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Corrosion Science
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Improving intergranular corrosion resistance in a nickel-free and
manganese-bearing high-nitrogen austenitic stainless steel through
grain boundary character distribution optimization
F. Shi
a
, P.C. Tian
a
, N. Jia
b
, Z.H. Ye
a
, Y. Qi
a,b
, C.M. Liu
b
, X.W. Li
a,b,∗
a
b
Institute of Materials Physics and Chemistry, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China
a r t i c l e
i n f o
a b s t r a c t
Grain boundary character distribution (GBCD) and the effect of GBCD optimization on the intergran-
ular corrosion (IGC) of a cold rolled and subsequently annealed nickel-free and manganese-bearing
high-nitrogen austenitic stainless steel were investigated. The results show that the fraction of low
coincidence site lattice boundaries increases from 47.3% for the solid solution treated specimen to 83.3%
for the specimen cold-rolled by 7% and then annealed at 1423 K for 10 min. 3 boundaries of high fraction
effectively interrupt the connectivity of non-corrosion-resisting boundaries (special boundaries like 9,
27, etc. and general high angle boundaries) network, thus improving the IGC resistance.
© 2016 Elsevier Ltd. All rights reserved.
Article history:
Received 13 October 2015
Received in revised form 11 February 2016
Accepted 11 February 2016
Available online xxx
Keywords:
A. Stainless steel
B. Weight loss
B. SEM
C. Intergranular corrosion
C. Welding
1. Introduction
As austenite-stabilizing elements, nitrogen and manganese are
introduced into the austenitic stainless steel (ASS) to replace more
expensive nickel, which can significantly reduce the material cost
[1,2].
In addition, nitrogen is a more effective solution strengthen-
ing element compared with carbon. The addition of nitrogen to the
nickel-free and manganese-bearing high-nitrogen austenitic stain-
less steel (HNASS) can not only increase the fine grain strengthening
effect and improve the strength, but also retain the toughness
of the steel. Compared to most ASSs, HNASS provides a higher
strength. For instance, the yield strength and tensile strength of
HNASS can exceed 200–350% of the AISI200 and AISI300 series
stainless steels without sacrificing toughness in the annealed con-
dition
[3–5].
Moreover, the addition of nitrogen also improves the
resistance to localized corrosion of ASSs
[6–9].
Therefore, as one
of the main directions in the structural steel adjustment, great
attention has been focused on this kind of resource-saving stainless
steels
[10–13].
∗
Corresponding author at: Institute of Materials Physics and Chemistry, School
of Materials Science and Engineering, Northeastern University, Shenyang 110819,
China.
E-mail address:
xwli@mail.neu.edu.cn
(X.W. Li).
http://dx.doi.org/10.1016/j.corsci.2016.02.019
0010-938X/© 2016 Elsevier Ltd. All rights reserved.
However, a high nitrogen content may cause the precipitation
of nitride during thermal processing, welding and service at high
temperatures in such advanced steels. Studies by Ogawa et al.
[14]
indicated that the precipitation of nitride would occur after aging
for 2s in the temperature range between 1173 K and 1373 K in
a 23Cr-4Ni-2Mo-1N HNASS. It is very difficult to avoid the pre-
cipitation in HNASS during thermal and/or mechanical processes.
The nitride precipitation can cause serious intergranular corrosion
(IGC) tendency in the heat affected zone (HAZ), which directly limits
the practical applications of such advanced steels.
Studies by Pumphrey
[15]
and Randle
[16]
have demonstrated
that IGC strongly depends on the crystallographic nature and
atomic structure of grain boundaries (GBs). Low energy GBs such
as low coincidence site lattice (CSL) boundaries exhibit a strong
resistance to IGC and are called “special boundaries (SBs)”. The con-
cept of “Grain boundary design and control” proposed by Watanabe
[17],
i.e., grain boundary engineering (GBE), is such a scheme for
improving the resistance to GB failure of materials by means of
increasing the proportion of SBs in grain boundary character dis-
tributions (GBCD)
[18,19].
GBE has been widely applied as a new method to solve the
problems of GB failure
[20,21],
especially IGC, in recent years
[22–29].
However, the investigations on the GBCD optimization
prohibiting the IGC are mainly focused on some conventional ASSs,
such as 304 ASS and 316 ASS, in which IGC is induced by the
Please cite this article in press as: F. Shi, et al., Improving intergranular corrosion resistance in a nickel-free and manganese-
bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization,
Corros. Sci.
(2016),
http://dx.doi.org/10.1016/j.corsci.2016.02.019
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F. Shi et al. / Corrosion Science xxx (2016) xxx–xxx
in a 23Cr-4Ni-2Mo-1N HNASS. Their research findings suggested
that the GBCD optimization suppressed the nitride precipitation in
the HAZ during welding, but concrete corrosion results were not
given and the mechanism of GBE for improving the IGC problem
was not elaborated. Our previous work has studied the GBCD opti-
mization technology and discussed the optimization mechanism in
nickel-free and manganese-bearing HNASS
[31].
The present work
focuses on the effect of GBCD optimization on IGC in this kind of
HNASS, in order to provide a new solution for solving the problem
of IGC caused by precipitation of nitride during welding in the steel.
2. Experimental procedures
The experimental steel 18Cr-18Mn-0.63N (wt%) used in the
present study was melted by induction furnace and electroslag
remelting (ESR) furnace both filled with N
2
, and cast into water-
cooled mould. After forging at 1473 K, the cast ingot was hot-rolled
at 1373 K into a plate with 6 mm thickness and then cold rolled into
plates with 4 mm thickness. The chemical composition of the exper-
imental steel is listed in
Table 1.
The cold-rolled plates were heated
at 1323 K for 1 h followed by water quenching (thereafter referred
Fig. 1.
Effect of cold rolling reduction on the fraction of SBs in the HNASS.
precipitation of carbides
[22,23,26–29].
So far, there are few reports
on the GBE applied for HNASS to prohibit the IGC by the precipita-
tion of nitride. Kokawa et al.
[30]
have ever utilized the GBE method
Fig. 2.
EBSD-reconstructed images of low CSL boundaries in the BM and those specimens cold-rolled to different deformation amounts and then annealed at 1423 K for
10 min. (a) BM, = 0, (b) = 3%, (c) = 5%, (d) = 7%, and (e) = 10%.
Please cite this article in press as: F. Shi, et al., Improving intergranular corrosion resistance in a nickel-free and manganese-
bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization,
Corros. Sci.
(2016),
http://dx.doi.org/10.1016/j.corsci.2016.02.019
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3
N
0.63
C
0.056
S
0.0029
P
0.0013
Al
0.02
Fe
Balanced
Table 1
Chemical composition of the experimental steel (wt%).
Cr
17.97
Mn
18.0
Table 2
The proportions of various SBs for the BM and those specimens after cold rolling at different thickness reductions and annealing at 1423 K for 10 min.
GBE treatment process
Solid solution (BM)
r3%-a1423 K/10 min
r5%-a1423 K/10 min
r7%-a1423 K/10 min
r10%-a1423 K/10 min
3 (%)
42.6
44.4
70.3
73.1
65.2
1.2
1.5
6.7
6.1
6.3
9 (%)
0.2
0.3
3.8
3.2
3.6
27 (%)
Other low CSL grain boundaries (%)
3.3
3.6
1.2
0.9
1.3
Total SBs (%)
47.3
49.8
82.0
83.3
76.4
to as solid solution treatment). The solution-treated specimen was
termed here as the base material (BM). The BM was cold-rolled by 3,
5, 7 and 10% in thickness reduction, and then annealed at 1423 K for
10 min in vacuum atmosphere and quickly cooled in water. For the
electron back scatter diffraction (EBSD) measurements, the sample
surfaces were subsequently electropolished in a solution of HClO
4
:
CH
3
CH
2
OH = 8: 100 (volume fraction) under 30 V for 15 s. The GBCD
of specimens was examined by orientation imaging microscopy
(OIM) system, which is attached to a JEOL JSM 7001F field emis-
sion scanning electron microscope (FESEM). EBSD analyses were
performed on at least four different areas (each 817
×
612 m
2
)
for each specimen, with a step size of 1 m. The fractions of the
different grain boundary types were determined on the basis of
the length fraction by dividing the number of pixels of a particular
boundary by that of the entire grain boundaries. Grain boundaries
with
≤
29 were regarded as the coincidence site lattice (CSL)
boundaries with low energy
[17,18],
and the others are random
boundaries with high energy. The Brandon criterion was used to
assess the CSL boundaries
[32].
The IGC properties were evalu-
ated by performing oxalic acid electrochemical corrosion testing
and ferric sulfate-sulfuric acid testing after sensitization at 1123 K
for 2 h
[33–35].
The specimen was etched in a 10% oxalic acid
solution under 1 A/cm
2
current density for 90 s by oxalic acid elec-
trochemical corrosion testing
[36].
The microstructures sensitized
were observed by FESEM, and the microstructures after oxalic acid
electrochemical corrosion were observed by Olympus GX 71 opti-
cal microscopy (OM) and the corresponding EBSD measurement by
FESEM was performed. Ferric sulfate-sulfuric acid test was carried
out in a boiling solution of 6.8 mol/L H
2
SO
4
plus 0.1 mol/L Fe
2
(SO
4
)
3
[36].
The size, surface area and weight of original specimens were
firstly tested, and then they were immersed into the corrosion solu-
tion for different time periods (8–48 h). After each corrosion test,
the specimen was taken out for cleaning and drying, and subse-
quently reweighed. According to the following formula
[22,36],
the
weight loss curve is obtained.
w
t
=
W
0
−
W
t
,
S
(1)
specimens cold-rolled at 5% and 7% reductions are higher, exceed-
ing 80%. The highest
f
SBs
is around 83.3% in the specimen cold-rolled
at 7% reduction and annealed at 1423 K for 10 min (herein called
r7%-a1423 K/10 min specimen for short).
The EBSD-reconstructed images for low CSL boundaries and
the proportions of various CSL boundaries for the BM and the
specimens cold-rolled to different reductions and then annealed
at 1423 K for10 min are shown in
Fig. 2
and
Table 2,
respec-
tively. The SBs are shown in color and the general high angle
boundaries (HABs) are shown in black. It is apparent that the r3%-
a1423 K/10 min specimen (f
SBs
49.8%) shows a high connectivity
of the general HABs network, as seen in
Fig. 2(b),
which is basi-
cally similar to the case of BM specimen (f
SBs
47.3%) as indicated
in
Fig. 2(a).
However, for the r10%-a1423 K/10 min specimen (f
SBs
76.4%), the connectivity of the general HABs network becomes
greatly weakened, as shown in
Fig. 2(e).
Furthermore, from
Fig. 2(c)
and (d) it can be observed that the connectivity of the general
HABs network has been seriously interrupted in those specimens
exceeding 80% for the proportion of
CSL, especially in the r7%-
a1423 K/10 min specimen (f
SBs
83.3%). As seen in
Fig. 2,
the so-called
grain-cluster reported by some researchers
[22,25,37]
also occurs
in the experimental steel. The grain-cluster was referred to such a
grain encircled by the general HABs and containing a large amount
of
3
n
boundaries within the grain
[25].
The size of the grain-
CSL boundaries
cluster gradually increases as the proportion of
increases (Fig.
2(c)–(e))
and the size is the largest in the r7%-
a1423 K/10 min specimen. The large-sized grain-cluster becomes
a common feature of the microstructures after GBCD optimization
[22].
where
w
t
is weight loss,
W
0
the weight of the specimen before
test,
W
t
the weight of specimen after test, and
S
is the total surface
area of the specimen. The surfaces and cross-sections of the tested
specimens were then observed by FESEM.
3. Results
3.1. Effect of the GBCD optimization on SBs
Fig. 1
shows the effect of the cold-rolling reduction on the
fraction of special boundaries (f
SBs
) in the HNASS during thermo-
mechanical processing. The
f
SBs
in all specimens after GBCD
optimization increases compared with that in the BM. The
f
SBs
in the
Fig. 3.
Triple-junction distribution for the BM and the specimens cold-rolled at
different deformation amounts and subsequently annealed at 1423 K for 10 min.
Please cite this article in press as: F. Shi, et al., Improving intergranular corrosion resistance in a nickel-free and manganese-
bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization,
Corros. Sci.
(2016),
http://dx.doi.org/10.1016/j.corsci.2016.02.019
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Fig. 4.
SEM images obtained after sensitization for the BM and the specimens cold rolled at different deformation amounts and subsequent annealing at 1423 K for 10 min.
(a) BM, = 0, (b) = 3%, (c) = 5%, (d) = 7%, and (e) = 10%.
Fig. 5.
SEM observation and EDS results of the BM: (a) Morphology of precipitate, (b) EDS of precipitate, and (c) EDS of matrix.
Please cite this article in press as: F. Shi, et al., Improving intergranular corrosion resistance in a nickel-free and manganese-
bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization,
Corros. Sci.
(2016),
http://dx.doi.org/10.1016/j.corsci.2016.02.019
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5
Precipitate (wt%)
7.58
8.78
47.39
11.63
24.62
Matrix (wt%)
7.83
–
14.40
17.65
60.12
Precipitate (at%)
22.37
22.19
32.30
7.51
15.63
Matrix (at%)
28.02
–
11.90
13.81
46.27
Table 3
The element contents of precipitate and matrix.
Element
C
N
Cr
Mn
Fe
Fig. 6.
Optical micrographs showing the grain boundary morphologies subjected to oxalic acid electrolytic corrosion in the sensitized BM and those specimens cold-rolled
to different deformation amounts and annealed at 1423 K for 10 min: (a) BM, = 0, (b) = 3%, (c) = 5%, (d) = 7%, and (e) = 10%.
Table 4
Comparisons of fractions of SBs and precipitates as well as corrosion extents after oxalic acid electrolytic corrosion in the BM and GBE treated specimens.
GBE treatment process
solid solution (BM)
r3%-a1423 K/10 min
r5%-a1423 K/10 min
r7%-a1423 K/10 min
r10%-a1423 K/10 min
Total SBs (%)
47.3
±
1.8
49.8
±
2.1
82.0
±
1.5
83.3
±
1.9
76.4
±
2.7
The fraction of precipitates (%)
5.2
±
0.5
2.9
±
0.2
0.8
±
0.04
0.4
±
0.02
2.2
±
0.3
Corrosion extent
SE
ME
SL
NE
ME
Please cite this article in press as: F. Shi, et al., Improving intergranular corrosion resistance in a nickel-free and manganese-
bearing high-nitrogen austenitic stainless steel through grain boundary character distribution optimization,
Corros. Sci.
(2016),
http://dx.doi.org/10.1016/j.corsci.2016.02.019

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