Phase Transformation and Carbide Precipitation of Functional Gradient Semi-solid 9Cr18 Steel

doi:10.1007/s40195-018-0723-7

Abstract

Abstract:

The unique phase transformation and carbide evolution in 9Cr18 steel were investigated during semi-solid forming and subsequent heat treatment. The functional gradient thixoforging 9Cr18 component was divided into inner area and edge area. Microstructure evolution was different at each area. After semi-solid cooling, the solid particles in the inner area were retained as meta-austenite. During annealing, M₂₃C₆ carbide began to precipitate when temperature reached 700 °C. Martensite transformation occurred when temperature reached 800 °C. The occurrence of M₂₃C₆ carbide and martensite structure would be harmful to the mechanical properties of inner area. In the edge area, the liquid underwent eutectic transformation to form bar-shape M₇C₃ carbide and secondary austenite after semi-solid cooling. The width of bar-shape carbide would decrease during annealing. By controlling the carbide evolution, we could tailor the functional gradient material with required property.

Key words: Semi-solid forming, Solid/liquid phase, Annealing, Carbide precipitation

Yong-Jin Wang, Ren-Bo Song, Ren-Feng Song. Phase Transformation and Carbide Precipitation of Functional Gradient Semi-solid 9Cr18 Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(8): 823-830.

Figures/Tables 10

Fig. 1 Temperature-time cycle of the whole process

Fig. 2Microstructures of thixoforging specimen a, and after different heat treatments: b550 °C, c 800 °C

Fig. 3 a Dilatometric curve of inner area of thixoforging specimen; b DSC curve of 9Cr18 billet

Fig. 4 XRD patterns from the inner area of specimens under different heat treatments

Fig. 5 Microhardness evolution of thixoforging specimen under different heat treatments

Fig. 6 TEM micrograph of thixoforging specimen a and after different heat treatments: b200 °C, c 550 °C, inner area (insets show the corresponding SAED pattern)

Fig. 7 a TEM micrograph of thixoforging specimen annealed at 700 °C, inner area (insets show the corresponding SAED pattern); b EDS analysis of M23C6

Fig. 8 TEM micrograph of thixoforging specimen annealed at a 800 °C, b 1050 °C, inner area (insets show the corresponding SAED pattern of the matrix)

Fig. 9 TEM micrographs of thixoforging specimen a and after different heat treatments: b550 °C, c 800 °C in the edge area, d EDS analysis of M7C3 (insets show the corresponding SAED pattern)

Fig. 10 a Compressive stress-strain curves of specimens after different heat treatments, b the analysis of mechanical values

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