- Email: [email protected]
Effect of QPQ nitriding time on wear and corrosion behavior of 45 carbon steel
Applied Surface Science 261 (2012) 411–414
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of QPQ nitriding time on wear and corrosion behavior of 45 carbon steel Wei Cai a,b , Fanna Meng a,b , Xinyan Gao a,b , Jing Hu a,b,∗ a b
Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou 213164, China School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China
a r t i c l e
i n f o
Article history: Received 21 May 2012 Received in revised form 3 August 2012 Accepted 7 August 2012 Available online 13 August 2012 Keywords: 45 steel Corrosion resistance Wear resistance Microhardness QPQ salt bath
a b s t r a c t QPQ salt bath treatment of 45 steel was conducted by nitriding at 565 ◦ C for various time (60 min, 90 min, 120 min, 150 min and 180 min), followed by the same post-oxidation process with heating temperature of 430 ◦ C and holding duration of 40 min. Characterization of modified surface layers was made by means of optical microscopy, microhardness test, X-ray diffraction analysis, corrosion and wear resistance test. The results showed the formation of a very thin Fe3 O4 oxide layer during post-oxidation on the top of the bright nitrides compound layer formed during nitriding. The maximum microhardness value of 630 HV0.01 was obtained after nitriding at 565 ◦ C for 120 min, which was two times higher than that of the untreated sample. The corrosion and wear resistance of 45 steel could be significantly improved by QPQ complex salt bath treatment, and the optimum nitriding duration for improving the wear and corrosion resistance was 120 min and 90 min, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The 45 steel is widely applied in industry due to its low cost and good combination mechanical properties. But its application is highly constrained in the field of high requirement for wear resistance and especially, corrosion resistance. It is established that Quench-Polish-Quench complex salt bath (QPQ) surface modification technology can improve its both wear resistance and corrosion resistance. The essence of QPQ salt bath technology is low temperature salt bath nitriding + salt bath oxidation or low temperature salt bath carbonitriding + salt bath oxidation. The engineering of QPQ salt bath mainly consists of six steps: (1) degreasing, (2) pre-heating, (3) nitriding, (4) oxidizing, (5) washing and (6) oil immersion [1]. The goal of each step is as below; degreasing is to remove the dirt, pre-heating is to remove water on the components, and also produce a thin oxide layer on the surface of the components. This thin oxide layer will benefit nitriding process. Nitriding and oxidizing are to produce the nitrided layer and oxide layer on the surface of metal, which are the key steps of the whole process. The outer layer of QPQ treated samples is an oxide layer which could be Fe3 O4 , Fe2 O3 or FeO or a combination of them. Among the three kinds of oxide, only magnetite Fe3 O4 can highly improve the wear and corrosion resistance of the surface due to its low friction coefficient and chemical stability [2]. It is
∗ Corresponding author at: Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou 213164, China. Tel.: +86 0519 86330065. E-mail address: [email protected] (J. Hu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.024
reported that the corrosion resistance of the treated components could be higher than that treated by chrome plating and nickel plating [3]. Moreover, the corrosion resistance of carbon steel treated by QPQ could be as good as stainless steels in some applications. In addition, a nitrided layer and a diffusion layer beneath the oxide layer could improve the hardness and wear resistance of the surface. Therefore, carbon steel after suitable QPQ treatment may replace stainless steels in some fields, and with much better combination mechanical properties. For the past few years, there have been some researches about the influence of QPQ treatment on the microstructure and properties of various materials [4–7]. However, systematic investigation of the effect of QPQ complex treatment on the microstructure, wear resistance and corrosion resistance of 45 carbon steel was not yet presented in the literature. This research was initially required by a compressor industry; the main goal is to investigate if 45 steel components after suitable QPQ treatment can have good corrosion resistance in water lubricant environments instead of machine oil lubricants used previously, the purpose of the lubricants changing from machine oil to water is to meet the requirements of green manufacturing without carbon releasing and also decrease running cost. If the corrosion resistance of 45 steel in water immersing test can be highly improved, it can imply that it is possible to still keep using 45 steel as the components in compressor when lubricant is changed from machine oil to water, which could bring out obvious advantages than stainless steels, initially planed to use, since 45 steel has much better combination mechanical properties, better machinability and lower cost comparing with stainless steels. Therefore, the aim of the present study is focused on the corrosion and wear resistance of 45 steel treated by QPQ treatment to
412
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
Fig. 1. Cross sectional microstructure of 45 steel specimens treated at 565 ◦ C for different nitriding time: (a) 60 min, (b) 90 min, (c) 120 min, (d) 180 min.
determine the optimum process conditions. The modified surface was studied in terms of optical microscopy, X-ray diffraction (XRD), micro-hardness test, wear resistance and corrosion resistance test.
2. Materials and methods The material used in this study was 45 steel with the nominal composition (wt.%) of: 0.42–0.5% C, 0.17–0.37% Si, 0.5–0.8% Mn, and balance Fe. Samples were cut in the size of 10 mm × 10 mm × 10 mm for optical microscopy, microhardness test and corrosion test, and in 32 mm diameter and 5 mm thick discs for wear test. Then the samples were quenched at 850 ◦ C and tempered at 580 ◦ C. All the flat surfaces of each sample were ground using various grades of SiC paper and polished to a mirror finish, and then ultrasonically cleaned in deionized water and alcohol for 5 min before QPQ treatment. The samples were processed in tailormade QPQ salt bath furnace with the CNO− concentration of 35%. In order to investigate the effect of nitriding time on the surface characteristics, QPQ salt bath nitriding was conducted at the same temperature of 565 ◦ C for various time (60 min, 90 min, 120 min, 150 min and 180 min), followed by the same post-oxidation process with heating temperature of 430 ◦ C and holding duration of 40 min. The polish process was omitted in this research by simulating the real production process. The nitriding process was conducted using the nitride salt. The N atom came from the dissociation of CNO− , and the C atom came from the dissociation of CNO− simultaneously. The post-oxidation process was conducted using the oxide salt. The formula of nitride salt and oxide salt was kept in secret by the manufacturing factory. The cross sectional morphology was studied by optical metallography (XUG-05). The phase constituents were determined by X-ray diffraction (XRD) (Dmax 2500) using Cu–K␣ radiation. Hardness measurements were made in a HXD-1000TMC microhardness tester, with the test load of 10 g and the holding duration of 15 s. Each hardness value was determined by averaging at least 5 measurements.
The corrosion resistance was evaluated by hot water immersing test according to the cooperator’s requirements based on the potential real application, the temperature of hot-water was hold at 90 ◦ C during the test. The standard of initial rust was designed by referring to ASTM B117-03. The initial rusting time of each sample was recorded. The wear tests were carried out on a MMW-1A Wear Test Machine under ambient condition (20 ± 2 ◦ C and 50%RH). During the test, a 6 mm diameter GCr15 steel ball rotated at a speed of 200 rpm on the surface of the disc samples for the testing time of 60 min. The test load was 50 N. The friction coefficient was continuously recorded during the test and the weight of the samples was measured by a balance accurate to 0.1 mg for calculating the weight loss. 3. Results and discussion 3.1. Metallographic observations Fig. 1 shows the cross sectional metallographs of 45 steel specimens QPQ treated at 565 ◦ C for different nitriding time with the same post-oxidation process (430 ◦ C, 40 min). From the outmost surface to the core, a thin oxide film, compound layer (bright layer), diffusion layer were formed. The oxide film, whose thickness is too thin to be clearly observed by optical microscopy, was produced in the post-oxidation process, which could further improve the corrosion resistance and wear resistance when existing in the form of magnetite Fe3 O4 [8]. The compound layer, also called bright layer is obviously observed and its thickness increases from 5 m to 16 m with the nitriding duration from 60 min to 180 min, and with a porous or loosen structure at the top of the compound layer, which is getting more and more looser with its thickness increase. The compound layer is an important part of the QPQ treated samples and plays an important role in improving the wear resistance and corrosion resistance. The diffusion layer between bright layer and radical center is hard to be clearly distinguished by optical microscopy as shown in Fig. 1. Its main components are solid solution or supersaturated solid solution of the N and C atoms in ␣-Fe,
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
(b)
413
650
Fe3O4 Fe3N
untreated 60min 90min 120min 150min 180min
600
Hardness(HV0.01)
550
Intensity
Nitrocarburized only
(a)
500 450 400 350 300 250
With post oxidation
200 0
20
40
60
80
100
2
50
100
Depth (μ m)
150
200
Fig. 3. The surface hardness-depth profile of 45 steel specimens untreated and QPQ treated at 565 ◦ C for different nitriding time.
Fig. 2. XRD diffraction patterns of 45 steel samples nitrided at 565 ◦ C for 120 min (a) with post-oxidation at 430 ◦ C for 40 min and (b) without post-oxidation.
which could bring about an improvement in fatigue strength when compared with an untreated material [9].
diffusion zone grows coarser, resulting in the decrease of surface hardness as well.
3.2. XRD analysis
3.4. Wear resistance analysis
Fig. 2 presents the XRD diffraction patterns of 45 steel samples nitrided for 120 min followed by post-oxidation (QPQ complex salt bath treatment) and without post-oxidation. Fig. 2(a) shows that the surface layer of QPQ treated samples composed of Fe3 O4 and -Fe3 N, and (b) indicates that the surface layer is mainly composed of -Fe3 N. The reason why Fe peaks disappear is that the compound layer becomes thicker and substrate influence is no more obvious [10]. The main difference between the two processes with postoxidation and without post-oxidation is the formation of Fe3 O4 . As agreed that -Fe3 N has much higher hardness and excellent wear resistance comparing with base metal, and magnetite Fe3 O4 formed in the post-oxidation process can further improve the wear and corrosion resistance of the surface due to its low friction coefficient and chemical stability [2].
The variation of weight loss of 45 steel samples untreated and QPQ treated under different conditions is shown in Fig. 4. It can be clearly seen that the weight loss of the untreated sample is much higher than that of the nitrided samples, reaching around 0.14 g. Whereas, the weight loss of samples after QPQ salt bath treatment decreases dramatically, and with the extension of nitriding time, the weight loss reduces gradually, reaching the minimum of 0.0029 g when nitrided for 120 min. This is because after QPQ complex salt bath treatment, the surface layer is made of -Fe3 N with high hardness and Fe3 O4 with low friction coefficient, which results in excellent wear resistance. The wear resistance changing regularity with the nitriding time is in good agreement with the hardness profile as shown in Fig. 3.
3.3. Hardness analysis 0.14 0.12 0.10
Weight loss(g)
Fig. 3 shows the evolution of the cross section microhardness for different nitriding time. In this figure, the profile that the hardness values decrease as the depth increase is the distinguished diffusion feature, which can be used to evaluate the diffusion layer or the effective hardening depth. The maximum microhardness values are obtained on the subsurface of the samples, due to the loosen structure formed on the outermost surface. The surface hardness reaches the maximum of 630 HV0.01 when nitrided for 120 min, which is almost 3 times as high as that of untreated sample of 230 HV0.01 . With the longer nitriding duration, the surface hardness rises gradually at first and declines slightly when nitrided time is exceeded 120 min. The reason of this phenomenon is as below, with nitriding time prolonged, the thickness of compound layer becomes much thicker, leading to the increase of surface hardness. While as the nitriding duration getting too long (exceeding 120 min), the sponge-like or columnar porous areas, called loosen structure, formed on the top of the compound layer is getting more and more serious, leading to lower hardness. Also with the prolonged nitriding time, the grain size in the compound layer and
0.08 0.06 0.04 0.02 0.00 untreated
60
90
120
150
180
Nitriding time(min) Fig. 4. The weight loss after wear test for 45 steel samples untreated and nitrided for different time.
414
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
is improved accordingly. While with the nitriding time further increasing, the mircopores existing on the external loosen layer turn to contact each other and microcracks are formed, and then the compound layer becomes too loose to form dense Fe3 O4 film, resulting in the corrosion medium accessing to the substrate easily, therefore, accelerating the localized corrosion, which leading to a decline in the overall corrosion resistance [11]. The conclusion could be drawn that the optimal nitriding time was 90 min from the point of corrosion resistance in water.
250
Initial rust-producing time(h)
With post oxidation 200
150
100
4. Conclusions Nitrocarburized only 50
0 0
30
60
90
120
150
180
Nitriding time(min) Fig. 5. The relationship between initial rusting time and nitriding time.
3.5. Corrosion resistance analysis In order to evaluate the corrosion resistance in water as the cooperator’s requirements, the hot-water immersing test was performed for 45 steel samples treated in different conditions. Fig. 5 shows the initial rusting time for samples untreated and QPQ treated with different nitriding time. As can be seen from Fig. 5, the initial rusting time for untreated samples is only around 10 min, which indicates that 45 steel has very poor corrosion resistance in usual water, and cannot be used if lubricants change from machine oil to water without suitable surface modification. Also, it clearly shows that the initial rusting time for QPQ treated specimens becomes much longer than that of the untreated, indicating that the corrosion resistance can be greatly improved by QPQ surface modification. This can be attributed to the formation of the oxide film and the nitrided layer. With the extension of nitriding time, the initial rusting time increases, reaching the maximum of 216 h when nitrided for 90 min. However, when nitriding time is exceeded 90 min, the initial rusting time reduces remarkably. The possible reason is that with the extension of nitriding time, the thickness of compound layer increases, and the mircopores existing on the top of the compound layer are separated, which is beneficial for increasing the superficial area of the oxidized layer, so the corrosion resistance
(1) A thin oxide film, compound layer (bright layer) and diffusion layer were formed on the surface of 45 steel after QPQ treatment, and the thickness of the compound layer increases from 5 m to 16 m with the nitriding duration increasing from 60 min to 180 min. (2) The surface layer of 45 steel after QPQ treatment was composed of Fe3 O4 and -Fe3 N. (3) The surface hardness of 45 steel increased significantly after QPQ treatment, reaching the maximum of 630 HV0.01 , almost 3 times as high as that of untreated sample of 230 HV0.01 , and the maximum cross section microhardness of 45 steel after QPQ treatment were obtained on the subsurface of the samples, due to the loosen structure formed on the outermost surface. (4) The optimum nitriding time for improving the wear and corrosion resistance of 45 steel was 120 min and 90 min, respectively. References [1] C.F. Yeung, K.H. Lau, H.Y. Li, D.F. Luo, Journal of Materials Processing Technology 66 (1997) 249–252. [2] M.A.J. Somers, E.J. Mittemeijer, Metallurgical and Materials Transactions A 21 (1990) 901–912. [3] H.Y. Li, et al., The QPQ Complex Salt-Bath Treatment Technology, China Machine Press, Beijing, 1997, p. 64 (in Chinese). [4] G.-j. Li, Q. Peng, C. Li, et al., Surface and Coatings Technology 202 (2008) 2865–2870. [5] G.-j. Li, Q. Peng, C. Li, et al., Materials Characterization 59 (2008) 1359–1363. [6] N. Krishnaraj, K.J.L. Iyer, S. Sundaresan, Wear 210 (1997) 237–244. [7] G.-j. Li, J. Wang, Q. Peng, Journal of Materials Processing Technology 207 (2008) 187–192. [8] H.Y. Li, D.F. Luo, C.F. Yeung, K.H. Lau, Journal of Materials Processing Technology 69 (1997) 45–49. [9] X. Guangyao, H. Bolin, Z. Rui, Key Engineering Materials 373–374 (2008) 260–263. [10] P. Jacquet, J.B. Coudert, P. Lourdin, Surface and Coatings Technology 205 (2011) 4064–4067. [11] Y.-h. Li, D.-f. Luo, S.-x. Wu, Solid State Phenomena 18 (2006) 131–136.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effect of QPQ nitriding time on wear and corrosion behavior of 45 carbon steel Wei Cai a,b , Fanna Meng a,b , Xinyan Gao a,b , Jing Hu a,b,∗ a b
Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou 213164, China School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China
a r t i c l e
i n f o
Article history: Received 21 May 2012 Received in revised form 3 August 2012 Accepted 7 August 2012 Available online 13 August 2012 Keywords: 45 steel Corrosion resistance Wear resistance Microhardness QPQ salt bath
a b s t r a c t QPQ salt bath treatment of 45 steel was conducted by nitriding at 565 ◦ C for various time (60 min, 90 min, 120 min, 150 min and 180 min), followed by the same post-oxidation process with heating temperature of 430 ◦ C and holding duration of 40 min. Characterization of modified surface layers was made by means of optical microscopy, microhardness test, X-ray diffraction analysis, corrosion and wear resistance test. The results showed the formation of a very thin Fe3 O4 oxide layer during post-oxidation on the top of the bright nitrides compound layer formed during nitriding. The maximum microhardness value of 630 HV0.01 was obtained after nitriding at 565 ◦ C for 120 min, which was two times higher than that of the untreated sample. The corrosion and wear resistance of 45 steel could be significantly improved by QPQ complex salt bath treatment, and the optimum nitriding duration for improving the wear and corrosion resistance was 120 min and 90 min, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The 45 steel is widely applied in industry due to its low cost and good combination mechanical properties. But its application is highly constrained in the field of high requirement for wear resistance and especially, corrosion resistance. It is established that Quench-Polish-Quench complex salt bath (QPQ) surface modification technology can improve its both wear resistance and corrosion resistance. The essence of QPQ salt bath technology is low temperature salt bath nitriding + salt bath oxidation or low temperature salt bath carbonitriding + salt bath oxidation. The engineering of QPQ salt bath mainly consists of six steps: (1) degreasing, (2) pre-heating, (3) nitriding, (4) oxidizing, (5) washing and (6) oil immersion [1]. The goal of each step is as below; degreasing is to remove the dirt, pre-heating is to remove water on the components, and also produce a thin oxide layer on the surface of the components. This thin oxide layer will benefit nitriding process. Nitriding and oxidizing are to produce the nitrided layer and oxide layer on the surface of metal, which are the key steps of the whole process. The outer layer of QPQ treated samples is an oxide layer which could be Fe3 O4 , Fe2 O3 or FeO or a combination of them. Among the three kinds of oxide, only magnetite Fe3 O4 can highly improve the wear and corrosion resistance of the surface due to its low friction coefficient and chemical stability [2]. It is
∗ Corresponding author at: Key Laboratory of Advanced Metal Materials of Changzhou City, Changzhou University, Changzhou 213164, China. Tel.: +86 0519 86330065. E-mail address: [email protected] (J. Hu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.024
reported that the corrosion resistance of the treated components could be higher than that treated by chrome plating and nickel plating [3]. Moreover, the corrosion resistance of carbon steel treated by QPQ could be as good as stainless steels in some applications. In addition, a nitrided layer and a diffusion layer beneath the oxide layer could improve the hardness and wear resistance of the surface. Therefore, carbon steel after suitable QPQ treatment may replace stainless steels in some fields, and with much better combination mechanical properties. For the past few years, there have been some researches about the influence of QPQ treatment on the microstructure and properties of various materials [4–7]. However, systematic investigation of the effect of QPQ complex treatment on the microstructure, wear resistance and corrosion resistance of 45 carbon steel was not yet presented in the literature. This research was initially required by a compressor industry; the main goal is to investigate if 45 steel components after suitable QPQ treatment can have good corrosion resistance in water lubricant environments instead of machine oil lubricants used previously, the purpose of the lubricants changing from machine oil to water is to meet the requirements of green manufacturing without carbon releasing and also decrease running cost. If the corrosion resistance of 45 steel in water immersing test can be highly improved, it can imply that it is possible to still keep using 45 steel as the components in compressor when lubricant is changed from machine oil to water, which could bring out obvious advantages than stainless steels, initially planed to use, since 45 steel has much better combination mechanical properties, better machinability and lower cost comparing with stainless steels. Therefore, the aim of the present study is focused on the corrosion and wear resistance of 45 steel treated by QPQ treatment to
412
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
Fig. 1. Cross sectional microstructure of 45 steel specimens treated at 565 ◦ C for different nitriding time: (a) 60 min, (b) 90 min, (c) 120 min, (d) 180 min.
determine the optimum process conditions. The modified surface was studied in terms of optical microscopy, X-ray diffraction (XRD), micro-hardness test, wear resistance and corrosion resistance test.
2. Materials and methods The material used in this study was 45 steel with the nominal composition (wt.%) of: 0.42–0.5% C, 0.17–0.37% Si, 0.5–0.8% Mn, and balance Fe. Samples were cut in the size of 10 mm × 10 mm × 10 mm for optical microscopy, microhardness test and corrosion test, and in 32 mm diameter and 5 mm thick discs for wear test. Then the samples were quenched at 850 ◦ C and tempered at 580 ◦ C. All the flat surfaces of each sample were ground using various grades of SiC paper and polished to a mirror finish, and then ultrasonically cleaned in deionized water and alcohol for 5 min before QPQ treatment. The samples were processed in tailormade QPQ salt bath furnace with the CNO− concentration of 35%. In order to investigate the effect of nitriding time on the surface characteristics, QPQ salt bath nitriding was conducted at the same temperature of 565 ◦ C for various time (60 min, 90 min, 120 min, 150 min and 180 min), followed by the same post-oxidation process with heating temperature of 430 ◦ C and holding duration of 40 min. The polish process was omitted in this research by simulating the real production process. The nitriding process was conducted using the nitride salt. The N atom came from the dissociation of CNO− , and the C atom came from the dissociation of CNO− simultaneously. The post-oxidation process was conducted using the oxide salt. The formula of nitride salt and oxide salt was kept in secret by the manufacturing factory. The cross sectional morphology was studied by optical metallography (XUG-05). The phase constituents were determined by X-ray diffraction (XRD) (Dmax 2500) using Cu–K␣ radiation. Hardness measurements were made in a HXD-1000TMC microhardness tester, with the test load of 10 g and the holding duration of 15 s. Each hardness value was determined by averaging at least 5 measurements.
The corrosion resistance was evaluated by hot water immersing test according to the cooperator’s requirements based on the potential real application, the temperature of hot-water was hold at 90 ◦ C during the test. The standard of initial rust was designed by referring to ASTM B117-03. The initial rusting time of each sample was recorded. The wear tests were carried out on a MMW-1A Wear Test Machine under ambient condition (20 ± 2 ◦ C and 50%RH). During the test, a 6 mm diameter GCr15 steel ball rotated at a speed of 200 rpm on the surface of the disc samples for the testing time of 60 min. The test load was 50 N. The friction coefficient was continuously recorded during the test and the weight of the samples was measured by a balance accurate to 0.1 mg for calculating the weight loss. 3. Results and discussion 3.1. Metallographic observations Fig. 1 shows the cross sectional metallographs of 45 steel specimens QPQ treated at 565 ◦ C for different nitriding time with the same post-oxidation process (430 ◦ C, 40 min). From the outmost surface to the core, a thin oxide film, compound layer (bright layer), diffusion layer were formed. The oxide film, whose thickness is too thin to be clearly observed by optical microscopy, was produced in the post-oxidation process, which could further improve the corrosion resistance and wear resistance when existing in the form of magnetite Fe3 O4 [8]. The compound layer, also called bright layer is obviously observed and its thickness increases from 5 m to 16 m with the nitriding duration from 60 min to 180 min, and with a porous or loosen structure at the top of the compound layer, which is getting more and more looser with its thickness increase. The compound layer is an important part of the QPQ treated samples and plays an important role in improving the wear resistance and corrosion resistance. The diffusion layer between bright layer and radical center is hard to be clearly distinguished by optical microscopy as shown in Fig. 1. Its main components are solid solution or supersaturated solid solution of the N and C atoms in ␣-Fe,
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
(b)
413
650
Fe3O4 Fe3N
untreated 60min 90min 120min 150min 180min
600
Hardness(HV0.01)
550
Intensity
Nitrocarburized only
(a)
500 450 400 350 300 250
With post oxidation
200 0
20
40
60
80
100
2
50
100
Depth (μ m)
150
200
Fig. 3. The surface hardness-depth profile of 45 steel specimens untreated and QPQ treated at 565 ◦ C for different nitriding time.
Fig. 2. XRD diffraction patterns of 45 steel samples nitrided at 565 ◦ C for 120 min (a) with post-oxidation at 430 ◦ C for 40 min and (b) without post-oxidation.
which could bring about an improvement in fatigue strength when compared with an untreated material [9].
diffusion zone grows coarser, resulting in the decrease of surface hardness as well.
3.2. XRD analysis
3.4. Wear resistance analysis
Fig. 2 presents the XRD diffraction patterns of 45 steel samples nitrided for 120 min followed by post-oxidation (QPQ complex salt bath treatment) and without post-oxidation. Fig. 2(a) shows that the surface layer of QPQ treated samples composed of Fe3 O4 and -Fe3 N, and (b) indicates that the surface layer is mainly composed of -Fe3 N. The reason why Fe peaks disappear is that the compound layer becomes thicker and substrate influence is no more obvious [10]. The main difference between the two processes with postoxidation and without post-oxidation is the formation of Fe3 O4 . As agreed that -Fe3 N has much higher hardness and excellent wear resistance comparing with base metal, and magnetite Fe3 O4 formed in the post-oxidation process can further improve the wear and corrosion resistance of the surface due to its low friction coefficient and chemical stability [2].
The variation of weight loss of 45 steel samples untreated and QPQ treated under different conditions is shown in Fig. 4. It can be clearly seen that the weight loss of the untreated sample is much higher than that of the nitrided samples, reaching around 0.14 g. Whereas, the weight loss of samples after QPQ salt bath treatment decreases dramatically, and with the extension of nitriding time, the weight loss reduces gradually, reaching the minimum of 0.0029 g when nitrided for 120 min. This is because after QPQ complex salt bath treatment, the surface layer is made of -Fe3 N with high hardness and Fe3 O4 with low friction coefficient, which results in excellent wear resistance. The wear resistance changing regularity with the nitriding time is in good agreement with the hardness profile as shown in Fig. 3.
3.3. Hardness analysis 0.14 0.12 0.10
Weight loss(g)
Fig. 3 shows the evolution of the cross section microhardness for different nitriding time. In this figure, the profile that the hardness values decrease as the depth increase is the distinguished diffusion feature, which can be used to evaluate the diffusion layer or the effective hardening depth. The maximum microhardness values are obtained on the subsurface of the samples, due to the loosen structure formed on the outermost surface. The surface hardness reaches the maximum of 630 HV0.01 when nitrided for 120 min, which is almost 3 times as high as that of untreated sample of 230 HV0.01 . With the longer nitriding duration, the surface hardness rises gradually at first and declines slightly when nitrided time is exceeded 120 min. The reason of this phenomenon is as below, with nitriding time prolonged, the thickness of compound layer becomes much thicker, leading to the increase of surface hardness. While as the nitriding duration getting too long (exceeding 120 min), the sponge-like or columnar porous areas, called loosen structure, formed on the top of the compound layer is getting more and more serious, leading to lower hardness. Also with the prolonged nitriding time, the grain size in the compound layer and
0.08 0.06 0.04 0.02 0.00 untreated
60
90
120
150
180
Nitriding time(min) Fig. 4. The weight loss after wear test for 45 steel samples untreated and nitrided for different time.
414
W. Cai et al. / Applied Surface Science 261 (2012) 411–414
is improved accordingly. While with the nitriding time further increasing, the mircopores existing on the external loosen layer turn to contact each other and microcracks are formed, and then the compound layer becomes too loose to form dense Fe3 O4 film, resulting in the corrosion medium accessing to the substrate easily, therefore, accelerating the localized corrosion, which leading to a decline in the overall corrosion resistance [11]. The conclusion could be drawn that the optimal nitriding time was 90 min from the point of corrosion resistance in water.
250
Initial rust-producing time(h)
With post oxidation 200
150
100
4. Conclusions Nitrocarburized only 50
0 0
30
60
90
120
150
180
Nitriding time(min) Fig. 5. The relationship between initial rusting time and nitriding time.
3.5. Corrosion resistance analysis In order to evaluate the corrosion resistance in water as the cooperator’s requirements, the hot-water immersing test was performed for 45 steel samples treated in different conditions. Fig. 5 shows the initial rusting time for samples untreated and QPQ treated with different nitriding time. As can be seen from Fig. 5, the initial rusting time for untreated samples is only around 10 min, which indicates that 45 steel has very poor corrosion resistance in usual water, and cannot be used if lubricants change from machine oil to water without suitable surface modification. Also, it clearly shows that the initial rusting time for QPQ treated specimens becomes much longer than that of the untreated, indicating that the corrosion resistance can be greatly improved by QPQ surface modification. This can be attributed to the formation of the oxide film and the nitrided layer. With the extension of nitriding time, the initial rusting time increases, reaching the maximum of 216 h when nitrided for 90 min. However, when nitriding time is exceeded 90 min, the initial rusting time reduces remarkably. The possible reason is that with the extension of nitriding time, the thickness of compound layer increases, and the mircopores existing on the top of the compound layer are separated, which is beneficial for increasing the superficial area of the oxidized layer, so the corrosion resistance
(1) A thin oxide film, compound layer (bright layer) and diffusion layer were formed on the surface of 45 steel after QPQ treatment, and the thickness of the compound layer increases from 5 m to 16 m with the nitriding duration increasing from 60 min to 180 min. (2) The surface layer of 45 steel after QPQ treatment was composed of Fe3 O4 and -Fe3 N. (3) The surface hardness of 45 steel increased significantly after QPQ treatment, reaching the maximum of 630 HV0.01 , almost 3 times as high as that of untreated sample of 230 HV0.01 , and the maximum cross section microhardness of 45 steel after QPQ treatment were obtained on the subsurface of the samples, due to the loosen structure formed on the outermost surface. (4) The optimum nitriding time for improving the wear and corrosion resistance of 45 steel was 120 min and 90 min, respectively. References [1] C.F. Yeung, K.H. Lau, H.Y. Li, D.F. Luo, Journal of Materials Processing Technology 66 (1997) 249–252. [2] M.A.J. Somers, E.J. Mittemeijer, Metallurgical and Materials Transactions A 21 (1990) 901–912. [3] H.Y. Li, et al., The QPQ Complex Salt-Bath Treatment Technology, China Machine Press, Beijing, 1997, p. 64 (in Chinese). [4] G.-j. Li, Q. Peng, C. Li, et al., Surface and Coatings Technology 202 (2008) 2865–2870. [5] G.-j. Li, Q. Peng, C. Li, et al., Materials Characterization 59 (2008) 1359–1363. [6] N. Krishnaraj, K.J.L. Iyer, S. Sundaresan, Wear 210 (1997) 237–244. [7] G.-j. Li, J. Wang, Q. Peng, Journal of Materials Processing Technology 207 (2008) 187–192. [8] H.Y. Li, D.F. Luo, C.F. Yeung, K.H. Lau, Journal of Materials Processing Technology 69 (1997) 45–49. [9] X. Guangyao, H. Bolin, Z. Rui, Key Engineering Materials 373–374 (2008) 260–263. [10] P. Jacquet, J.B. Coudert, P. Lourdin, Surface and Coatings Technology 205 (2011) 4064–4067. [11] Y.-h. Li, D.-f. Luo, S.-x. Wu, Solid State Phenomena 18 (2006) 131–136.