- Email: [email protected]
The effect of nitriding temperature on hardness and microstructure of die steel pre-treated by ultrasonic cold forging technology
Materials and Design 49 (2013) 392–399
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of nitriding temperature on hardness and microstructure of die steel pre-treated by ultrasonic cold forging technology Dingshun She a,b, Wen Yue a,b,⇑, Zhiqiang Fu a, Yanhong Gu c, Chengbiao Wang a, Jiajun Liu d a
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, China c School of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China d Mechanical Engineering Department, Tsinghua University, Beijing 100084, China b
a r t i c l e
i n f o
Article history: Received 5 October 2012 Accepted 2 January 2013 Available online 16 January 2013 Keywords: Ferrous metals and alloys Surface treatment Microstructure
a b s t r a c t To promote the formation and growth of nitride layer, a befitting surface nano-crystallization process was introduced as a pre-treatment of nitriding, and an optimized nitriding temperature was investigated. A modification layer with a depth of 350 lm was formed by ultrasonic cold forging technology (UCFT) on AISI D2 surface. A series of plasma nitriding experiments for both treated and untreated samples were conducted at various temperatures ranging from 350 °C to 550 °C for 4 h. The influences of nitriding temperature on the hardness, microstructure, morphology and composition of the sample surface were investigated by micro-hardness tester, optical microscope (OM), 3D profile-meter and scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The diffusion of nitrogen and the formation of nitrides were markedly improved by UCFT pre-treatment. A thicker and harder nitrided layer was formed at a high nitriding temperature. However, as the nitriding temperature increased to 550 °C, a stronger sputter occurred on the sample surface. The results presents that it is an optimized process to be pre-treated by UCFT and nitriding at 520 °C for 4 h. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Plasma nitriding, one of well-established commercial surface modification techniques, has been widely used to enhance wear and corrosion behaviour of moulds [1–3]. Nevertheless, reducing energy consumption still presents a major challenge to surface engineers and researchers, and it also acts as a driving force for the optimization of plasma nitriding process [4]. The thickness, composition and properties of a nitrided layer are dependent on the nitriding conditions as well as on the material [1,5,6]. Therefore, there are two ways to improve the plasma nitriding process: one is to find a complex surface modification technology to accelerate the chemical reaction of the material surface and the other is to develop an optimal technological plasma nitriding parameter [1,5–8]. It is reported that the surface layer possesses ultrafine grains with a large number of grain boundaries and defect densities, which may act as fast atomic diffusion channels, then the gas nitriding temperature could be reduced [6–9]. Li et al. [7] reported that the surface mechanical attrition treatment (SMAT) plus plasma nitriding can be successfully used to further improve the properties of AISI 4140 and AISI 316L steel. Ultrasonic cold forging ⇑ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China. Tel.: +86 10 82320255; fax: +86 10 82322624. E-mail addresses: [email protected], [email protected] (W. Yue). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.003
technology (UCFT) utilizes ultrasonic vibration energy to induce severe plastic deformation to a material surface [10,11]. It could produce outstanding surface hardness, surface roughness (0.08– 0.5 lm) and an effect layer with a certain depth (about 300 lm) [10–14]. Therefore, UCFT could be expected as a highly promising pre-treatment means of plasma nitriding. Meanwhile, the plasma nitriding temperature plays another basic role on the formation of nitrided layer [15–18]. However, few works on the die steel pre-treated by UCFT and then plasma nitriding are reported so far. In this study, UCFT was introduced as a pre-treatment of plasma nitriding. Plasma nitriding processes performed at temperature of 350, 450, 500, 520, and 550 °C for 4 h. The effects of plasma nitriding temperature on the modification layer structural properties, including surface morphology, hardness profile, diffusion layer thickness, compound layer thickness and phase content, were investigated. It aims to find an optimized nitriding temperature to the surface combined modification process.
2. Experimental procedure 2.1. Materials A commercial available AISI D2 die steel cylinder (£32 mm 105 mm) with hardness of about 200 HV was prepared. Table 1 shows its chemical composition.
393
D. She et al. / Materials and Design 49 (2013) 392–399 Table 1 Nominal composition of AISI D2 steel (wt.%).
Table 2 UCFT treatment parameters.
Element
C
Cr
Si
Mn
Mo
V
P6
S6
Content (wt.%)
1.50
12.00
0.25
0.45
1.00
0.35
0.025
0.01
Vibration frequency (KHZ)
Feed rate Amplitude Load Spindle (N) speed (rpm) (mm/ (lm) rev)
Number Tip diameter of shots per mm2 (mm)
20
30
2.38
100
30
0.07
21,840
2.2. Ultrasonic cold forging technology treatment The sketch of the UCFT device is shown in Fig. 1. The principle of the device was developed by Alekhine and Alekhine [14]. It transforms ultrasonic vibratory energy into tens of thousands of strikes per second. The output power is about 1 kW. The dynamic load is 1.5–5 times higher than static load. The total load acting on the specimen is the sum of the static load and the sinusoidal function of the dynamic load. Table 2 shows the UCFT treatment parameters. 2.3. Plasma nitriding Both the UCFT and Un-UCFT samples were cut to 10 mm 10 mm 15 mm plates. The samples were ultrasonically cleaned in distilled water for 10 min, and then with acetone for another 10 min. After cleaning, the samples were placed into a LDM 2-25 plasma nitriding furnace. When vacuum chamber was evacuated to ultimate vacuum and the leakage rate of the equipment was less than or equal to 2 Pa per 15 min, all the nitrided samples were treated for 4 h in an NH3 containing atmosphere and a voltage of 650 V. The treatment conditions for various samples are given in Table 3. During the plasma nitriding, the samples were connected to the cathode, and the furnace wall acted as the anode.
Table 3 Treatment conditions of samples. Sample number
UCFT or not
Nitriding temperature (°C)
Pressure (Pa)
Un-UCFT + Un-nitrided Un-UCFT + 350 Un-UCFT + 400 Un-UCFT + 450 Un-UCFT + 500 Un-UCFT + 520 Un-UCFT + 550
Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT
(Un-nitrided) 350 400 450 500 520 550
– 250 400 520 550 580 600
UCFT + Un-nitrided UCFT + 350 UCFT + 400 UCFT + 450 UCFT + 500 UCFT + 520 UCFT + 550
UCFT UCFT UCFT UCFT UCFT UCFT UCFT
(Un-nitrided) 350 400 450 500 520 550
– 250 400 520 550 580 600
2.4. Performance and analysis test The surface roughness and surface morphology of the UCFT + Un-nitrided samples were characterized by 3D profiler
Fig. 1. Configuration of UCFT device.
Fig. 2. SEM morphologies of the cross-section of the UCFT + Un-nitrided samples.
394
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 3. XRD diffraction profiles of the before and after UCFT treatment.
Fig. 5. The hardness distribution of the cross-section of the Un-nitrided samples.
(Nano-Map-D). According to the ISO 4287:1997 standard [19], the measurement length for the surface roughness is about 0.75 mm. For the roughness measurements, the tests were repeated ten times and the average was taken. Scanning electron microscope (JSM-7001 F) was used to investigate surface and cross-section morphologies of the samples. Optical microscopy (OM) was used to determine the thickness of the diffusion and compound layer. The samples were cut in cross-section with a CNC wire-cut electric discharge machine, and then mounted in the dental base acrylic resin. After being polished, and then etched in the 4% Nital, the samples were conducted with metallographic examinations. X-ray diffraction (XRD) was carried out on a D/max X-ray diffractometer using a Cu-Ka radiation source (wavelength of 1.5406 Å) to determine the phases in the modified layer. Each sample was conducted with a 2h range of 30–90°, an increment of 0.04°/step and a time per step of 1.5 s. The element distribution along with the depth of the cross-section of the nitrided samples was characterized by a HORIBA EMAX energy dispersive X-ray spectroscopy (EDS).
The surface hardness and the micro-hardness variation on the cross-section of samples were evaluated using a MH-6 Vickers micro-hardness tester at a load of 200 gf with 5 s dwell time. According to the ASTM E384-11e1 standard, the testing machine was verified before the hardness test. The repeatability (R) and the error (E) in the performance of testing machine met the requirements (R < 4%, E < 2%). The hardness measurements were repeated five times and the average was plotted. 3. Results and discussion 3.1. UCFT pre-treatment Cross-section image of the UCFT + Un-nitrided samples was shown in Fig. 1. It can be seen that micro-structural morphology of severe plastic deformation layer is quite different from that of the matrix. An effective layer caused by severe plastic deformation with a depth of about 350 lm was formed. Fig. 3 illustrates the XRD pattern obtained from the Un-UCFT + Un-nitrided samples and the UCFT + Un-nitrided samples. The
Fig. 4. 3D surface profile of the UCFT + Un-nitrided samples.
D. She et al. / Materials and Design 49 (2013) 392–399
395
Fig. 6. SEM micrographs of the surface of the samples nitrided at 520 °C for 4 h, (a) Un-UCFT + 520 and (b) UCFT + 520.
Table 4 The average size of nitride nano-particles covered on the surface of the nitrided samples. Samples number
Average size (nm)
Samples number
Average size (nm)
Un-UCFT + 350 Un-UCFT + 400 Un-UCFT + 450 Un-UCFT + 500 Un-UCFT + 520 Un-UCFT + 550
– 321 547 874 951 987
UCFT + 350 UCFT + 400 UCFT + 450 UCFT + 500 UCFT + 520 UCFT + 550
107 242 384 521 606 908
Bragg diffraction peaks of the UCFT + Un-nitrided samples are broader than those of the Un-UCFT + Un-nitrided samples. This result is due to the gain refinement effect and the lattice dislocation on the atomic-level [7]. The average grain size was calculated from line broadening of the Bragg diffraction peaks of ferrite a-Fe (1 1 0), (2 2 0) and (2 1 1) based on Fourier transform method (FTM) [20]. Using the Scherer and Wilson equation [21], the average grain size of the surface of UCFT + Un-nitrided samples was calculated to be about 20.4 nm. It is confirmed that a surface layer with ultrafine grains was formed by UCFT. The 3D profiler image of the UCFT + Un-nitrided samples is given in Fig. 4. It is observed that the surface of the UCFT + Un-nitrided sample appears some microgrooves. Using the 3D profiler accompanying program, the surface roughness Ra of the UCFT + Un-nitrided samples was measured to be about 380 nm. The hardness distribution of the cross-section of Un-UCFT + Unnitrided samples along the depth is shown in Fig. 5. The surface hardness of the UCFT + Un-nitrided samples is 520 HV, which is
about 2.6 times of that of the Un-UCFT + Un-nitrided samples (200 HV). The micro-hardness of the UCFT + Un-nitrided samples is a gradient change along the depth from about 530 HV on the top surface to about 200 HV in the matrix. The thickness of hard layer is about 400 lm. According to the Hall–Petch law, the smaller the grain size the harder the material, the results may be related to the average grain size gradient change along the depth [5].
3.2. The microstructure and morphology of nitrided samples The surfaces of the nitrided samples are covered by the compact plasma nitriding layers composed of a number of nano-particles, which is shown in Fig. 6. During plasma nitriding, there were high concentration of nitrogen ions and the presence of weighty nonionized nitrogen molecules in plasma media [1]. In addition the surface sputtering increased and the lower energy nitrogen atoms tend to form molecular N2 on the surface [15]. The combination of these effects and the agglomeration of nitride precipitates lead to the formation of micro-pores. To calculate the average sizes of nitride nano-particles, the number of nitride nano-particles in a unit area was counted in the SEM images of nitriding samples. The average sizes of nitride nano-particles covered on the surface of the samples are shown in Table 4. It can be found that with the increase of nitriding temperature the nitrided nano-precipitates are easy to aggregate and form some large-size particles. Besides, the nitride nano-particles distributed on the UCFT samples are obviously smaller than the particles on the surfaces of Un-UCFT samples treated under the same nitriding condition. The principal reason may be that the grain boundaries and defect densities accelerate the formation of nitride precipitates and retard the growth of nitride particle.
Fig. 7. SEM micrographs of the surface of the samples nitrided at 350 °C for 4 h, (a) Un-UCFT + 350 and (b) UCFT + 350.
396
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 8. SEM micrographs of the surface of the samples nitrided at 550 °C for 4 h, (a) Un-UCFT + 550 and (b) UCFT + 550.
Fig. 9. OM cross-sectional observations of the nitrided samples, (a) Un-UCFT + 520 and (b) UCFT + 520, (c) Un-UCFT + 450 and (d) UCFT + 450.
Table 5 Thickness of nitriding layer of samples. Un-UCFT samples Temperature (°C) Pressure (Pa) Compound layer (lm) Diffusion layer (lm)
350 250 0 20
400 400 0 52
UCFT samples 450 520 0 73
500 550 6 78
520 580 10 81
Fig. 7 shows the surface micrographs of the samples nitrided at 350 °C. Unlike the Un-UCFT + 350 samples, a great number of nitride particles can be obviously seen on the surface of UCFT + 350 samples. It is implied that the UCFT pre-treatment enhanced nitriding process and reduced the nitriding temperature. The surfaces of Un-UCFT + 550 and UCFT + 550 samples appear to be irregular and jagged, as shown in Fig. 8, This result indicates that a greater amount of sputtering occurred at the nitriding temperature of 550 °C, which resulted in the surface damage. Fig. 9 shows the cross-section morphologies of the nitrided samples. A white compound layer (upper part) as well as a dark diffusion layer (lower part) can be seen in the cross-section of samples. The thickness of the nitriding layer is listed in Table 5. It
550 600 12 90
350 250 0 40
400 400 0 74
450 520 8 80
500 550 15 87
520 580 21 105
550 600 12 110
shows that the thickness of nitriding layer is influenced by the nitriding temperature. An increased nitriding temperature resulted in an increase of the thickness of the layer. This result could be related to the fact that a higher temperature could promote the diffusion of nitrogen. Meanwhile, under the same nitriding condition, a thicker compound layer as well as thicker diffusion layer can be easily formed on the UCFT samples. Nonetheless, the compound layer of the UCFT + 550 samples is only 12 lm, which is thinner than that of the UCFT + 520 samples. It may be resulted from the fact that the compound layer is etched by a greater amount of sputtering at 550 °C, which is coordinated with the results in Fig. 8. In addition, for both samples nitrided at 450 °C, there is no white compound layer on the surface of Un-UCFT + 450 samples.
D. She et al. / Materials and Design 49 (2013) 392–399
397
Fig. 10. EDS line analysis along the cross-sections of the nitrided samples (a) 520 °C nitrided samples and (b) 550 °C nitrided samples.
Fig. 11. XRD patterns for samples treated at various condition, (a) Un-UCFT samples and (b) UCFT samples.
It may be attributed to the fact that the samples pre-treated by UCFT could enhance the nitriding kinetics for lots of grain boundaries and lattice dislocations caused by severe plastic deformation [1,4–9,17,18]. 3.3. Composition and phase analysis of nitrided samples Fig. 10 shows the distribution of N elements along the depth of the cross-sections of the nitrided samples. The concentration of nitrogen in the nitrided layer decreases along with the depth from surface to interior. Besides, the UCFT samples tend to have a deeper nitrogen distribution with higher intensity than the Un-UCFT samples nitrided at the same temperature. Fig. 11 shows the XRD patterns of the samples treated at various temperatures. As nitriding temperature increased, the intensity of peaks for e-Fe2–3N, c0 -Fe4N and CrN phases increased. Besides, the nitrided UCFT samples tend to possess stronger peaks of e-Fe2–3N, c0 -Fe4N and CrN phases than the Un-UCFT samples treated under the same nitriding temperature. It is also a proof that the surface of UCFT samples makes nitride layer easy to be formed. The Un-UCFT + 350 samples contain strong diffraction peaks for the ferrite a-Fe, as well as weak diffraction peaks of e-Fe2–3N. It is in good agreement with the result shown in Fig. 7a that nitride
Fig. 12. The surface micro-hardness of the samples.
nano-particle can be hardly observed on the surface of the UnUCFT + 350 samples. In addition, conspicuous c0 -Fe4N phase peaks could be identified in the UCFT + 450 samples, however, in the
398
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 13. The micro-hardness variation on the cross-section of samples, (a) Un-UCFT samples and (b) UCFT samples.
Un-UCFT + 450 samples it could not be found. The white compound layer consisting of c0 -Fe4N or e-Fe2–3N phases is formed when the nitrogen potential exceeds the critical value [9,18]. This result can explain why there is no white compound layer on the surface of the Un-UCFT + 450 samples shown in Fig. 9a. 3.4. Micro-hardness of nitrided layer The surface micro-hardness of samples is listed in Fig. 12. The nitrided UCFT samples tend to have a harder surface than the nitrided Un-UCFT samples after nitriding under the same nitriding condition. Meanwhile, higher nitriding temperature tends to form a harder surface. It verified that a nano-structure layer induced by UCFT makes a hard nitriding layer form. Nonetheless, the UCFT + 520 samples present the highest hardness (1350 HV). The results must be due to the following reasons: First of all, a higher nitriding temperature makes more nitride form. However, at 550 °C, the compound layer was etched by ion sputter (as shown in Fig. 8). Secondly, a high nitriding temperature converts a greater number of e-Fe2–3N into c0 -Fe4N (as shown in Fig. 11). Moreover, c0 -Fe4 N phase has an fcc crystalline structure and shows lower hardness with respect to e-Fe2–3N phase with hcp crystalline structure [15,18]. In addition, the samples nitrided at 550 °C (it is near to the tempering temperature of AISI D2 steel) were further tempered. Further tempering makes the reduction of residual stress and structural transformation occurring, which result in the decrease of surface hardness. Fig. 13 shows the micro-hardness variation on the cross-section of samples. It can be seen that a gradient modification layer was formed and a higher nitriding temperature tends to form a thicker and harder modification layer. Meanwhile, the UCFT samples tend to have a thicker layer. Its thickness is about 400 lm, which is nearly equal to the thickness of the effective layer of the UCFT sample (as shown in Fig. 2). As the results shown above, the higher temperature results in an increased diffusion coefficient. Besides, a great number of grain boundaries and high defect densities play a decisive role in the formation of the nitride nano-particles and the growth of nitrided layer for UCFT samples. Accordingly, the combination of these effects and ion sputter occurring at 550 °C lead to the consequence that pre-treated by UCFT and then nitriding at 520 °C for 4 h is an optimized complex surface modification process to AISI D2 die steel. 4. Conclusions To sum up, the UCFT can be introduced as a pre-treatment means of plasma nitriding to effectively enhance the properties
of AISI D2 die steel. Pre-treated by UCFT, the surface of AISI D2 steel samples tends to form a thicker and harder nitride layer with smaller nitrided nano-particles and more nitride phases (e-Fe2–3N, c0 Fe4N and CrN). A higher nitriding temperature promotes the formation of the hard phases (e-Fe2–3N, c0 -Fe4N and CrN). Meanwhile, to some extent, a high nitriding temperature also results in a thicker and harder nitrided layer. However, with the nitriding temperature increased to 550 °C, a stronger sputter occurred on the surface of nitrided samples, which depleted the compound layer. In the case of nitriding at 520 °C for 4 h after UCFT pre-treatment can be regarded as an optimized process. In this way, a gradient modification layer was formed with the thickness of 400 lm. The micro-hardness gradually decreases from around 1350 HV of the surface to around 200 HV in the matrix.
Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51275494), the Fundamental Research Funds for the Central Universities (2012ZY51), the Program for Key International Science and Technology Cooperation Project of China (2010DFR50070) and the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF11B04).
References [1] Ferkel H, Glatzler M, Estrin Y, Valiev RZ, Blawert C, Mordike BL. RF plasma nitriding of severely deformed iron-based alloys. Mater Sci Eng A 2003;348: 100–10. [2] Öztürk O, Onmousß O, Williamson D L. Microstructural, mechanical and corrosion characterization of nitrogen-implanted plastic injection mould steel. Surf Coat Techno 2005;196:341–8. [3] Oliveira SD, Tschiptschin AP, Pinedo CE. Simultaneous plasma nitriding and ageing treatments of precipitation hardenable plastic mould steel. Mater Dse 2007;28:1714–8. [4] Tong WP, Han Z, Wang LM, Lu J, Lu K. Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment. Surf Coat Technol 2008;202:4957–63. [5] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1989;33:223–315. [6] Tong WP, Liu CZ, Wang W, Tao NR, Wang ZB, Zuo L, et al. Gas nitriding of iron with a nanostructured surface layer. Scripta mater 2007;57:533–6. [7] Li Y, Wang L, Zhang DD, Shen L. The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel. Appl Surf Sci 2010;257:979–84. [8] Tong WP, Zhang HW, Tao NR, Wang ZB, Lu J, Lu K. Low-temperature nitriding by means of SMAT. Trans Mater Heat Treat 2004;25:301–6. [9] Tong WP, Tao NR, Wang ZB, Zhang HW, Lu J, Lu K. The formation of e-Fe2–3N phase in a nanocrystalline Fe. Scripta Mater 2004;50:647–50. [10] Amanov A, Cho IS, Pyoun YS, Lee CS, Park IG. Micro-dimpled surface by ultrasonic nanocrystal surface modification and its tribological effects. Wear 2012;286–287:136–44.
D. She et al. / Materials and Design 49 (2013) 392–399 [11] Suh CM, Song GH, Suh MS, Pyoun YS. Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology. Mater Sci Eng A 2007;443:101–6. [12] Mordyuk BN, Milman Yu V, Iefimov MO, Prokopenko GI, Silberschmidt VV, Danylenko MI, et al. Characterization of ultrasonically peened and laser shock peened surface layers of AISI 321 stainless steel. Surf Coat Technol 2008;202:4875–83. [13] Cao XJ, Pyou YS, Murakami R. Fatigue properties of a S45C steel by subjected to uitrasonic nanocrystal surface modification. Appl Surf Sci 2010;256:6297–303. [14] Alekhine VP, Alekhine OV. The final processing of constructional steel and tool steel: surface nanocrystallization technology. Mech Eng Technol 2007;4 C:2–13. [15] Rad HF, Amadeh A, Moradi H. Wear assessment of plasma nitrided AISI H11 steel. Mater Des 2011;32:2635–43.
399
[16] Li Y, Wang L, Xu J, Zhang D. Plasma nitriding of AISI 316 L austenitic stainless steels at anodic potential. Surf Coat Technol 2012;206:2430–7. [17] Larisch B, Brusky U, Spies HJ. Plasma nitriding of stainless steels at low temperatures. Surf Coat Technol 1999;116–119:205–11. [18] Sirin SY, Kaluc E. Structural surface characterization of ion nitrided AISI 4340 steel. Mater Des 2012;36:741–7. [19] ISO 4287. Geometrical Product Specification (GPS). Surface texture: profile method-terms, definitions, and surface texture parameters; 1997. [20] Ida T, Shimazaki S, Hibino H, Toraya H. Diffraction peak profiles from spherical crystallites with lognormal size distribution. J Appl Cryst 2003;36: 1107–15. [21] Scardi P, Leoni M, Delhez R. Line broadening analysis using integral breadth methods: a critical review. J Appl Cryst 2004;37:381–90.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of nitriding temperature on hardness and microstructure of die steel pre-treated by ultrasonic cold forging technology Dingshun She a,b, Wen Yue a,b,⇑, Zhiqiang Fu a, Yanhong Gu c, Chengbiao Wang a, Jiajun Liu d a
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, China c School of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China d Mechanical Engineering Department, Tsinghua University, Beijing 100084, China b
a r t i c l e
i n f o
Article history: Received 5 October 2012 Accepted 2 January 2013 Available online 16 January 2013 Keywords: Ferrous metals and alloys Surface treatment Microstructure
a b s t r a c t To promote the formation and growth of nitride layer, a befitting surface nano-crystallization process was introduced as a pre-treatment of nitriding, and an optimized nitriding temperature was investigated. A modification layer with a depth of 350 lm was formed by ultrasonic cold forging technology (UCFT) on AISI D2 surface. A series of plasma nitriding experiments for both treated and untreated samples were conducted at various temperatures ranging from 350 °C to 550 °C for 4 h. The influences of nitriding temperature on the hardness, microstructure, morphology and composition of the sample surface were investigated by micro-hardness tester, optical microscope (OM), 3D profile-meter and scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The diffusion of nitrogen and the formation of nitrides were markedly improved by UCFT pre-treatment. A thicker and harder nitrided layer was formed at a high nitriding temperature. However, as the nitriding temperature increased to 550 °C, a stronger sputter occurred on the sample surface. The results presents that it is an optimized process to be pre-treated by UCFT and nitriding at 520 °C for 4 h. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Plasma nitriding, one of well-established commercial surface modification techniques, has been widely used to enhance wear and corrosion behaviour of moulds [1–3]. Nevertheless, reducing energy consumption still presents a major challenge to surface engineers and researchers, and it also acts as a driving force for the optimization of plasma nitriding process [4]. The thickness, composition and properties of a nitrided layer are dependent on the nitriding conditions as well as on the material [1,5,6]. Therefore, there are two ways to improve the plasma nitriding process: one is to find a complex surface modification technology to accelerate the chemical reaction of the material surface and the other is to develop an optimal technological plasma nitriding parameter [1,5–8]. It is reported that the surface layer possesses ultrafine grains with a large number of grain boundaries and defect densities, which may act as fast atomic diffusion channels, then the gas nitriding temperature could be reduced [6–9]. Li et al. [7] reported that the surface mechanical attrition treatment (SMAT) plus plasma nitriding can be successfully used to further improve the properties of AISI 4140 and AISI 316L steel. Ultrasonic cold forging ⇑ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, China. Tel.: +86 10 82320255; fax: +86 10 82322624. E-mail addresses: [email protected], [email protected] (W. Yue). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.003
technology (UCFT) utilizes ultrasonic vibration energy to induce severe plastic deformation to a material surface [10,11]. It could produce outstanding surface hardness, surface roughness (0.08– 0.5 lm) and an effect layer with a certain depth (about 300 lm) [10–14]. Therefore, UCFT could be expected as a highly promising pre-treatment means of plasma nitriding. Meanwhile, the plasma nitriding temperature plays another basic role on the formation of nitrided layer [15–18]. However, few works on the die steel pre-treated by UCFT and then plasma nitriding are reported so far. In this study, UCFT was introduced as a pre-treatment of plasma nitriding. Plasma nitriding processes performed at temperature of 350, 450, 500, 520, and 550 °C for 4 h. The effects of plasma nitriding temperature on the modification layer structural properties, including surface morphology, hardness profile, diffusion layer thickness, compound layer thickness and phase content, were investigated. It aims to find an optimized nitriding temperature to the surface combined modification process.
2. Experimental procedure 2.1. Materials A commercial available AISI D2 die steel cylinder (£32 mm 105 mm) with hardness of about 200 HV was prepared. Table 1 shows its chemical composition.
393
D. She et al. / Materials and Design 49 (2013) 392–399 Table 1 Nominal composition of AISI D2 steel (wt.%).
Table 2 UCFT treatment parameters.
Element
C
Cr
Si
Mn
Mo
V
P6
S6
Content (wt.%)
1.50
12.00
0.25
0.45
1.00
0.35
0.025
0.01
Vibration frequency (KHZ)
Feed rate Amplitude Load Spindle (N) speed (rpm) (mm/ (lm) rev)
Number Tip diameter of shots per mm2 (mm)
20
30
2.38
100
30
0.07
21,840
2.2. Ultrasonic cold forging technology treatment The sketch of the UCFT device is shown in Fig. 1. The principle of the device was developed by Alekhine and Alekhine [14]. It transforms ultrasonic vibratory energy into tens of thousands of strikes per second. The output power is about 1 kW. The dynamic load is 1.5–5 times higher than static load. The total load acting on the specimen is the sum of the static load and the sinusoidal function of the dynamic load. Table 2 shows the UCFT treatment parameters. 2.3. Plasma nitriding Both the UCFT and Un-UCFT samples were cut to 10 mm 10 mm 15 mm plates. The samples were ultrasonically cleaned in distilled water for 10 min, and then with acetone for another 10 min. After cleaning, the samples were placed into a LDM 2-25 plasma nitriding furnace. When vacuum chamber was evacuated to ultimate vacuum and the leakage rate of the equipment was less than or equal to 2 Pa per 15 min, all the nitrided samples were treated for 4 h in an NH3 containing atmosphere and a voltage of 650 V. The treatment conditions for various samples are given in Table 3. During the plasma nitriding, the samples were connected to the cathode, and the furnace wall acted as the anode.
Table 3 Treatment conditions of samples. Sample number
UCFT or not
Nitriding temperature (°C)
Pressure (Pa)
Un-UCFT + Un-nitrided Un-UCFT + 350 Un-UCFT + 400 Un-UCFT + 450 Un-UCFT + 500 Un-UCFT + 520 Un-UCFT + 550
Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT Un-UCFT
(Un-nitrided) 350 400 450 500 520 550
– 250 400 520 550 580 600
UCFT + Un-nitrided UCFT + 350 UCFT + 400 UCFT + 450 UCFT + 500 UCFT + 520 UCFT + 550
UCFT UCFT UCFT UCFT UCFT UCFT UCFT
(Un-nitrided) 350 400 450 500 520 550
– 250 400 520 550 580 600
2.4. Performance and analysis test The surface roughness and surface morphology of the UCFT + Un-nitrided samples were characterized by 3D profiler
Fig. 1. Configuration of UCFT device.
Fig. 2. SEM morphologies of the cross-section of the UCFT + Un-nitrided samples.
394
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 3. XRD diffraction profiles of the before and after UCFT treatment.
Fig. 5. The hardness distribution of the cross-section of the Un-nitrided samples.
(Nano-Map-D). According to the ISO 4287:1997 standard [19], the measurement length for the surface roughness is about 0.75 mm. For the roughness measurements, the tests were repeated ten times and the average was taken. Scanning electron microscope (JSM-7001 F) was used to investigate surface and cross-section morphologies of the samples. Optical microscopy (OM) was used to determine the thickness of the diffusion and compound layer. The samples were cut in cross-section with a CNC wire-cut electric discharge machine, and then mounted in the dental base acrylic resin. After being polished, and then etched in the 4% Nital, the samples were conducted with metallographic examinations. X-ray diffraction (XRD) was carried out on a D/max X-ray diffractometer using a Cu-Ka radiation source (wavelength of 1.5406 Å) to determine the phases in the modified layer. Each sample was conducted with a 2h range of 30–90°, an increment of 0.04°/step and a time per step of 1.5 s. The element distribution along with the depth of the cross-section of the nitrided samples was characterized by a HORIBA EMAX energy dispersive X-ray spectroscopy (EDS).
The surface hardness and the micro-hardness variation on the cross-section of samples were evaluated using a MH-6 Vickers micro-hardness tester at a load of 200 gf with 5 s dwell time. According to the ASTM E384-11e1 standard, the testing machine was verified before the hardness test. The repeatability (R) and the error (E) in the performance of testing machine met the requirements (R < 4%, E < 2%). The hardness measurements were repeated five times and the average was plotted. 3. Results and discussion 3.1. UCFT pre-treatment Cross-section image of the UCFT + Un-nitrided samples was shown in Fig. 1. It can be seen that micro-structural morphology of severe plastic deformation layer is quite different from that of the matrix. An effective layer caused by severe plastic deformation with a depth of about 350 lm was formed. Fig. 3 illustrates the XRD pattern obtained from the Un-UCFT + Un-nitrided samples and the UCFT + Un-nitrided samples. The
Fig. 4. 3D surface profile of the UCFT + Un-nitrided samples.
D. She et al. / Materials and Design 49 (2013) 392–399
395
Fig. 6. SEM micrographs of the surface of the samples nitrided at 520 °C for 4 h, (a) Un-UCFT + 520 and (b) UCFT + 520.
Table 4 The average size of nitride nano-particles covered on the surface of the nitrided samples. Samples number
Average size (nm)
Samples number
Average size (nm)
Un-UCFT + 350 Un-UCFT + 400 Un-UCFT + 450 Un-UCFT + 500 Un-UCFT + 520 Un-UCFT + 550
– 321 547 874 951 987
UCFT + 350 UCFT + 400 UCFT + 450 UCFT + 500 UCFT + 520 UCFT + 550
107 242 384 521 606 908
Bragg diffraction peaks of the UCFT + Un-nitrided samples are broader than those of the Un-UCFT + Un-nitrided samples. This result is due to the gain refinement effect and the lattice dislocation on the atomic-level [7]. The average grain size was calculated from line broadening of the Bragg diffraction peaks of ferrite a-Fe (1 1 0), (2 2 0) and (2 1 1) based on Fourier transform method (FTM) [20]. Using the Scherer and Wilson equation [21], the average grain size of the surface of UCFT + Un-nitrided samples was calculated to be about 20.4 nm. It is confirmed that a surface layer with ultrafine grains was formed by UCFT. The 3D profiler image of the UCFT + Un-nitrided samples is given in Fig. 4. It is observed that the surface of the UCFT + Un-nitrided sample appears some microgrooves. Using the 3D profiler accompanying program, the surface roughness Ra of the UCFT + Un-nitrided samples was measured to be about 380 nm. The hardness distribution of the cross-section of Un-UCFT + Unnitrided samples along the depth is shown in Fig. 5. The surface hardness of the UCFT + Un-nitrided samples is 520 HV, which is
about 2.6 times of that of the Un-UCFT + Un-nitrided samples (200 HV). The micro-hardness of the UCFT + Un-nitrided samples is a gradient change along the depth from about 530 HV on the top surface to about 200 HV in the matrix. The thickness of hard layer is about 400 lm. According to the Hall–Petch law, the smaller the grain size the harder the material, the results may be related to the average grain size gradient change along the depth [5].
3.2. The microstructure and morphology of nitrided samples The surfaces of the nitrided samples are covered by the compact plasma nitriding layers composed of a number of nano-particles, which is shown in Fig. 6. During plasma nitriding, there were high concentration of nitrogen ions and the presence of weighty nonionized nitrogen molecules in plasma media [1]. In addition the surface sputtering increased and the lower energy nitrogen atoms tend to form molecular N2 on the surface [15]. The combination of these effects and the agglomeration of nitride precipitates lead to the formation of micro-pores. To calculate the average sizes of nitride nano-particles, the number of nitride nano-particles in a unit area was counted in the SEM images of nitriding samples. The average sizes of nitride nano-particles covered on the surface of the samples are shown in Table 4. It can be found that with the increase of nitriding temperature the nitrided nano-precipitates are easy to aggregate and form some large-size particles. Besides, the nitride nano-particles distributed on the UCFT samples are obviously smaller than the particles on the surfaces of Un-UCFT samples treated under the same nitriding condition. The principal reason may be that the grain boundaries and defect densities accelerate the formation of nitride precipitates and retard the growth of nitride particle.
Fig. 7. SEM micrographs of the surface of the samples nitrided at 350 °C for 4 h, (a) Un-UCFT + 350 and (b) UCFT + 350.
396
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 8. SEM micrographs of the surface of the samples nitrided at 550 °C for 4 h, (a) Un-UCFT + 550 and (b) UCFT + 550.
Fig. 9. OM cross-sectional observations of the nitrided samples, (a) Un-UCFT + 520 and (b) UCFT + 520, (c) Un-UCFT + 450 and (d) UCFT + 450.
Table 5 Thickness of nitriding layer of samples. Un-UCFT samples Temperature (°C) Pressure (Pa) Compound layer (lm) Diffusion layer (lm)
350 250 0 20
400 400 0 52
UCFT samples 450 520 0 73
500 550 6 78
520 580 10 81
Fig. 7 shows the surface micrographs of the samples nitrided at 350 °C. Unlike the Un-UCFT + 350 samples, a great number of nitride particles can be obviously seen on the surface of UCFT + 350 samples. It is implied that the UCFT pre-treatment enhanced nitriding process and reduced the nitriding temperature. The surfaces of Un-UCFT + 550 and UCFT + 550 samples appear to be irregular and jagged, as shown in Fig. 8, This result indicates that a greater amount of sputtering occurred at the nitriding temperature of 550 °C, which resulted in the surface damage. Fig. 9 shows the cross-section morphologies of the nitrided samples. A white compound layer (upper part) as well as a dark diffusion layer (lower part) can be seen in the cross-section of samples. The thickness of the nitriding layer is listed in Table 5. It
550 600 12 90
350 250 0 40
400 400 0 74
450 520 8 80
500 550 15 87
520 580 21 105
550 600 12 110
shows that the thickness of nitriding layer is influenced by the nitriding temperature. An increased nitriding temperature resulted in an increase of the thickness of the layer. This result could be related to the fact that a higher temperature could promote the diffusion of nitrogen. Meanwhile, under the same nitriding condition, a thicker compound layer as well as thicker diffusion layer can be easily formed on the UCFT samples. Nonetheless, the compound layer of the UCFT + 550 samples is only 12 lm, which is thinner than that of the UCFT + 520 samples. It may be resulted from the fact that the compound layer is etched by a greater amount of sputtering at 550 °C, which is coordinated with the results in Fig. 8. In addition, for both samples nitrided at 450 °C, there is no white compound layer on the surface of Un-UCFT + 450 samples.
D. She et al. / Materials and Design 49 (2013) 392–399
397
Fig. 10. EDS line analysis along the cross-sections of the nitrided samples (a) 520 °C nitrided samples and (b) 550 °C nitrided samples.
Fig. 11. XRD patterns for samples treated at various condition, (a) Un-UCFT samples and (b) UCFT samples.
It may be attributed to the fact that the samples pre-treated by UCFT could enhance the nitriding kinetics for lots of grain boundaries and lattice dislocations caused by severe plastic deformation [1,4–9,17,18]. 3.3. Composition and phase analysis of nitrided samples Fig. 10 shows the distribution of N elements along the depth of the cross-sections of the nitrided samples. The concentration of nitrogen in the nitrided layer decreases along with the depth from surface to interior. Besides, the UCFT samples tend to have a deeper nitrogen distribution with higher intensity than the Un-UCFT samples nitrided at the same temperature. Fig. 11 shows the XRD patterns of the samples treated at various temperatures. As nitriding temperature increased, the intensity of peaks for e-Fe2–3N, c0 -Fe4N and CrN phases increased. Besides, the nitrided UCFT samples tend to possess stronger peaks of e-Fe2–3N, c0 -Fe4N and CrN phases than the Un-UCFT samples treated under the same nitriding temperature. It is also a proof that the surface of UCFT samples makes nitride layer easy to be formed. The Un-UCFT + 350 samples contain strong diffraction peaks for the ferrite a-Fe, as well as weak diffraction peaks of e-Fe2–3N. It is in good agreement with the result shown in Fig. 7a that nitride
Fig. 12. The surface micro-hardness of the samples.
nano-particle can be hardly observed on the surface of the UnUCFT + 350 samples. In addition, conspicuous c0 -Fe4N phase peaks could be identified in the UCFT + 450 samples, however, in the
398
D. She et al. / Materials and Design 49 (2013) 392–399
Fig. 13. The micro-hardness variation on the cross-section of samples, (a) Un-UCFT samples and (b) UCFT samples.
Un-UCFT + 450 samples it could not be found. The white compound layer consisting of c0 -Fe4N or e-Fe2–3N phases is formed when the nitrogen potential exceeds the critical value [9,18]. This result can explain why there is no white compound layer on the surface of the Un-UCFT + 450 samples shown in Fig. 9a. 3.4. Micro-hardness of nitrided layer The surface micro-hardness of samples is listed in Fig. 12. The nitrided UCFT samples tend to have a harder surface than the nitrided Un-UCFT samples after nitriding under the same nitriding condition. Meanwhile, higher nitriding temperature tends to form a harder surface. It verified that a nano-structure layer induced by UCFT makes a hard nitriding layer form. Nonetheless, the UCFT + 520 samples present the highest hardness (1350 HV). The results must be due to the following reasons: First of all, a higher nitriding temperature makes more nitride form. However, at 550 °C, the compound layer was etched by ion sputter (as shown in Fig. 8). Secondly, a high nitriding temperature converts a greater number of e-Fe2–3N into c0 -Fe4N (as shown in Fig. 11). Moreover, c0 -Fe4 N phase has an fcc crystalline structure and shows lower hardness with respect to e-Fe2–3N phase with hcp crystalline structure [15,18]. In addition, the samples nitrided at 550 °C (it is near to the tempering temperature of AISI D2 steel) were further tempered. Further tempering makes the reduction of residual stress and structural transformation occurring, which result in the decrease of surface hardness. Fig. 13 shows the micro-hardness variation on the cross-section of samples. It can be seen that a gradient modification layer was formed and a higher nitriding temperature tends to form a thicker and harder modification layer. Meanwhile, the UCFT samples tend to have a thicker layer. Its thickness is about 400 lm, which is nearly equal to the thickness of the effective layer of the UCFT sample (as shown in Fig. 2). As the results shown above, the higher temperature results in an increased diffusion coefficient. Besides, a great number of grain boundaries and high defect densities play a decisive role in the formation of the nitride nano-particles and the growth of nitrided layer for UCFT samples. Accordingly, the combination of these effects and ion sputter occurring at 550 °C lead to the consequence that pre-treated by UCFT and then nitriding at 520 °C for 4 h is an optimized complex surface modification process to AISI D2 die steel. 4. Conclusions To sum up, the UCFT can be introduced as a pre-treatment means of plasma nitriding to effectively enhance the properties
of AISI D2 die steel. Pre-treated by UCFT, the surface of AISI D2 steel samples tends to form a thicker and harder nitride layer with smaller nitrided nano-particles and more nitride phases (e-Fe2–3N, c0 Fe4N and CrN). A higher nitriding temperature promotes the formation of the hard phases (e-Fe2–3N, c0 -Fe4N and CrN). Meanwhile, to some extent, a high nitriding temperature also results in a thicker and harder nitrided layer. However, with the nitriding temperature increased to 550 °C, a stronger sputter occurred on the surface of nitrided samples, which depleted the compound layer. In the case of nitriding at 520 °C for 4 h after UCFT pre-treatment can be regarded as an optimized process. In this way, a gradient modification layer was formed with the thickness of 400 lm. The micro-hardness gradually decreases from around 1350 HV of the surface to around 200 HV in the matrix.
Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51275494), the Fundamental Research Funds for the Central Universities (2012ZY51), the Program for Key International Science and Technology Cooperation Project of China (2010DFR50070) and the Tribology Science Fund of State Key Laboratory of Tribology (SKLTKF11B04).
References [1] Ferkel H, Glatzler M, Estrin Y, Valiev RZ, Blawert C, Mordike BL. RF plasma nitriding of severely deformed iron-based alloys. Mater Sci Eng A 2003;348: 100–10. [2] Öztürk O, Onmousß O, Williamson D L. Microstructural, mechanical and corrosion characterization of nitrogen-implanted plastic injection mould steel. Surf Coat Techno 2005;196:341–8. [3] Oliveira SD, Tschiptschin AP, Pinedo CE. Simultaneous plasma nitriding and ageing treatments of precipitation hardenable plastic mould steel. Mater Dse 2007;28:1714–8. [4] Tong WP, Han Z, Wang LM, Lu J, Lu K. Low-temperature nitriding of 38CrMoAl steel with a nanostructured surface layer induced by surface mechanical attrition treatment. Surf Coat Technol 2008;202:4957–63. [5] Gleiter H. Nanocrystalline materials. Prog Mater Sci 1989;33:223–315. [6] Tong WP, Liu CZ, Wang W, Tao NR, Wang ZB, Zuo L, et al. Gas nitriding of iron with a nanostructured surface layer. Scripta mater 2007;57:533–6. [7] Li Y, Wang L, Zhang DD, Shen L. The effect of surface nanocrystallization on plasma nitriding behaviour of AISI 4140 steel. Appl Surf Sci 2010;257:979–84. [8] Tong WP, Zhang HW, Tao NR, Wang ZB, Lu J, Lu K. Low-temperature nitriding by means of SMAT. Trans Mater Heat Treat 2004;25:301–6. [9] Tong WP, Tao NR, Wang ZB, Zhang HW, Lu J, Lu K. The formation of e-Fe2–3N phase in a nanocrystalline Fe. Scripta Mater 2004;50:647–50. [10] Amanov A, Cho IS, Pyoun YS, Lee CS, Park IG. Micro-dimpled surface by ultrasonic nanocrystal surface modification and its tribological effects. Wear 2012;286–287:136–44.
D. She et al. / Materials and Design 49 (2013) 392–399 [11] Suh CM, Song GH, Suh MS, Pyoun YS. Fatigue and mechanical characteristics of nano-structured tool steel by ultrasonic cold forging technology. Mater Sci Eng A 2007;443:101–6. [12] Mordyuk BN, Milman Yu V, Iefimov MO, Prokopenko GI, Silberschmidt VV, Danylenko MI, et al. Characterization of ultrasonically peened and laser shock peened surface layers of AISI 321 stainless steel. Surf Coat Technol 2008;202:4875–83. [13] Cao XJ, Pyou YS, Murakami R. Fatigue properties of a S45C steel by subjected to uitrasonic nanocrystal surface modification. Appl Surf Sci 2010;256:6297–303. [14] Alekhine VP, Alekhine OV. The final processing of constructional steel and tool steel: surface nanocrystallization technology. Mech Eng Technol 2007;4 C:2–13. [15] Rad HF, Amadeh A, Moradi H. Wear assessment of plasma nitrided AISI H11 steel. Mater Des 2011;32:2635–43.
399
[16] Li Y, Wang L, Xu J, Zhang D. Plasma nitriding of AISI 316 L austenitic stainless steels at anodic potential. Surf Coat Technol 2012;206:2430–7. [17] Larisch B, Brusky U, Spies HJ. Plasma nitriding of stainless steels at low temperatures. Surf Coat Technol 1999;116–119:205–11. [18] Sirin SY, Kaluc E. Structural surface characterization of ion nitrided AISI 4340 steel. Mater Des 2012;36:741–7. [19] ISO 4287. Geometrical Product Specification (GPS). Surface texture: profile method-terms, definitions, and surface texture parameters; 1997. [20] Ida T, Shimazaki S, Hibino H, Toraya H. Diffraction peak profiles from spherical crystallites with lognormal size distribution. J Appl Cryst 2003;36: 1107–15. [21] Scardi P, Leoni M, Delhez R. Line broadening analysis using integral breadth methods: a critical review. J Appl Cryst 2004;37:381–90.