E-Journal of Advanced Maintenance Vol.1 (2009) 44-51
Japan Society of Maintenology

Examination of Relation between IASCC Susceptibility
and Magnetic Property
Shigeru TAKAYA1,*, Yuji NAGAE1, Tsunemitsu YOSHITAKE1, Yoshiyuki NEMOTO2,Junichi NAKANO2, Fumiyoshi UENO2, Kazumi AOTO1 and Takashi TSUKADA2
1
2

Japan Atomic Energy Agency, 4002 Narita, O-arai, Ibaraki 311-1393, Japan
Japan Atomic Energy Agency, 2-4 Shirane, Shirakata, Tokai, Ibaraki 319-1195, Japan.

(Received; October 31st, 2008)

Abstract. Comparison between the magnetic flux density and the IASCC susceptibility evaluated by SSRT teston neutron-irradiated model alloys revealed that there is a relation between them regardless of minor differencesin chemical composition if the contribution of neutron irradiation to SCC is significant. Furthermore, themagnetic flux densities of two kinds of unirradiated materials simulating neutron-irradiated degradations weremeasured, and it was shown that the magnetic flux density will change if both of segregation along grainboundary and defects are induced due to neutron irradiation. These results show the possibility ofnon-destructive estimation of the susceptibility to IASCC by measuring magnetic flux density.

KEYWORDS: IASCC susceptibility, magnetic flux density, non-destructive estimation, neutron irradiation

1.

Introduction

Irradiation-assisted stress corrosion cracking (IASCC) is one of the critical issues of using
austenitic stainless steel as in-core structural material in light-water nuclear fission reactors [1], as
well as in supercritical water-cooled fast breeder reactors—candidate next-generation reactors.
Development of a non-destructive method to estimate the susceptibility to IASCC will enhance thesafety of these reactors. In a previous study [2], our group showed that for neutron-irradiated highpurity type 304 stainless steel (HP304), its heat doped with C (HP304/C), and high purity type 316
stainless steel (HP316), there is a relation between the susceptibility to IASCC and the magnetic flux
density. This suggests that it is possible to use a magnetic method for non-destructive estimation of
the susceptibility to IASCC. One of the purposes of this study is to examine whether this relation isapplicable to other irradiated materials. For this purpose, the samples of neutron-irradiated HP304
doped with minor elements were subjected to magnetic flux density measurement by a flux-gate
sensor, and the results were compared with the susceptibility to IASCC reported in the references[3–6]. The other purpose is to investigate the mechanism of magnetic flux density change resulting
from neutron irradiation. It is well known that neutron irradiation depletes some elements such as Crand Fe and enriches others such as Ni and Si in extremely narrow regions (<10 nm width) along grainboundaries, and this Cr depletion is believed to be important in the IASCC mechanism [7, 8]. In aprevious study [9], these local changes in chemical composition were assumed as the cause ofmagnetic property change. We prepared two kinds of unirradiated materials for the magnetic flux
density measurement to study the influence of segregation on magnetic property. One is SUS 304
stainless steel (304 SS) with a very narrow Cr depleted region, produced by the special heat treatment
described below, along grain boundaries, and the other is a model alloy whose mean matrix chemicalcomposition is similar to local chemical composition at a grain boundary in neutron-irradiated 304 SS.
In addition, the evolution of defect microstructures during irradiation is also considered to promote
IASCC susceptibility [7]. As for the model alloy, to observe the effect of defects on magnetic property*Corresponding Author: Tel:+81-29-267-4141, Fax:+81-29-267-3761, E-mail: takaya.shigeru@jaea.go.jp
ISSN – 1883 – 9894/09 © 2009 – JSM and the authors. All rights reserved.

44S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Propertychanges, tensile test at 923 K was also conducted, and the fractured samples were subjected tomagnetic flux density measurement.
2.

Experimental Procedure

2.1.

Materials

2.1.1. Neutron-irradiated model alloys
HP304 doped with Si (HP304/Si), P (HP304/P), and S (HP304/S) were investigated. Theirchemical compositions are shown in Table 1. These alloys were solution annealed by heat treatment at1373 K for 0.5 h followed by water quenching. In previous studies [3–6], specimens for slow strain
rate tensile (SSRT) test (dimensions shown in Fig. 1) were machined from these alloys, and neutron
irradiated up to two dose levels, about 1 and 5 dpa, in the Japan Research Reactor No. 3 (JRR-3).
After neutron irradiation, they were submitted to SSRT tests. The irradiation conditions are shown inTable 2 [3–6]. For the magnetic flux density measurement in this study, the fractured specimens after
the SSRT tests were provided, and specimens as shown in Fig. 2 were cut from the grip sections.
2.1.2. 304SSThe chemical composition of 304 SS is shown in Table 1. Three plates, A, B, and C, wereprepared and heat treated at conditions shown in Table 3. The objectives of heat treatment aftersolution annealing are as follows. (1) Sensitization: to precipitate Cr carbide at grain boundaries andproduce a relatively wide Cr-depleted region near the Cr carbide, (2) Healing: to extinguish wide
Cr-depleted regions with Cr carbide left at grain boundaries, (3) Aging: to grow the Cr carbide
gradually and produce narrow Cr-depleted regions [8, 10]. After each heat treatment, the plates werewater quenched to maintain high-temperature microchemistry. The Cr concentrations at grainboundaries and depletion widths estimated using a field emission transmission electron microscopewith an energy dispersed X-ray spectrometer were about 10 wt% and about 50 nm for plate B, and 13
wt% and a few nanometres for plate C, respectively [2]. In plate C, very narrow Cr depletion resulting
from neutron irradiation is successfully simulated. However, it is noted that segregation of otherelements such as Ni and Si is not simulated. A specimen with a size of 10 × 10 × 3 mm was cut fromeach plate for the magnetic flux density measurement.
2.1.3. Model alloys simulating local chemical composition in neutron irradiated 304 SS
The chemical compositions of the model alloys, which are called 13Cr-20Ni-2Si and10Cr-30Ni-3Si, are shown in Table 1. A plate with a size of 6 × 6 × 3 mm and a tensile test specimen

Specimen

CTable 1

Chemical Compositions of Specimen Materials (wt%)

SiMn

PS

CrNiMoTiAlN

FeHP304/Si

0.003

0.69

1.36

0.001

0.001418.01

12.24-

<0.01

0.10

0.0014

Bal.

HP304/P

0.006

0.03

1.40

0.017

0.0011

18.60

12.56

-

0.01

0.110.0016

Bal.

HP304/S

0.002

0.03

1.41

0.001

0.0318

18.32

12.47

-

0.01

0.07

0.0012

Bal.

HP304*

0.003

0.01

1.36

0.001

0.0014

18.17

12.27

-

0.01

0.16

0.0014

Bal.

HP304/C*

0.098

0.03

1.39

0.001

0.0020

18.30

12.50

-

<0.01

0.11

0.0016

Bal.

HP316*

0.004

0.02

1.40

<0.001

0.001

17.21

13.50

2.50

0.01

0.100.0020

Bal.

304SS forheat treatment

0.04

0.54

0.77

0.0210.002

18.4

9.0

0.10

0.01

<0.01

0.04

Bal.

10Cr-30Ni-3Si

0.034

3.02

1.02-

-

10.3

30.0

0.003

Bal.

13Cr-20Ni-2Si

0.034

2.00

1.02-

-

13.2

20.0

0.003

Bal.

---

* Materials used in the previous study [2]45S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property
Table 2 Irradiation Conditions in JRR-3
Dose level

Temperature (K)

1 dpa
5 dpa

513±10543±20

Fluence
E>1MeV (n/m2) E<0.68eV (n/m2)
6.7 x 1024
3.0 x 10253.5 x 1025
1.7 x 1026

Table 3 Conditions of heat treatment
ID

Solution-Annealing

Sensitization

Healing

Aging

A1323 K × 0.25 h---B1323 K × 0.25 h923 K × 25 h--C

1323 K × 0.25 h923 K × 25 h1173 K × 25 h773 K × 20 h

φ4This part was used in this study.

φ115φ６All specimens were water-quenched after each heat treatment.

GL2455
Fig. 1. Dimensions of SSRT testpiece irradiated in JRR-3 (mm).

Fig. 2. Dimensions of sample (shadow part) for measurement of magnetic flux density
cut from grip section of irradiated SSRT testpiece (mm).

46

S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property

186R
8Measuring area

GL = 301081290

Fig. 3. Dimensions of tensile test specimen: t = 3 mm (mm).

illustrated in Fig. 3 were machined from each alloy. The tensile test specimens were subjected totensile tests at 923 K in air. The strain rate was 0.3%/min until the strain reached 3%, and then it was7.5%/min till fracture.
2.2.

Measurement of Magnetic Flux Density

The specimens were demagnetized using an alternating magnetic field before measurements, andthen magnetized in the direction perpendicular to the measurement plane (z direction) by a pair ofpermanent magnets. The magnetic field at the centre of those magnets is about 0.3 /μ0 A/m. μ0 is
magnetic permeability in a vacuum. The measurement planes on the irradiated samples and thefractured tensile test specimen are shown in Figs. 2 and 3, respectively. For the other plate samples,the measurement planes are the largest planes on those samples. The z component of magnetic flux
density was measured using a flux-gate sensor. The magnetic flux density resolution is less than 1 μT.
The distance between the sensor and the sample surface was about 0.5 mm for the irradiatedspecimens and 1 mm for the unirradiated ones. To suppress scattering of measured data resulting froma change in the environmental magnetic flux density, the differences between the magnetic flux
densities with and without samples are presented in this paper.
3.

Results and Discussion

3.1.

Neutron irradiated model alloys

Results of the SSRT tests were reported in detail elsewhere [3–6]. Figure 4 shows thecomparison between the fraction of inter-granular SCC on the fractured SSRT test sample surface(%IGSCC), which is one of parameters used to estimate the susceptibility to SCC, and the z
component of the magnetic flux density at the centre of the measured area. The results obtained in aprevious study [2] are also plotted in this figure. The chemical compositions for the materials used in
the previous study are shown in Table 1. Figure 4 shows a close relationship between the magneticflux density and %IGSCC except for HP304/P and HP304/S. The magnetic flux density for any of themodel alloys increases with dose level, but the values are different to each other even at the same doselevel. On the other hand, %IGSCC also shows similar tendency except for HP304/P and HP304/S. As
a result, they have the relationship regardless of minor differences in chemical composition. As for
HP304/P and HP304/S, %IGSCC did not increase with dose level, unlike the others. The contributionof neutron irradiation to SCC appears to be rather small in those alloys, with dose levels used in thisstudy. From these results, it should be careful to estimate the susceptibility to IASCC of materials
doped with P and S by a magnetic method.
3.2.

304 SS with Cr depleted region along grain boundaries

Table 4 shows the measured results of the z component of magnetic flux density at the centre ofthe measuring area of heat-treated 304 SS samples. The magnetic flux density for sensitized sample B

47

S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property

Fig. 4. Relationship between %IGSCC and magnetic flux density for model alloys irradiated in JRR-3.

Table 4 Magnetic flux density for heat treatment 304 SS samples
IDAB
C

Final heat treatment
Solution annealing
Sensitization
Aging

Magnetic flux density (μT)
6.3
24
1.2

is much larger than that for solution annealed sample A. This tendency is the same as in our previous
study, which has been demonstrated by magnetic force microscopy and scanning electron microscopy
with the electron back-scatter pattern technique: phase transformation from paramagnetic austenite
phase to ferromagnetic martensite phase occurs along grain boundaries in sensitized 304 SS [11]. The
martensitic transformation is thought to be caused by Cr depletion near grain boundaries. Thetemperature at which martensitic transformation starts, the Ms point, is given by the following
equation [12]:Ms (K) = 775 − 810[% C] − 1230[% N] − 13[% Mn] − 30[% Ni] − 12[% Cr]− 54[% Cu] − 46[%Mo](1)This equation predicts that the Ms point of the solution-annealed sample A is about 190 K, but it
becomes higher than the room temperature when Cr concentration decreases to about 10 wt% or less.
In such Cr-depleted regions in the sensitized sample B, martensitic transformation may occur in thecooling process after heat treatment.
On the other hand, for the aged sample C, which has a very narrow Cr-depleted region similar toa neutron-irradiated sample, the magnetic flux density was much smaller than that for the sensitizedsample B, and a little smaller than that for the solution-annealed sample A. These results show that
local magnetisation was not introduced along grain boundaries in very narrow Cr-depleted regions in
the aged sample C or the amount of introduced magnetization was too small to be measured by aflux-gate sensor. This aged sample C simulates only the Cr-depletion profile at grain boundaries in48

S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property

neutron-irradiated 304 SS, and segregation of Ni and other elements at grain boundaries was not
simulated. As evident from Equation 1, increase in Ni concentration at grain boundaries lowers theMs point, and makes it more difficult to introduce magnetization by martensitic transformation. Fromthese results, it is difficult to explain the magnetic flux density change for neutron-irradiated materials
shown in Fig. 4 solely by Cr depletion in very narrow regions.
One of the reasons for the smaller magnetic flux density of the aged sample C being comparedwith the solution-annealed sample A is that the amount of ferromagnetic δ-ferrite phase, which iscontained slightly in solution-annealed sample, decreases during heat treatment because of theprecipitation of non-magnetic Cr carbide in the δ-ferrite phase [13].
3.3.

Model alloy simulating local chemical composition in neutron irradiated 304 SS

Figure 5 (a) and (b) shows the distribution of the magnetic flux density for as-received samplesof 13Cr-20Ni-2Si and 10Cr-30Ni-3Si, respectively. The sample area is represented by the dottedframes in Fig. 5, but the difference between the magnetic flux density inside and outside the frames ishardly recognized for both alloys. This result shows that the magnetic flux density does not change
only by a change in local chemical composition even if Ni is enriched. Figure 6 (a) and (b) shows thedistributions of the magnetic flux density for the parallel parts of the fractured tensile test samples of
13Cr-20Ni-2Si and 10Cr-30Ni-3Si, respectively. Unlike the results for the as-received samples, we
can easily recognize that the parallel parts of the fractured tensile test samples were magnetized
uniformly for both alloys. The magnetic flux density after the tensile test for 10Cr-30Ni-3Si, which
simulated more severe segregation, was larger than that for 13Cr-20Ni-2Si. These results suggest that,in addition to segregation, some defects are needed for change in the magnetic property ofneutron-irradiated materials. In general, defects contribute to material hardening through interactions
with dislocations. It was reported that irradiation hardening increases %IGSCC [7]. Minor differencesin chemical compositions of alloys will affect on the behaviours of segregation and hardening during
irradiation. It is one of the reasons why the IASCC susceptibilities of the model alloys are not thesame even at the same dose level in Fig. 4. On the other hand, it was shown that the magnetic property
also depends on both of segregation and hardening. This similar tendency will leads to therelationship between the IASCC susceptibility and the magnetic flux density regardless of minor
differences in chemical compositions of alloys, and further investigation is required.

(a) 13Cr-20Ni-2SiFig. 5.

(b) 10Cr-30Ni-3Si

Distribution of Magnetic flux densities for as-received model alloys simulating
local chemical composition in neutron irradiated 304 SS.

49

S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property(a) 13Cr-20Ni-2Si(b) 10Cr-30Ni-3Si
Fig.6.

4.

Distribution of Magnetic flux densities for model alloys simulating local chemicalcomposition in neutron irradiated 304 SS after tensile tests at 923 K.

Conclusion

The relationship between IASCC susceptibility and the magnetic flux density was examined, and,the mechanism for the magnetic flux density change resulting from neutron irradiation wasinvestigated using unirradiated materials. We obtained the following results.
1. A close relationship exists between %IGSCC and magnetic flux density regardless of minor
differences in chemical compositions of alloys if the contribution of neutron irradiation to SCC
is significant.
2. Change in the magnetic flux density measured for neutron-irradiated materials is not induced
solely by segregation at grain boundaries.
3. Some defects are needed for the magnetic property change in neutron-irradiation induced
segregation region.
These results show the possibility of non-destructive estimation of the susceptibility to IASCC by amagnetic method.

50

S. Takaya et al./Examination of Relation
between IASCC Susceptibility and Magnetic Property

Acknowledgement
Present study includes the result of “Development of diagnostic method of in-core structurematerial degradation for super critical water cooled fast breeder reactor” entrusted to Japan Atomic
Energy Agency by the Ministry of Education, Culture, Sports, Science and Technology of Japan(MEXT).

References[1] T. Shoji, S. Suzuki and K.S. Raja, Current status and future of IASCC research, J. Nucl. Mater.,258-263 (1998) 241-251.
[2] F. Ueno et al., A study on degradation of structural materials used under the irradiationEnvironment in Nuclear Reactors, Japan Atomic Energy Research Institute – Japan Nuclear
Cycle Development Institute Joint Research Report, JAERI-Research 2005-023 / JNC TY9400
2005-013 (2005), in Japanese.
[3] T. Tsukada et al., stress corrosion cracking of neutron irradiated type 304 stainless steels,
Proceedings and Symposium Discussions of 7th International Symposium on Environmental
Degradation of Materials in Nuclear Power Systems –Water Reactors, ANS (1995) 1009-1019.
[4] T. Tsukada, Y. Miwa, H. Nakajima, T. Kondo, Effects of minor elements on IASCC of type 316
model stainless steels, Proceedings of 8th International Symposium on Environmental
Degradation of Materials in Nuclear Power Systems –Water Reactors, ANS (1997) 795-802.
[5] J. Nakano et al., Effects of silicon, carbon and molybdenum additions on IASCC of neutron
irradiated austenitic stainless steels, J. Nucl. Mater., 329-333 (2004) 643-647.
[6] J. Nakano, Y. Nemoto, M. Numata, T. Taguchi, T. Tsukada, to be published in Proc. of 2007 fallmeeting of atomic energy society of Japan (2007), in Japanese.
[7] S.M. Bruemmer et al., Radiation-induced material changes and susceptibility to intergranular
failure of light-water-reactor core internals, J. Nucl. Mater., 274 (1999) 299-314.
[8] S.M. Brummer, B.W. Arey and L.A. Charlot, Influence of chromium depletion on intergranular
stress corrosion cracking of 304 stainless steel, Corrosion, 48 (1992) 42-49.
[9] J. Morisawa, M. Otaka, M. Kodama, T. Kato, S. Suzuki, Detection of intergranular cracking
susceptibility due to hydrogen in irradiated austenitic stainless steel with a superconducting
quantum interference device (SQUID) sensor, J. Nucl. Mater., 302 (2002) 66-71.
[10] S. Shimanuki, K. Nakata, Effect of Grain baoundary Cr concentration, profiles on stresscorrosion cracking susceptibility in 304 stainless steel, Tetsu to Hagane, 83-10 (1993) 49-54, inJapanese.
[11] S. Takaya, T. Suzuki, Y. Matsumoto, K. Demachi, M. Uesaka, Estimation of stress crrosion
cracking sensitivity of type 304 stainless steel by magnetic force microscope, J. Nucl. Mater.,
327 (2004) 19-26.
[12] F.B. Pickering, Physical metallurgy and design of steels, Appl. Sci. Publ. Ltd., London (1978)
228.
[13] T. Asayama, S. Hasebe, Weld metal creep-fatigue life prediction by modeling the microstructure
degradation due to the exposure to high temperature and load, Nucl. Eng. Des., 195 (2000)
197-210.

51