Gradient Elastic-Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel

doi:10.1007/s40195-018-0710-z

Abstract

Abstract:

The gradient mechanical properties, variation of stress with strain and surface cracking behavior of expanded austenite developed on 316L austenitic stainless steel were investigated by nanoindentation tests, X-ray residual stress analysis and scanning electron microscope observation in four-point bending tests. The results show that the plastic properties of the carburizing layer including true initial yield strengths and strain hardening exponents increase significantly from substrate to surface, while the true elastic modulus just improves slightly. Due to the onset of plastic flow, the residual stresses are almost equivalent to the true initial yield strengths from surface to the depth of ~ 10 μm. The results of four-point bending tests show that surface stress increases linearly with the increase in strain until the strain reaches ~ 1.0%, after that the plastic yield happens. The expanded austenite surface layer is brittle, and the cracks will be created at the strain of ~ 1.4%. The cracking stress is about ~ 2.4 GPa.

Key words: Low-temperature gaseous carburization, Expanded austenite, Nanoindentation, Gradient elastic-plastic properties, Four-point bending

Yong Jiang, Yang Li, Yun-Fei Jia, Xian-Cheng Zhang, Jian-Ming Gong. Gradient Elastic-Plastic Properties of Expanded Austenite Layer in 316L Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(8): 831-841.

Figures/Tables 18

Fig. 1 Procedure of nanoindentation tests

Fig. 2 Schematic of loading device of four-point bending test

Fig. 3 Cross-sectional OM image of the 316L ASS treated by LTGC process

Fig. 4 Carbon content profiles along cross section depth direction of the 316L ASS treated by LTGC process

Fig. 5 Load-displacement curves at different depths away from the carburizing sample surface: a 4 μm, b 14 μm, c 23 μm, d 33 μm

Fig. 6 Impression in situ SPM images at different depths away from the carburizing sample surface: a 4 μm, b 14 μm, c 23 μm, d 33 μm

Fig. 7 H a, E b under different loads at different depths away from the carburizing sample surface

Fig. 8 Scale dependence of H a, E b at different depths away from the carburizing sample surface

Fig. 9 Distributions of H0 a, h H * b, E0, c, h E * , d along cross section depth direction of carburizing layer

Fig. 10 Distribution of H0/E0 along cross section depth direction of carburizing layer

Fig. 11 σy a, n b under different loads at different depths away from carburizing sample surface

Fig. 12 Relationship between σy and h obtained from reverse analysis: a 4 μm, b 14 μm, c23 μm, d 33 μm

Fig. 13 Distributions of σy0 and σre along cross section depth direction of the carburizing layer

Fig. 14 Cross-sectional SEM image of the 316L ASS treated by LTGC process

Fig. 15 Distributions of n a, εy b, hc/hmax, c and σy/E, d along cross section depth direction of carburizing layer

Fig. 16 Scaling relationships between hc/h and σy/E for representative values of n [30]

Fig. 17 Surface stress-strain curves for the 316L ASS treated by LTGC process

Fig. 18 SEM images of carburizing layer surface after four-point bending tests: a applied strain 1.41%, b applied strain 1.31%

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