Journal Pre-proofs Laser cladding and in-situ nitriding of martensitic stainless steel coating with striking performance Baichun Li, Hongmei Zhu, Changjun Qiu, Xiaokang Gong PII: DOI: Reference:
S0167-577X(19)31460-0 https://doi.org/10.1016/j.matlet.2019.126829 MLBLUE 126829
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Materials Letters
Received Date: Revised Date: Accepted Date:
14 September 2019 15 October 2019 16 October 2019
Please cite this article as: B. Li, H. Zhu, C. Qiu, X. Gong, Laser cladding and in-situ nitriding of martensitic stainless steel coating with striking performance, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.126829
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Laser cladding and in-situ nitriding of martensitic stainless steel coating with striking performance Baichun Li, Hongmei Zhu*, Changjun Qiu*, Xiaokang Gong Provincial Key Laboratory of Advanced Laser Manufacturing Technology, University of South China, Hengyang 421001, China
E-mail:
[email protected],
[email protected]
Abstract: The nitrogen (N) alloying in traditional bulk martensitic stainless steel (MSS) necessitates high-pressure and high-temperature treatment because of its extremely low solubility. Herein, we report a novel strategy for in-situ nitriding MSS coating with N content up to 0.14 wt.% by laser cladding technique without harsh conditions. The resulting coating is composed of martensite, austenite, nano-precipitates M23C6 and M2N, which exhibits striking mechanical properties with tensile strength of 1900 MPa and elongation of 7.3% as well as excellent corrosion resistance, exceptionally superior to the reported laser cladded MSS coatings. This may be ascribed to the combination impacts of grain refinement, nano-precipitation, interstitial solid solution and dislocation strengthening. Keywords: laser processing; in-situ nitriding; nitrogen-bearing MSS coating; microstructure
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1. Introduction Laser cladding metallic materials has attracted substantial attention due to its inherent merits in metallurgical bonding, small heat-affected zone, excellent process control and high efficiency [1]. Martensitic stainless steel (MSS) is one of the most potential alloys for coatings owing to its high strength and moderate corrosion resistance [2], nevertheless, still problematic in poor ductility and low toughness due to the strength-ductility tradeoff [3]. As well known, nitrogen (N) addition has been widely employed in austenitic SS and duplex SS to improve mechanical properties and corrosion resistance [4-5]. However, due to extremely low solubility of N in MSS (0.045 wt.% at one bar, 1600°C), traditional bulk N-bearing MSS with N content of 0.0045~0.52 wt.% was usually manufactured by casting or forging under high pressure (0.5~5 MPa) and hightemperature post-treatment above 900°C[5-7]. To our best knowledge, the fabrication of N-bearing MSS coating has not been studied to date. Herein, we develop a novel strategy, for the first time, to obtain in-situ nitriding MSS coatings by a cost-effective and high-efficiency laser cladding technique. This work will be beneficial for laser forming high-strength steel coatings/components with excellent corrosion resistance. 2. Experimental A FL-1500 1.5 kW fiber laser, with a coaxial powder feeding and water-cooling system, was used for coating preparation as followings: laser power 1.2 kW, laser spot
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diameter 2 mm, traverse speed 480 mm/min, overlap rate 50% and powder delivery rate 6.5 g/min. High-purity N2 (6N) was used as both a shielding gas and a carrier gas with a flow rate of 10 L/min. AISI 420 MSS powders were atomized in an Ar atmosphere with a mean diameter of ~75 µm, supplied by Changsha Tianjiu Materials Ltd.. A total of 10 cladding layers, with a dimension of 80 mm ×40 mm × 4 mm, were deposited on AISI D2 harden steel with grain size of 5~20 µm. The substrate was sandblasted and cleaned by acetone prior to the laser cladding process. The chemical compositions of substrate, powder and coating were analyzed by emission spectrometry (Table 1). The elemental distribution in the laser-cladded specimen was investigated by an EPMA8050G
field
emission
electron
microprobe
microanalyzer
(EPMA).
The
characterization for microstructure and properties was conducted similarly as reported by Ref. [8]. Table 1 Chemical compositions (wt.%) Element
C
Cr
Si
Mn
Mo
V
Ni
N
Fe
Substrate
1.49
11.95
0.27
0.26
0.40
0.15
0.29
-
Bal.
Powder
0.21
13.25
1.00
1.15
-
-
0.58
-
Bal.
Coating
0.19
13.20
0.98
1.10
-
-
0.51
0.14
Bal.
3. Results and Discussion Fig. 1 shows phase analysis and microstructure of the laser-cladded specimen. The coating is composed of martensite (M) and trace austenite (A) (Fig. 1a). Noticeably, the M peaks shift leftward ~2° compared with standard diffraction peaks shown in ICCD
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#06-0696. This is caused by interstitial N atoms expanding M lattice structure and interplanar distance [5, 9]. However, no diffraction peaks of carbides and nitrides were found probably due to their lower contents beyond detection limit.
Fig. 1 (a) XRD pattern, (b) SEM micrograph, (c~f) TEM analysis: (c) Bight-field (BF) image, (d) Dark-field (DF) image and selected area diffraction (SAD) pattern of A, (e) DF image and SAD pattern of nano-precipitates, and (f) dislocations.
The cross-section SEM micrograph (Fig. 1b) shows a good metallurgical bonding between the coating and the substrate with few voids and cracks, evidenced by extremely compact and fine microstructure (inset). This is because the N addition in MSS can remarkably decrease the Ms temperature and the peritectic reaction rate, which effectively restrains A growth and hence promotes M nucleation to refine M lath [10-13]. Fig. 1c-f reveals TEM in-depth insights on the resulting N-bearing MSS coating. The lath-shaped M exists as a matrix, over which a little A is distributed (Fig. 1c-d).
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This may be caused by the insufficient M phase transformation and the austenitepromoting element N addition in the laser cladding process [10-11,14]. Interestingly, massive nano-precipitates (25~35 nm) were homogeneously dispersed on M matrix (Fig. 1e). Meanwhile, the indexed SAD pattern indicates that nitride M2N coexisted with carbide M23C6 in the N-bearing MSS coating (M represents Cr, Fe etc.), similar as the main precipitates produced by treating bulk N-bearing MSS >500°C [11,15]. The N addition could promote nitride precipitation while suppress coarse eutectic carbide simultaneously, owing to higher A stability upon quenching and higher binging energy of Cr-N than Cr-C [5,7,11,15]. Nevertheless, nano-precipitates M2N and M23C6 were difficult to distinguish merely by morphologies (Fig. 1e), due to rapid-cooling laser cladding process. In addition, massive dislocations were observed in the lath-shaped M matrix, due to rapid heat dissipation and high stress by laser cladding [3,16]. The coexistence of nano-precipitates and dislocations is beneficial for the coating strengthening by impeding lattice movement.
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Fig. 2 EPMA result of the laser-cladded specimen: (a) back-scattering electron (BSE) image; and (b~f) elemental distribution of Fe, Cr, Ni, C and N in sequence.
Fig. 2 reveals the elemental line scanning results of the laser-cladded specimens. In contrast to similar elemental concentrations of Fe, Cr and Ni (Fig. 2b-d) as listed in Tab. 1, the C and N contents in the substrate were much higher than the actual values (Fig. 2e-f), probably due to the presence of organic contaminations on the surface induced by specimen preparation. Given no elemental N in the substrate, it can be deduced that the N content in the resulting coating is approximately 0.14 wt.%, consistent with the emission spectrometry results (Table 1), which is mainly contributed by in-situ N2 nitriding by laser cladding. Furthermore, a transition zone with a width of 125 μm was observed, indicating little influence of the element diffusion from the substrate on the phase constitution and microstructure of the N-bearing MSS coating (Fig. 1).
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Fig. 3 (a) microhardness curve, (b) tensile curve, (c) fractography, and (d) polarization curves by referring bulk AISI 420 MSS.
Fig. 3a shows the microhardness variation curve across the coating, bonding zone, and substrate. The average microhardness of the N-bearing MSS coating is 610 HV, much higher than those of MSS coatings (450~580 HV) [8,17-18]. Noticeably, the coating microhardness reveals a negligible fluctuation with small errors, indicating that a homogeneous microstructure was formed (Fig. 2b). Fig. 3b shows the tensile curves that ultimate tensile strength, yield strength and elongation are 1990 MPa, 1750 MPa and 7.3%, respectively, equivalent to those of traditional bulk high-nitrogen MSS [10,12], while superior to those of laser cladded MSS [8,17,19]. The corresponding fractography has a fibrous region surrounded by overload shear lips (Fig. 3c). The fracture surface (inset of Fig. 3c) of the N-bearing MSS coating includes small cleavage
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planes, tearing ridges, dimples and microcracks, consistent with moderate ductility of 7.3% (Fig. 3b). The reasons accounting for the striking mechanical properties of the N-bearing MSS coating could be associated with: (1) Grain refinement. The N addition can refine the prior A grain and the resulting M lath by reducing Ms temperature and peritectic reaction rate [10-11,13], improving both strength and ductility simultaneously; (2) Nano-precipitation. The scattered nano-precipitates of carbide M23C6 and nitride M2N (Fig. 1e) can significantly enhance the mechanical properties [20,21]; (3) Interstitial solid solution. The N addition can facilitate the solid solution of interstitial C and N atoms in the MSS coating [13]; and (4) Dislocation strengthening. The high-density dislocation (Fig. 1f) is vital to enhance the tensile property of the resulting coatings. Surprisingly, the corrosion resistance of the resulting N-bearing MSS coating is significantly higher than that of the commercial AISI 420 MSS (Fig. 3d). This could be ascribed to two aspects: i) the coarse carbides M23C6, preferentially acting as pitting initiation sites were reduced due to N addition in MSS. The severe Cr depletion was alleviated, and the corrosion resistance was improved consequently [5,7,11,15]; and ii) the N addition could promote Cr enrichment, especially Cr2O3 and CrN in the passive film, and thereby improve protective property and stability [5,7]. 4. Conclusions The N-bearing MSS coating has been successfully prepared by laser cladding and in-situ nitriding processes. A few new findings are demonstrated as:
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i)
Novel fabrication method has been developed to obtain N-bearing MSS coating, for the first time, by a cost-effective and high-efficiency laser cladding technique under N2 flow.
ii) Extraordinarily high N-content coatings up to ~0.14 wt.% have been achieved successfully by laser cladding and in-situ nitriding, without high-pressure and high-temperature treatment utilized in traditional bulk MSS. iii) Exemplary mechanical properties and superior corrosion resistance have been obtained in the N-bearing MSS coatings, including microhardness (610 HV), ultimate tensile strength (1990 MPa), yield strength (1750 MPa) and elongation (7.3%). The mechanisms accounting for the striking performance of the N-bearing MSS coating have been elucidated by grain refinement, nano-precipitation, interstitial solid solution and dislocation strengthening. This work will potentially facilitate laser manufacturing high-strength steel coatings/components with excellent corrosion resistance. Acknowledgements This work was supported by National Key Research and Development Program of China (No. 2018YFB1105803) and Graduate research innovation project of Hunan province (No. CX20190731).
References [1] D. C. Saha, E. Biro, A. P. Gerlich, N. Y. Zhou, Scripta Mater. 121(2016)18-22.
9
[2] B. Mahmoudi, M. J. Torkamany, A. S. R. Aghdam, J. Sabbaghzade, Mater. Des. 31(2010) 2553-2560. [3] B. B. He, B. Hu, H. W. Yen, G. J. Cheng, Z. K. Wang, H. W. Luo, M. X. Huang, Science. 357(2017)1029-1032. [4] X. Qi, H. Mao, Y. Yang, Corros. Sci. 120(2017)90-98. [5] H. Feng, Z. Jiang, H. Li, P. Lu, S. Zhang, H. Zhu, B. Zhang, T. Zhang, D. Xu, Z. Chen, Corros. Sci. 144 (2018)288-300. [6] J. W. Simmons, Mater. Sci. Eng. A. 207(1996)159-169. [7] Z. Jiang, H. Feng, H. Li, H. Zhu, S. Zhang, B. Zhang, Y. Han, T. Zhang, D. Xu, Mater. 10 (2017)861. [8] H. Zhu, Y. Li, B. Li, Z. Zhang, C. Qiu, Coat. 8(2018)451. [9] C. X. Li, T. Bell, Corros. Sci. 48(2006)2036-2049. [10] R. Fan, M. Gao, Y. Ma, X. Zha, X. Hao, K. Liu, J. Mater. Sci. Technol. 28(2012)1059-1066. [11] X. P. Ma, L. J. Wang, B. Qin, C. M. Liu, S. V. Subramanian, Mater. Des. 34(2012)74-81. [12] J. B. Wang, Y. F. Zhou, X. L. Xing, S. Liu, C. C. Zhao, Y. L. Yang, Q. X. Yang, Surf. Coat. Technol. 294(2016)115-121. [13] M. Wendler, M. Hauser, O. Fabrichnaya, L. Krüger, A. Weiß, J. Mola, Mater. Sci. Eng. A. 645(2015)28-39. [14] L. Costa, R. Vilar, T. Reti, A. M. Deus, Acta Mater. 53(2005)3987-3999. [15] T. H. Lee, S. J. Kim, Y. C. Jung, Metall. Mater. Trans. A. 31(2000)1713-1723. [16] D. C. Saha, E. Biro, A. P. Gerlich, Y. Zhou, Mater. Sci. Eng. A. 673(2016)467-475.
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[17] S. D. Sun, D. Fabijanic, C. Barr, Q. Liu, K. Walker, N. Matthews, N. Orchowski, M. Easton, M. Brandt, Surf. Coat. Technol. 333(2018)210-219. [18] S.D. Sun, D. Fabijanic, A. Ghaderi, M. Leary, J. Toton, S. Sun, M. Brandt, M. Easton, Surf. Coat. Technol. 296(2016)76-87. [19] J. Sun, T. Jiang, Y. Sun, Y. Wang, Y. Liu, J. Alloys Compd. 698(2017)390-399. [20] M. I. Isik, A. Kostka, V. A. Yardley, K. G. Pradeep, M. J. Duarte, P. P. Choi, G. Eggeler, Acta Mater. 90(2015)94-104. [21] J. Y. Maetz, T. Douillard, S. Cazottes, C. Verdu, X. Kleber, Micron. 84(2016)43-53.
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Highlights:
A cost-effective and high-efficiency laser cladding method for N-bearing MSS coating
Increase N content to 0.14% in MSS coating by laser cladding and in-situ nitriding
Generate coating with martensite, austenite, and nano-precipitates M23C6 and M2N
Achieve striking mechanical property with strength of 1900 MPa and elongation of 7.3%
Achieve excellent corrosion resistance superior greatly to traditional AISI 420 MSS
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Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled” Laser cladding and in-situ nitriding of martensitic stainless steel coating with striking performance”.
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