1 School of Maerials Science and Engineering Shenyang University of Technology, Shenyang 110870, China 2 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
Sihan CHEN,Tian LIANG,Long ZHANG,Yingche MA,Zhengjun LIU,Kui LIU. Study on Evolution Mechanism of bcc Phase During Solution Treatment in 6%Si High Silicon Austenitic Stainless Steel. Acta Metall Sin, 2017, 53(4): 397-405.
After decades of development, high silicon austenitic stainless steels are widely concerned about due to their excellent corrosion resistance and good mechanical properties. Till now, 4%Si high silicon stainless steel has been widely used, but it is not doing well under the condition of high temperature and strong oxidizing medium. 6%Si high silicon austenitic stainless steels can resist in the strong oxidizing medium when the temperature is up to 100 ℃. But the increasing of Si may lead to the increasing of precipitation such as bcc phase, which may cause hot cracks during heat processing. As a result, obtaining a temperature range which is without precipitation is essential. The bcc phase evolution mechanism of 6%Si as-cast high silicon austenitic stainless steel under different solid solution treatment temperature was investigated by means of OM, SEM, XRD and TEM in this work. In order to study the precipitation and re-dissolution of bcc phase, the distribution of alloy elements, morphology and crystal structure of the bcc phase were analyzed under different solution treatments. Moreover, the heat-treated schedules were made based on the experimental results. The results indicated that the solid solution treatment temperatures had a great influence on the microstructure of 6%Si high silicon austenitic stainless steel. The precipitates existed in the as-cast structure were mainly bcc phase with a lattice constant of 0.8747 nm, rich in Mo, Si and Ni elements, and distributed in grain interior and grain boundary. The bcc phase redissolved during the solution when the temperature was between 1050~1200 ℃ for 2 h. The contents of Mo, Si and Ni increased with the rising solution temperature. Furthermore, the bcc phase re-precipitated when the test specimen was heat treated at 1250 ℃ for 2 h. The re-precipitated phase has the same composition with that in the as-cast structure. Thus the optimal solid solution treatment temperature of 6%Si high silicon austenitic stainless is 1100~1200 ℃ for 2 h.
Fig.1 Mass fraction of phases as a function of temperature in 6%Si high silicon austenitic stainless steels at thermodynamic equilibrium state (1—γ, 2—G, 3—Laves, 4—M6C, 5—bcc, 6—liquid, 7—Cr3Si, 8—σ) (a) whole graph (mass fraction ranged from 0 to 100%) (b) amplification graph (mass fraction ranged from 0 to 10%)
Fig.2 Solidification characteristics of bcc phase in 6%Si high silicon austenitic stainless steels at thermodynamic equilibrium state
Fig.3 Low (a, b) and high (c, d) magnified OM images of edge (1/2 radius) (a, c) and core (b, d) in as-cast 6%Si high silicon stainless steel
Fig.4 SE (a) and BSE (b) images of as-cast 6%Si high silicon stainless steel
Fig.5 XRD spectrum of as-cast 6%Si high silicon stainless steel
Fig.6 TEM image and SAED pattern (inset) of precipitate of as-cast 6%Si high silicon stainless steel
Phase
Si
Cr
Mn
Fe
Ni
Mo
Total
bcc
10.28±0.38
20.47±1.07
1.96±0.79
35.92±1.35
26.50±1.01
4.86±0.31
100.00
γ
5.51±0.39
19.22±0.37
1.22±0.15
50.14±1.13
22.48±0.34
0.93±0.15
100.00
Table 1 Chemical compositions of precipitations in as-cast 6%Si high silicon stainless steel(mass fraction / %)
Fig.7 SEM images of the bcc phase in 6%Si high silicon stainless steel after solution treatment at 1050 ℃ (a), 1100 ℃ (b), 1150 ℃ (c), 1200 ℃ (d) and 1250 ℃ (e, f) for 120 min
Fig.8 Volume fraction of precipitates under different solution treatment temperatures
Temperature
Point
Si
Cr
Mn
Fe
Ni
Mo
Total
℃
1050
1
8.82±0.41
23.57±0.60
1.23±0.25
40.34±1.94
17.63±0.91
8.40±1.35
100
1100
2
9.00±0.50
23.79±0.59
1.38±0.22
38.25±0.57
18.43±0.52
9.16±0.49
100
1250
3
11.74±1.10
18.27±1.41
1.66±0.40
32.46±1.89
26.87±1.42
4.64±1.50
100
Table 2 EDS analyses of bcc phases in Fig.7 (mass fraction / %)
Fig.9 XRD spectrum of solution treatment sample(1250 ℃, 120 min) of 6%Si high silicon stainless steel
Fig.10 TEM image and SAED pattern (inset) of 6%Si high silicon stainless steel after heat treated at 1250 ℃ for 120 min
Fig.11 DSC curves of 6%Si high silicon austenitic stainless steel
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SONG Xueyan; LEI Yongquan; ZHANG Xiaobin; ZHANG Ze; CHEN Lixin; YANG Xiaoguang; LU Guanglie; ZHANG Wenkui; WANG Qidong (Department of Materials Science and Engineering; Zhejiang University ; Hangzhou; 310027)(Beijing Laboratory of Electron Microscopy; Center for Condensed Matter Physics; The Chinese Academy of Sciences; P. O. Box 2724; Beijing 100080)(Centeral Laboratory of Hangzhou University; Hangzhou 310028)Correspondent: SONG Xueyan; Tel. (0571) 7951406; Fax: (0571)7951152;E-mail: msecheny@dial. zju. edu. cn. EFFECT OF Ti ON THE MICROSTRUCTURE AND ELECTROCHEMICAL PROPERTIES OF Zr-Mn-V-Ni ALLOYS[J]. 金属学报, 1998, 34(9): 977-982.
