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Effect of NaCl and CaCl2 additives on NaNO3 bath nitriding of steel
Materials Science and Engineering A 527 (2010) 1048–1051
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of NaCl and CaCl2 additives on NaNO3 bath nitriding of steel Min-ku Lee a , Dong-sam Kim a , Sung-chul Kim a , Sang-won Han a , Insoo Kim b , Dong Nyung Lee c,∗ a
Iljin Light Metal Co., Ltd. 446-1, Kwaerang-Ri, Jungnam-Myun, Hwasung-Si, Gyunggi-Do 445-963, Republic of Korea School of Advanced Materials Science and Systems Engineering, Kumoh National Institute of Technology, Gumi-Si, Gyung-Buk 730-701, Republic of Korea c Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 July 2009 Received in revised form 9 September 2009 Accepted 10 September 2009
Keywords: Sodium nitrate Nitriding IF steel Hardness Tensile properties
a b s t r a c t A study has been made of nitriding of interstitial-free steel sheet specimens in the sodium nitrate bath. The nitriding rate increased when the bath contained a small amount of sodium chloride and calcium chloride. Accordingly, the tensile strengths and hardness values of the steel specimens nitrided in the chloride containing bath were higher than those of the same steel specimens nitrided in the pure nitrate bath when the nitriding time and temperature were controlled so that the excessive formation of brittle compound Fe4 N could be prevented. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
[11,12]:
Pure liquid nitriding of steel using the potassium nitrate salt (KNO3 ) bath was first introduced in 2005 [1]. Since then, nitriding in the nitrate bath has been further studied [2,3]. The source of nitrogen in the nitriding was attributed to nitrogen, nitrogenic oxide (NO), and nitrogen dioxide (NO2 ) generated from the decomposition of KNO3 at temperatures above about 500 ◦ C [4–10]. It was suggested that NO and NO2 as well as nitrogen generated from the decomposition of KNO3 could participate in nitriding and oxidation reactions [3]. The diffusion behavior of nitrogen in IF steel was similar to that in ˛-iron with the nitrogen concentration at the surface of steel specimen being approximated by the solid solubility of nitrogen in ˛-Fe, in case of nitriding at 650 ◦ C [1–3]. Pronounced increases in tensile strength with reasonable elongations could be achieved by the nitrate bath nitriding [2,3]. In the light of these facts, the nitrate-nitriding is regarded as a promising method for introducing nitrogen into iron and steel, though surface oxidation cannot be avoided. The surface oxidation can cause not only a slight loss of steel but also a reduction in nitriding rate because of the surface oxide layer that hampers the nitrogen sources reaching steel. The thermal decomposition behavior of sodium nitrite (NaNO2 ) and sodium nitrate (NaNO3 ) is similar to that of KNO3 . Above 450 ◦ C noticeable thermal decomposition of sodium nitrite occurs thus
5NaNO2 → 3NaNO3 + N2 + Na2 O
∗ Corresponding author. Tel.: +82 2 880 7093; fax: +82 2 887 6388. E-mail address: [email protected] (D.N. Lee). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.09.021
(1)
If oxygen is present, the nitrite can oxidize to the nitrate at similar temperatures: 2NaNO2 + O2 → 2NaNO3
(2)
Oxygen may be present from air ingress or the decomposition of the nitrates at high temperatures. Sodium nitrate begins to decompose above 500 ◦ C according to the following reactions: 4NaNO3 → 5O2 + 2N2 + 2Na2 O
(3)
or 2NaNO3 → 2NaNO2 + O2
(4)
Another possible decomposition, at fairly high temperatures, involves the formation of nitrogen oxides [13,14]: 2NaNO2 → Na2 O + NO + NO2
(5)
It follows from the above reactions that the NaNO3 bath may be used to nitride steels. The objective of this study is to nitride IF steel in the NaNO3 bath instead of the KNO3 bath and to examine a possible oxide breaking ability of chlorides such as sodium chloride (NaCl) and calcium chloride (CaCl2 ) in the nitrate bath.
M.-k. Lee et al. / Materials Science and Engineering A 527 (2010) 1048–1051
Fig. 1. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 bath at 650 ◦ C for 2–12 h.
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Fig. 2. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath containing at 650 ◦ C for 30 min to 5 h.
2. Experimental method The material to be nitrided was deep drawing quality (DDQ) interstitial free (IF) steel (0.0020% C, 0.006% Si, 0.053% Mn, 0.01% P, 0.0096% S) sheets of 0.8 mm thickness from POSCO. The nitriding bath was composed of commercial purity NaNO3 and additives such as NaCl and CaCl2 . The salt baths were kept at given temperatures in an electric furnace. After the nitriding, all the samples were quenched in cold water to remove the surface scale. The tensile specimens of 6 mm × 25 mm in gauge dimensions, in accordance with ASTM E 8M-99, with the tension axis parallel to the rolling direction were ground and polished up to a bright surface before tensile test. Tension tests were conducted on an Instron testing machine at a crosshead speed of 5 mm min−1 at room temperature. The Vickers hardness tests were conducted under a load of 300 g to obtain hardness profiles through the thickness of the steel specimens. The optical microstructures of nitrided specimens and scanning electron micrographs (SEM) of fracture surfaces of nitrided specimens were obtained.
3. Results and discussion Fig. 1 shows the hardness values of steel sheets nitrided in NaNO3 bath at 650 ◦ C for 2–12 h. The specimen denoted by ‘0 h’ indicates the starting steel sheet. The hardness of steel specimen increases by about two times after nitriding for 2 h and about three times after nitriding for 12 h. The hardness increases with nitriding time and is almost uniform through the thickness. This implies that the nitrogen diffusion rate is so high that the nitrogen concentration in the specimens is almost the same through the thickness under the experimental conditions. As the nitriding time increases, the nitrogen concentration increases, which in turn increases the hardness of specimens. However, the hardness appears to be saturated when nitrided for about 8 h. Figs. 2–4 show the hardness values of steel sheets nitrided in NaNO3 bath containing 2.44% NaCl at 650, 600, and 550 ◦ C, respectively. When nitrided in the NaNO3 –2.44% NaCl bath at 650 ◦ C, the hardness values of specimens increase substantially compared with the case without NaCl (Fig. 1). As the nitriding temperature decreases, both the nitriding reaction rate at the surface and the diffusion rate of nitrogen decrease, resulting in a decrease in hardness, as expected. It is also noted that the difference in hardness between the surface and center layers increases with decreasing nitriding temperature and time, indicating that the decreasing rate of diffusion is higher than that of nitriding reaction at the surface.
Fig. 3. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath at 600 ◦ C for 30 min to 5 h.
The tensile properties of steel sheets nitrided in the NaNO3 bath at 600 and 650 ◦ C are given in Table 1. When nitrided at 600 ◦ C, the tensile strength increases from 291 to 476 MPa and the elongation decreases from 80 to 32% with increasing nitriding time to 8 h. At the nitriding temperature of 650 ◦ C, the tensile strength increases to the maximum value of 887 MPa and the elongation decreases to 19%, when nitrided for 6 h. As the nitriding time exceeds 6 h, the tensile strength and the elongation decrease possibly due to the formation of brittle compound Fe4 N [1–3].
Fig. 4. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath at 500 ◦ C for 30 min to 5 h.
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M.-k. Lee et al. / Materials Science and Engineering A 527 (2010) 1048–1051
Table 1 Tensile properties of DDQ steel sheets nitrided in NaNO3 bath at 600 and 650 ◦ C for 2–24 h. YS refers to 0.2% offset yield strength, TS tensile strength, UE uniform elongation, TE total elongation. N600-2 h indicates specimen nitrided at 600 ◦ C for 2 h. Specimen
YS, MPa
TS, MPa
UE, %
TE, %
Non-nitrided N600-2 h N600-4 h N600-6 h N600-8 h N650-2 h N650-4 h N650-6 h N650-8 h N650-12 h N650-24 h
142 347 397 383 456 484 564 418 240 557 397
291 384 427 459 476 553 725 887 862 790 473
53 33 29 26 24 18 14 19 11 6 8
80 47 43 35 32 22 15 19 11 6 8
Comparison of the tensile properties in Table 1 with those in Table 2 indicates that the steel sheet specimens nitrided in the NaNO3 –4.8% NaCl bath have higher strengths than those nitrided in the NaNO3 bath at given nitriding temperature and time. It is worth noting that a tensile property of 1000 MPa in tensile strength and 16% in elongation is obtained at 650 ◦ C for 1 h in the NaCl added nitrate bath. This indicates that the addition of about 5% NaCl accelerates nitriding. The mechanism of the increase in nitriding rate in the NaCl added bath is not known yet. It is inferred that the surface oxide layer formed during nitriding reacts with the chloride to form a kind of flux and partially removed, whereby nitriding agents easily approach and react with steel. An increase in the NaCl concentration in the bath does not necessarily increase the tensile strength (compare the tensile strengths of N4.8Na650-30 min, N25Na650-30 m, and N33Na650-30 m). The effect of the NaCl concentration on the nitriding rate is complex. We can infer two competing effects of NaCl on the nitriding rate. Table 2 Tensile properties of DDQ steel sheets nitrided in NaNO3 bath containing NaCl and/or CaCl2 at 600 and 650 ◦ C for 10 m (min) to 6 h. Numerical values preceding Na and Ca indicate % of NaCl and CaCl2 in NaNO3 bath. Specimen
YS, MPa
N4.8Na600-30 m N4.8Na600-1 h N4.8Na600-2 h N4.8Na600-3 h N4.8Na600-4 h N4.8Na600-5 h N4.8Na650-30 m N4.8Na650-1 h N4.8Na650-2 h N4.8Na650-3 h N4.8Na650-4 h N4.8Na650-5 h N25Na650-10 m N25Na650-15 m N25Na650-20 m N25Na650-30 m N33Na650-1 h N33Na650-2 h N33Na650-3 h N33Na650-4 h N33Na650-5 h N33Na650-6 h N1.2Na1.2Ca650-30 m N1.2Na1.2Ca650-1 h N1.2Na1.2Ca650-2 h N1.2Na1.2Ca650-3 h N1.2Na1.2Ca650-4 h N1.2Na1.2Ca650-5 h N20Ca650-1 h N20Ca650-2 h N20Ca650-3 h
330 369 383 411 466 486 677 696 790 827 821 721 106 552 119 601 665 643 159 137 154 308 412 239 447 499 226 558 636 269 143
TS, MPa 402 438 472 502 548 573 887 1001 939 962 932 880 585 727 781 857 847 643 777 666 634 308 481 495 542 602 657 675 734 704 451
UE, %
TE, %
21 19 17 15 13 11 15 15 11 5 4 7 28 23 22 21 4 2 2 2 2 1 14 11 10 7 8 8 3 1 1
31 26 24 19 16 13 17 16 13 5 4 7 34 27 25 22 4 2 2 2 2 1 19 15 13 10 10 10 3 1 1
Fig. 5. Tensile strength as a function of elongation for nitrided specimens. N, N4.8Na, N25Na, N33Na, N1.2Na1.2Ca, and N20Ca refer to specimens nitrided in NaNO3 , NaNO3 –4.8%NaCl, NaNO3 –25%NaCl, NaNO3 –33%NaCl, NaNO3 –1.2%NaCl–1.2%CaCl2 , and NaNO3 –20%CaCl2 baths, respectively. () Non-nitrided specimen.
An increase in the NaCl concentration decreases the concentration of NaNO3 , which is likely to decrease the nitriding rate. On the other hand, an increase in NaCl concentration increases the oxide removing power, which is likely to increase the nitriding rate. Comparison of the tensile properties (887 MPa in tensile strength and 17% in elongation) of N4.8Na650-30 m with those (857 MPa and 22%) of N25Na650-30 m indicates that the decreasing nitriding-rate effect of NaCl seems to be prevalent because the lower tensile strength and the higher elongation reflect a lower nitriding rate. Even when the nitriding rate is high, the tensile strength can be low because of the excessive formation of Fe4 N. In this case, the elongation is low. For example, N33Na650 and N20Ca650 specimens show decreases in tensile strength and elongation with increasing nitriding time. The addition of 1.2% NaCl and 1.2% CaCl2 does little influence the nitriding rate. Fig. 5 shows the tensile strength as a function of elongation for specimens in Tables 1 and 2. The correlation between the two properties is not good. The mechanical properties depend on the state
Fig. 6. Optical microstructure of DDQ steel nitrided in NaNO3 –4.8% NaCl bath at 650 ◦ C for 1 h.
M.-k. Lee et al. / Materials Science and Engineering A 527 (2010) 1048–1051
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specimen nitrided for 1 h and about 25 m thick in the specimen nitrided for 5 h, look different from those of the inner regions. Xray diffraction data did not show a difference in phase between the surface layer and the inner region. It is interesting to note that grain boundaries cannot be clearly seen in the surface layers (Figs. 6 and 7). It is speculated that the nitrogen evolution rate is so high in the early stage that the nitrogen diffusion rate through lattice is not much different from that along grain boundaries. As the diffusion depth increases, the nitrogen concentration decreases and the diffusion along the boundaries become increasingly favored. 4. Conclusions Nitriding of steel is possible in sodium nitrate baths above 500 ◦ C. Nitriding agents are nitrogen and nitrogen oxides generated from the decomposition of the nitrate as in the potassium nitrate bath. Existence of chlorides such as sodium chloride and calcium chloride in the NaNO3 bath accelerates nitriding, possibly due to its role of breaking the oxide layer formed on the surface of steel. The optimum concentration of sodium chloride in the nitrate bath seems to be about 5%. When nitrided at 650 ◦ C for 1 h, IF steel specimen showed a tensile strength of 1000 MPa and an elongation of about 16%. However, this specimen shows brittle fracture features, requiring further study to obtain better mechanical properties. Acknowledgement This work was supported by Advanced Technology Center project 2008-10031476. Fig. 7. SEM micrographs of DDQ steel nitrided in NaNO3 –4.8% NaCl bath at 650 ◦ C for (a) 1 h and (b) 5 h.
(solution or compound) of nitrogen in steel even at the same nitrogen concentration. The compound state is likely to deteriorate the elongation more than the tensile strength. Therefore, nitriding conditions should be carefully controlled for the best combination of strength and elongation. Fig. 6 shows an optical microstructure of the cross section of a specimen nitrided in the NaNO3 –4.8%NaCl bath at 650 ◦ C for 1 h. Fig. 7 shows SEM micrographs of tensile fractured specimens nitrided in the NaNO3 –4.8%NaCl bath at 650 ◦ C for 1 and 5 h. The optical microstructure shows small dark spots in interiors and dark, thick boundaries of the grains, which are thought to be associated with Fe4 N particles and layers. The fractographs in Fig. 7 show typical cleavage fracture and intergranular fracture, which are features of brittle fracture. This can be attributed to the brittle Fe4 N phase formed in interiors and boundaries of the grains [2]. The microstructures of the surface layers, about 10 m thick in the
References [1] [2] [3] [4] [5] [6] [8] [9] [10] [11] [12]
Y.Z. Shen, K.H. Oh, D.N. Lee, Scripta Mater. 53 (2005) 1345. Y.Z. Shen, K.H. Oh, D.N. Lee, ISIJ Int. 46 (2006) 111. Y.Z. Shen, K.H. Oh, D.N. Lee, Mater. Sci. Eng. A434 (2006) 314. E.S. Freeman, J. Am. Chem. Soc. 79 (1957) 838. T.M. Oza, J. Indian Chem. Soc. 22 (1945) 173. V.J. Szper, K.Z. Fiszmon, Anorg. Allg. Chem. 206 (1932) 257. T.M. Oza, S.A. Patel, J. Indian Chem. Soc. 31 (1954) 520. K.H. Stern, J. Phys. Chem. Ref. Data 1 (1972) 747. E.S. Freeman, J. Phys. Chem. 60 (1956) 1487. G. Von Noy, H. Kuhnlee, Sonderdr. Fachzeitschr. Warme 85 (4/5) (1979). Hitec: Properties, Uses, Storage and Handling, issued by Coastal Chemical Co., P.O.Box C, Abbeville, LA 70510. [13] G.J. Janz, R.T.P. Tomkins, Physical Properties Data Compilations Relevant to Energy Storage, IV, Molten Salts: Data on Additional Single and Multicomponent Salt Systems, U.S. Dept. of Energy, National Standard Reference Data System, National Bureau of Standards (NSRDS-NBS 61), U.S. Government Printing Office, Washington D.C., 1981. [14] G.J. Janz, C.B. Allen, N.P. Bansal, R.M. Murphy, R.T.P. Tomkins, Physical Properties Data Compilations Relevant to Energy Storage, II, Molten Salts: Data on Additional Single and Multicomponent Salt Systems, U.S. Dept. of Energy, National Standard Reference Data System, National Bureau of Standards (NSRDS-NBS 61), U.S. Government Printing Office, Washington D.C., 1979.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of NaCl and CaCl2 additives on NaNO3 bath nitriding of steel Min-ku Lee a , Dong-sam Kim a , Sung-chul Kim a , Sang-won Han a , Insoo Kim b , Dong Nyung Lee c,∗ a
Iljin Light Metal Co., Ltd. 446-1, Kwaerang-Ri, Jungnam-Myun, Hwasung-Si, Gyunggi-Do 445-963, Republic of Korea School of Advanced Materials Science and Systems Engineering, Kumoh National Institute of Technology, Gumi-Si, Gyung-Buk 730-701, Republic of Korea c Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 12 July 2009 Received in revised form 9 September 2009 Accepted 10 September 2009
Keywords: Sodium nitrate Nitriding IF steel Hardness Tensile properties
a b s t r a c t A study has been made of nitriding of interstitial-free steel sheet specimens in the sodium nitrate bath. The nitriding rate increased when the bath contained a small amount of sodium chloride and calcium chloride. Accordingly, the tensile strengths and hardness values of the steel specimens nitrided in the chloride containing bath were higher than those of the same steel specimens nitrided in the pure nitrate bath when the nitriding time and temperature were controlled so that the excessive formation of brittle compound Fe4 N could be prevented. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
[11,12]:
Pure liquid nitriding of steel using the potassium nitrate salt (KNO3 ) bath was first introduced in 2005 [1]. Since then, nitriding in the nitrate bath has been further studied [2,3]. The source of nitrogen in the nitriding was attributed to nitrogen, nitrogenic oxide (NO), and nitrogen dioxide (NO2 ) generated from the decomposition of KNO3 at temperatures above about 500 ◦ C [4–10]. It was suggested that NO and NO2 as well as nitrogen generated from the decomposition of KNO3 could participate in nitriding and oxidation reactions [3]. The diffusion behavior of nitrogen in IF steel was similar to that in ˛-iron with the nitrogen concentration at the surface of steel specimen being approximated by the solid solubility of nitrogen in ˛-Fe, in case of nitriding at 650 ◦ C [1–3]. Pronounced increases in tensile strength with reasonable elongations could be achieved by the nitrate bath nitriding [2,3]. In the light of these facts, the nitrate-nitriding is regarded as a promising method for introducing nitrogen into iron and steel, though surface oxidation cannot be avoided. The surface oxidation can cause not only a slight loss of steel but also a reduction in nitriding rate because of the surface oxide layer that hampers the nitrogen sources reaching steel. The thermal decomposition behavior of sodium nitrite (NaNO2 ) and sodium nitrate (NaNO3 ) is similar to that of KNO3 . Above 450 ◦ C noticeable thermal decomposition of sodium nitrite occurs thus
5NaNO2 → 3NaNO3 + N2 + Na2 O
∗ Corresponding author. Tel.: +82 2 880 7093; fax: +82 2 887 6388. E-mail address: [email protected] (D.N. Lee). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2009.09.021
(1)
If oxygen is present, the nitrite can oxidize to the nitrate at similar temperatures: 2NaNO2 + O2 → 2NaNO3
(2)
Oxygen may be present from air ingress or the decomposition of the nitrates at high temperatures. Sodium nitrate begins to decompose above 500 ◦ C according to the following reactions: 4NaNO3 → 5O2 + 2N2 + 2Na2 O
(3)
or 2NaNO3 → 2NaNO2 + O2
(4)
Another possible decomposition, at fairly high temperatures, involves the formation of nitrogen oxides [13,14]: 2NaNO2 → Na2 O + NO + NO2
(5)
It follows from the above reactions that the NaNO3 bath may be used to nitride steels. The objective of this study is to nitride IF steel in the NaNO3 bath instead of the KNO3 bath and to examine a possible oxide breaking ability of chlorides such as sodium chloride (NaCl) and calcium chloride (CaCl2 ) in the nitrate bath.
M.-k. Lee et al. / Materials Science and Engineering A 527 (2010) 1048–1051
Fig. 1. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 bath at 650 ◦ C for 2–12 h.
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Fig. 2. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath containing at 650 ◦ C for 30 min to 5 h.
2. Experimental method The material to be nitrided was deep drawing quality (DDQ) interstitial free (IF) steel (0.0020% C, 0.006% Si, 0.053% Mn, 0.01% P, 0.0096% S) sheets of 0.8 mm thickness from POSCO. The nitriding bath was composed of commercial purity NaNO3 and additives such as NaCl and CaCl2 . The salt baths were kept at given temperatures in an electric furnace. After the nitriding, all the samples were quenched in cold water to remove the surface scale. The tensile specimens of 6 mm × 25 mm in gauge dimensions, in accordance with ASTM E 8M-99, with the tension axis parallel to the rolling direction were ground and polished up to a bright surface before tensile test. Tension tests were conducted on an Instron testing machine at a crosshead speed of 5 mm min−1 at room temperature. The Vickers hardness tests were conducted under a load of 300 g to obtain hardness profiles through the thickness of the steel specimens. The optical microstructures of nitrided specimens and scanning electron micrographs (SEM) of fracture surfaces of nitrided specimens were obtained.
3. Results and discussion Fig. 1 shows the hardness values of steel sheets nitrided in NaNO3 bath at 650 ◦ C for 2–12 h. The specimen denoted by ‘0 h’ indicates the starting steel sheet. The hardness of steel specimen increases by about two times after nitriding for 2 h and about three times after nitriding for 12 h. The hardness increases with nitriding time and is almost uniform through the thickness. This implies that the nitrogen diffusion rate is so high that the nitrogen concentration in the specimens is almost the same through the thickness under the experimental conditions. As the nitriding time increases, the nitrogen concentration increases, which in turn increases the hardness of specimens. However, the hardness appears to be saturated when nitrided for about 8 h. Figs. 2–4 show the hardness values of steel sheets nitrided in NaNO3 bath containing 2.44% NaCl at 650, 600, and 550 ◦ C, respectively. When nitrided in the NaNO3 –2.44% NaCl bath at 650 ◦ C, the hardness values of specimens increase substantially compared with the case without NaCl (Fig. 1). As the nitriding temperature decreases, both the nitriding reaction rate at the surface and the diffusion rate of nitrogen decrease, resulting in a decrease in hardness, as expected. It is also noted that the difference in hardness between the surface and center layers increases with decreasing nitriding temperature and time, indicating that the decreasing rate of diffusion is higher than that of nitriding reaction at the surface.
Fig. 3. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath at 600 ◦ C for 30 min to 5 h.
The tensile properties of steel sheets nitrided in the NaNO3 bath at 600 and 650 ◦ C are given in Table 1. When nitrided at 600 ◦ C, the tensile strength increases from 291 to 476 MPa and the elongation decreases from 80 to 32% with increasing nitriding time to 8 h. At the nitriding temperature of 650 ◦ C, the tensile strength increases to the maximum value of 887 MPa and the elongation decreases to 19%, when nitrided for 6 h. As the nitriding time exceeds 6 h, the tensile strength and the elongation decrease possibly due to the formation of brittle compound Fe4 N [1–3].
Fig. 4. Vickers hardness test (load: 300 g) results of cross sections of DDQ steel sheets nitrided in NaNO3 –2.44% NaCl bath at 500 ◦ C for 30 min to 5 h.
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M.-k. Lee et al. / Materials Science and Engineering A 527 (2010) 1048–1051
Table 1 Tensile properties of DDQ steel sheets nitrided in NaNO3 bath at 600 and 650 ◦ C for 2–24 h. YS refers to 0.2% offset yield strength, TS tensile strength, UE uniform elongation, TE total elongation. N600-2 h indicates specimen nitrided at 600 ◦ C for 2 h. Specimen
YS, MPa
TS, MPa
UE, %
TE, %
Non-nitrided N600-2 h N600-4 h N600-6 h N600-8 h N650-2 h N650-4 h N650-6 h N650-8 h N650-12 h N650-24 h
142 347 397 383 456 484 564 418 240 557 397
291 384 427 459 476 553 725 887 862 790 473
53 33 29 26 24 18 14 19 11 6 8
80 47 43 35 32 22 15 19 11 6 8
Comparison of the tensile properties in Table 1 with those in Table 2 indicates that the steel sheet specimens nitrided in the NaNO3 –4.8% NaCl bath have higher strengths than those nitrided in the NaNO3 bath at given nitriding temperature and time. It is worth noting that a tensile property of 1000 MPa in tensile strength and 16% in elongation is obtained at 650 ◦ C for 1 h in the NaCl added nitrate bath. This indicates that the addition of about 5% NaCl accelerates nitriding. The mechanism of the increase in nitriding rate in the NaCl added bath is not known yet. It is inferred that the surface oxide layer formed during nitriding reacts with the chloride to form a kind of flux and partially removed, whereby nitriding agents easily approach and react with steel. An increase in the NaCl concentration in the bath does not necessarily increase the tensile strength (compare the tensile strengths of N4.8Na650-30 min, N25Na650-30 m, and N33Na650-30 m). The effect of the NaCl concentration on the nitriding rate is complex. We can infer two competing effects of NaCl on the nitriding rate. Table 2 Tensile properties of DDQ steel sheets nitrided in NaNO3 bath containing NaCl and/or CaCl2 at 600 and 650 ◦ C for 10 m (min) to 6 h. Numerical values preceding Na and Ca indicate % of NaCl and CaCl2 in NaNO3 bath. Specimen
YS, MPa
N4.8Na600-30 m N4.8Na600-1 h N4.8Na600-2 h N4.8Na600-3 h N4.8Na600-4 h N4.8Na600-5 h N4.8Na650-30 m N4.8Na650-1 h N4.8Na650-2 h N4.8Na650-3 h N4.8Na650-4 h N4.8Na650-5 h N25Na650-10 m N25Na650-15 m N25Na650-20 m N25Na650-30 m N33Na650-1 h N33Na650-2 h N33Na650-3 h N33Na650-4 h N33Na650-5 h N33Na650-6 h N1.2Na1.2Ca650-30 m N1.2Na1.2Ca650-1 h N1.2Na1.2Ca650-2 h N1.2Na1.2Ca650-3 h N1.2Na1.2Ca650-4 h N1.2Na1.2Ca650-5 h N20Ca650-1 h N20Ca650-2 h N20Ca650-3 h
330 369 383 411 466 486 677 696 790 827 821 721 106 552 119 601 665 643 159 137 154 308 412 239 447 499 226 558 636 269 143
TS, MPa 402 438 472 502 548 573 887 1001 939 962 932 880 585 727 781 857 847 643 777 666 634 308 481 495 542 602 657 675 734 704 451
UE, %
TE, %
21 19 17 15 13 11 15 15 11 5 4 7 28 23 22 21 4 2 2 2 2 1 14 11 10 7 8 8 3 1 1
31 26 24 19 16 13 17 16 13 5 4 7 34 27 25 22 4 2 2 2 2 1 19 15 13 10 10 10 3 1 1
Fig. 5. Tensile strength as a function of elongation for nitrided specimens. N, N4.8Na, N25Na, N33Na, N1.2Na1.2Ca, and N20Ca refer to specimens nitrided in NaNO3 , NaNO3 –4.8%NaCl, NaNO3 –25%NaCl, NaNO3 –33%NaCl, NaNO3 –1.2%NaCl–1.2%CaCl2 , and NaNO3 –20%CaCl2 baths, respectively. () Non-nitrided specimen.
An increase in the NaCl concentration decreases the concentration of NaNO3 , which is likely to decrease the nitriding rate. On the other hand, an increase in NaCl concentration increases the oxide removing power, which is likely to increase the nitriding rate. Comparison of the tensile properties (887 MPa in tensile strength and 17% in elongation) of N4.8Na650-30 m with those (857 MPa and 22%) of N25Na650-30 m indicates that the decreasing nitriding-rate effect of NaCl seems to be prevalent because the lower tensile strength and the higher elongation reflect a lower nitriding rate. Even when the nitriding rate is high, the tensile strength can be low because of the excessive formation of Fe4 N. In this case, the elongation is low. For example, N33Na650 and N20Ca650 specimens show decreases in tensile strength and elongation with increasing nitriding time. The addition of 1.2% NaCl and 1.2% CaCl2 does little influence the nitriding rate. Fig. 5 shows the tensile strength as a function of elongation for specimens in Tables 1 and 2. The correlation between the two properties is not good. The mechanical properties depend on the state
Fig. 6. Optical microstructure of DDQ steel nitrided in NaNO3 –4.8% NaCl bath at 650 ◦ C for 1 h.
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specimen nitrided for 1 h and about 25 m thick in the specimen nitrided for 5 h, look different from those of the inner regions. Xray diffraction data did not show a difference in phase between the surface layer and the inner region. It is interesting to note that grain boundaries cannot be clearly seen in the surface layers (Figs. 6 and 7). It is speculated that the nitrogen evolution rate is so high in the early stage that the nitrogen diffusion rate through lattice is not much different from that along grain boundaries. As the diffusion depth increases, the nitrogen concentration decreases and the diffusion along the boundaries become increasingly favored. 4. Conclusions Nitriding of steel is possible in sodium nitrate baths above 500 ◦ C. Nitriding agents are nitrogen and nitrogen oxides generated from the decomposition of the nitrate as in the potassium nitrate bath. Existence of chlorides such as sodium chloride and calcium chloride in the NaNO3 bath accelerates nitriding, possibly due to its role of breaking the oxide layer formed on the surface of steel. The optimum concentration of sodium chloride in the nitrate bath seems to be about 5%. When nitrided at 650 ◦ C for 1 h, IF steel specimen showed a tensile strength of 1000 MPa and an elongation of about 16%. However, this specimen shows brittle fracture features, requiring further study to obtain better mechanical properties. Acknowledgement This work was supported by Advanced Technology Center project 2008-10031476. Fig. 7. SEM micrographs of DDQ steel nitrided in NaNO3 –4.8% NaCl bath at 650 ◦ C for (a) 1 h and (b) 5 h.
(solution or compound) of nitrogen in steel even at the same nitrogen concentration. The compound state is likely to deteriorate the elongation more than the tensile strength. Therefore, nitriding conditions should be carefully controlled for the best combination of strength and elongation. Fig. 6 shows an optical microstructure of the cross section of a specimen nitrided in the NaNO3 –4.8%NaCl bath at 650 ◦ C for 1 h. Fig. 7 shows SEM micrographs of tensile fractured specimens nitrided in the NaNO3 –4.8%NaCl bath at 650 ◦ C for 1 and 5 h. The optical microstructure shows small dark spots in interiors and dark, thick boundaries of the grains, which are thought to be associated with Fe4 N particles and layers. The fractographs in Fig. 7 show typical cleavage fracture and intergranular fracture, which are features of brittle fracture. This can be attributed to the brittle Fe4 N phase formed in interiors and boundaries of the grains [2]. The microstructures of the surface layers, about 10 m thick in the
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