Effects of Temperature and Strain Rate on Solid-/Liquid-Phase Flow Behavior of 9Cr18 Steel During Thixoforging

doi:10.1007/s40195-017-0544-0

Abstract

Abstract:

A thixoforging process of the 9Cr18 steel was conducted in a designed setup, and a kind of multi-diameter component was fabricated. The effects of the forming temperature and the strain rate on the solid-/liquid-phase flow behavior were discussed. The results showed that functional gradient properties of the 9Cr18 steel could be obtained after thixoforging. Changes of microstructure along radial direction could be obtained. Solid austenite was retained after fast cooling, and the liquid film enriched in alloying elements was extruded outside to form a dendrite skin layer. As temperature increased, more molten liquid formed during thixoforging. A heterogeneous flow phenomenon was activated as free liquid channels were formed. The macro-separation of solid and liquid phases was critical for the formation of functional gradient properties. Above 1300 °C, full dendrite skin layer could be formed. The strain rate affected the thixotropic property via influencing the deformation time of thixoforging. In the presence of lower strain rates, there was more time for the flow of liquid metal, which was the key to the extension of the thixotropic stage. High temperatures and low strain rates contributed to the formation of full skin layer for the designed specimen. The average thickness of skin layer for current specimen could be over 1000 μm when thixoforged at 1340 °C and under a strain rate of 0.02 s^-1.

Key words: 9Cr18 steel, Thixoforging, Processing parameters, Functional gradient property

Yong-Jin Wang, Ren-Bo Song, Ya-Ping Li, Qiang Chen. Effects of Temperature and Strain Rate on Solid-/Liquid-Phase Flow Behavior of 9Cr18 Steel During Thixoforging[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(6): 567-575.

Figures/Tables 14

Fig. 1 Microstructure of 9Cr18 semi-solid billet

Fig. 2 DSC curves of 9Cr18 billet a and liquid fraction-temperature curve obtained by DSC b

Fig. 3 Schematic illustration of the thixoforging setup. 1 AC supply; 2 copper electrode; 3 punch; 4 tantalum piece; 5 ceramic die; 6 R-type thermocouple; 7 semi-solid specimen

Fig. 4 Temperature diagram of the thixoforging process

Table 1 Experimental parameters of thixoforging

Condition	Values
Temperature (°C)	1280	1300	1320	1340
Corresponding liquid fraction (f_l)	0	12.9%	18.1%	25.3%
Strain rate (s^-1)	0.02	0.05	0.1	1
Corresponding ram speed (mm/s)	0.6	1.5	3	30

Fig. 5 Microstructure of thixoforging specimen at 1340 °C, 0.02 s-1, a-f are positions in the illustrated specimen, respectively

Fig. 6 EBSD mapping results of the inner area of a specimen thixoforged at 1340 °C, 0.02 s-1: a IPF image; b phase image (blue: austenite; red: martensite)

Fig. 7 Microstructures of the boundary area of a specimen thixoforged at 1340 °C, 0.02 s-1: a solid/liquid boundary; b partial zoom of a; c EDS line scanning of line marked in a

Fig. 8 Load-displacement curves during thixoforging at 0.05 s-1 at different temperatures

Fig. 9 Microstructure of the edge area of specimens thixoforged at 0.05 s-1: a 1280 °C; b 1300 °C; c 1320 °C; d 1340 °C

Fig. 10 Average thickness of the skin layer of specimens thixoforged at: a different temperatures under 0.05 s-1; b different strain rates under 1340 °C (average thickness: value obtained from the average of three edge areas as shown in Fig. 5c-e)

Fig. 11 Inner area of a specimen thixoforged at 1300 °C

Fig. 12 Load-displacement curves during thixoforging at 1340 °C under different strain rates

Fig. 13 Microstructure of thixoforging specimen at the edge area, thixoforged at 1340 °C: a 0.02 s-1; b 0.05 s-1; c 0.1 s-1; d 1 s-1

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