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Acta Metall Sin  2018, Vol. 54 Issue (1): 39-46    DOI: 10.11900/0412.1961.2017.00209
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Deformation Behavior and Microstructure Evolution of 9Cr18 Alloy During Semi-Solid Compression
Yongjin WANG1,2, Renbo SONG1(), Renfeng SONG3
1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
2 Institute of Industrial Science, The University of Tokyo, Tokyo 1538505, Japan
3 Ansteel Mining Engineering Corporation, Anshan 114004, China
Cite this article: 

Yongjin WANG, Renbo SONG, Renfeng SONG. Deformation Behavior and Microstructure Evolution of 9Cr18 Alloy During Semi-Solid Compression. Acta Metall Sin, 2018, 54(1): 39-46.

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Abstract  

The compression behavior during semi-solid state is a fundamental basis for the following rheoforming or thixoforming. Coexist of solid/liquid phase leads to the unique deformation behavior. The chemical composition at each phase is different from conventional forming process. Deformation behavior and microstructure evolution are determined by various effects such as initial state, heating, cooling, etc. In this work, the semi-solid compression tests of 9Cr18 as hot-rolled material and semi-solid billet were conducted, respectively. Microstructure evolution during heating, semi-solid state, deformation and cooling was investigated by OM and SEM. Solid/liquid flow behavior and the relationship of stress-strain were analyzed. The results showed the preparation of semi-solid billet is essential for the uniformity of solid particle and liquid phase, which would help to demonstrate the flow behavior. Only heating the as hot-rolled material to semi-solid led to the banded precipitation of liquid phase. The banded melting of as hot-rolled material made it hard for liquid phase to connect with each other. Liquid flow only happened in partial area and plastic deformation of solid particles was the main deformation behavior. The stress increased at the final stage. As for semi-solid billet, solid particles and liquid film coexisted uniformly. Macro separation of solid/liquid occurred as deformation came into thixotropic stage. Liquid flew towards outside and solid particles rotated, thus leading to the decrease of stress. Microstructure evolution at semi-solid state was different from conventional heat treatment. Solid austenite particles at semi-solid state could dissolve more alloying elements than normal austenization (1050 ℃). This phenomenon would help to improve the stability of austenite and over-saturated meta-austenite was obtained after cooling. The special microstructure evolution during semi-solid state might provide a possible way to design a new heat treatment procedure.

Key words:  9Cr18 alloy      semi-solid state      mechanical property      microstructure evolution     
Received:  01 June 2017     
ZTFLH:  TG142.71  
Fund: Supported by National Natural Science Foundation of China (No.51175036)

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00209     OR     https://www.ams.org.cn/EN/Y2018/V54/I1/39

Fig.1  Schematic of semi-solid compression test
Fig.2  Initial microstructures of as hot-rolled state (a) and semi-solid billet (b)
Fig.3  Thermo-Calc results of Fe-C-Cr alloy(a) pseudo-binary cut (b) property diagram of phases
Fig.4  Microstructures after rapid cooling from semi-solid state(a) as hot-rolled material (b) semi-solid billet
Fig.5  True stress-true strain curves of 9Cr18 as hot-rolled material at 1 s-1 (a) and 1300 ℃ (b)
Fig.6  True stress-true strain curves of 9Cr18 semi-solid billet at 1 s-1 (a) and 1300 ℃ (b)
Fig.7  Microstructures in the center of 9Cr18 as hot-rolled material at 1300 ℃, 1 s-1 after the semi-solid reduction rates of 20% (a), 40% (b) and 60% (c) (Insets show the configurations of compressed specimens)
Fig.8  Microstructures in the center (a~c) and at the edge (d~f) of 9Cr18 semi-solid billet at 1300 ℃, 1 s-1 after the semi-solid reduction rates of 20% (a, d), 40% (b, e) and 60% (c, f) (Insets show the configurations of compressed specimens)
Fig.9  Inverse pole figures of 9Cr18 traditional heat treatment specimen (a) and semi-solid billet after deformation (b)
Fig.10  EDS results of 9Cr18 traditional martensite lath (a) and semi-solid particles (b) (Insets show the microstructures of related area, E—energy)
Fig.11  Illustration of microstructure evolution for different initial states
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