Materials Science Forum Vols 654-656 (2010) pp 2515-2518
© (2010) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.654-656.2515
Online: 2010-06-30
A Study of Toughness Degradation in CA6NM Stainless Steel
Yoshitaka Iwabuchi1, a and Isao Kobayashi1, b
1
Kushiro National College of Technology, Otanoshike-nishi, Kushiro City Hokkaido, Japan
a
iwa@mech.kushiro-ct.ac.jp, bikoba@mech.kushiro-ct.ac.jp
Keywords: Stainless steel, toughness degradation, intergranular fracture, carbide precipitation,
carbon solubility, mosaic-like marking
Abstract. The mechanism of toughness degradation during slow cooling in the austenite range was
studied in CA6NM stainless steel, 13% Cr-4% Ni soft martensitic stainless steel. The variation of
toughness, fracture mode and microstructural features were examined by means of cooling rate and
isothermal heating in the austenite range together with chemical composition. Toughness
degradation was referred to as the increases of FATT and intergranular fracture when those steels
were cooled slowly after austenitizing and isothermally heated in the austenite range. The
embrittlement was found to be related the intergranular fracture and the precipitation of carbide
along prior austenite grain boundaries. Its fracture surface was characterized by mosaic-like
markings when the carbide precipitation got to increase. Reducing carbon, silicon and phosphorus
and increasing molybdenum improve the toughness degradation.
Introduction
The soft martensitic stainless steels have lower carbon and increased nickel contents comparing
with normal martensitic stainless steels [1]. The transformation behavior of these steels is
characterized by the delayed pearlite transformation, the suppression of the intermediate phase and,
consequently, primarily a martensite transformation. As the transformation to pearlite is further
delayed by Ni and effectively suppressed by 3% Ni or more, furnace cooling is sufficient to bring
about complete transformation to martensite. Proper choice of the chemical composition and heat
treatment can produce a good combination of strength and toughness.
However impact notch toughness has been found to deteriorate in these steels, when slowly cooled
from or isothermally heated in the austenite range. Such toughness deterioration known as grain
boundary embrittlement impose many practical problem [2]. The problem of grain boundary
embrittlement of these steels has been with us for several years, and the study of it now appears to
be entering the final stage. This paper discussed several factors affecting intergranular failure along
prior austenite grain boundary of type CA6NM stainless cast steel with special emphasis on heat
treatment and alloying elements.
Experimental Precedures
Table 1 Chemical composition of steels used for this study (mass %)
The experimental alloys used Des ignation
C
Si
Mn
P
S
Ni
Cr
Mo
were based on CA6NM steel, soft
CA6NM
＜ 0.06
＜ 1.00
＜1.00
＜0.030
＜0.030
3.50-4.00 11.50-14.00
0.40-1.00
Standard
0.04
0.45
0.66
0.022
0.024
3.80
12.12
0.14
martensitic stainless cast steel. In
Rang e
0.04-0.08 0.07-0.45
0.007-0.049
0.01-0.34
order to study the effect of
chemical composition on grain boundary embrittlement, 50 kg laboratory induction melts were
made and poured into cold setting sand molds of size 85×260×160 mm3. The influence of C, Si, Mo
and P on metallographic and mechanical properties was investigated using 11 heats of which
chemical composition is given in Table 1. Austenitization was carried out at 1223K, which was
considered high enough to allow complete dissolution of carbides without excessive grain
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coarsening. A typical heat treatment to obtain the desired properties is austenitizing at 1223K for
28.8ks, followed by air cooling to ambient temperature and tempering for 36ks at 893K. In order to
study the embrittlement in the austenite range, some steel were cooled at the various cooling rates
of 0.0014 to 0.17 K/s from 1223K, and the others were exposed to an isothermal treatment of
holding at 923K for 7.2 to 360ks during cooling from 1223K. Fractured surfaces of Charpy impact
specimens were examined with a scanning electron microscope. Charpy V impact tests were carried
out at the temperature between 173 and 373K.
Results
Figure 1 shows the variation of 50% ductile-fracture appearance transition temperature (FATT) as a
function of the cooling rate from 1223K. When the cooling rate is increased, FATT is abruptly
lowered and then slightly decreased. The effect of C, Si, Mo and P on the toughness is depicted in
Fig.2, indicating the shift of FATT to high temperature side with increasing C, Si and P contents. It
is noted that lowering C, Si and P contents and adding Mo decrease FATTs.
500
150
solid line : intergranular fracture
broken line : intragranular fracture
400
P
]K
[ 50
T
T
A
F
fo
tfi 0
S
]
K
[
T 300
T
A
F
200
100
C
100
Si
-50
10-3
10-2
Cooling Rate [K/s]
10-1
Fig. 1 Variation of FATT of the steels with
cooling rate
-100
Mo
0
0.1
0.2
0.3
Increment of Alloying Elements [mass%]
0.4
Fig. 2 Variation of the shift of FATT with
alloying elements
Figure 3 is the scanning electron fractgraphs of brittle fracture appearance of Charpy impact
specimens tested at a temperature corresponding to FATT, indicating the typical mode of
intergranular fracture along the austenite grain boundaries. It reveals that the fracture surface is
characterized by mosaic-like markings as the cooling rate decrease, although the rapidly cooled
specimens show the normal intergranular appearance with the flat and smooth surface.
Figure 4 shows the variation in fracture mode of three specimens having 0.007, 0.026 and 0.049%P
with isothermal holding time at 923K. It is clear that the intergranular fracture mode is increased
with increasing isothermal-heating time and P content.
Figure 5 shows the transmission
electron
micrographs
of
carbon
extraction replicas taken from the
intergranular fractured surface and the
polished-etched cross section of the
specimens with 0.04%C cooled at
1.4K/ks from 1223K and followed by
50μ
50μm
tempering at 873K for 36ks. It is noted
that the prior austenite grain boundary is
Fig. 3 Scanning electron fractographs of brittle fracture
covered by carbide precipitates which
appearance of impact specimens with 0.06%C
Materials Science Forum Vols. 654-656
2517
100
0.049%P
]
%
[
e
r
u
t
c
a
r
F
r
a
l
u
n
a
r
g
r
e
t
In
80
0.026%P
60
40
0.007%P
20
0
104
105
Isothermal Holding Time [s]
1μm
106
Fig. 4 The fraction of intergranular fracture
vs. isothermal holding time at 923K
were identified as type Cr23C6 carbide. A large
amount of intergranular carbide precipitates are
unavoidable where the slow cooling and the
long-period isothermal heating in the austenite
range are performed respectively. From the
transmission electron micrographs of carbon
replicas of the specimens with different
amounts of P, which are isothermally held at
923K for 86.4ks during cooling from 1223K
and followed by tempering at 853K, it is noted
that with increasing P contents, the carbide
precipitates are unambiguously observed along
the prior austenite grain boundary.
Figure 6 shows the relation between the fraction
of intergranular fracture and the corrosion loss
after immersing in a boiled 4.4% HNO3
solution. It is noted that the magnitude of
intergranular fracture becomes large with
increasing corrosion loss.
5μm
Fig. 5 Transmission electron micrograph of
carbon extraction replicas.
100
]
%
[
e
r
u
t
c
a
r
F
r
a
l
u
n
a
r
g
r
e
t
n
I
80
60
40
20
0
30
40
50
60
2
Corrosion Loss [mg/cm ･d]
Fig. 6 The fraction of intergranular fracture vs.
weight loss after immersion in 4.4%HNO3
Discussion
Extensive studies [3, 4] have been performed on the carbide precipitation at grain boundaries in
austenitic stainless steels. The rate determining process of the carbide precipitation has been
theoretically discussed by Arai [5] using austenitic stainless steel. Since an only austenite phase
exists until a martensite transformation is brought about, it is right to discuss the carbide
precipitation in the austenite range for CA6NM steel as well as austenitic stainless steels.
The amount of carbide is expressed as eq. (1), where the rate determining process is the rate of
carbide reaction.
(1)
ΔC = (C0-Cs)｛1-exp(3kt/r0)｝
where ΔC : amount of C, C0 : initial C content, Cs : C solubility, k : reaction rate constant, t : time,
r0 : austenite grain diameter assuming they have a spherical geometry.
Area fraction of carbide precipitates, A0 was measured as 0.095%, which was converted into
0.0015% of weight fraction, using that the volume fraction is A0√A0, and the densities of austenite
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and (Cr ･ Fe)23C6 are 7.9 and 6.92 g/cm3. A
1100
time-temperature- Δ C (0.0015%) diagram of
0.08%C
13%Cr-4%Ni steel is obtained from eq.(1) as
shown in Fig.7. These theoretical curves also
0.06%C
1000
correspond to the critical cooling rate at which the
0.04%C
]K
[
mosaic-like marking appeared on the fractured
er
ut 900
grain boundary surface.
ar
It is clear that coarse carbides are observed
ep
me
densely on prior austenite grain boundaries in the
T
specimens of high amounts of P. Banerjee [6]
800
reported that P was substituted for metallic
elements and included in M23C6 carbide as
700
(Cr18Fe4P1)C6 leading the reduction of lattice
1
10
100
1000
10000
constant, and consequently it gave the ease of the
Time [ks]
carbide nucleation. The basic statistical technique
employed was multiple regression analysis for the
Fig.7 Time-temperature –∆C(0.0015%C) diagram
correlation in FATTs with cooling rate, R0 and
obtained by theoretical calculation.
carbon content. The result of regression analysis
gives eq. (2).
FATT(K) = 2586×(%C) – 91×log〔R0 (K/s) – 47 …………(2)
The regression equation satisfactorily predicts FATT. Although uncertainty exists in the role of
carbide concerning the toughness, increasing carbide content and decreasing cooling rate in the
austenite range promote the carbide precipitation and also coarsen its size, consequently lower the
absorbed energy.
The precipitation and growth of M23C6 carbide, which plays the important role on the
toughness, has been studied [3] in austenitic stainless steels, and there was parallel relation between
the carbide and the matrix. Supposing that 13%Cr-4%Ni steels have the same relation as austenitic
stainless steels concerning the carbide precipitation, mosaic-like marking on the grain boundary
surface is presumed to be the marks of M23C6 carbide precipitated in parallel to each other.
Summary
The experimental results obtained in this work lead to the following conclusion.
(1) Carbide precipitated in the austenite range causes adverse effect on the toughness and gives
the grain boundary embrittlement with mosaic-like marking.
(2) Toughness degrades by slow cooling and isothermal heating in the range between 823 and
1023K during cooling from austenitizing temperature.
(3) The embrittlement is related to the intergranular fracture and the precipitation of carbides
along prior austenite grain boundaries.
(4) Susceptibility to the embrittlement is improved by reducing C, Si and P, and adding Mo
improves toughness as the result of retarding carbide precipitation along grain boundaries.
References
[1]
[2]
[3]
[4]
[5]
[6]
Soursny H. and Sauer H: Giesserei, 58(1971), 775
C.Leymonie, M.C.Ottman and R.Risache: PowerIndustry Research, (1982), 17
L.K.Singhal and J.W.Martin: Trans.Metall.Soc.AIME, 242(1968), 814
C.S.Tedmor Jr. and D.A.Vermilyea: Metall. Trans., (1970), 2043
H.Arai: Tetsu-to-Hagane, 56(1970), 44
S.K.Banerjee, CJ.McMahon Jr. and H.C.Feng: Metall. Trans. A, 9(1978), 237
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10.4028/www.scientific.net/MSF.654-656
A Study of Toughness Degradation in CA6NM Stainless Steel
10.4028/www.scientific.net/MSF.654-656.2515