Influence mechanism of boron segregation on the microstructure evolution and hot ductility of super austenitic stainless steel S32654

doi:10.1016/j.jmst.2021.11.005

Abstract

Abstract:

Influence mechanism of B segregation on the microstructure evolution and hot ductility of S32654 at 850-1250 °C was systematically investigated through experimental research and theoretical calculation. The results demonstrated that the segregation of B at grain boundary (GB) played different roles in the microstructure evolution and hot ductility at various temperatures. At 850 °C, B segregation inhibited Mo segregation at the GB and enhanced the GB cohesion. At 900-950 °C, B segregation restricted the diffusion and segregation of Mo to the GB, inhibiting the precipitation of σ phase. At 1000-1050 °C, B segregation accelerated the dislocation accumulation and limited the GB migration, promoting the nucleation and inhibiting the growth of DRX grains. At 1100-1150 °C, B has little effect on the DRX due to sufficient energy supply by higher temperature. Under the above beneficial effects of B, the hot ductility of S32654 was improved to varying degrees at 850-1150 °C. However, as the temperature increased to 1200-1250 °C, B segregation decreased the solidus temperature and enhanced the liquefaction cracking tendency, resulting in a deterioration of the hot ductility.

Key words: Super austenitic stainless steel, Boron, Hot ductility, Grain boundary segregation, Atom probe tomography

Jiangtao Yu, Shucai Zhang, Huabing Li, Zhouhua Jiang, Hao Feng, Panpan Xu, Peide Han. Influence mechanism of boron segregation on the microstructure evolution and hot ductility of super austenitic stainless steel S32654[J]. J. Mater. Sci. Technol., 2022, 112: 184-194.

Figures/Tables 17

Table 1. Chemical compositions (wt.%) of the two S32654 steels used in this study.

Steel	C	Si	Mn	P	S	Cr	Ni	Mo	Cu	N	B	Fe
B0	0.013	0.38	2.95	0.005	0.002	24.48	22.51	7.32	0.48	0.50	0	bal.
B20	0.013	0.39	2.97	0.005	0.002	24.46	22.53	7.33	0.49	0.50	0.0020	bal.

Fig. 1. Thermal schedule of the hot tensile tests.

Fig. 2. Typical microstructures of the two S32654 steels after solution treatment at 1200 °C for 300 s: (a) B0 and (b) B20.

Fig. 3. (a) Hot ductility and (b) peak stress (σp) of S32654 steels versus temperature.

Fig. 4. SEM images of fracture morphology of S32654 steels: (a) B0-1000 °C, (b) B0-1250 °C, (c) B20-1000 °C and (d) B20-1250 °C.

Fig. 5. Cross-section microstructure near the fracture surface of S32654 steels strained at different temperatures: (a) B0-950 °C, (b) B0-1000 °C, (c) B0-1150 °C, (d) B0-1250 °C, (e) B20-950 °C, (f) B20-1000 °C, (g) B20-1150 °C and (h) B20-1250 °C.

Fig. 6. SEM images of re-solidified liquid zone near the fracture surface of S32654 steels strained at 1250 °C: (a) B0 and (b) B20.

Fig. 7. SEM and TEM images of cross-section microstructures near the fracture surface of S32654 steels strained at different temperatures: (a) B0-950 °C, (b) B0-1000 °C, (c) σ phase in B0-950 °C, (d) B20-950 °C, (e) B20-1000 °C and (f) σ phase in B20-950 °C.

Fig. 8. Size distribution of the intergranular precipitates in the B0 and B20 steels strained at different temperatures: (a) 950 °C and (b) 1000 °C.

Fig. 9. (a-f) Representative inverse pole maps and grain average misorientation maps of microstructures near the fracture surface in the B0 and B20 steels strained at different temperatures (the black arrows indicate the tensile direction), (g) color code and (h) volume fraction of different types of grains.

Fig. 10. Variation of (a) number density of DRX grain and (b) DRX grain size in the B0 and B20 steels strained at different deformation temperatures.

Fig. 11. APT analysis of element distribution across the GB in the B20 steel deformed at 950 °C: (a) 3D atomic distribution maps and (b) 1D concentration profile of elements.

Fig. 12. (a) Simulational models of FCC-Fe Σ5(210) structure (1-7 indicate the sites of B and Mo atoms), (b) segregation energy of B and Mo atom at different sites, (c) separation energy and (d) theoretical tensile strength.

Table 2. Fracture energy and theoretical tensile peak stress for different systems.

System-Σ5(210)	Fracture energy (J/m²)	Theoretical tensile peak stress (GPa)
Fe-Mo	5.93	34.99
Fe-Mo (B)	6.10	36.80

Fig. 13. Effect of B and Mo content on the driving force for σ phase.

Fig. 14. KAM analyses of S32654 steels strained at different temperatures: (a) B0-1000 °C, (b) B20-1000 °C, (c) B0-1150 °C and (d) B20-1150 °C.

Fig. 15. Effect of B content on the solidus and liquidus temperatures of S32654.

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