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Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures
Austenitic stainless steels have an excellent combination of mechanical and physical properties for load-bearing structures of large superconducting magnets for plasma containment in magnetic fusion experiments. To assess their relative suitability fracture toughness, fatigue crack growth, and tensile properties data for five austenitic steels at 295, 76, and 4 K have been obtained. The steels were AISI 304, 316, 304LN, and 316LN, and an Fe-21cr-12Ni-5Mn alloy with a higher nitrogen content than the other four grades. The two principal findings were the systematic variation of yield strength with nitrogen content and a systematic inverse correlation between fracture toughness and yield strength. Data from previous studies are reviewed which confirm the trends of the present data.
Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures D.T. Read and R.P. Reed Nitrogen-strengthened austenitic stainless steels have attracted interest as cryogenic structural materials because of their superior strength but data on their toughness at cryogenic temperatures are scarce) As cryogenic structures become larger and magnetic fields more intense, materials with higher strengths than the widely used alloy AISI 304 are necessary. In fact, nitrogen-strengthened AISI 304LN and 316LN have been selected for respectively, the magnet cases for the Magnetic'Fusion Test Facility at Lawrence Livermore Laboratory and a toroidal field coil being built for the Large Coil Project. Clearly, development of a data base on the structural properties of nitrogen-strengthened austenitic stainless steels is needed to support materials selection and design for the large cryogenic magnet structures of the near future.
Materials Chemical compositions are given in Table 1. The AISI 304 and 316 are from the Oak Ridge National Laboratory reference heats whose mechanical properties have been extensively characterized2'3 at room and higher temperatures for several product forms. The AISI 304L, 304LN and 316LN are from commercial heats. The Fe-21Cr-12Ni-5Mn alloy, designated Nitronic 50 by the manufacturer, (this tradename is used only for clarity. No approval or endorsement of any commercial product by NBS is implied) was solution-annealed in the laboratory. Final heat-treatment parameters and. measured grain sizes and hardnesses are given in Table 2. All microstructures were equiaxed.
Specimens Fracture toughness and fatigue crack growth data were obtained using compact specimens. All specimens were 2.54 cm (1 in) thick. Small, round tensile specimens oriented transverse to the rolling direction were used. Their gauge length was 2.54 cm (1 in) and the diameter of the gauge section was 0.4 cm (0.15 in). The fracture toughness specimens were in the TL orientation. The authors are at The Fracture and Deformation Division, National Bureau of Standards, Boulder, Colorado 80303, USA. Work supported by DOE/OFE. Contribution of NBS not subject to copyright. Paper received 6 June 1980.
0011-2275/81/007415-03 C R Y O G E N I C S . J U L Y 1981
Techniques Toughness. All toughness data reported here were obtained using a variant of the single specimen J-integral technique 4, with at least two complete tests for each material and temperature. It has been shown s that the critical stress intensity value obtained using the J-integral technique (denoted KIt(J)) is given by6:
KIc(J')
=
I EJIc](1 -/)2)[
~
(1)
where E is Young's modulus, Jlc is the measured critical J-integral value, and v is Poisson's ratio. For the AISI 304 alloy, the minimum specimen thickness required for a valid Jic test at 4 K was approximately 2.5 cm, while a valid KIc test required a thickness of about 220 cm. This is an extreme case because this alloy has a very high toughness and a very low yield strength. Nevertheless the thickness requirements for the J1c test are generally much less severe than for the KIc test. Despite the comparatively small specimen thicknesses required for J-integral testing, none of the room temperature tests proved valid because of insufficient specimen thickness. Similarly, many of the liquid nitrogen tempera. ture tests were invalid.
Results and discussion Fracture toughness is plotted against yield strength for selected stainless steels I at 4 K in Fig. 1. Both fracture toughness and tensile data for this plot were obtained on specimens from the same plate. The selected data represent wrought and annealed austenitic stainless steels. No weldments nor sensitized nor otherwise improperly heat-treated alloys have been included. Fig. 1 shows that there is a linear relationship between yield strength and fracture toughness for austenitic stainless steels at 4 K. All weldments fall well below the trend line shown in Fig. 1. The relationship between fracture toughness and yield strength has been suggested by others. 7 No satisfactory explanation for this relationship has been offered. However this trend may be significant if it represents, for 4 K use, the metal quality attainable with current metallurgical
$02.00 © 1981 IPC BusinessPress Ltd. 415
Table 1. Chemical compositions of the materials used in the present study in weight percent, as supplied by the manufacturers except for AISI 304, supplied by O R N L analysis 2 Designation
Cr
Mn
Mo
P
S
Si
C
N
AISI 304
18.44
9.72
1.28
0.32
0.030
0.016
0.48
0.051
0.031
AISI 304LN
18.85
8.60
1.74
0.38
0.38
0.003
0.51
0.24
0.13
AISI 316
17.25
13.48
1,86
2.34
0.024
0.019
0.58
0.057
0.030
AISI 316LN
17.40
13.90
1,58
2.50
0.021
0.021
0.48
0.016
0.16
Fe-21Cr-12Ni-5Mn
21.15
12.37
4,96
2.17
0.026
0.015
0.49
0.041
0.310
AISI 304L
18.54
9.27
1.78
n.a.
0.019
0.012
0.50
0.021
0.075
AISI 304L
18.7
9.2
1.74
0.38
0.029
0.006
0.37
0.021
0.096
AISI 304 AISI 316 AISI 304LN Fe-21Cr-12Ni-5Mn-X AISI 316LN
Table 2.
Ni
Designation
Hardness, Rockwell B
Grain Diameter, microns
ASTM Grain Size Number
AISI 304
75
210
1.5
AISI 304 LN
79
57
5.5
AISI 316
79
65
5.0
AISI 316 LN
83
105
3.5
Fe-25Cr-1 2Ni-5Mn* 93
160
2.4
Yield strength, ksi I00 200 I i t
"~o
E 300 -
~
I
t
o
e
o
•
0 | AISI 3 0 4 L ( O . O 9 6 w t % N ) 0 0 AISI 304L(0.075 wt %N ) 2000Yield Ultimate strength strength
300
"%
,
oo
,
*A non-standard heat treatment was used for this material. It was annealed for 1.5 h at 1150°C and water quenched.
0
o
• • •
o
Hardness and grain size of the materials
~
I000
- 300
+l'standarddeviation
O.
.E E
20o ="
e-
9 ,,
200 -
°° " X ~ °
Selected 4K data 0 304 (N) | o 3,6 (.)
0
c
rl
0
g
~
o
~. -
I 0 0 ~ n Other austenitJc stainless steels - - L i n e o r least squares fit
,oo
I
I
400
I
I
800
All the fatigue crack growth rates observed in the present study were within the band of previously observed results for austenitic stainless steelsJ Temperature dependences of the yield and ultimate tensile
416
0
250
4K o 304 (+N) A 316 ( + N ) o Other
I
techniques. Perhaps better properties will be attainable or perhaps the trend represents the limit of what can be achieved in wrought, annealed stainless steels. Further research is needed to determine whether and how better combinations of strength and toughness can be obtained.
I 300
Temperature, K
n
I I I I 0 12OO 16OO 2OOO Yield strength, MPa Fig. 1 Fracturetoughnessat 4 K of selected wrought and annealed austenitic stainlesssteels plotted againsttheir 4 K yield strengths. The line is a linear least squaresfit
0
I 200
Fig. 2 Temperature dependencesof the yield and ultimate strengths of the annealedaustenitic stainlesssteels
1600 0 /
I I00
200
1200
g.
]
e
800
--
150
i
OO
8
,oo >-
40O ,~_~.t "6
. ._._._.,._ . ------'--"
0 0
295K.
. I
- 50
I
I
I
0.1
0.2
0.3
o 0.4
Weight percent nitrogen Fig. 3 Yield strength at 4 K for annealed austenitic stainless steels plotted against weight percent nitrogen. Behaviour trends at 76 and 295 K are roughly indicated by the dashed lines
CRYOGENICS. JULY 1981
strengths are shown in Fig. 2. The alloys that have higher 4 K yield strength have stronger temperature dependences. The effect o f nitrogen content on the 4 K yield strength is displayed in Fig. 3. Data are included from other studiesJ 'a At higher temperatures the solid solution strengthening effect is not as pronounced. A consistent increase in yield strength with increasing nitrogen content is apparent. It appears that austenitic stainless steels can be tailor-made for 4 K applications by adjusting the nitrogen concentration to produce a desired yield strength-fracture toughness combination.
2
3
4
5
6
References Read, D.T., Reed, R.P: Toughness, Fatigue crack growth, and Tensile properties of three nitrogen-strengthened stainless steels at cryogenic temperatures, Fickett, F.R., Reed, R.P., (eds), Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures-I, NBSIR 78-884, National Bureau of Standards (1978) 93-154
CRYOGENICS
. JULY
1981
7
Swindeman, R.W., MeAfee, WJ., Sikka, V.K., Product form variability in the mechanical behavior of type 304 stainless steel at 593°C, Reproducibility and Accuracy of Mechanical Tests, ASTM STP 626, American Society for Testing and Materials, Philadelphia, PA, 1977, (41-64) Sikka, V.K., Product form characterization of reference heat of type 316 stainless steel, ORNL-5384, Oak Ridge National Laboratory, Oak Ridge, TN, (1978)
Clarke,G.A., Andrews, W.R., Paris, P.C., Sehmidt, D.W., Single specimen tests for JIc determination, mechanics of crack growth, ASTM STP 590, Americal Society for Testing and Materials, Philadelphia, PA, (1976) (27-42) Read, D.T., Reed, R.P., Effects of specimen thickness on fracture toughness of an aluminium alloy, Intn J. Fracture 13 (1977) 201-213 Landes, J.D., Begley, J.A., Test results form J-integral studies: an attempt to establish a Jlc testing procedure, Fracture Analysis, ASTM STP 560, American Society for Testing and Materials, Philadelphia, PA, (1974) 170-186
Gerberieh,W.W., Stout, M., Jatavallabhula,K., Atteridge, D., Acoustic emission interpretation of ductile fracture processes, Intn JFracture 15 (1977) 491-514
8
Voyer, R., Weil, L., Tensile and creep properties of a high nitrogen content 18/10 (AISI 304L) stainless steel at cryogenic temperatures, Adv Cryo Eng 11 (1965) 447-452
417
Fracture and strength properties of selected austenitic stainless steels at cryogenic temperatures D.T. Read and R.P. Reed Nitrogen-strengthened austenitic stainless steels have attracted interest as cryogenic structural materials because of their superior strength but data on their toughness at cryogenic temperatures are scarce) As cryogenic structures become larger and magnetic fields more intense, materials with higher strengths than the widely used alloy AISI 304 are necessary. In fact, nitrogen-strengthened AISI 304LN and 316LN have been selected for respectively, the magnet cases for the Magnetic'Fusion Test Facility at Lawrence Livermore Laboratory and a toroidal field coil being built for the Large Coil Project. Clearly, development of a data base on the structural properties of nitrogen-strengthened austenitic stainless steels is needed to support materials selection and design for the large cryogenic magnet structures of the near future.
Materials Chemical compositions are given in Table 1. The AISI 304 and 316 are from the Oak Ridge National Laboratory reference heats whose mechanical properties have been extensively characterized2'3 at room and higher temperatures for several product forms. The AISI 304L, 304LN and 316LN are from commercial heats. The Fe-21Cr-12Ni-5Mn alloy, designated Nitronic 50 by the manufacturer, (this tradename is used only for clarity. No approval or endorsement of any commercial product by NBS is implied) was solution-annealed in the laboratory. Final heat-treatment parameters and. measured grain sizes and hardnesses are given in Table 2. All microstructures were equiaxed.
Specimens Fracture toughness and fatigue crack growth data were obtained using compact specimens. All specimens were 2.54 cm (1 in) thick. Small, round tensile specimens oriented transverse to the rolling direction were used. Their gauge length was 2.54 cm (1 in) and the diameter of the gauge section was 0.4 cm (0.15 in). The fracture toughness specimens were in the TL orientation. The authors are at The Fracture and Deformation Division, National Bureau of Standards, Boulder, Colorado 80303, USA. Work supported by DOE/OFE. Contribution of NBS not subject to copyright. Paper received 6 June 1980.
0011-2275/81/007415-03 C R Y O G E N I C S . J U L Y 1981
Techniques Toughness. All toughness data reported here were obtained using a variant of the single specimen J-integral technique 4, with at least two complete tests for each material and temperature. It has been shown s that the critical stress intensity value obtained using the J-integral technique (denoted KIt(J)) is given by6:
KIc(J')
=
I EJIc](1 -/)2)[
~
(1)
where E is Young's modulus, Jlc is the measured critical J-integral value, and v is Poisson's ratio. For the AISI 304 alloy, the minimum specimen thickness required for a valid Jic test at 4 K was approximately 2.5 cm, while a valid KIc test required a thickness of about 220 cm. This is an extreme case because this alloy has a very high toughness and a very low yield strength. Nevertheless the thickness requirements for the J1c test are generally much less severe than for the KIc test. Despite the comparatively small specimen thicknesses required for J-integral testing, none of the room temperature tests proved valid because of insufficient specimen thickness. Similarly, many of the liquid nitrogen tempera. ture tests were invalid.
Results and discussion Fracture toughness is plotted against yield strength for selected stainless steels I at 4 K in Fig. 1. Both fracture toughness and tensile data for this plot were obtained on specimens from the same plate. The selected data represent wrought and annealed austenitic stainless steels. No weldments nor sensitized nor otherwise improperly heat-treated alloys have been included. Fig. 1 shows that there is a linear relationship between yield strength and fracture toughness for austenitic stainless steels at 4 K. All weldments fall well below the trend line shown in Fig. 1. The relationship between fracture toughness and yield strength has been suggested by others. 7 No satisfactory explanation for this relationship has been offered. However this trend may be significant if it represents, for 4 K use, the metal quality attainable with current metallurgical
$02.00 © 1981 IPC BusinessPress Ltd. 415
Table 1. Chemical compositions of the materials used in the present study in weight percent, as supplied by the manufacturers except for AISI 304, supplied by O R N L analysis 2 Designation
Cr
Mn
Mo
P
S
Si
C
N
AISI 304
18.44
9.72
1.28
0.32
0.030
0.016
0.48
0.051
0.031
AISI 304LN
18.85
8.60
1.74
0.38
0.38
0.003
0.51
0.24
0.13
AISI 316
17.25
13.48
1,86
2.34
0.024
0.019
0.58
0.057
0.030
AISI 316LN
17.40
13.90
1,58
2.50
0.021
0.021
0.48
0.016
0.16
Fe-21Cr-12Ni-5Mn
21.15
12.37
4,96
2.17
0.026
0.015
0.49
0.041
0.310
AISI 304L
18.54
9.27
1.78
n.a.
0.019
0.012
0.50
0.021
0.075
AISI 304L
18.7
9.2
1.74
0.38
0.029
0.006
0.37
0.021
0.096
AISI 304 AISI 316 AISI 304LN Fe-21Cr-12Ni-5Mn-X AISI 316LN
Table 2.
Ni
Designation
Hardness, Rockwell B
Grain Diameter, microns
ASTM Grain Size Number
AISI 304
75
210
1.5
AISI 304 LN
79
57
5.5
AISI 316
79
65
5.0
AISI 316 LN
83
105
3.5
Fe-25Cr-1 2Ni-5Mn* 93
160
2.4
Yield strength, ksi I00 200 I i t
"~o
E 300 -
~
I
t
o
e
o
•
0 | AISI 3 0 4 L ( O . O 9 6 w t % N ) 0 0 AISI 304L(0.075 wt %N ) 2000Yield Ultimate strength strength
300
"%
,
oo
,
*A non-standard heat treatment was used for this material. It was annealed for 1.5 h at 1150°C and water quenched.
0
o
• • •
o
Hardness and grain size of the materials
~
I000
- 300
+l'standarddeviation
O.
.E E
20o ="
e-
9 ,,
200 -
°° " X ~ °
Selected 4K data 0 304 (N) | o 3,6 (.)
0
c
rl
0
g
~
o
~. -
I 0 0 ~ n Other austenitJc stainless steels - - L i n e o r least squares fit
,oo
I
I
400
I
I
800
All the fatigue crack growth rates observed in the present study were within the band of previously observed results for austenitic stainless steelsJ Temperature dependences of the yield and ultimate tensile
416
0
250
4K o 304 (+N) A 316 ( + N ) o Other
I
techniques. Perhaps better properties will be attainable or perhaps the trend represents the limit of what can be achieved in wrought, annealed stainless steels. Further research is needed to determine whether and how better combinations of strength and toughness can be obtained.
I 300
Temperature, K
n
I I I I 0 12OO 16OO 2OOO Yield strength, MPa Fig. 1 Fracturetoughnessat 4 K of selected wrought and annealed austenitic stainlesssteels plotted againsttheir 4 K yield strengths. The line is a linear least squaresfit
0
I 200
Fig. 2 Temperature dependencesof the yield and ultimate strengths of the annealedaustenitic stainlesssteels
1600 0 /
I I00
200
1200
g.
]
e
800
--
150
i
OO
8
,oo >-
40O ,~_~.t "6
. ._._._.,._ . ------'--"
0 0
295K.
. I
- 50
I
I
I
0.1
0.2
0.3
o 0.4
Weight percent nitrogen Fig. 3 Yield strength at 4 K for annealed austenitic stainless steels plotted against weight percent nitrogen. Behaviour trends at 76 and 295 K are roughly indicated by the dashed lines
CRYOGENICS. JULY 1981
strengths are shown in Fig. 2. The alloys that have higher 4 K yield strength have stronger temperature dependences. The effect o f nitrogen content on the 4 K yield strength is displayed in Fig. 3. Data are included from other studiesJ 'a At higher temperatures the solid solution strengthening effect is not as pronounced. A consistent increase in yield strength with increasing nitrogen content is apparent. It appears that austenitic stainless steels can be tailor-made for 4 K applications by adjusting the nitrogen concentration to produce a desired yield strength-fracture toughness combination.
2
3
4
5
6
References Read, D.T., Reed, R.P: Toughness, Fatigue crack growth, and Tensile properties of three nitrogen-strengthened stainless steels at cryogenic temperatures, Fickett, F.R., Reed, R.P., (eds), Materials Studies for Magnetic Fusion Energy Applications at Low Temperatures-I, NBSIR 78-884, National Bureau of Standards (1978) 93-154
CRYOGENICS
. JULY
1981
7
Swindeman, R.W., MeAfee, WJ., Sikka, V.K., Product form variability in the mechanical behavior of type 304 stainless steel at 593°C, Reproducibility and Accuracy of Mechanical Tests, ASTM STP 626, American Society for Testing and Materials, Philadelphia, PA, 1977, (41-64) Sikka, V.K., Product form characterization of reference heat of type 316 stainless steel, ORNL-5384, Oak Ridge National Laboratory, Oak Ridge, TN, (1978)
Clarke,G.A., Andrews, W.R., Paris, P.C., Sehmidt, D.W., Single specimen tests for JIc determination, mechanics of crack growth, ASTM STP 590, Americal Society for Testing and Materials, Philadelphia, PA, (1976) (27-42) Read, D.T., Reed, R.P., Effects of specimen thickness on fracture toughness of an aluminium alloy, Intn J. Fracture 13 (1977) 201-213 Landes, J.D., Begley, J.A., Test results form J-integral studies: an attempt to establish a Jlc testing procedure, Fracture Analysis, ASTM STP 560, American Society for Testing and Materials, Philadelphia, PA, (1974) 170-186
Gerberieh,W.W., Stout, M., Jatavallabhula,K., Atteridge, D., Acoustic emission interpretation of ductile fracture processes, Intn JFracture 15 (1977) 491-514
8
Voyer, R., Weil, L., Tensile and creep properties of a high nitrogen content 18/10 (AISI 304L) stainless steel at cryogenic temperatures, Adv Cryo Eng 11 (1965) 447-452
417