS OF COLD WORK AND AGING TREATMENTS ON THE ...envdeg2015.org/final-proceedings/ENVDEG/papers/ENVDEG062.pdf · More work need to be done to clarify the possible effects of cold

17th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water ReactorsAugust 9-13, 2015, Ottawa, Ontario, Canada

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EFFECTS OF COLD WORK AND AGING TREATMENTS ON THE

MICROSTRUCTURE AND STRESS CORROSION CRACKING INITIATION

BEHAVIOR OF SOLUTION ANNEALED ALLOY 690

Wenjun Kuang1, Cody Miller2, Mike Kaufman2, Talukder Alam3, Bharat Gwalani3, Rajarshi Banerjee3,

Gary S. Was1

1University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 481052Colorado Schools of Mines, 201 Hill Hall, Golden, CO 80401

3University of North Texas, UNT Research Park, 3940 N. Elm, Denton, TX 76207

ABSTRACT

There is concern that long-term ageing treatment may adversely affect the SCC resistance of this alloy

through changes in sub-structure or phase composition. This study was aimed at investigating the effects

of aging treatments on the SCC initiation behavior of both solution annealed (SA) and SA + cold rolled

Alloy 690 using slow strain rate tensile (SSRT) technique. The aging treatments were 3000 and 10000 h

at 350 and 475 oC. For the non-cold rolled samples, no significant difference in the degree of crack

initiation was found between different aging conditions, suggesting that those aging treatments did not

induce microstructure changes that affect the SCC initiation susceptibility. For the cold rolled samples,

the twin boundary structure may be damaged after cold rolling. The aging treatments induced the

restructuring of the dislocation substructure, which was indicated by the changes in mechanical

properties. For samples aged at 350 oC, the SCC initiation susceptibility decreased with increasing aging

time. The SCC initiation susceptibility increased significantly after aging at 475 oC which may be related

to carbide precipitation on grain boundaries.

Keywords: Alloy 690, aging treatment, cold work, crack initiation, twin, carbides

1. INTRODUCTION

Alloy 690 is now widely used in nuclear power plants as a replacement for Alloy 600 due to its superior

resistance to stress corrosion cracking (SCC) [1-3]. However, there is concern that aging treatment at

temperature close to the operating temperature in nuclear power plant could induce microstructure

changes in this alloy, i.e. formation of ordered phase Ni2Cr, which may adversely affect the SCC

behavior. Delabrouille et. al. [4] first reported the precipitation of Ni2Cr in an alloy 690 containing 7.2%

Fe aged at 420 oC for up to 70000 h. Marucco [5] investigated the phase transformations during

long-term aging of Ni-Fe-Cr alloys in the temperature range of 450-600 oC and found that these alloys

underwent three microstructure changes: atomic ordering based on Ni2Cr, and precipitation of two

embrittling phases-α’-Cr and σ-(FeCr). Young et.al [6] studied the kinetics of long range ordering in

Ni-Cr and Ni-Cr-Fe alloys and their results revealed that the development of long range order increased

the yield strength and promoted brittle, intergranular fracture. It was also pointed out that lattice

contraction associated with long range ordering could cause appreciable internal stress. Nevertheless,

there is still lack of direct data showing the effects of microstructure changes induced by aging

treatments on the SCC behavior of those Ni base alloys.


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The effect of cold work on the microstructure change during aging has not been thoroughly studied.

Delabrouille et. al. [4] found that the precipitation of Ni2Cr was accelerated by cold work due to the

increased diffusion rate of elements. However, Young et.al [6] thought that cold work might delay the

onset of ordering by destroying initial regimes of order. Furthermore, cold work also induces changes on

the grain boundary structure, which may affect the SCC susceptibility. More work need to be done to

clarify the possible effects of cold work.

In this work, cold rolled and non-cold rolled Alloy 690 samples were aged at 350 and 475 oC for 3000

and 10000 h. The SCC initiation susceptibility of those aged samples was evaluated in 360 oC

hydrogenated water using slow strain rate tensile (SSRT) test technique. The results were discussed in

relation with precipitation, mechanical property and grain boundary structure.

2. EXPERIMENTAL

2.1 Material

The chemical composition of Alloy 690 used in this work is 57.6 wt.% Ni, 32.7% Cr, 8.64% Fe, 0.25%

Mn, 0.315% Al, 0.08% Si and 0.02% C. The material was a forged bar with a diameter of 185 mm. It

was solution annealed at 1100 oC for 1 h and then water quenched. Some samples were cold rolled (CR)

to 20% thickness reduction, resulting in a sheet of approximately 8 mm in thickness. Solution annealed

sample conditions were designated with “XX” and solution annealed and cold rolled samples are

designated with “CR”. Both conditions were aged at 350 and 475 oC for 3000 and 10000 h. The aging

conditions were indicated in the sample designations. For example, K-CR-350-10000 denotes the cold

rolled sample that was aged at 350 oC for 10000 h. The cold rolled sheet was machined into round tensile

bars with the sample axis in the cold rolling direction. The gage section of the tensile bar is 20 mm in

length and 2 mm in diameter. Samples were mechanically abraded up to 4000 grit and then

electropolished for 30 seconds at 30 V in a solution of 10% (volume fraction) perchloric acid in

methanol solution. Some coupons were also prepared for carbide analysis using the same procedure.

Grain boundary carbides were characterized with SEM on FEI Helios Nanolab 650. The imaged surfaces

were perpendicular to the transverse direction of cold rolled bulk sample. EBSD tests were also

performed on this instrument. The acceleration voltage was 20 keV and the electron current was 6.4 nA.

The binning size used was 4×4 and the step size was 1 μm. The scanned surface was also perpendicular

to the transverse direction and the scanned area was 400×600 μm.

2.2 Apparatus and Methodology

The tensile bars were strained in 360 oC high purity water containing 18 cm3 (STP) H2/kg H2O which is

approximately the electrochemical potential at the Ni/NiO boundary. The inlet resistivity was above 18

MΩ·cm and the outlet resistivity was above 5 MΩ·cm. A recirculating water loop equipped with a 4 L

stainless steel autoclave was used to maintain the required water environment. Both inlet and outlet

dissolved oxygen concentrations and water conductivities were monitored during the test. Ion exchange

columns were used to purify the water continuously. Samples were strained with a servo-electric system

via a crosshead that was capable of loading a maximum of 4 tensile bars. For non-cold worked samples,

the tensile bars were strained to 7% at a rate of 5×10-8/s while for cold worked samples, due to their

limited uniform elongation, they were strained at a lower rate of 1×10-8/s in order to initiate cracking.


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After the straining tests, the gage sections of tensile bars were examined with a JOEL JSM-6480

scanning electron microscope (SEM). More than 40 equally spaced areas on the uniformly strained

gauge section were imaged at 1000×. The images were magnified to 4500× to characterize

intergranular (IG) cracks. Crack lengths were measured and the crack numbers were counted.

Characterization of cracking was always done at or near the point of uniform strain. Beyond this point,

strain is confined to the necked region in which the strain rate increased rapidly. From those data, crack

length per unit area, crack density (number of cracks per unit area) and averaged crack length were

calculated. These parameters especially crack length per unit area have long been used to assess the SCC

initiation susceptibility of materials [7, 8].

3. RESULTS

Figure 1 shows the SEM images of different aged samples. The non-aged samples and the samples aged

at 350 oC have sparse precipitates on grain boundaries (Figure 1-a1, b1 a2 and b2) regardless of whether

the sample was cold rolled or not. For samples aged at 475 oC, the non-cold worked samples show

precipitates on random high angle boundaries (Figure 1-c1 and d1), not on twin boundaries, while the

cold-worked samples show semi-continuous precipitates not only on high angle boundaries, but also on

previous coherent annealing twin boundaries (Figure 1-c2 and d2). So aging treatments at 475 oC

resulted in precipitation on random high angle boundaries and also on previous twin boundaries in cold

worked samples. Figure 2 shows the TEM micrograph and the diffraction pattern of precipitate of

K-CR-475-10000. The structure of precipitates was identified to be M23C6.

Figure 3 shows the stress-strain curves and yield strengths of solution annealed and aged (XX) samples.

All the samples show serrations on the stress-strain curve, which are due to dynamic strain aging (Figure

3a). The yield strengths were extracted and shown in Figure 3b. The yield strength increased a little after

3000 h aging at both temperatures and then decreased slightly after 10000 h aging. Overall, the variation

is small.

Figure 4 shows some intergranular cracks found on those samples. Those cracks are shallow and within a

single grain domain because they are still in the stage of initiation. Figure 5 shows the crack density,

average crack length and the crack length per unit area for solution annealed and aged samples. The

variation among different samples is insignificant and still within the error bar. So it seems that aging

treatments did not affect the SCC initiation susceptibility of solution annealed 690 significantly.

Figure 6 shows the stress-strain curves and yield strengths of solution annealed, cold rolled and aged

(CR) samples. All the samples show significant dynamic strain aging. Sample K-CR-00-00 was strained

to failure. K-CR-350-3000 and K-CR-350-10000 were strained beyond necking point. K-CR-475-3000

and K-CR-475-10000 were uniformly strained to 3.55% and 3.66% respectively. The extracted yield

strengths are shown in Figure 6b. Different aging treatments resulted in different yield strengths. At both

temperatures, the yield strength decreased after aging treatments. For the same aging time, the sample

aged at 350 oC has higher yield strength than that aged at 475 oC.


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Figure 7 shows some intergranular cracks found on those samples. The sample surfaces were all

decorated with spinel oxide particles. The cracks are shallow and within single grain domain as they were

in the initiation stage. Figure 8 shows the cracking results from those samples. The uniform plastic strains

were indicated in Figure 8c. For sample aged at 350 oC, all the parameters used to characterize cracking

susceptibility decreased after aging treatment although the applied uniform plastic strains on the aged

samples are higher. On the contrary, for samples aged at 475 oC, all the parameters increase abruptly.

Given that the samples aged at 475 oC were uniformly strained more than the non-aged sample, it is better

to compare those samples with K-CR-350-3000 because they were uniformly strained to similar levels.

The comparison indicates that aging treatments at 475 oC increased the SCC initiation susceptibility of

cold rolled 690 significantly.

4. DISCUSSION

4.1 Effect of aging treatments on non-cold rolled samples

The aging treatments at 350 oC have little effect on the grain boundary carbide precipitation of solution

annealed samples (Figure 1a1 and b1). As the aging temperature was increased to 475 oC,

semi-continuous carbides started to form on random high angle boundaries (Figure 1c1 and d1). The

stress-strain curves of those samples are similar and the yield strength changed only slightly (Figure 3).

The cracks formed following straining are shallow but clear and most of them are intergranular. A small

amount of transgranular cracks initiate from slip band which are mainly due to aggressive loading

condition. Those transgranular cracks were excluded for measurement. The cracking results measured by

the three parameters are similar although there are some variations (Figure 5). Among the three

parameters, crack length per unit area provides the most complete description of SCC initiation

susceptibility of material and the differences between those samples are within the error bars. So it seems

that for solution annealed and non-cold rolled samples, the aging treatments at both temperatures have

little effects on the SCC initiation susceptibility although some intergranular carbides formed at high

angle boundaries after aging treatments at 475 oC.

4.2 Effect of aging treatments on cold rolled samples

The aging treatments at 350 oC did not cause any precipitation on the grain boundaries or slip bands in

cold-rolled sample (Figure 1-b2). Notable carbide formation was found after aging treatments at 475 oC

(Figure 1-c2 and d2). The carbide formed not only on high angle random boundaries, but also on slip

bands. Interestingly, the prior coherent twins were also decorated with semi-continuous carbides.

Normally twins are very resistant to carbide precipitation [9, 10]. So it may be that the twin boundary

structure was damaged during cold rolling and behaved like random high angle boundaries. The

stress-strain curves of those samples show some difference (Figure 6a). The aged samples have lower

yield strengths and larger uniform elongations compared to the un-aged sample. The samples aged at 475oC show lower yield strengths and larger uniform elongations than those aged at 350 oC and the yield

strength decreases with increasing aging time (Figure 6b). The softening of cold-rolled sample after aging

treatments was mainly due to the restructuring of dislocations.


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The cracks found on those cold rolled samples were mainly intergranular (Figure 7). Some of the previous

twin boundaries which were transformed during cold rolling were also found to crack. Those twins are

previous annealing twins which appear as long, straight and paired boundaries and could be easily

distinguished. The three parameters used to characterize the cracking results show similar trend (Figure 8).

The three parameters decreased for samples aged at 350 oC as the aging time increased, although the

applied strains on aged sample were larger than the un-aged one. This trend is very consistent with that of

yield strength. So it is believed that reduced cold work after aging treatments decreased the SCC initiation

susceptibility of material if other microstructure features are not significantly changed. Interestingly, for

samples aged at 475 oC, the cracking parameters increase significantly although the yield strengths of

those samples are lower. It may be that in addition to residual cold work, the microstructure changes

induced during aging at 475 oC also affect the SCC behavior of cold-rolled samples. In this case, directly

comparing the non-aged sample with those aged at 475 oC is not appropriate because both the residual

cold work and microstructure changed and the effects of those two factors seems to be in opposite

directions. A more valid comparison is between samples aged at 350 oC and 475 oC as the difference of

residual cold work between them are smaller. It should be noted here that the uniform plastic strains

applied on those sample are not the same. Nevertheless, CR-350-3000 was strained to 3.29% which is

comparable to the uniform strains applied on those aged at 475 oC and could be used as a reference

sample. The crack lengths per unit area of those samples aged at 475 oC are significantly higher than that

of CR-350-3000. So aging at 475 oC induced some microstructure changes that reduced the SCC

resistance of this alloy in spite of the reduced residual strain. Considering the major difference in

microstructure between samples aged at 350 oC and 475 oC is the grain boundary precipitation, it is

speculated that the grain boundary carbide precipitation at 475 oC may be the primary cause of reduction

SCC resistance in cold-rolled samples. It is generally accepted that carbides on grain boundaries can

increase the SCC resistance of alloy 690. It should be noted that those carbides are normally formed by

thermal treatment around 700 oC. Moreover, the non-cold rolled samples also have such carbide

precipitation after aging at 475 oC but didn’t show significant change in the SCC behavior. More work is

still needed to characterize the microstructure changes induced by aging treatment at 475 oC in cold-rolled

samples to explain the SCC susceptibility changes.

5. CONCLUSIONS

The effects of aging treatments on the microstructure and SCC initiation susceptibility of non-cold rolled

and cold rolled alloy 690 were studied. Aging treatments at 475 oC induced carbide precipitation at grain

boundaries. Carbides also formed on prior annealing twins in cold-rolled samples. Aging treatments have

little effects on the mechanical properties and SCC initiation susceptibility of non-cold rolled samples.

For cold-rolled samples, aging treatments caused softening of materials and the softening increased with

aging temperature. Aging at 350 oC decreased the SCC susceptibility of cold-rolled sample mainly due to

the reduced residual strain. Aging at 475 oC increased the SCC susceptibility significantly in spite of

further reduced residual strain. Such increase of SCC susceptibility may be related to the carbides

precipitation induced by aging.


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ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial support through the DOE I-NERI program contract

2011-01-K. The authors would like to thank Young Suk Kim and Sung Soo Kim from Korea Atomic

Energy Research Institute for providing the materials for this study, and Alex Flick from the University of

Michigan for his assistance with preparation of the high temperature autoclave systems.

REFERENCES

[1] K. Smith, A. Klein, P. Saint-Paul, J. Blanchet. “Inconel 690: A Material with Improved Corrosion

Resistance for PWR Steam Generator Tubes”, in: proceedings of the second international symposium on

environmental degradation of materials in nuclear power systems-water reactors, American Nuclear

Society, Monterey, California, 1985, pp. 319-328.

[2] J. J. Kai, G. P. Yu, C. H. Tsai, M. N. Liu, S. C. Yao. “The effects of heat treatment on the chromium

depletion, precipitate evolution, and corrosion resistance of INCONEL alloy 690”, Metall.Trans. A Vol.

20, 1989, pp. 2057-2067.

[3] J. J. Kai, C. H. Tsai, G. P. Yu. “The IGSCC, sensitization, and microstructure study of Alloys 600 and

690”, Nucl. Eng. Des. Vol. 144, 1993, pp. 449-457.

[4] F. Delabrouille, D. Renaud, F. Vaillant, J. Massoud. “Long range ordering of alloy 690”, in: 14th

international conference on environmental degradation of materials in nuclear power systems, Virginia

Beach, VA, 2009.

[5] A. Marucco. “Phase transformations during long-term ageing of Ni-Fe-Cr alloys in the temperature

range 450–600 °C”, Materials Science and Engineering: A Vol. 194, 1995, pp. 225-233.

[6] G. A. Young, J. D. Tucker, D. R. Eno. “The kinetics of long range ordering in Ni-Cr and Ni-Cr-Fe

alloys”, in: 16th international conference on environmental degradation of materials in nuclear power

systems-water reactors, Asheville, North Carolina, USA, 2013.

[7] T. Moss, G. S. Was. “Accelerated stress corrosion crack initiation of Alloy 690 and Alloy 600 in high

temperature hydrogenated water”, in: 16th international conference on environmental degradation of

materials in nuclear power systems-water reactors, Asheville, North Carolina, USA, 2013.

[8] S. Teysseyre, G. S. Was. “Stress Corrosion Cracking of Austenitic Alloys in Supercritical Water”,

Corrosion Vol. 62, 2006, pp.1100-1116.

[9] Y. S. Lim, J. S. Kim, H. P. Kim, H. D. Cho. “The effect of grain boundary misorientation on the

intergranular M23C6 carbide precipitation in thermally treated Alloy 690”, Journal of Nuclear Materials

Vol. 335, 2004, pp. 108-114.

[10] B. Tang, L. Jiang, R. Hu, Q. Li. “Correlation between grain boundary misorientation and M23C6

precipitation behaviors in a wrought Ni-based superalloy”, Materials Characterization Vol. 78, 2013, pp.

144-150.


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Figure 1. SEM images of (a1) K-XX-00-00, (b1) K-XX-350-10000, (c1) K-XX-475-3000, (d1)

K-XX-475-10000, (a2) K-CR-00-00, (b2) K-CR-350-10000, (c2) K-CR-475-3000 and (d2)

K-CR-475-10000.

(a1) (a2)

(b1) (b2)

(c1) (c2)

(d1) (d2)

5 µm

4 µm5 µm

5 µm 5 µm

5 µm

5 µm 5 µm


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Figure 2. TEM image of K-CR-475-10000 and the diffraction pattern of precipitate on grain

boundary.

(a) (b)

Figure 3. (a) Stress-strain curves and (b) yield strengths of solution annealed and aged samples

strained to 7% at 5×10-8 s-1 in 360 oC water containing 18 cm3 (STP) H2/kg H2O.

M23C6


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Figure 4. Cracks on (a) K-XX-00-00, (b) K-XX-350-3000, (c) K-XX-475-3000 and (d)

K-XX-475-10000.

Loading direction

(a) (b)

(c) (d)


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(a) (b)

(c)

Figure 5. (a) Crack density, (b) average crack length and (c) crack length per unit area of solution

annealed and aged samples.

(a) (b)

Figure 6. (a) Stress-strain curves and (b) yield strengths of solution annealed, cold rolled and

aged samples strained at 1×10-8 s-1 in 360 oC water containing 18 cm3 (STP) H2/kg H2O.


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Figure 7. Cracks on (a) K-CR-00-00, (b) K-CR-350-3000, (c) K-CR-475-3000 and (d)

K-CR-475-10000.

(a) (b)

(c) (d)


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(a) (b)

(c)

Figure 8. (a) Crack density, (b) average crack length and (c) crack length per unit area of solution

annealed, cold rolled and aged samples (the applied uniform plastic strains were indicated in the

figure).

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EFFECTS OF COLD WORK AND AGING TREATMENTS ON THE ...envdeg2015.org/final-proceedings/ENVDEG/papers/ENVDEG062.pdf · More work need to be done to clarify the possible effects of cold