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Acta Metall Sin  2016, Vol. 52 Issue (6): 641-648    DOI: 10.11900/0412.1961.2015.00590
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EVOLUTION BEHAVIOR OF LAVES PHASE IN FB2 MARTENSITIC STAINLESS STEEL DURING WELDING
Kejian LI1,Zhipeng CAI1,2,3(),Yifei LI1,Jiluan PAN1
1 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China .
2 State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.
3 Collabrative Innovation Center of Advanced Nuclear Energy Technology, Tsinghua University, Beijing 100084, China
Cite this article: 

Kejian LI,Zhipeng CAI,Yifei LI,Jiluan PAN. EVOLUTION BEHAVIOR OF LAVES PHASE IN FB2 MARTENSITIC STAINLESS STEEL DURING WELDING. Acta Metall Sin, 2016, 52(6): 641-648.

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Abstract  

Elevating steam parameters is the key to enhance the efficiency of fossil power plants, reducing fuel consumption and noxious emission. Therefore, a lot of new creep resistant martensitic stainless steels have been developed, among which FB2 steel (a new 9%Cr martensitic stainless steel) is the most promising candidate for manufacturing steam turbine rotors operated at temperature range from 600 ℃ to 650 ℃. In the present work, the evolution behavior of Laves phase in the as received FB2 steel was studied by thermal simulation technique. Firstly, some sparse micron-sized particles of Laves phase were observed in as received FB2 steel by SEM. It was concluded that the large Laves phase particles formed in casting due to dendritic segregation. Then constitutional liquation resulting from eutectic reaction between Laves phase and γ-Fe in the heating process of welding thermal simulation was found, suggesting a liquation crack tendency in heat affected zone of FB2 steel. In the specimens experiencing thermal simulation, some eutectic microstructures were observed by SEM. Based on the results of EDS analysis and SAED, the two eutectic constituents were identified as χ phase and γ-Fe. At last, the reason for the difference in morphology of eutectic microstructures between specimens experiencing thermal simulation with different peak temperatures was analyzed.

Key words:  FB2 steel      Laves phase      constitutional liquation      χ       phase      hot crack tendency      welding     
Received:  16 November 2015     
Fund: Supported by Shanghai Science and Technology Committee (No.13DZ1101500) and Tribology Science Fund of State Key Laboratory of Tribology of Tsinghua University (No.SKLT2015A02)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00590     OR     https://www.ams.org.cn/EN/Y2016/V52/I6/641

Fig.1  Process of preparing the TEM specimen of eutectic microstructure by focused ion beam (FIB), sequence is (a~f)
Fig.2  SEM-SE image (a) and EDS result (b) of Laves phases (arrows) in FB2 steel in as received condition
Fig.3  SEM images of Laves phase in specimens and EDS analysis results (insets) after thermal simulation at peak temperature 1150 ℃ (a) and 1200 ℃ (b)
Fig.4  SEM images of specimen experiencing peak temperature of 1250 ℃

(a) discontinuous band of microstructure, some of which presents eutectic morphology (Arrows show the discontinuous eutectic microstructures and inset shows the highly magnified image)

(b) discretely distributed eutectic microstructures (Arrows show particles surrounding the netlike microstructures and inset shows EDS analysis result)

Fig.5  SEM images of specimen experiencing peak temperature of 1350 ℃

(a) continuous eutectic microstructure (Inset shows the highly magnified image, and arrows show the strip shaped microstructures at grain boundaries)

(b) micro topography of eutectic microstructure (Inset shows the EDS analysis result of eutectic microstructures, and arrows show the strip shaped microstructures at grain boundaries)

Fig.6  Equilibrium solidification path of the netlike eutectic constituent and II in Fig.7a
Fig.7  TEM results of eutectic microstructure in specimen experiencing thermal simulation with peak temperature of 1350 ℃ (a) bright field TEM image of eutectic microstructure (Arrows 1~5 show the netlike eutectic constituent and arrows I~IV show the other eutectic constituent inside the net mesh) (b) SAED patterns of points 1, 2, I
Material Fe Cr Mo Ni Ref.
CF-8M austenitic stainless steel 45 26 20 4 [22]
316 austenitic stainless steel 52 21 22 5 [25]
FB2 martensitic stainless steel EDS 1 52.60 23.28 20.36 0 This work
EDS 2 54.15 22.83 20.03 0 This work
Table 1  Comparison of chemical composition of χ phase in references and the eutectic constituent in this work (mass fraction / %)
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