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Influence of nitrogen-induced grain refinement on mechanical properties of nitrogen alloyed type 316LN stainless steel
Journal of Nuclear Materials 420 (2012) 473–478
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Influence of nitrogen-induced grain refinement on mechanical properties of nitrogen alloyed type 316LN stainless steel Dae Whan Kim ⇑ Nuclear Materials Development Division, Korea Atomic Energy Research Institute, 1045 Daedeog daero, Yuseong, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2011 Accepted 2 November 2011 Available online 11 November 2011
a b s t r a c t Tensile, fatigue, and creep tests were conducted to investigate the effect of grain refinement by the addition of nitrogen on mechanical properties of nitrogen alloyed type 316LN stainless steel. Grain size was reduced from 100 lm to 47 lm as nitrogen concentration was increased from 0.04% (N04) to 0.10% (N10). When nitrogen concentration was increased, there was a 20% increase in yield stress and a 14% increase in UTS, respectively. Elongation was not significantly changed with increasing nitrogen concentration. As nitrogen concentration was increased, there was a 41% increase in fatigue life and an approximately sixfold increase in the time to rupture. As grain size was reduced from 100 lm to 47 lm, there was an 8% increase in yield stress and a 3% increase in UTS, respectively. Elongation was little changed with decreasing grain size. As grain size was reduced from 100 lm to 47 lm, there was a 9% increase in fatigue life and a 23% increase in the time to rupture. The grain refinement achieved by the addition of nitrogen improved the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel but was not the main mechanism for improvement of mechanical properties. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steel is a prospective structural material for liquid metal reactor (LMR) because of its resistance to neutron irradiation and corrosion and good mechanical properties such as high temperature strength, ductility, and toughness. Carbide precipitated in type 316 stainless steel results in a reduction of corrosion resistance and creep rupture ductility. Nitrogen alloyed type 316LN stainless steel has been studied as LMR structural material because nitrogen is the most effective solid solution hardening element, beneficial to the corrosion and creep resistance by a reduction of precipitation at grain boundaries, and it decreases grain size [1–8]. It has been reported that yield strength, fatigue, and creep properties are improved with increasing nitrogen concentration [1,2,6,7,9–11]. Improvement of mechanical properties achieved by the addition of nitrogen is related to short range order (SRO), the change of microstructure from cellular dislocation structure to planar dislocation structure, the decrease of grain size and carbide precipitation at grain boundaries, the retardation of dynamic strain aging (DSA), and so on [1,2,12–19]. The relative importance of these factors is dependent on the temperature and the stress applied during the period of the test.
⇑ Tel.: +82 42 868 2046; fax: +82 42 868 8549. E-mail address: [email protected] 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.11.001
In this study, to evaluate separately the effect of nitrogen concentration and grain refinement induced by the addition of nitrogen on mechanical properties, mechanical properties were quantitatively evaluated as a function of nitrogen concentration and grain size. The relative importance of grain refinement induced by the addition of nitrogen is compared with the significance of other factors. 2. Experimental procedure 2.1. Specimen Laboratory ingots of nitrogen alloyed type 316LN stainless steel containing different levels of nitrogen concentration, 0.04% (N04) and 0.10% (N10), were prepared by vacuum induction melting and hot rolled to a thickness of 15 mm. Chemical compositions are given in Table 1. Alloys were solution annealed at 1100 °C and water quenched. Grain sizes of N04 and N10 are shown in Fig. 1 and grain sizes are measured using the line intercept method in accordance with ASTM E112. Grain size was reduced from 100 lm to 47 lm as nitrogen concentration was increased from 0.04% to 0.1%. In order to evaluate separately the effects of nitrogen concentration and grain refinement induced by the addition of nitrogen on mechanical properties, several specimens of N10 were heat treated at 1050–1100 °C for 1–10 h to change grain size. Using this technique, grain sizes in the range of 38–135 lm were achieved.
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
Table 1 Chemical composition of nitrogen alloyed type 316LN stainless steel (wt.%). Spec. ID
C
Si
Mn
Ni
Cr
Mo
N
P
S
Grain size (lm)
N04 N10
0.018 0.019
0.67 0.63
0.95 0.97
12.21 12.35
17.78 17.26
2.36 2.41
0.04 0.10
0.007 0.008
0.002 0.004
100 47
Fig. 1. Grain size after solution annealing at 1100 °C for: (a) N04 and (b) N10.
Table 2 Dimension of specimens used for tensile, fatigue, and creep test. Testing method
Gage length (mm)
Diameter (mm)
Tension Fatigue Creep
25 8 30
4 7 6
700
N04 N10
N04 N10
600
300
UTS, MPa
Yield stress, MPa
400
Specimens were taken in the rolling direction for mechanical tests. The surface of the specimens was polished with a 1000 grit sand paper along the specimen axis. The dimensions of specimens for tensile, fatigue and creep tests are listed in Table 2.
200
100
500 400 300 200
0 0
200
600
400
800
0
400
200
o
600
800
o
Temperature, C
Temperature, C
(a)
(b) 80
N04 N10
Elongation, %
70 60 50 40 30 0
200
400
600
800
o
Temperature, C
(c)
Fig. 2. Tensile properties as a function of temperature for the two levels of nitrogen concentration: (a) Yield stress, (b) UTS, and (c) elongation.
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
700 RT o 200 C o 400 C 600 oC
400
600
UTS, MPa
Yield stress, MPa
500
300 200 100
500 400 RT o 200 C o 400 C o 600 C
300
0 0
50
100
150
200
200 0
50
Grain size, μm
100
200
150
Grain size, μm
(a)
(b)
100
Elongation, %
80 60 40 RT o 200 C o 400 C
20
o
600 C
0 0
50
100
150
200
Grain size, μm
(c)
Fig. 3. Tensile properties of N10 as a function of grain size: (a) Yield stress, (b) UTS, and (c) elongation.
N04 N10
1.5
Tensile peak stress, MPa
Total strain range, %
2
1
o
T=600 C
0.5 2 10
3
10
4
10
400
N04 N10
300 200 100 o
T=600 C Δε t =1.0%
0 0 10
2.2. Test conditions Tensile tests were carried out in the temperature range of RT – 700 °C under the displacement control operation mode. Low cycle fatigue (LCF) tests were conducted at 600 °C under axial strain control mode using fully reversed triangular waveform. A strain gage was directly attached to the specimen. The total strain was applied in the range from 0.8% to 1.5%. A strain rate used in tensile test and LCF test was 2 10 3/s. The fatigue life was defined as the number of cycles corresponding to a 25% reduction in the peak tensile saturation stress. Creep tests were conducted at 600 °C using a constant load method. The steady state creep rate was calculated from the steady state region of a creep curve. All tests in this study were conducted in an air environment and the temperature was maintained constant within ±2 °C during the period of the test. The specimens were held at the test temperature for 1 h before starting the test.
1
10
2
10
3
10
4
Cycles to failure
Cycles to failure Fig. 4. Effect of nitrogen concentration on fatigue life.
10
Fig. 5. Effect of nitrogen concentration on cyclic peak stress.
2.3. Dislocation structures The thin foils for transmission electron microscopy (TEM) observation were cut perpendicular to the loading axis at a distance of 1 mm away from the fractured surfaces and then electropolished in a solution containing 5% perchloric acid and 95% acetic acid. Dislocation structures were examined using JEOL 2000FX2 TEM operating at an acceleration voltage of 200 kV. 3. Results and discussion 3.1. Tensile properties Tensile properties are shown as a function of temperature for the two levels of nitrogen concentration in Fig. 2. Tensile strength increased with increasing nitrogen concentration. The contribution
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
(a)
(b)
10
4
10
3
0.8% 1.0% 1.5%
o
10
T=600 C
2
0
50
100
150
200
Grain size, μm Fig. 7. Fatigue life of N10 as a function of grain size.
of nitrogen to tensile strength was made up of two components; thermal component associated with solid solution hardening at temperatures below 300 °C and athermal component associated with grain refinement and short range order (SRO) by the strong interaction between Cr and N at temperatures above 300 °C [1–3]. As nitrogen concentration was increased, there was a 20% increase in yield stress and a 14% increase in UTS, respectively. Elongation was not significantly changed with increasing nitrogen concentration. Tensile properties are shown as a function of grain size in Fig. 3. Yield stress and UTS increased with decreasing grain size. When grain size was decreased from 100 lm to 47 lm, the variation in yield stress and UTS is calculated from the slopes of the curves in Fig. 3. As grain size was decreased, there was an 8% increase in yield stress and a 3% increase in UTS, respectively. Elongation was little changed with decreasing grain size. The effect of grain refinement on the increase in tensile strength was much less than that of nitrogen concentration. SRO was more effective than grain refinement for the increase of tensile strength of nitrogen alloyed type 316LN stainless steel at high temperature. 3.2. Fatigue properties Fatigue life is shown for the two levels of nitrogen concentration in Fig. 4. As nitrogen concentration was increased, there was
400
Tensile peak stress, MPa
Cycles to failure
Fig. 6. Dislocation structures in samples LCF tested at Det = 1.0% and 600 °C for (a) N04 and (b) N10.
38 μm 75 μm 135 μm
300
200
100 Δεt =1.0% o
600 C
0 0 10
10
1
10
2
10
3
10
4
Cycles to failure Fig. 8. Effect of grain size on cyclic peak stress of N10.
a 41% increase in fatigue life. Fatigue life has been related to dislocation structure, DSA, strain induced martensite, grain size, and precipitation [12–15,17–21]. Cyclic peak stress is shown for the two levels of nitrogen concentration in Fig. 5. The alloys exhibited cyclic hardening before about 200 cycles followed by a regime of stable cyclic peak stress. The cyclic peak stress of N10 was higher than that of N04 because nitrogen increased tensile strength as shown in Fig. 2. It has been reported that DSA is responsible for the hardening at an early stage of fatigue life because DSA increases strength [17–19]. Nitrogen decreased this hardening and increased fatigue life at high temperature because nitrogen retarded DSA, which is known to decrease fatigue life [19]. Strain induced martensite was not detected after LCF test because LCF tests were carried out at 600 °C. Dislocation structures in samples tested at total strain range of 1.0% are shown for two levels of nitrogen concentration in Fig. 6. The results exhibited cellular dislocation structure for N04 and planar dislocation structure for N10. The change of dislocation structure achieved by the addition of nitrogen increased fatigue life because the planar arrays were more resistant to fatigue [2,13,14]. Carbide was precipitated after 50 h at 600 °C in type 316L stainless steel and the time required for carbide precipitation increased with increasing nitrogen concentration [8]. Cr2N could be precipitated in austenitic stainless steel having more than 0.16% nitrogen
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
N04 N10
400
Time to rupture, hrs
Applied stress, MPa
500
300
200
10
4
10
3
10
2
101
230 MPa 240 MPa 260 MPa 300 MPa
o
T=600 C
100 0 10
10
1
10
2
10
3
10
10
4
0
0
50
-4
10
-5
10
-6
10
-7
10
-8
N04 N10
o
T=600 C
100
200
300
150
400 500
10
-5
200
10
-6
10
-7
10
-8
Applied stress, MPa
3.3. Creep properties The time to rupture and the steady state creep rate are shown for the two levels of nitrogen concentration in Fig. 9. As nitrogen concentration was increased, there was an approximately sixfold increase in the time to rupture. The time to rupture was dependent on carbide precipitation and cavity formation at grain boundaries [7]. Nitrogen decreased carbide precipitation at grain boundaries [8] and increased the time to rupture. The time to rupture and the steady state creep rate is shown as a function of grain size in Fig. 10. The time to rupture and the steady state creep rate increased with decreasing grain size. When grain size is decreased from 100 lm to 47 lm, the time to rupture and the steady state creep rate are calculated from the slope of curves in Fig. 10. As grain size was decreased, there was a 23% increase in the time to rupture and a 17% increase in the steady state creep rate. It is considered that the effect of nitrogen on the in-
T=600o C 0
50
100
150
200
Grain size, μm
Fig. 9. Effect of nitrogen concentration on the time to rupture and the steady state creep rate.
concentration [22]. The duration of LCF test was about 8 h at total strain range of 0.8% and this time was not sufficient for carbide precipitation. The precipitate was not observed by TEM at this test condition. It is believed that carbide precipitation does not affect fatigue life and cyclic peak stress. Fatigue life is shown as a function of grain size in Fig. 7. Fatigue life increased with decreasing grain size. When grain size is decreased from 100 lm to 47 lm, the variation in fatigue life is calculated from the slopes of curves in Fig. 7. As grain size was decreased, there was a 9% increase in fatigue life. The effect of nitrogen concentration on the increase in fatigue life was greater than that of grain refinement. Cyclic peak stress is not significantly affected by grain size as shown in Fig. 8. The effect of nitrogen concentration on the increase in fatigue life of nitrogen alloyed type 316LN stainless steel at high temperature was attributed to the retardation of DSA and the change of dislocation structure rather than grain refinement induced by the addition of nitrogen.
230 MPa 240 MPa 260 MPa 300 MPa
-1
10
100
Grain size, μm
Steady state creep rate, s
Steady state creep rate, s
-1
Time to rupture, hrs
o
T=600 C
Fig. 10. The time to rupture and the steady state creep rate as a function of grain size for N10.
crease in the time to rupture and the steady state creep rate is attributed to the increase of the strength and the decrease of carbide precipitation at grain boundaries rather than grain refinement induced by the addition of nitrogen. 4. Conclusions Tensile, fatigue and creep tests were conducted to evaluate the effect of grain refinement induced by the addition of nitrogen on the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel. The results are as follows: 1. Grain size is decreased from 100 lm to 47 lm as nitrogen concentration is increased from 0.04% to 0.10%. 2. As nitrogen concentration is increased from 0.04% to 0.10%, there is a 20% increase in yield stress and a 14% increase in UTS. When grain size is decreased from 100 lm to 47 lm, there is an 8% increase in yield stress and a 3% increase in UTS. 3. There is a 41% increase in fatigue life as nitrogen concentration is increased. As grain size is decreased, there is a 9% increase in fatigue life. 4. As nitrogen concentration is increased, there is an approximately sixfold increase in the time to rupture. As grain size is decreased, there is a 23% increase in the time to rupture. 5. The grain refinement induced by the addition of nitrogen improves the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel but is not the main mechanism.
References [1] M.L.G. Byrnes, M. Grujicic, W.S. Owen, Acta Metall. 7 (1987) 1853–1862. [2] J.W. Simmons, Mater. Sci. Eng. A207 (1996) 159–169. [3] J.O. Nilsson, T. Thorvaldsson, Scandinavian J. Metall. 15 (1985) 83–89.
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[14] J.O. Nilson, Scripta Metall. 17 (1983) 593–596. [15] J.B. Vogt, S. Degallaix, J. Foct, Int. J. Fatigue 6 (1984) 211–215. [16] T. Nakazawa, H. Abo, M. Tanino, H. Komatsu, T. Nishida, M. Tashimo, Effect of nitrogen and carbon on creep properties of type 316 stainless steel, in: J. Foct, A. Hendry (Eds.), High Nitrogen Steels, The Institute of Metals, Lille France, 1989, pp. 218–224. [17] V.S. Srinivasan, R. Sandhya, K. Bhanu Sankara Rao, S.L. Mannan, K.S. Raghavan, Int. J. Fatigue 13 (1991) 471–478. [18] M. Valsan, D. Sundararaman, K. Bhanu Sankara Rao, S.L. Mannan, Met. Trans. 26A (1995) 1207–1219. [19] D.W. Kim, W.S. Ryu, J.H. Hong, S.K. Choi, J. Nucl. Mat. 254 (1998) 226–233. [20] D. Baudry, A. Pineau, Mater. Sci. Eng. 28 (1977) 229–242. [21] A.G. Pineau, R.M. Pelloux, Met. Trans. 5 (1974) 1103–1112. [22] P. Shankar, D. Sundararaman, S. Tanganathan, Scripta Metall. 31 (1994) 589– 593.
Contents lists available at SciVerse ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Influence of nitrogen-induced grain refinement on mechanical properties of nitrogen alloyed type 316LN stainless steel Dae Whan Kim ⇑ Nuclear Materials Development Division, Korea Atomic Energy Research Institute, 1045 Daedeog daero, Yuseong, Daejeon 305-353, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 March 2011 Accepted 2 November 2011 Available online 11 November 2011
a b s t r a c t Tensile, fatigue, and creep tests were conducted to investigate the effect of grain refinement by the addition of nitrogen on mechanical properties of nitrogen alloyed type 316LN stainless steel. Grain size was reduced from 100 lm to 47 lm as nitrogen concentration was increased from 0.04% (N04) to 0.10% (N10). When nitrogen concentration was increased, there was a 20% increase in yield stress and a 14% increase in UTS, respectively. Elongation was not significantly changed with increasing nitrogen concentration. As nitrogen concentration was increased, there was a 41% increase in fatigue life and an approximately sixfold increase in the time to rupture. As grain size was reduced from 100 lm to 47 lm, there was an 8% increase in yield stress and a 3% increase in UTS, respectively. Elongation was little changed with decreasing grain size. As grain size was reduced from 100 lm to 47 lm, there was a 9% increase in fatigue life and a 23% increase in the time to rupture. The grain refinement achieved by the addition of nitrogen improved the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel but was not the main mechanism for improvement of mechanical properties. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Austenitic stainless steel is a prospective structural material for liquid metal reactor (LMR) because of its resistance to neutron irradiation and corrosion and good mechanical properties such as high temperature strength, ductility, and toughness. Carbide precipitated in type 316 stainless steel results in a reduction of corrosion resistance and creep rupture ductility. Nitrogen alloyed type 316LN stainless steel has been studied as LMR structural material because nitrogen is the most effective solid solution hardening element, beneficial to the corrosion and creep resistance by a reduction of precipitation at grain boundaries, and it decreases grain size [1–8]. It has been reported that yield strength, fatigue, and creep properties are improved with increasing nitrogen concentration [1,2,6,7,9–11]. Improvement of mechanical properties achieved by the addition of nitrogen is related to short range order (SRO), the change of microstructure from cellular dislocation structure to planar dislocation structure, the decrease of grain size and carbide precipitation at grain boundaries, the retardation of dynamic strain aging (DSA), and so on [1,2,12–19]. The relative importance of these factors is dependent on the temperature and the stress applied during the period of the test.
⇑ Tel.: +82 42 868 2046; fax: +82 42 868 8549. E-mail address: [email protected] 0022-3115/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2011.11.001
In this study, to evaluate separately the effect of nitrogen concentration and grain refinement induced by the addition of nitrogen on mechanical properties, mechanical properties were quantitatively evaluated as a function of nitrogen concentration and grain size. The relative importance of grain refinement induced by the addition of nitrogen is compared with the significance of other factors. 2. Experimental procedure 2.1. Specimen Laboratory ingots of nitrogen alloyed type 316LN stainless steel containing different levels of nitrogen concentration, 0.04% (N04) and 0.10% (N10), were prepared by vacuum induction melting and hot rolled to a thickness of 15 mm. Chemical compositions are given in Table 1. Alloys were solution annealed at 1100 °C and water quenched. Grain sizes of N04 and N10 are shown in Fig. 1 and grain sizes are measured using the line intercept method in accordance with ASTM E112. Grain size was reduced from 100 lm to 47 lm as nitrogen concentration was increased from 0.04% to 0.1%. In order to evaluate separately the effects of nitrogen concentration and grain refinement induced by the addition of nitrogen on mechanical properties, several specimens of N10 were heat treated at 1050–1100 °C for 1–10 h to change grain size. Using this technique, grain sizes in the range of 38–135 lm were achieved.
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
Table 1 Chemical composition of nitrogen alloyed type 316LN stainless steel (wt.%). Spec. ID
C
Si
Mn
Ni
Cr
Mo
N
P
S
Grain size (lm)
N04 N10
0.018 0.019
0.67 0.63
0.95 0.97
12.21 12.35
17.78 17.26
2.36 2.41
0.04 0.10
0.007 0.008
0.002 0.004
100 47
Fig. 1. Grain size after solution annealing at 1100 °C for: (a) N04 and (b) N10.
Table 2 Dimension of specimens used for tensile, fatigue, and creep test. Testing method
Gage length (mm)
Diameter (mm)
Tension Fatigue Creep
25 8 30
4 7 6
700
N04 N10
N04 N10
600
300
UTS, MPa
Yield stress, MPa
400
Specimens were taken in the rolling direction for mechanical tests. The surface of the specimens was polished with a 1000 grit sand paper along the specimen axis. The dimensions of specimens for tensile, fatigue and creep tests are listed in Table 2.
200
100
500 400 300 200
0 0
200
600
400
800
0
400
200
o
600
800
o
Temperature, C
Temperature, C
(a)
(b) 80
N04 N10
Elongation, %
70 60 50 40 30 0
200
400
600
800
o
Temperature, C
(c)
Fig. 2. Tensile properties as a function of temperature for the two levels of nitrogen concentration: (a) Yield stress, (b) UTS, and (c) elongation.
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
700 RT o 200 C o 400 C 600 oC
400
600
UTS, MPa
Yield stress, MPa
500
300 200 100
500 400 RT o 200 C o 400 C o 600 C
300
0 0
50
100
150
200
200 0
50
Grain size, μm
100
200
150
Grain size, μm
(a)
(b)
100
Elongation, %
80 60 40 RT o 200 C o 400 C
20
o
600 C
0 0
50
100
150
200
Grain size, μm
(c)
Fig. 3. Tensile properties of N10 as a function of grain size: (a) Yield stress, (b) UTS, and (c) elongation.
N04 N10
1.5
Tensile peak stress, MPa
Total strain range, %
2
1
o
T=600 C
0.5 2 10
3
10
4
10
400
N04 N10
300 200 100 o
T=600 C Δε t =1.0%
0 0 10
2.2. Test conditions Tensile tests were carried out in the temperature range of RT – 700 °C under the displacement control operation mode. Low cycle fatigue (LCF) tests were conducted at 600 °C under axial strain control mode using fully reversed triangular waveform. A strain gage was directly attached to the specimen. The total strain was applied in the range from 0.8% to 1.5%. A strain rate used in tensile test and LCF test was 2 10 3/s. The fatigue life was defined as the number of cycles corresponding to a 25% reduction in the peak tensile saturation stress. Creep tests were conducted at 600 °C using a constant load method. The steady state creep rate was calculated from the steady state region of a creep curve. All tests in this study were conducted in an air environment and the temperature was maintained constant within ±2 °C during the period of the test. The specimens were held at the test temperature for 1 h before starting the test.
1
10
2
10
3
10
4
Cycles to failure
Cycles to failure Fig. 4. Effect of nitrogen concentration on fatigue life.
10
Fig. 5. Effect of nitrogen concentration on cyclic peak stress.
2.3. Dislocation structures The thin foils for transmission electron microscopy (TEM) observation were cut perpendicular to the loading axis at a distance of 1 mm away from the fractured surfaces and then electropolished in a solution containing 5% perchloric acid and 95% acetic acid. Dislocation structures were examined using JEOL 2000FX2 TEM operating at an acceleration voltage of 200 kV. 3. Results and discussion 3.1. Tensile properties Tensile properties are shown as a function of temperature for the two levels of nitrogen concentration in Fig. 2. Tensile strength increased with increasing nitrogen concentration. The contribution
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D.W. Kim / Journal of Nuclear Materials 420 (2012) 473–478
(a)
(b)
10
4
10
3
0.8% 1.0% 1.5%
o
10
T=600 C
2
0
50
100
150
200
Grain size, μm Fig. 7. Fatigue life of N10 as a function of grain size.
of nitrogen to tensile strength was made up of two components; thermal component associated with solid solution hardening at temperatures below 300 °C and athermal component associated with grain refinement and short range order (SRO) by the strong interaction between Cr and N at temperatures above 300 °C [1–3]. As nitrogen concentration was increased, there was a 20% increase in yield stress and a 14% increase in UTS, respectively. Elongation was not significantly changed with increasing nitrogen concentration. Tensile properties are shown as a function of grain size in Fig. 3. Yield stress and UTS increased with decreasing grain size. When grain size was decreased from 100 lm to 47 lm, the variation in yield stress and UTS is calculated from the slopes of the curves in Fig. 3. As grain size was decreased, there was an 8% increase in yield stress and a 3% increase in UTS, respectively. Elongation was little changed with decreasing grain size. The effect of grain refinement on the increase in tensile strength was much less than that of nitrogen concentration. SRO was more effective than grain refinement for the increase of tensile strength of nitrogen alloyed type 316LN stainless steel at high temperature. 3.2. Fatigue properties Fatigue life is shown for the two levels of nitrogen concentration in Fig. 4. As nitrogen concentration was increased, there was
400
Tensile peak stress, MPa
Cycles to failure
Fig. 6. Dislocation structures in samples LCF tested at Det = 1.0% and 600 °C for (a) N04 and (b) N10.
38 μm 75 μm 135 μm
300
200
100 Δεt =1.0% o
600 C
0 0 10
10
1
10
2
10
3
10
4
Cycles to failure Fig. 8. Effect of grain size on cyclic peak stress of N10.
a 41% increase in fatigue life. Fatigue life has been related to dislocation structure, DSA, strain induced martensite, grain size, and precipitation [12–15,17–21]. Cyclic peak stress is shown for the two levels of nitrogen concentration in Fig. 5. The alloys exhibited cyclic hardening before about 200 cycles followed by a regime of stable cyclic peak stress. The cyclic peak stress of N10 was higher than that of N04 because nitrogen increased tensile strength as shown in Fig. 2. It has been reported that DSA is responsible for the hardening at an early stage of fatigue life because DSA increases strength [17–19]. Nitrogen decreased this hardening and increased fatigue life at high temperature because nitrogen retarded DSA, which is known to decrease fatigue life [19]. Strain induced martensite was not detected after LCF test because LCF tests were carried out at 600 °C. Dislocation structures in samples tested at total strain range of 1.0% are shown for two levels of nitrogen concentration in Fig. 6. The results exhibited cellular dislocation structure for N04 and planar dislocation structure for N10. The change of dislocation structure achieved by the addition of nitrogen increased fatigue life because the planar arrays were more resistant to fatigue [2,13,14]. Carbide was precipitated after 50 h at 600 °C in type 316L stainless steel and the time required for carbide precipitation increased with increasing nitrogen concentration [8]. Cr2N could be precipitated in austenitic stainless steel having more than 0.16% nitrogen
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3.3. Creep properties The time to rupture and the steady state creep rate are shown for the two levels of nitrogen concentration in Fig. 9. As nitrogen concentration was increased, there was an approximately sixfold increase in the time to rupture. The time to rupture was dependent on carbide precipitation and cavity formation at grain boundaries [7]. Nitrogen decreased carbide precipitation at grain boundaries [8] and increased the time to rupture. The time to rupture and the steady state creep rate is shown as a function of grain size in Fig. 10. The time to rupture and the steady state creep rate increased with decreasing grain size. When grain size is decreased from 100 lm to 47 lm, the time to rupture and the steady state creep rate are calculated from the slope of curves in Fig. 10. As grain size was decreased, there was a 23% increase in the time to rupture and a 17% increase in the steady state creep rate. It is considered that the effect of nitrogen on the in-
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concentration [22]. The duration of LCF test was about 8 h at total strain range of 0.8% and this time was not sufficient for carbide precipitation. The precipitate was not observed by TEM at this test condition. It is believed that carbide precipitation does not affect fatigue life and cyclic peak stress. Fatigue life is shown as a function of grain size in Fig. 7. Fatigue life increased with decreasing grain size. When grain size is decreased from 100 lm to 47 lm, the variation in fatigue life is calculated from the slopes of curves in Fig. 7. As grain size was decreased, there was a 9% increase in fatigue life. The effect of nitrogen concentration on the increase in fatigue life was greater than that of grain refinement. Cyclic peak stress is not significantly affected by grain size as shown in Fig. 8. The effect of nitrogen concentration on the increase in fatigue life of nitrogen alloyed type 316LN stainless steel at high temperature was attributed to the retardation of DSA and the change of dislocation structure rather than grain refinement induced by the addition of nitrogen.
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crease in the time to rupture and the steady state creep rate is attributed to the increase of the strength and the decrease of carbide precipitation at grain boundaries rather than grain refinement induced by the addition of nitrogen. 4. Conclusions Tensile, fatigue and creep tests were conducted to evaluate the effect of grain refinement induced by the addition of nitrogen on the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel. The results are as follows: 1. Grain size is decreased from 100 lm to 47 lm as nitrogen concentration is increased from 0.04% to 0.10%. 2. As nitrogen concentration is increased from 0.04% to 0.10%, there is a 20% increase in yield stress and a 14% increase in UTS. When grain size is decreased from 100 lm to 47 lm, there is an 8% increase in yield stress and a 3% increase in UTS. 3. There is a 41% increase in fatigue life as nitrogen concentration is increased. As grain size is decreased, there is a 9% increase in fatigue life. 4. As nitrogen concentration is increased, there is an approximately sixfold increase in the time to rupture. As grain size is decreased, there is a 23% increase in the time to rupture. 5. The grain refinement induced by the addition of nitrogen improves the high temperature mechanical properties of nitrogen alloyed type 316LN stainless steel but is not the main mechanism.
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