ISSN 2466-2232 (Print)

ISSN 2466-2100 (Online)

Effect of the Holding Time during Solution Heat Treatment on

Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

Eun-Jong Oh*, Dong-Hwa Lee*, Sung-Woo Cho*, Yun-Il Choi**,†and Ki-Woo Nam***
*Nuclear Business Group, Doosan Heavy Industries & Construction Co. Ltd., Changwon, 51711, Korea
**Corporate R&D Institute, Doosan Heavy Industries & Construction Co. Ltd., Changwon, 51711, Korea
***Depart. of Materials Science & Engineering, Pukyong National University, Busan, 48513, Korea

†Corresponding author : yunil.choi@doosan.com

(Received November 5, 2019 ; Revised December 10, 2019 ; Accepted March 4, 2020)

Abstract
The holding time during solution heat treatment of unstabilized austenitic stainless steels as specified in the nu-
clear regulatory requirements was investigated. The sensitized 2.54cm thick specimens held at 675℃ for 1 h were
rejected by ASTM A262 test, due to the large amount of chromium carbide precipitated in the form of 50~300nm
particles at the grain boundaries. They also showed about 10.8% of DOS in the DL-EPR test. However, solution
heat treatment of the sensitized specimens at 1,038℃ and 1,121℃ for at least 1 min resulted in the complete dis-
solution of chromium carbide into the grains, and they passed ASTM A262 test and showed less than 0.01% of DOS
in the DL-EPR test. As a result of solution heat treatment at 1,038℃ for 5 h of the 25.4cm thick specimen sensitized
at 675℃ for 10 h, it passed ASTM A262 and DL-EPR test at any position in the specimen thickness. While the
specimen surface showed a step structure without the precipitation of chromium carbide and a DOS less than 0.01%,
towards the center, a dual structure was observed. It exhibited about 0.6% of DOS due to the longer exposure time to
the sensitization range of 427~816℃. Considering the minimum time in which the chromium carbide precipitated at
the grain boundary at 1,038℃ was completely dissolved into the grain, and the maximum delay time for the center
of the specimen to reach 1,038℃ rather than the surface, the holding time for complete solution heat treatment to the
center was found to be up to 2 min per 2.54cm of material thickness. The solution heat treatment for 0.5~1.0 h per
2.54cm of material thickness at 1,038~1,121℃, which is employed in the nuclear power industry, was proven to pre-
vent grain boundary corrosion by inhibiting the sensitization of unstabilized austenitic stainless steels.

Key Words : Austenitic stainless steel, Solution heat treatment, Holding time, Intergranular corrosion

requirements, exposure to the sensitization temperature

1. Introduction range of 427 to 816℃ is restricted in order to prevent
the intergranular corrosion occurring in the chromium
Austenitic stainless steels have excellent corrosion re- loss region near the grain boundary from the precip-
sistance, heat resistance, fatigue resistance, machi- itation of chromium carbide (Cr23C6). In addition, sol-
nability, weldability and cost efficiency due to its mate- ution heat treatment, which is held at 1,038~1,121℃
rial properties with high strength, toughness, and ductil- for 0.5~1.0 hours per 2.54cm of material thickness and
ity properties, accounting for more than 90% of stainless then rapidly cooled, is proposed to inhibit the sensitiza-
steels used in the nuclear power industry1). However, for tion of the material. However, the ASME Section II
nuclear facility components made from unstabilized Part A code, which is applied for the production of un-
austenitic stainless steels, such as type 304 and 316, stabilized stainless steel in the actual industrial site,
rather than low-carbon grade stainless steels or stabi- specifies only the lower limit for the temperature of sol-
lized stainless steels, such as type 321 and 347, in the ution heat treatment, such as 1,038℃ at minimum, for
Safety Analysis Report, one of the nuclear regulatory most materials, while the holding time for heat treat-

Journal of Welding and Joining, Vol.38 No.3(2020) pp278-288

https://doi.org/10.5781/JWJ.2020.38.3.7
Effect of the Holding Time during Solution Heat Treatment on Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

Uniform holding Effective holding To evaluate the effect on the solution heat treatment
Temp. time(ⓐ) time(ⓑ) holding time, 10 coupons of 2.54cm in thickness, 18cm
p. in width and 20cm in length were prepared.
p.

tem
p.
tem

tem
r To evaluate the uniform holding time according to the
nte
Ce
ce
ace

Rapid material thickness, one coupon with a thickness of

fa
rn

r
Su

cooling 25.4cm, a width of 48cm and a length of 48 cm was

Fu

prepared. At this time, the coupon thickness of 25.4cm

Time
is based on the consideration of the maximum thickness
Heating time Holding time(ⓒ) of austenitic stainless steels used in the nuclear power
industry, and the coupon with a width and length great-
Fig. 1 Schematic diagram of solution heat treatment cycle
er than this thickness was prepared to exclude the effect
on the thickness direction.
ment is not suggested. In this regard, the necessity of
The above coupons were heated at a rate of 100℃/h,
demonstration tests has emerged to establish the stand-
held at 675℃ for 1 hour per 2.54cm of coupon thick-
ards for specific solution heat treatment conditions2).
ness, and then water cooled for sensitization heat
Accordingly, we investigated the effect of intergranular
treatment. For comparative evaluation before and after
corrosion according to the solution heat treatment hold-
ing time and material thickness was investigated, and the solution heat treatment, the specimens subjected to
the time at which the center of the material reaches the sensitization heat treatment were collected from the
solution heat treatment was analyzed as in Fig. 1. In the surface of the coupons before conducting the solution
heat treatment cycle, uniform holding time (ⓐ) at heat treatment.
which the solution heat treatment temperature is It is known that as the stress applied to the material in-
reached up to the center of the material, and the effec- creases, the resulting strain increases the chromium dif-
tive holding time (ⓑ) at which the chromium carbide fusion rate, leading to the acceleration of the chromium
was completely dissolved at the grain boundary after carbide precipitation at the grain boundary and increase
reaching the solution heat treatment temperature were in the sensitization, but the effect of external stress is
experimented to determine the solution heat treatment excluded in this experiment3).
holding time (ⓒ). These study results are expected to
be the experimental reference data for the nuclear regu-
2.1.2 Solution heat treatment
latory requirements and also to provide an opportunity
For the investigation of the effect on the solution heat
in which the effectiveness of solution heat treatment
treatment holding time, the coupons with a thickness of
conditions applied in the nuclear power industry is
2.54cm subjected to sensitization heat treatment at 675℃.
demonstrated.
for 1 hour were held for 1 minute, 5 minutes, 10 minutes,
15 minutes, and 30 minutes at a minimum temperature
2. Experimental Method
of 1,038℃. and a maximum temperature of 1,121℃,
2.1 Experiment preparation and then water cooled, and the heating rate was 100℃
/h. At this time, the specimens were taken from the sur-
2.1.1 Specimen and sensitization heat treatment face area of the coupons to minimize the effect from
This experiment was performed under conservative the thickness.
conditions reflecting the phenomenon that sensitization In order to evaluate the uniform holding time accord-
is more likely to occur as the carbon content is in- ing to the material thickness, a 25.4cm coupon sub-
creased using austenitic stainless steel specimens of jected to sensitization heat treatment at 675℃ for 10
0.074wt.%C, which is higher than 0.065wt.%C, the hours was heated with a heating rate of 50℃/h to a
regulatory requirements on the maximum carbon con- minimum temperature of 1,038℃ of solution heat treat-
tent of the nuclear power industry. The chemical com- ment specified in nuclear regulatory requirements, then
positions of test coupons were analyzed with an inductively it was held for 5 hours and water cooled. At this time,
coupled plasma optical emission spectrometer, and the the minimum temperature was set considering that the
results are shown in Table 1.
lower the heat treatment temperature, the longer it takes
Table 1 Chemical compositions(wt.%) of test coupons to apply uniform heat up to the center of the material,
and the test was conducted under conservative con-
Type C Si Mn P S Ni Cr
ditions by applying 0.5 hour per 2.54cm, the minimum
A182
F304H
0.074 0.455 1.405 0.031 0.013 8.44 18.53 holding time of nuclear regulatory requirements. After

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 Eun-Jong Oh, Dong-Hwa Lee, Sung-Woo Cho, Yun-Il Choi and Ki-Woo Nam

2.2.2 Microstructure observation and composition

analysis
Chromium carbide existed in the grain boundary ac-

25.4cm
cording to the sensitization heat treatment and solution
heat treatment was observed through TEM (Transmission
electron microscopy) images of replica specimens, and
through SAD (Selected area diffraction) patterns and
12.7cm

EDS (Energy dispersive X-ray spectroscopy) analysis,

the types and compositions of grain boundary precip-

m c
.0
48
48.0cm
itate were analyzed.

Fig. 2 Specimen sampling location for evaluation by sol- 2.2.3 Intergranular corrosion by electrochemical po-
ution heat treatment per material thickness larization
To supplement the ASTM A262 intergranular corro-
sion test, which has a limitation in the quantitative rep-
the solution heat treatment, as shown in Fig. 2, specimens resentation of the degree of sensitization, a DL-EPR (Double
were collected at the coupon surface and at depths of Loop Electrochemical Potentiokinetic Reactivation) test
2.54cm, 5.08cm, 7.62cm, 10.16cm and 12.70cm spaced 1 was performed according to ASTM G108 and ISO-
inch apart from the coupon surface. In addition, before 12732 requirements5,6). The specimen was polished to
the solution heat treatment, the thermocouples were 1µm with silicone carbide paper and alumina paste, and
separately installed on the surface and at the center of then ultrasonic cleaning was performed in ethanol and
the coupon so that the difference between the surface distilled water for 5 minutes each. The specimen was
and the center can be measured for the time it took to immersed in 1L of 0.5M H2SO4+0.01M KSCN solution
reach 1,038℃, the solution heat treatment temperature, at 30℃ aerated to simulate the environment during op-
and the time exposed to the sensitization temperature eration of the nuclear facility components, and then
range of 427 to 816℃ in the cooling process. from a potential -50mV lower than the natural corro-
sion potential (EOC) measured for 30 minutes of open
2.2 Test and analysis methods circuit delay time, anodic polarization was performed up
to +200mVSCE, at a constant scanning rate of 1.667mV/s,
2.2.1 Intergranular corrosion by chemical immersion and then reverse scanning was performed7). Saturated
In order to evaluate the effect of chromium carbide calomel electrodes (SCE) and high- purity carbon rods
precipitation of austenitic stainless steel on intergranular were used as reference and counter electrodes,
corrosion, an intergranular corrosion test was con- respectively. The ratio (Ir/Ip) of the maximum current
ducted in accordance with requirements of ASTM (Ip) for the activation during the forward anodic polar-
A262 Practice A and E specified by nuclear regulatory ization to the maximum current (Ir) for the selective re-
requirements4). activation of chromium loss region during reverse po-
In the ASTM A262 Practice A test, 25 × 25 × 12mm larization was used as a measure to calculate the degree
specimen was immersed in a 10% oxalic acid solution of sensitization (DOS) that represents the degree of the
intergranular corrosion3,8,9).
at room temperature in which 100g of oxalic acid
(C2H2O4·2H2O) was dissolved in 900ml of distilled wa-
2.2.4 Finite Element Method (FEM)
ter, current was applied for 90 seconds with the current
After the surface of the material reached the solution
density of 1A/㎠, and then the degree of intergranular
heat treatment temperature of 1,038℃, the uniform
corrosion on the surface was observed by an optical mi-
holding time at which the central part reached the same
croscope with 250 times magnification. In the ASTM temperature was analyzed for evaluation using ANSYS
A262 Practice E test, after dissolving 100g of copper program as shown in Fig. 3. In the first stage, an ana-
sulfate (CuSO4·5H2O) in 700ml of distilled water, 100ml lytical methodology was established through the creat-
of sulfuric acid (H2SO4) was added, and 80 × 15 × ing of a model for test validation, heat transfer analysis,
3mm specimen was immersed in a solution diluted to comparison with thermocouple measurement results,
1,000ml with distilled water and boiled for 15 hours. correction of initial conditions, and confirmation of
Then, a bending test was performed to observe whether analysis property values. In the second stage, the time
grain boundary cracks were generated by an optical mi- to reach the solution heat treatment temperature at the
croscope with 60 times magnification. surface and at the center was calculated by creating a

280 Journal of Welding and Joining, Vol. 38, No. 3, 2020

Effect of the Holding Time during Solution Heat Treatment on Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

Stage-1 Methodology establish

Create a model for test validation
ANSYS
Establish initial conditions R15.07
for high temperature
Perform heat transfer analysis properties & heat transfer
coefficient
Review & comparison with test results
Result analysis & initial
condition correction
Confirmation of analysis property
value & method establishment

Review & comparison with test results

Stage-2 Result output

Surface
Create a model for calculating Results
Center
Perform heat transfer analysis

Result evaluation
Fig. 4 The model geometry of finite element method
Fig. 3 Schematic sequence diagram of uniform holding
time analysis by ANSYS (ver. 15.07) program
model for evaluating the uniform holding time, and Fig.
4 shows the model geometry of the finite element anal-
model for calculating results, analyzing heat transfer ysis model.
and evaluating the results. For the analysis of uniform As for the property values applied to the uniform
time to reach the solution heat treatment temperature, holding time analysis, the material properties of auste-
convective heat transfer analysis considering natural nitic stainless steel (304 SS) of ASME Section III Part
convection phenomenon in the heat treatment furnace D were used as initial conditions and for the area ex-
was performed and for the analysis input, the temper- ceeding the temperature range provided by the ASME
ature heating rate in the heat treatment furnace was code, a quadratic polynomial was assumed for the
applied. As for the analysis result, the time to reach the curve fitting to apply for the initial analysis2). In order
solution heat treatment temperature at each of surface to determine the analytical properties of final applica-
and center was derived and the uniform holding time, which tion, the thermocouple measurement results were com-
is the difference between the two times, was calculated. pared with the initial analysis results and the initial
Twelve analytical models with different thicknesses properties were modified to simulate the thermocouple
(T) and widths (W) were selected to form a three-di- measurement results. At this time, the thermal con-
mensional rectangular shape to encompass the speci- ductivity and specific heat value of 2,000°F were modi-
men size and heat transfer characteristics to which sol- fied, and at the temperature below that, the properties
ution heat treatment was performed. As for the length provided by the ASME code were applied. Heat treat-
(L), 5 times the width (W) was applied so as not to af- ment heating rate was applied at 50℃/h, convection heat
fect the analysis result. Table 2 shows the analytical transfer coefficient was applied at 0.009BTU/hr·in2·℉.
Table 2 Evaluation analytical models of uniform holding
time during solution heat treatment (1,038℃, 5h) 3. Test Results and Discussion
Analytical models Thickness Width Length 3.1 Effect evaluation of solution heat treatment
Type No. (T, cm) (W, cm) (L, cm) holding time
TM1* 25.4 50.8 50.8
The high intergranular corrosion resistance of auste-
RM1 38.1 2.54 12.7
nitic stainless steel required by the nuclear power in-
RM2 38.1 12.7 63.5
dustry is greatly affected by the process parameters of
RM3 38.1 25.4 127.0 solution heat treatment during the material production
Models for RM4 38.1 38.1 190.5 process that determines the solubility of precipitates
calculating
RM5 25.4 2.54 12.7 near the grain boundary1,10). Therefore, ASTM A262
results
RM6 25.4 12.7 63.5 Practice A and E, TEM, and DL-EPR tests have been
*TM1 : Model
RM7 25.4 25.4 127.0 conducted to investigate the effect of heat treatment
for test
verification RM8 12.7 2.54 12.7 temperature of 1,038~1,121℃, the solution heat treat-
RM9 12.7 6.35 31.75 ment condition specified in the nuclear regulatory re-
RM10 12.7 12.7 63.5 quirements, and the holding time of 0.5~1.0 per 2.54cm
RM11 2.54 1.27 6.35 on the intergranular corrosion.
RM12 2.54 2.54 12.7 In the specimen with the thickness of 2.54cm sub-

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 Eun-Jong Oh, Dong-Hwa Lee, Sung-Woo Cho, Yun-Il Choi and Ki-Woo Nam

(a) (b)

(c) (d)

Fig. 5 Etch structures on the sensitized 2.54cm-thick

specimen (675℃ for 1 h, WC) by ASTM A262
Practice A test (×250)

Fig. 7 TEM analysis results of the sensitized 2.54cm-

thick specimen (675℃ for 1 h, WC). (a) dark field
image (x7k), (b) mapping on 'A' (20k), (c) EDS
spectra on 'B', (d) SAD pattern on 'B', respectively

(a) (b)

Fig. 6 Bent area view of the sensitized 2.54cm-thick speci-

men (675℃ for 1 h, WC) by ASTM A262 Practice
E test (×60)

jected to sensitization at 675℃ for 1 hour before per- (c) (d)

forming solution heat treatment, ditch structure, in which
the sensitization by chromium carbide continuously
precipitated at the grain boundary is suspected, was ob-
served in the ASTM A262 Practice A test, as shown in
Fig. 5. Also, as in Fig. 6, in the ASTM A262 Practice E
test, micro cracks in micro fissure form were observed (e)
in the bent areas on which the bending test was conducted.
It is known that the formation of these micro fissures is
due to the ditch structure caused by intergranular corro-
sion11).
For analyzing the causes of forming the ditch structure
and micro cracks in the form of micro fissures observed Fig. 8 Etch structures of the 2.54cm-thick specimens sol-
in the ASTM A262 practice test, TEM analysis was ution heat treated at 1,038℃ for (a) 1min, (b)
performed as shown in Fig. 7, and it was possible to 5min, (c) 10min, (d) 15min, (e) 30min, respectively,
by ASTM A262 Practice A test (×250)
verify the presence of chromium carbide (Cr23C6) of the
FCC structure clustered in the form of nanoparticles
with a diameter of about 50~300 nm at the grain solved and a step structure, which is the conforming
boundary. From this result, it is thought that the ditch structure that can pass the test was observed. Also, as in
structure and micro cracks identified in the surface of Fig. 10 and Fig. 11, in the ASTM A262 Practice E test,
the sensitized specimen is caused by the precipitation micro cracks of micro-fissure-type were not observed
of chromium carbide at the grain boundary. in the bent areas of the bending test.
On the other hand, in all specimens subjected to sol- The result of the observation of step structure without
ution heat treatment for 1 minute, 5 minutes, 10 mi- micro cracks in the ASTM A262 Practice test was con-
nutes, 15 minutes and 30 minutes at 1,038℃ and 1,121℃, sistent with the TEM analysis result of Fig. 12, in which
as shown in Fig. 8 and Fig. 9, chromium carbide at the chromium carbide precipitated at the grain boun-
grain boundary which was continuously precipitated in dary was completely dissolved into the grains and dis-
the ASTM A262 Practice A test was completely dis- appeared through solution heat treatment at 1,038℃ for

282 Journal of Welding and Joining, Vol. 38, No. 3, 2020

Effect of the Holding Time during Solution Heat Treatment on Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

(a) (b) (a) (b)

(c) (d)
(c) (d)

(e)
(e)

Fig. 9 Etch structures on the 2.54cm-thick specimens sol-

ution heat treated at 1,121℃ for (a) 1min, (b)
Fig. 11 Bent area view of the 2.54cm-thick specimens sol-
5min, (c) 10min, (d) 15min, (e) 30min, respectively,
ution heat treated at 1,121℃ for (a) 1min, (b)
by ASTM A262 Practice A test (×250)
5min, (c) 10min, (d) 15min, (e) 30min, respectively,
by ASTM A262 Practice E test (×60)
(a) (b)

(c) (d)

(a) (b)

(e)

(c) (d)

Fig. 10 Bent area view of the 2.54cm-thick specimens

solution heat treated at 1,038℃ for (a) 1min, (b)
5min, (c) 10min, (d) 15min, (e) 30min, respectively,
by ASTM A262 Practice E test (×60).

1 minute or longer.
(e)
In order to analyze the correlation with ASTM A262
test according to sensitization heat treatment and sol- Fig. 12 TEM micrographs of the 2.54cm-thick specimens
ution heat treatment and to quantify DOS, a DL-EPR solution heat treated at 1,038℃ for (a) 1min, (b)
test was performed. As shown in Fig. 13 and Fig. 14, 5min, (c) 10min, (d) 15min, (e) 30min, respectively,
the corrosion potential of all specimens subjected to (×20k, ×7k)

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 Eun-Jong Oh, Dong-Hwa Lee, Sung-Woo Cho, Yun-Il Choi and Ki-Woo Nam

0.3 102
Material : 304H Material : 304H Sensitized at 675℃ for 1 hr
675℃, 1hr 101 Sensitized(>5%)
0.2 Solution Temp. at 1038℃
1038℃, 1min
Partially Sensitized(1~5%)

DOS(%)
1038℃, 5min 100 Solution Temp. at 1121℃
0.1
1038℃, 10min Not Sensitized(<1%)
10-1
Potential(VSCE)

0.0 1038℃, 15min

1038℃, 30min 10-2
-0.1
10-3
-0.2 0 5 10 15 20 25 30
Solutioning time(min)
-0.3

-0.4 Fig. 15 DOS (Ir/Ip×100%) of the sensitized and solution

heat treated, 2.54cm-thick specimens calculated
-0.5
from Figs. 13 and 14
-0.6
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100
the ASTM A262 test results in which complete step
Current density(A cm-2)
structure was formed and no micro cracks of micro fis-
Fig. 13 DL-EPR curves of the 2.54cm-thick specimens sure type were observed. AS for the DOS evaluation,
sensitized at 675℃ for 1 h (WC) and solution with the application of ISO-12732 standard, the stand-
heat-treated at 1,038℃ for 1~30 min (WC) ards were set to the occurrence of sensitization (DOS
over 5%), partial sensitization (DOS 1~5%) and no sen-
0.3 sitization (DOS less than 1%)6).
Material : 304H 675℃, 1hr
0.2
1121℃, 1min
1121℃, 5min 3.2 Evaluation of uniform holding time according
0.1
1121℃, 10min to material thickness
Potential(VSCE)

0.0 1121℃, 15min

1121℃, 30min As described above, an demonstration test on the sen-
-0.1
sitization with a maximum thickness of 25.4cm, which
-0.2 is the maximum thickness of austenitic stainless steel
-0.3 used in the nuclear power industry, was performed, and
-0.4 through the test, DOS was evaluated according to the
location of each material thickness to verify the appro-
-0.5
priateness of 0.5 hours per 2.54cm, the minimum hold-
-0.6
ing time of solution heat treatment specified in nuclear
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100
regulatory requirements.
Current density(A cm-2)
In the specimen with 25.4cm thickness artificially sen-
Fig. 14 DL-EPR curves of the 2.54cm-thick specimens sitized for 10 hours at 675℃ before the solution heat
sensitized at 675℃ for 1 h (WC) and solution treatment, as shown in Fig. 16, ditch structure, in which
heat-treated at 1,121°C for 1~30 min (WC) sensitization by chromium carbide continuously pre-
cipitated at the grain boundary is suspected, was ob-
solution heat treatment was about -0.4VSCE, regardless served in the ASTM A262 Practice A test. Also, as shown
of the heat treatment temperature and holding time, and in Fig. 17, micro cracks with a micro fissure-type were
anodic dissolution reaction occurred from this corro- observed in the bent areas where the bend test was con-
sion potential to about -0.2VSCE, the basic passivation
potential. Thereafter, the potential reached about+0.2VSCE,
a passivation region accompanied by a decrease in cur-
rent density. During the reverse potential scanning, two
or more current peaks were observed by hydrogen re-
duction reaction and anodic dissolution reaction
As a result of calculating DOS (Ir/Ip×100%), as shown
in Fig. 15, sensitized specimens in which ditch struc-
ture and micro cracks in the form of micro fissures
were observed showed DOS of about 10.8%, and for all
specimens with solution heat treatment of 1 minute or Fig. 16 Etch structures on the sensitized 25.4cm-thick
longer, DOS was reduced to about 0.01%, indicating no specimen (675℃ for 10 h, WC) by ASTM A262
occurrence of sensitization, and this was consistent with Practice A test (×250)

284 Journal of Welding and Joining, Vol. 38, No. 3, 2020

Effect of the Holding Time during Solution Heat Treatment on Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

(a) (b)

(c) (d)
Fig. 17 Bent area view of the sensitized 25.4cm-thick
specimen (675℃ for 10 h, WC) by ASTM A262
Practice E test (×60)

(a) (b)
(e) (f)

(c) (d)

Fig. 19 Bent area view of the 25.4cm-thick specimen sol-

ution heat treated at 1,038℃ for 5h. (a) 0cm(sur-
face), (b) 2.54cm, (c) 5.08cm, (d) 7.62cm, (e)
10.16cm, and (f) 12.70cm away from the surface,
(e) (f) respectively, by ASTM A262 Practice E test (×60)

0.4
0.3 Material : 304H Distance from the surface
0cm 2.54cm
0.2 5.08cm 7.62cm
10.16cm 12.70cm
0.1
Potential(VSCE)

Fig. 18 Etch structures of the 25.4cm-thick specimen sol-

0.0
ution heat treated at 1,038℃ for 5h. (a) 0cm(sur-
face), (b) 2.54cm, (c) 5.08cm, (d) 7.62cm, (e) -0.1
10.16cm, and (f) 12.70cm away from the surface, -0.2
respectively, by ASTM A262 Practice A test (×250)
-0.3

-0.4
ducted in the ASTM A262 Practice E test.
-0.5
On the other hand, in the ASTM A262 Practice A test
-0.6
of a specimen subjected to solution heat treatment for 5 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100
hours at 1,038℃, step structure considered as an ac-
Current density(A cm-2)
cepted structure, which is a conforming structure with
no precipitation of chromium carbide at the grain boun- Fig. 20 DL-EPR curves of the 25.4cm-thick specimen
dary was observed, but dual structure considered as an solution heat treated at 1,038℃ for 5h with the
accepted structure, another conforming structure formed distance from the surface
by the discontinuous formation of chromium carbide
due to the tendency of increasing precipitation of chro- served in the bent areas of the bending test in ASTM
mium carbide getting closer to the center part, was ob- A262 Practice E test, for all positions by thickness of
served as in Fig. 18. This phenomenon is thought to the specimen, as shown in Fig. 19.
have caused by longer exposure to the sensitization In order to analyze the correlation with ASTM A262
temperature range due to slower cooling rate at the cen- test by quantifying DOS of discontinuous chromium
ter than the surface in the water cooling process of sol- carbide formed according to the location per thickness
ution heat treatment. However, although dual structure of the material subjected to solution heat treatment, a
was observed with the move from the surface to the DL-EPR test was performed. As shown in Fig. 20, the
center, no micro cracks in micro fissure type was ob- corrosion potential was about -0.4VSCE, regardless of

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 Eun-Jong Oh, Dong-Hwa Lee, Sung-Woo Cho, Yun-Il Choi and Ki-Woo Nam

102 The measurement results show that, during the cooling

Material : 304H
10 1 Sensitized(>5%) process, the surface is briefly exposed for 1.7 minutes
Partially sensitized(1~5%) to the sensitization temperature range of 427~816℃, while
DOS(%)

100
10-1
the central part is exposed for a long time of 15.3 minutes,
10-2
Not sensitized(<1%) support the test results, in which discontinuous dual struc-
ture was formed with the increase in the precipitation
10-3
0 2.54 5.08 7.62 10.16 12.70 amount of the chromium carbide with a move closer to
Distance from the surface (cm) the center part and DOS was increased with its value
Fig. 21 DOS (Ir/Ip×100%) of the 25.4cm-thick specimen converging to about 0.6%.
solution heat-treated at 1,038℃ for 5h with the However, considering that on the surface, the step
distance from the surface calculated from Fig. 20 structure with no precipitation of chromium carbide at
all was observed and DOS of less than 0.01% was
the location per thickness, and anodic dissolution re- measured, and that the part directly affected by inter-
action occurred from this corrosion potential to about granular corrosion is the surface of the material, the re-
-0.2VSCE, the basic passivation potential. Thereafter, the sults in the center part with the formation of dual struc-
potential reached about+0.2VSCE, a passivation region ture and DOS of less than 0.01% are thought to have no
accompanied by a decrease in current density. During effect on the for the nuclear facility components in the
the reverse potential scanning, two or more current environment of those operation.
peaks were observed. As for the results of finite element analysis on the uni-
As a result of DL-EPR test, as shown in Fig. 21, the form holding time for 12 analytical models according
DOS on the surface was less than 0.01%, but it was to the thickness and width of the material, Fig. 23
confirmed that the DOS increased with getting closer to shows an example of the analytical model RM3 and the
the center, converging to about 0.6% at the depth of analysis results are presented in Fig. 24. In this case,
12.7cm. This is considered to be the result of being ex- the uniform holding time (ⓐ in Fig. 22) refers to the
posed to the sensitization temperature range for a lon- time required for the central part to reach 1,038℃ after
ger time because the cooling rate in the central part is
slower than that in the surface, similar to the results of Finite Element Method (T38.1-W25.4-L127.0cm, RM3)
1950
the ASTM A262 Practice A test. However, DOS less
Temperature(℉)

than 1% falls within the range of no occurrence of sen- 1900

sitization in the ISO-12732 standard, and given that

1850 Surface
there was no micro cracks observed in the ASTM A262 Center
Practice E test, the same results that although ditch 1800
structure was observed getting closer to the center in
1750
the ASTM A262 Practice A test, the observed structure
70000 71000 72000 7300074000 75000 76000 77000 78000 79000
was considered as accepted structure were confirmed in
the DL-EPR test. Time(s)
In addition, as shown in Fig. 22, the thermocouple is Fig. 23 FEM results of analytical model No. RM3
separately installed on the surface and the central part.

min/2.54cm
Temp ⓐ ⓑ ⓐ Uniform holding time 38.1cm-thick (RM1~4)
1.78min/2.54cm 25.4cm-thick (RM5~7)
(℃) (17.8min) ⓑ Effective holding time 4.0 3.74
1038 12.7cm-thick (RM8~10)
Surface Center 2.54cm-thick (RM11~12)
3.0
1.83
816℃ max.
2.0
816
1.0
427
427℃
ce

0.35
r

Surface
nte
rfa

Cent

0.5 max.
Su

Ce

0.10 0.20
er

1.7min max. Width

15.3min 0 (cm)
Time 0
Heating Holding (ⓒ) Cooling
0 1.27 2.54 6.35 12.7 25.4 38.1

Fig. 22 Schematic diagram of the 25.4cm-thick specimen Fig. 24 Schemaic diagram for FEM results of 12 evalua-
solution heat-treated at 1,038℃ for 5h measured tion analytical models of uniform holding time
by thermocouples during solution heat treatment at 1,038℃ for 5h

286 Journal of Welding and Joining, Vol. 38, No. 3, 2020

Effect of the Holding Time during Solution Heat Treatment on Intergranular Corrosion of Unstabilized Austenitic Stainless Steels

the surface has reached 1,038℃ divided by 2.54cm per 675℃ for 1 hour per 2.54cm of material thickness, and
material thickness. The arbitrarily specified thickness the detailed results are as follows.
or width of the material was not significant considering 1) 2.54cm thick coupons subjected to sensitization
the heat transfer occurs by the shortest distance, and as heat treatment at 675℃ for 1 hour were rejected in
the thinner of the thickness and width increased from ASTM A262 test due to a large amount of chromium
1.27cm to 38.1cm, the uniform holding time increased carbide precipitated in the form of grains 50 to 300nm
from 0 minutes to 3.74 minutes per 2.54cm. It was con- in diameter at the grain boundary and showed DOS of
firmed that the rate of increase in uniform holding time about 10.8% in the DL-EPR test. However, as a result
increased as the thickness of the material increased. In of solution heat treatment of the sensitized specimen at
this case, the uniform holding time for 25.4cm, the 1,038℃ and 1,121℃ for 1 minute, chromium carbide
maximum material thickness used in the nuclear power was completely dissolved into the grains, passed the
industry, was evaluated to be 1.83 minutes per 2.54cm. ASTM A262 test, and showed a DOS of 0.01% or less
This result was confirmed to be similar to the result in in the DL-EPR test.
the thermocouple measurement test shown in Fig. 22 in 2) As a result of performing solution heat treatment at
which the center part reaches the temperature 1,038℃ 675℃ for 5 hours at 1,038℃ of 25.4cm thick coupon
slower than the surface by 1.78 minutes (ⓐ) per 2.54cm. subjected to sensitization heat treatment for 10 hours,
Based on these results, as a result of confirming the the specimens passed the ASTM A262 test and the
holding time (ⓒ in Fig. 22) for solution heat treatment DL-EPR test regardless of its location per material
to the center of the material of 25.4cm, which is the thickness. On the surface, a step structure with no pre-
maximum material thickness used in the nuclear power cipitation of chromium carbide at all and DOS of
industry, after the center reaches 1,038℃ for 18.3 mi- 0.01% or less were observed. As moving toward the
nutes considering the uniform holding time (ⓐ in Fig. center, some dual structures were observed due to lon-
22) of 1.83 minutes per 2.54cm, considering 1 minute, ger exposure than in the surface to the sensitization
which is the effective holding time (ⓑ in Fig. 22) of temperature range of 427~816℃ in the cooling process,
solution heat treatment during which the chromium car- and DOS of about 0.6% was obtained. Nevertheless,
bide precipitated at the grain boundary is completely considering that the area directly affected by inter-
dissolved into the grains at 1,038℃ in the above effect granular corrosion is the surface part rather than the
evaluation of solution heat treatment holding time, the center part, it is thought that the nuclear facility compo-
holding time for complete solution heat treatment to the nents will not be affected in the operation environment.
center is calculated as about 19.3 minutes by addition. 3) Considering the holding time of 1 minute during
Therefore, from conservative viewpoint, the holding which the chromium carbide precipitated at the grain
time for complete solution heat treatment to the center boundary at 1,038℃ is completely dissolved into the
of the material was determined to be up to 2 minutes grains, as well as the analysis result in which the center
per 2.54cm of the material thickness. reached 1,038℃, the solution heat treatment temper-
ature, later than the surface by 18.3 minutes at the
4. Conclusion thickness of 25.4cm, the maximum thickness used in
the nuclear power industry, the holding time for com-
In this study, for the 0.74wt.%C stainless steel materi- plete solution heat treatment to the center of the materi-
al, which is higher than the maximum carbon content al is determined to be 2 minutes at maximum per
regulation requirement of the nuclear power industry at 2.54cm of the material thickness.
0.65wt.%C, a test to verify the holding time according
to the solution heat treatment temperature of the un- ORCID: Eun-Jong Oh: https://orcid.org/0000-0002-4907-7936
stabilized austenitic stainless steels specified in the ORCID: Dong-Hwa Lee: https://orcid.org/0000-0001-9082-1934
Safety Analysis Report, one of the nuclear regulatory ORCID: Sung-Woo Cho: https://orcid.org/0000-0002-1393-9880
requirements, was conducted, and an demonstration test ORCID: Yun-Il Choi: https://orcid.org/0000-0003-2929-0550
ORCID: Ki-Woo Nam: http://orcid.org/0000-0001-7019-358X
considering the thickness 25.4cm, the maximum mate-
rial thickness used in the nuclear power industry, was
also included. As a result of these tests, the solution References
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288 Journal of Welding and Joining, Vol. 38, No. 3, 2020