1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China 3 College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Cite this article:
Jialong TIAN,Yongcan LI,Wei WANG,Wei YAN,Yiyin SHAN,Zhouhua JIANG,Ke YANG. ALLOYING ELEMENT SEGREGATION EFFECT IN A MULTI-PHASE STRENGTHENED MARAGING STAINLESS STEEL. Acta Metall Sin, 2016, 52(12): 1517-1526.
Maraging stainless steels are the most widely used high strength stainless steels because of their excellent combination of high strength, superior corrosion resistance and good weldability. The typical heat treatment of maraging stainless steel consists of solution treatment and the following aging treatment. Aging treatment is the important process since it affects the steel's final properties and then determines its application prospect. Thus, understanding well the segregation behavior of alloying elements during the aging treatment plays a key role in developing the new maraging stainless steel with superior properties. In this work, segregation of alloying elements as well as its effect on mechanical properties and corrosion resistance of a multi-phase strengthened maraging stainless steel was studied by HRTEM and APT analyses. It was found that three precipitating species including Mo-rich R′ phase, η phase and Cr-rich α′ phase were identified in the steel. A unique core-shell structure with membrane-like R′ phase formed on the surface of η phase was identified however α′ phase distributed in the matrix separately. The core-shell structure enabled the maraging stainless steel a superior over-aging resistance and since aging time has reached 40 h, the characteristics of precipitations change little even aging time prolongs to 100 h. The corrosion test results indicated that the occurrence of α′ phase resulted in the formation of Cr-depleted zone and deteriorated the corrosion resistance seriously. In conclusion, the segregation behavior of alloying elements in maraging stainless steel has a significant effect on both mechanical property and corrosion resistance although some underlying mechanisms still haven't been understood well.
Fund: Supported by National Natural Science Foundation of China (No.51201160) and Science and Technology Innovation Foundation from Institute of Metal Research, Chinese Academy of Sciences (No.2015-ZD04)
Fig.1 Bright-field TEM images of maraging stainless steel after cryogenic treatment (a) and aging at 500 ℃ for 3 h (b)
Fig.2 Bright-field TEM (a) and HRTEM (b) images of maraging stainless steel after aging at 500 ℃ for 12 h, FFT result of η phase (c) and SAED pattern of the matrix (d)
Fig.3 Low (a) and high (b) magnified TEM images and corresponding EDS analyses of maraging stainless steel after aging at 500 ℃ for 100 h
Fig.4 Three-dimensional reconstructions of Co (a), Ni (b), Cr (c), Mo (d) and Ti (e) after cryogenic treatment
Fig.5 Distributions of precipitates in maraging stainless steel aged at 500 ℃ for 3 h (a), 12 h (b), 20 h (c), 40 h (d) and 100 h (e) (30 nm×30 nm×50 nm) (purple: Fe atom; blue: isosurface containing 30%Cr (atomic fraction); green: isosurface containing 35%(Ni+Ti); red: isosurface containing 15%Mo)
Aging time / h
Radius / nm
Number density / m-3
Atomic fraction of Ni / %
3
2.4
1.4×1023
59.63
12
3.5
7.9×1022
63.12
20
4.8
7.3×1022
63.34
40
5.7
3.8×1022
68.86
100
5.8
4.0×1022
69.58
Table 1 Evolution of η phase during the aging treatment at 500 ℃
Fig.6 One-dimensional concentration profile along the isosurface containing 35%(Ni+Ti) of maraging stainless steel aged at 500 ℃ for 12 h
Fig.7 Fluctuation of atomic concentration of Cr in maraging stainless steel during aging process
Fig.8 Macrostructures of maraging stainless steel after cryogenic treatment (a, c) and peak aging treatment (b, d) before (a, b) and after (c, d) salt spray test for 144 h
Fig.9 SEM images of maraging stainless steel after cryogenic treatment (a) and peak aging treatment (b) after salt spray test for 144 h
Fig.10 XPS concentration-depth profiles (a, c) and XPS spectra (b, d) of the passive (oxide) film on maraging stainless steel samples after cryogenic treatment (a, b) and peak aging treatment (c, d) after salt spray test for 144 h
Fig.11 Schematic of evolution of precipitations in maraging stainless steel during aging at 500 ℃
Fig.12 XRD spectra of the peak aged tensile specimens taken from the near fracture and clamped end
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