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Research on Fracture Behavior of High Nitrogen Austenitic Stainless Steels at Cryogenic Temperature
Proceedings of SineSwedish Structural Materials Symposium 2007
Research on Fracture Behavior of High Nitrogen Austenitic Stainless Steels at Cryogenic Temperature LI Hua-bing, JIANG Zhou-hua, ZHANG Zu-mi, XU Bao-yu (Northeastern University, School of Materials and Metallurgy, Liaoning, Shenyang, 11OOO4, China)
Abstract: This paper investigates the ductile to brittle transition (DBT) behavior of 19.55Cr-19.5Mn-2.26Mo-0.96N,
19.56Cr-19.4Mn-2.29Me0.82N and 19.84Cr18.9Mn-2.26Mo-0.88N steels from 77K to 290K by Charpy V-Notch (CVN) impact tests, and then reveals the possible fracture mechanism by SEM and "EM. With the concentration of nitrogen increasing, the ductility at low temperature decreases rapidly and the DBTT increases. The fracture modes are transgranular and intergranular fractures. Fracture facets along ( 1 1 1 ) and annealing twin boundary appear in the impact fractures at low temperature. River patterns which are the typical features of cleave fracture facets and tear ridges which are the typical feature of quasi-cleave fracture facets exist at cryogenic temperature. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple-shallow
dimple-tmixture of quasi-cleavage facet and dimple+cleavage facet.
Key words: ductile to brittle transition; impact fracture; cryogenic temperature; transgranular and intergranular
1. Introduction High nitrogen austenitic stainless steels have a
fracture mechanism of the steels in cryogenic temperature is interpreted.
favorable combination of strength and toughness at
2. Experimental Procedures
room temperature and excellent corrosion resistance. 1 8 0 -18Mn-2Mo-N type austenitic stainless steels have recently been investigated widely. Moreover, this type of steels for cryogenic structural materials has been applied. Because of the stable f.c.c. structure, metals and alloys with f.c.c. lattice are usually ductile over the whole temperature range. However, steels
The chemical compositions of high nitrogen austenitic stainless steels with above 0.8% nitrogen content investigated are shown in Table 1. The high nitrogen austenitic stainless steels used in this experiment are manufactured using vacuum induction furnace by adding nitrided ferroalloy under
with high contents of manganese and nitrogen"-*] are the exceptions. Brittle fracture behavior of these alloys at low temperature has first been reported in 1969 13] and later by several authors [4-61. In the present work, the ductile to brittle transition behavior of Fe-Cr-Mn-Mo series high nitrogen austenitic stainless steels are investigated, and the
nitrogen atmosphere, and using the electroslag furnace for remelting under nitrogen atmosphere. The steels are hot rolled to 12 cm and solution-treated at 1100°C for 60 minutes, followed by water quenching. Then ASTM standard V-notched Charpy impact test specimens are prepared from the longest part of these as-quenched specimens which is parallel to the rolling direction. Charpy impact tests are performed in the temperature
Table 1 Chemical commition of the Cr-Mn-Mo-N hiph nitrogen austenitic stainless steel, % Steel
C
Si
Cr
Mn
Mo
Ni
S
P
Al
0
N
A1
0.058
0.19
19.55
19.5
2.26
--
0.003
g.03
0.04
0.0048
0.96
A2
0.022
0.19
19.84
18.9
2.26
--
0.002
50.03
0.002
0.0042
0.88
A3
0.049
0.19
19.56
19.4
2.29
--
0.003
50.03
0.04
0.0050
0.82
Proceedings of Sino-Swedish Structural Materials Symposium 2007
SSX-550 operating at 15 kV. From the fracture
range of 77K to room temperature (RT) in order to obtain the DB'IT of the HNS investigated in this paper. The SEM and TEM methods are used to observe the morphology of the fracture facets in order to reveal the possible fracture mechanism. Thin foils for TEM which are either parallel or perpendicular to the fractured surface are prepared by mechanical grinding down to 80 pm, and finished by a twin-jet electrolytic polishing technique using an electrolyte of 8% perchloric acid in ethanol at -25'C. The electron micrmpy and analysts is used by TECNCI G2 20
photographs in Fig.3, the change of fracture patterns of A2 steel with ultra low carbon concentration is dimplehshallow dimple+mixture of quasi-cleavage fracture facet and dimple-cleavage fracture facet at different temperatures. The fracture morphology at RT shows favorable ductility and equiaxial dimples in Fig.2 (a). With decreasing the temperature, dimples become shallow and small at -40 "C [Fig.2 (b)]. At -90 "C which is determined as ductile to brittle temperature, dimples and quasi-cleavage facet exist together [Fig.2 (c)]. Some tear ridges which are the typical feature of quasi-cleave fracture appear. Many smooth facets which exist on cleavage fracture surface can be observed in Fig.2 (d). Some of SEM morphologies of this investigation which have been mentioned by the previous authors are also observed in this paper. For example, Fig.3 (a) shows a facet which fractures along { 1 1 1 } at -90 "C reported by Tobler et al. [41 Fig.3 (b) shows the typical facet which fractures along an annealing twin boundary at -140 "C reported by Liu S.C. er al. [71 There are three sets of parallel lines on the facet and the lines in different sets intersect at 60". It means that although many annealing twin grains after solution treatment can be observed by optical microscope, few morphologies of annealing twin grains can be seen on the fracture facets. Fig.3 (c) shows the deformation twinning on the fracture at -90 "C reported by Chen
E M at 200 kV.
.-
_--
dm
.m
+WlKt.bpUllb4pI,
wmr.mW1*lpDm
J
'
-
M b u r , l r o -w # c o _ ~ , L
.m
*
0
P
Tii
Fi.1 DBlT curves d A l , A2 and A3 steel
3. Rmseu#s and Discussion 3.1 WECztile tQ Brittle transition curves Fig.1 shows the temperature dependence of impact absorbed energies obtained from Al, A2 and A3 steels tested at various temperatures by Charpy impact tests. As seen, the ductile to brittle temperatures of Al, A2 and A3 are determined as -85 "C, -90"C and -95 "C respectively. The impact absorbed energy of A2 (with ultra low carbon concentration) is higher than the other two curves at RT. It may be due to the less carbon concentration than the other two steels. However, the ductility decreases rapidly and the brittleness exhibits obviously with increasing the nitrogen concentration at low temperature. This phenomenon is also observed from the SEM photographs at the followed test. Results show that the nitrogen concentration mainly affects the ductility and brittleness at cryogenic temperature. 3.2 SEM observationson fracture surfaces The SEM observations are carried out on a
K.M. er al. [*' Slipping traces in left deformation twinning grain and the twin boundary intersect at 70". The result is consistent with 70'32' in theoretical calculation. The width of the deformation twinning grain is about 5pm, and the slipping traces are deformed in the right matrix crystal. However, the other SEM morphologies of this investigation show different features which have been mentioned by the previous authors as follows: (1) River patterns which are the typical features of cleave fracture facets exist at cryogenic temperature in Fig.4 (a). Arrow A shows that river patterns start at grain boundary then converge and cross to the adjacent grain. Arrow B points to the inclusion, and river patterns 326
Proceedings of Sino-Swedish Structural Materials Symposium 2007
originate from the inclusion. (2) Arrow C points at an intergrangular crack in Fig.4 (b). In high Mn austenitic steels, intergranular fracture has been frequently observed at low temperature, reported by Tomota et al. '51 (3) Transgrangular fracture occurs along annealing twin plane which is an unusual morphology as shown in Fig.5 (a). With decreasing the temperature, HNS will exhibit more brittleness. (4) Tear ridges which are the typical feature of quasi-cleave fracture facets are observed in Fig.5 (b) at -90 'C. The ductile to brittle transition occurs, and the typical fracture pattern is
mixture of quasi-cleavage facet and dimple at that temperature. This morphology may conduce to absorb energy. 3.3 TEM investigations on fracture surfaces Typical examples of dislocation structures are developed under the fracture surfaces of A1 steel. They are perpendicular to the fracture surface presented in Fig.6. The dislocation density decreases obviously with decreasing the temperature. Dislocation is the leading way to deform at a relative higher temperature. This is why the impact absorbed energy at-196 "C is less
Fig.2 SEM morphologies of impact fracture facets at (a) 20
C,(b)-40 C, (c) -90 C and (d) -1%
C
Fig3 (a) Facet fractures along (1 1 1) I plane; (b) Facet fractures along an annealing twin boundary; (c) Facet fractures with deformation twinning 327
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Figs (a) Annealing twin plane at -1% C and (b) Tear ridges at -90 C of A1 steel
than the one at -85 C.Peter Mullner '91 thinks that with the temperature, the twin thickness as well as the dislocation density decreases. The SEM and TEM morphoIogies in this paper are consistent with Peter Miillne's report. Deformation twinning can be frequently observed on the place below the fracture surface, about 0.5mm away from it at -196 "C . Fig.7 (a) shows the morphology of deformation twins which are very thin at cryogenic temperature. This is different with Fig.3 (c)
because of the effect of decreasing temperature. The SAD pattern of Fig. 7 (a) indicates the deformation twinning in zone axis of matrix direction as shown in Fig. 7 (b).
4. Conclusions (1) The nitrogen concentration in the high
nitrogen austenitic stainless steels mainly affects the ductile to brittle transition behavior at cryogenic temperature. The ductile to brittle transition temperatures (DBTT)of the steels increase with
Fig.6 TEM morphologies of A1 steel with different dislocation densities at (a) -1% C and (b) -85 C 328
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Fig.7 BF image (a) and S A D patterns (b) for deformation twinniig observed in zone axis
obtained from impact specimens at -1% C
increasing the nitrogen concentration. (2) The change of fracture patterns of high nitrogen austenitic stainless steels is dimple+shallow dimple+mixture of quasi-cleavage fracture facet and dimple-deavage fracture facet. The fracture modes are transgranular and intergranularfractures. (3) River patterns which are the typical features of cleave fracture facets and tear ridges which are the typical features of quasi-cleave fracture facets exist at low temperatures. (4)Fracture facets along annealing twin boundary and cross the annealing twin plane can be observed in this paper. Deformation twinning can be frequently observed at cryogenic temperature.
Transitionin Austenitic Chromium-Manganese-Nitrogen Stainless Steels [J]. Trans. Metall. SOC.AIME 1969,245 (10): 214 1-2148, [4] Tobler R L, Meyn D. Cleavage-like Fracture along Slip Planes in Fe-18Cr-3Ni-13Mn-0.37N Austenitic Stainless Steel at Liquid Helium Temperature [J]. Metall. Trans. A, 1998,19A(6): 1626-1631. [5] Tomota Y, Endo S. Cleave-like Fracture at Low Temperatures in an 18Mn-18Cr-OSNAustenitic Steels [J].
ISU Int. 1990.30 (8): 656-662. [6]. You R K, Kao P W,Gan D. Mechanical Properties of Fe-30Mn-1OAl-lC-1Si Alloy [J]. Mater. Sci: Eng. 1989, A117: 141-148. [7] Liu SC, Hashida T, Takahashi H, etal. A Study on Fractography in the Low Temperature Brittle Fracture of an
References:
18Cr-18Mn-0.7N Austenitic Steel [J]. Metall. and Mater.
[ 11 Gavriljuk V G Nitrogen in Iron and Steel [J]. ISU Int, 1996,
T ~ sA,. 1998,29A(3): 791-798. [8] Chen K M,Dai Q X.CrystallographicFeatures of Deformed
36 (7): 738-745.
Microstructurein Austenitic Steel with High N Content [J].
[2] Uggowitzer P J, Magdowski R, Speide M 0. Nickel Free High Nitrogen Austenitic Steels [J]. ISU Int, 1996, 36 (7):
Journal of Iron and Steel Research, 1998, 10 (1): 38-42. [9] Peter Miillner. On the Ductile to Brittle Transition in
90 1-908. [3] Defilippi J D, Brickner K G Gilbert E M. Ductile-to-Brittle
Austenitic Steel [J], Mater. Sci. Eng. A 1997,234-236: 94-97.
329
Research on Fracture Behavior of High Nitrogen Austenitic Stainless Steels at Cryogenic Temperature LI Hua-bing, JIANG Zhou-hua, ZHANG Zu-mi, XU Bao-yu (Northeastern University, School of Materials and Metallurgy, Liaoning, Shenyang, 11OOO4, China)
Abstract: This paper investigates the ductile to brittle transition (DBT) behavior of 19.55Cr-19.5Mn-2.26Mo-0.96N,
19.56Cr-19.4Mn-2.29Me0.82N and 19.84Cr18.9Mn-2.26Mo-0.88N steels from 77K to 290K by Charpy V-Notch (CVN) impact tests, and then reveals the possible fracture mechanism by SEM and "EM. With the concentration of nitrogen increasing, the ductility at low temperature decreases rapidly and the DBTT increases. The fracture modes are transgranular and intergranular fractures. Fracture facets along ( 1 1 1 ) and annealing twin boundary appear in the impact fractures at low temperature. River patterns which are the typical features of cleave fracture facets and tear ridges which are the typical feature of quasi-cleave fracture facets exist at cryogenic temperature. The change of fracture patterns of high nitrogen austenitic stainless steels is dimple-shallow
dimple-tmixture of quasi-cleavage facet and dimple+cleavage facet.
Key words: ductile to brittle transition; impact fracture; cryogenic temperature; transgranular and intergranular
1. Introduction High nitrogen austenitic stainless steels have a
fracture mechanism of the steels in cryogenic temperature is interpreted.
favorable combination of strength and toughness at
2. Experimental Procedures
room temperature and excellent corrosion resistance. 1 8 0 -18Mn-2Mo-N type austenitic stainless steels have recently been investigated widely. Moreover, this type of steels for cryogenic structural materials has been applied. Because of the stable f.c.c. structure, metals and alloys with f.c.c. lattice are usually ductile over the whole temperature range. However, steels
The chemical compositions of high nitrogen austenitic stainless steels with above 0.8% nitrogen content investigated are shown in Table 1. The high nitrogen austenitic stainless steels used in this experiment are manufactured using vacuum induction furnace by adding nitrided ferroalloy under
with high contents of manganese and nitrogen"-*] are the exceptions. Brittle fracture behavior of these alloys at low temperature has first been reported in 1969 13] and later by several authors [4-61. In the present work, the ductile to brittle transition behavior of Fe-Cr-Mn-Mo series high nitrogen austenitic stainless steels are investigated, and the
nitrogen atmosphere, and using the electroslag furnace for remelting under nitrogen atmosphere. The steels are hot rolled to 12 cm and solution-treated at 1100°C for 60 minutes, followed by water quenching. Then ASTM standard V-notched Charpy impact test specimens are prepared from the longest part of these as-quenched specimens which is parallel to the rolling direction. Charpy impact tests are performed in the temperature
Table 1 Chemical commition of the Cr-Mn-Mo-N hiph nitrogen austenitic stainless steel, % Steel
C
Si
Cr
Mn
Mo
Ni
S
P
Al
0
N
A1
0.058
0.19
19.55
19.5
2.26
--
0.003
g.03
0.04
0.0048
0.96
A2
0.022
0.19
19.84
18.9
2.26
--
0.002
50.03
0.002
0.0042
0.88
A3
0.049
0.19
19.56
19.4
2.29
--
0.003
50.03
0.04
0.0050
0.82
Proceedings of Sino-Swedish Structural Materials Symposium 2007
SSX-550 operating at 15 kV. From the fracture
range of 77K to room temperature (RT) in order to obtain the DB'IT of the HNS investigated in this paper. The SEM and TEM methods are used to observe the morphology of the fracture facets in order to reveal the possible fracture mechanism. Thin foils for TEM which are either parallel or perpendicular to the fractured surface are prepared by mechanical grinding down to 80 pm, and finished by a twin-jet electrolytic polishing technique using an electrolyte of 8% perchloric acid in ethanol at -25'C. The electron micrmpy and analysts is used by TECNCI G2 20
photographs in Fig.3, the change of fracture patterns of A2 steel with ultra low carbon concentration is dimplehshallow dimple+mixture of quasi-cleavage fracture facet and dimple-cleavage fracture facet at different temperatures. The fracture morphology at RT shows favorable ductility and equiaxial dimples in Fig.2 (a). With decreasing the temperature, dimples become shallow and small at -40 "C [Fig.2 (b)]. At -90 "C which is determined as ductile to brittle temperature, dimples and quasi-cleavage facet exist together [Fig.2 (c)]. Some tear ridges which are the typical feature of quasi-cleave fracture appear. Many smooth facets which exist on cleavage fracture surface can be observed in Fig.2 (d). Some of SEM morphologies of this investigation which have been mentioned by the previous authors are also observed in this paper. For example, Fig.3 (a) shows a facet which fractures along { 1 1 1 } at -90 "C reported by Tobler et al. [41 Fig.3 (b) shows the typical facet which fractures along an annealing twin boundary at -140 "C reported by Liu S.C. er al. [71 There are three sets of parallel lines on the facet and the lines in different sets intersect at 60". It means that although many annealing twin grains after solution treatment can be observed by optical microscope, few morphologies of annealing twin grains can be seen on the fracture facets. Fig.3 (c) shows the deformation twinning on the fracture at -90 "C reported by Chen
E M at 200 kV.
.-
_--
dm
.m
+WlKt.bpUllb4pI,
wmr.mW1*lpDm
J
'
-
M b u r , l r o -w # c o _ ~ , L
.m
*
0
P
Tii
Fi.1 DBlT curves d A l , A2 and A3 steel
3. Rmseu#s and Discussion 3.1 WECztile tQ Brittle transition curves Fig.1 shows the temperature dependence of impact absorbed energies obtained from Al, A2 and A3 steels tested at various temperatures by Charpy impact tests. As seen, the ductile to brittle temperatures of Al, A2 and A3 are determined as -85 "C, -90"C and -95 "C respectively. The impact absorbed energy of A2 (with ultra low carbon concentration) is higher than the other two curves at RT. It may be due to the less carbon concentration than the other two steels. However, the ductility decreases rapidly and the brittleness exhibits obviously with increasing the nitrogen concentration at low temperature. This phenomenon is also observed from the SEM photographs at the followed test. Results show that the nitrogen concentration mainly affects the ductility and brittleness at cryogenic temperature. 3.2 SEM observationson fracture surfaces The SEM observations are carried out on a
K.M. er al. [*' Slipping traces in left deformation twinning grain and the twin boundary intersect at 70". The result is consistent with 70'32' in theoretical calculation. The width of the deformation twinning grain is about 5pm, and the slipping traces are deformed in the right matrix crystal. However, the other SEM morphologies of this investigation show different features which have been mentioned by the previous authors as follows: (1) River patterns which are the typical features of cleave fracture facets exist at cryogenic temperature in Fig.4 (a). Arrow A shows that river patterns start at grain boundary then converge and cross to the adjacent grain. Arrow B points to the inclusion, and river patterns 326
Proceedings of Sino-Swedish Structural Materials Symposium 2007
originate from the inclusion. (2) Arrow C points at an intergrangular crack in Fig.4 (b). In high Mn austenitic steels, intergranular fracture has been frequently observed at low temperature, reported by Tomota et al. '51 (3) Transgrangular fracture occurs along annealing twin plane which is an unusual morphology as shown in Fig.5 (a). With decreasing the temperature, HNS will exhibit more brittleness. (4) Tear ridges which are the typical feature of quasi-cleave fracture facets are observed in Fig.5 (b) at -90 'C. The ductile to brittle transition occurs, and the typical fracture pattern is
mixture of quasi-cleavage facet and dimple at that temperature. This morphology may conduce to absorb energy. 3.3 TEM investigations on fracture surfaces Typical examples of dislocation structures are developed under the fracture surfaces of A1 steel. They are perpendicular to the fracture surface presented in Fig.6. The dislocation density decreases obviously with decreasing the temperature. Dislocation is the leading way to deform at a relative higher temperature. This is why the impact absorbed energy at-196 "C is less
Fig.2 SEM morphologies of impact fracture facets at (a) 20
C,(b)-40 C, (c) -90 C and (d) -1%
C
Fig3 (a) Facet fractures along (1 1 1) I plane; (b) Facet fractures along an annealing twin boundary; (c) Facet fractures with deformation twinning 327
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Figs (a) Annealing twin plane at -1% C and (b) Tear ridges at -90 C of A1 steel
than the one at -85 C.Peter Mullner '91 thinks that with the temperature, the twin thickness as well as the dislocation density decreases. The SEM and TEM morphoIogies in this paper are consistent with Peter Miillne's report. Deformation twinning can be frequently observed on the place below the fracture surface, about 0.5mm away from it at -196 "C . Fig.7 (a) shows the morphology of deformation twins which are very thin at cryogenic temperature. This is different with Fig.3 (c)
because of the effect of decreasing temperature. The SAD pattern of Fig. 7 (a) indicates the deformation twinning in zone axis of matrix direction as shown in Fig. 7 (b).
4. Conclusions (1) The nitrogen concentration in the high
nitrogen austenitic stainless steels mainly affects the ductile to brittle transition behavior at cryogenic temperature. The ductile to brittle transition temperatures (DBTT)of the steels increase with
Fig.6 TEM morphologies of A1 steel with different dislocation densities at (a) -1% C and (b) -85 C 328
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Fig.7 BF image (a) and S A D patterns (b) for deformation twinniig observed in zone axis
obtained from impact specimens at -1% C
increasing the nitrogen concentration. (2) The change of fracture patterns of high nitrogen austenitic stainless steels is dimple+shallow dimple+mixture of quasi-cleavage fracture facet and dimple-deavage fracture facet. The fracture modes are transgranular and intergranularfractures. (3) River patterns which are the typical features of cleave fracture facets and tear ridges which are the typical features of quasi-cleave fracture facets exist at low temperatures. (4)Fracture facets along annealing twin boundary and cross the annealing twin plane can be observed in this paper. Deformation twinning can be frequently observed at cryogenic temperature.
Transitionin Austenitic Chromium-Manganese-Nitrogen Stainless Steels [J]. Trans. Metall. SOC.AIME 1969,245 (10): 214 1-2148, [4] Tobler R L, Meyn D. Cleavage-like Fracture along Slip Planes in Fe-18Cr-3Ni-13Mn-0.37N Austenitic Stainless Steel at Liquid Helium Temperature [J]. Metall. Trans. A, 1998,19A(6): 1626-1631. [5] Tomota Y, Endo S. Cleave-like Fracture at Low Temperatures in an 18Mn-18Cr-OSNAustenitic Steels [J].
ISU Int. 1990.30 (8): 656-662. [6]. You R K, Kao P W,Gan D. Mechanical Properties of Fe-30Mn-1OAl-lC-1Si Alloy [J]. Mater. Sci: Eng. 1989, A117: 141-148. [7] Liu SC, Hashida T, Takahashi H, etal. A Study on Fractography in the Low Temperature Brittle Fracture of an
References:
18Cr-18Mn-0.7N Austenitic Steel [J]. Metall. and Mater.
[ 11 Gavriljuk V G Nitrogen in Iron and Steel [J]. ISU Int, 1996,
T ~ sA,. 1998,29A(3): 791-798. [8] Chen K M,Dai Q X.CrystallographicFeatures of Deformed
36 (7): 738-745.
Microstructurein Austenitic Steel with High N Content [J].
[2] Uggowitzer P J, Magdowski R, Speide M 0. Nickel Free High Nitrogen Austenitic Steels [J]. ISU Int, 1996, 36 (7):
Journal of Iron and Steel Research, 1998, 10 (1): 38-42. [9] Peter Miillner. On the Ductile to Brittle Transition in
90 1-908. [3] Defilippi J D, Brickner K G Gilbert E M. Ductile-to-Brittle
Austenitic Steel [J], Mater. Sci. Eng. A 1997,234-236: 94-97.
329