J. Cent. South Univ. (2013) 20: 3354−3362 DOI: 10.1007/s11771-013-1859-2
Influence of chemical composition and cold deformation on aging precipitation behavior of high nitrogen austenitic stainless steels LI Hua-bing(李花兵), JIANG Zhou-hua(姜周华), FENG Hao(冯浩), ZHU Hong-chun(朱红春), ZHANG Zu-rui(张祖瑞) School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2013 Abstract: The influence of chemical composition and cold deformation on aging precipitation behavior of 18Cr-16Mn-2Mo-1.1N (HNS-A), 18Cr-16Mn-1.3N (HNS-B), 18Cr-18Mn-2Mo-0.96N (HNS-C) and 18Cr-18Mn-2Mo-0.77N (HNS-D) high nitrogen austenitic stainless steels was investigated. The results show that the “nose” temperatures and incubation periods of the initial time−temperature−precipitation (TTP) curves of aged HNSs are found to be 850 °C, 60 s; 850 °C, 45 s; 850 °C, 60 s and 900 °C, 90 s, respectively. Based on the analysis of SAD patterns, the coarse cellular Cr2N precipitate which presents a lamellar structure has a hexagonal structure of a=0.478 nm and c=0.444 nm. The χ phase corresponding to a composition of Fe36Cr12Mo10, is determined to be a body-centered cubic structure of a=0.892 nm. The precipitating sensitivity presents no more difference with the nitrogen content increasing from 0.77% to 0.96%, but exhibits so obviously that the cellular precipitates nearly overspread the whole field. The addition of Mo element can restrain the TTP curves moving left and down, which means decreasing the sensitivity of aging precipitation. With increasing the cold deformation, the sensitivity of precipitation increases obviously. Key words: high nitrogen austenitic stainless steel; aging precipitation; time−temperature−precipitation curve; chemical composition; cold deformation
1 Introduction High nitrogen steels and nitrogen-bearing stainless steels, specially high nitrogen austenitic stainless steels are becoming an important class of engineering materials due to their more excellent mechanical and corrosion resistance properties than those of the traditional stainless steels [1−2]. But some kinds of secondary phases are prone to form at elevated temperature in nitrogen-bearing austenitic stainless steels, which will deteriorate their mechanical properties and corrosion resistance [3−5]. The secondary phases of nitrogenbearing austenitic stainless steels in certain heat forming process are so complex that there are the maximum 18 kinds of precipitates [6]. For high nitrogen austenitic stainless steels, more nitrogen is added instead of nickel in order to stabilize the austenite structure and improve strength and corrosion resistance. The precipitates such as intergranular and cellular Cr2N phase for high nitrogen austenitic stainless steels could form more easily, which affects more drastically the mechanical properties and
corrosion resistance [7−14]. A lot of researches on the precipitation behavior and its effect on the properties of high nitrogen austenitic stainless steel focus on the steel with nitrogen content less than 1% (mass fraction). With the rapid development of pressurized melting equipment, more methods such as pressurized electroslag remelting (PESR) and pressurized induction melting are applied to manufacture higher nitrogen content austenitic stainless steels. The retaining ring steels such as P900N, P900NMo and P2000 were developed by PESR in Germany [15]. The 23Cr-4Ni-(0−2)Mo-(0.7−1.1)N and 23Cr-0Ni-2Mo-1.3N with excellent crevice corrosion resistance were also developed using PESR and pressurized induction melting methods in Japan [16−17]. Some high nitrogen austenitic steels with nitrogen content above 1% (mass fraction) has been explored in Northeastern University using PESR and pressurized induction melting equipments [18−19]. So the aging precipitation behavior of high nitrogen stainless steels with higher nitrogen content requires more systematically investigation. The molybdenum element is usually added into the austenitic stainless steels for
Foundation item: Project(51304041) supported by the National Natural Science Foundation of China; Project(N100402015) supported by Fundamental Research Funds for the Central Universities of China; Project(2012AA03A502) supported by the National High Technology Research and Development Program of China; Project supported by Program for Liaoning Innovative Research Team in University, China Received date: 2012−07−06; Accepted date: 2012−12−10 Corresponding author: JIANG Zhou-hua, Professor, PhD; Tel: +86−24−83686453; E-mail:
[email protected]
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enhancing their local corrosion resistance [20−21]. Specially, the synergistic effects of molybdenum and nitrogen in Mo-containing high nitrogen austenitic stainless steels can improve the formation of the bipolar films which bring a greater resistance to the ingression of chloride ion [22]. The molybdenum element also exhibits an effect on retraining the pearlite transformation [23]. But there are few literature reports about its effect on the aging precipitation behavior of high nitrogen austenitic stainless steel. The cold deformation strengthening process as an effective approach to obtain higher strength is usually used for manufacturing high nitrogen steel such as retaining ring [24]. When the high nitrogen steels with cold deformation microstructure are probably applied in the elevated temperature environment, the precipitation behavior of retaining ring with cold deformation microstructure could be complex and there may be a more obvious effect on the mechanical properties. So, it is necessary to investigate the effect of the cold deformation on the precipitation of high nitrogen austenitic stainless steel. In the present work, the effect of nitrogen and molybdenum content on the aging precipitation behavior of four kinds of high nitrogen austenitic stainless steel was researched. At the same time, the influence of cold deformation on the aging precipitation behavior of Mo-containing high nitrogen austenitic stainless steel was also investigated.
2 Experimental The chemical compositions of the investigated high nitrogen austenitic stainless steel materials 18Cr-16Mn2Mo-1.1N (HNS-A), 18Cr-16Mn-1.3N (HNS-B), 18Cr18Mn-2Mo-0.96N (HNS-C) and 18Cr-18Mn-2Mo0.77N (HNS-D) are given in Table 1. The HNS-A and HNS-B ingots with lower w(Mn) and higher w(N) above 1% were manufactured with a pressurized induction furnace under high purity nitrogen gas [18]. The HNS-C and HNS-D ingots were manufactured using a vacuum induction furnace by adding nitrided ferroalloy under nitrogen atmosphere and using an electroslag furnace for remelting under nitrogen atmosphere [25]. They were forged and hot rolled to a thickness of 6 mm. The HNS-A hot rolled plates were solution-treated at 1 100 °C for 90 min, HNS-B at 1 150 °C for 60 min, HNS-C and HNS-D at 1 100 °C for 60 min, followed by water
quenching to retain a homogeneous austenite structure. To obtain the initial time−temperature−precipitation (TTP) curve of aged HNS-A, HNS-B, HNS-C and HNS-D alloys which start with 0.05% precipitates volume fraction, the solution-treated specimens were aged at 650−950 °C for different time with salt bath and resistance furnace. The aging samples to obtain microstructures were treated with standard grinding and polishing techniques, and then electrolytic etched with 10% oxalic acid. The microstructures were observed using a Carl Zeiss optical microscope (OM) and a scanning electron microscope (SEM, SSX-550) at 15 kV. The Image-Pro Plus 5.0 (IPP 5.0) software for particle size calculation was used for statistical quantitative analysis of aging precipitates of the samples. The electronic microanalysis of transmission electron microscopy (TEM, TECNCI G2 20) at 200 kV was also carried out.
3 Results and discussion 3.1 Influence of nitrogen element 1) TTP curves of high nitrogen steels with different nitrogen contents Figure 1 shows the micrographs of HNS-A, HNS-C and HNS-D solution-treated under different conditions to obtain the single and homogeneous γ (austenite) phase without any intergranular precipitate except for a few inclusions. Figure 2 shows the results of quantitative analysis of the TTP curves obtained by Image-Pro Plus (IPP) 5.0 software. The “nose” temperature of precipitation of aged HNS-A and HNS-C is found to be 850 °C with an incubation period of 60 s. The “nose” temperature of precipitation of aged HNS-D is found to be 900 °C with an incubation period of 90 s. The results indicate that the “nose” temperature of HNS-A and HNS-C decreases with the increment of nitrogen content and the initial precipitating time becomes shortened. For the initial TTP curve which starts with 0.05% precipitates volume fraction, though the precipitating time of aged HNS-A at 900 °C is longer than that of HNS-C and HNS-D, it is shorter than that of HNS-C and HNS-D at 800 °C and 850 °C. This causes the formation of new “nose” temperature area, as shown in Fig. 2(a). With increasing the aging time, the TTP curve of aged HNS-A with 0.10% volume fraction of precipitates presents
Table 1 Chemical compositions of HNS-A, HNS-B, HNS-C and HNS-D (mass fraction, %) Alloy
C
Cr
Mn
Mo
Si
P
Al
Ni
O
N
Fe
HNS-A
0.050
18.02
16.10
1.96
0.03
≤0.03
