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Acta Metall Sin  2018, Vol. 54 Issue (1): 55-64    DOI: 10.11900/0412.1961.2017.00291
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Effect of Nitrogen Content on Precipitation Behavior and Mechanical Properties of Mn18Cr18NAustenitic Stainless Steel
Fengming QIN, Yajie LI, Xiaodong ZHAO, Wenwu HE, Huiqin CHEN()
School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China
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Mn18Cr18N austenitic stainless steel with excellent mechanical properties and corrosion resistance is widely used in nuclear industries, power plants and medicine field. However, precipitation of the second phases during hot deformation deteriorates the mechanical properties and hot formability. In order to clarify the precipitation behavior of this steel, the precipitation behavior and its influence on mechanical properties of Mn18Cr18N austenitic stainless steel with different nitrogen contents were investigated by JmatPro software, OM, SEM and TEM analytical methods. The results indicate that precipitation phases consist of Cr2N and a few M23C6, in which Cr2N preferentially precipitates along grain bound aries and then grows up to the interior of austenite grain by discontinuous cellular. With increasing of ageing temperature, the precipitation of Cr2N became more sensitive. When the nitrogen content increases to 0.7%, the most sensitive precipitation temperature of Cr2N is 750 ℃ with an incubation period of 10 min. However, M23C6 mainly precipitates by granular at austenitic grain boundaries and maintains cube-on-cube orientation relationship with adjacent austenite grain. The results of mechanical property test indicate that the precipitation of Cr2N has a negligible effect on strength and obvious deterioration on plasticity of Mn18Cr18N austenitic stainless steel. The precipitation of Cr2N after ageing treatment leads to remarkable decrease in elongation and reduction of area, and the elongation reduced from 52.9% to 27.7%. Meanwhile, fracture mode also transformed from ductile fracture to intergranular fracture and transgranular fracture with the increasing of Cr2N. TEM analysis shows that solution treatment sample reveals good plastic deformation ability and coordinates deformation by slip and twinning, simultaneously. Nevertheless, dislocations slipped, propagated and eventually piled up between lamellas of Cr2N and around granular M23C6 after ageing treatment, which induce the degeneration of the plastic deformation capacity of Mn18Cr18N austenitic stainless steel.

Key words:  Mn18Cr18N austenitic stainless steel      precipitation behavior      microstructure      mechanical property     
Received:  12 July 2017     
ZTFLH:  TG142.71  
Fund: Supported by National Natural Science Foundation of China (No.51575372), Natural Science Foundation of Shanxi Province (No.2014011015-4) and Science and Technology Research Plan (Industrial) Project of Shanxi Province (No.201603D121006-2)

Cite this article: 

Fengming QIN, Yajie LI, Xiaodong ZHAO, Wenwu HE, Huiqin CHEN. Effect of Nitrogen Content on Precipitation Behavior and Mechanical Properties of Mn18Cr18NAustenitic Stainless Steel. Acta Metall Sin, 2018, 54(1): 55-64.

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Fig.1  Schematic of tensile test specimen (unit: mm)
Fig.2  Phase diagrams of Mn18Cr18N steel with different nitrogen contents (a) 0.5N (b) 0.6N (c) 0.7N
Fig.3  Time-temperature-transformation (TTT) curves for Mn18Cr18N steel with different nitrogen
contents (a) 0.5N (b) 0.6N (c) 0.7N
Fig.4  Microstructures of Mn18Cr18N steel aged at different conditions(a~d) Mn18Cr18N0.5 steel aged at 700 ℃ for 2 h, 4 h, 8 h, 24 h (e~h) Mn18Cr18N0.6 steel aged at 750 ℃ for 2 h, 4 h, 8 h, 24 h (i~l) Mn18Cr18N0.7 steel aged at 750 ℃ for 10 min, 1 h, 4 h, 24 h
Fig.5  SEM images of Cr2N precipitation in Mn18Cr18N steel aged at 750 ℃ for 8 h (a) 0.5N (b) 0.6N (c) 0.7N
Fig.6  TEM images and EDS analyses of precipitations of Mn18Cr18N0.7 steel aged at 750 ℃ for 24 h(a) lamellar Cr2N and SAED pattern (inset) (b) M23C6 and SAED pattern (inset)(c) EDS of Cr2N (d) EDS of M23C6
Fig.7  Stress-strain curves of Mn18Cr18N steel(a) solution treated (b) aged at 750 ℃ for 8 h
Fig.8  SEM images of tensile fracture surfaces of Mn18Cr18N alloy with 0.5N (a, b), 0.6N (c) and 0.7N (d) solution treated (a) and aged at 750 ℃ for 8 h (b~d)
Fig.9  Microstructures of tensile fracture surfaces of Mn18Cr18N0.7 steel with solution treated (a) and aged at 750 ℃ for 1 h (b), 4 h (c) and 8 h (d)
Fig.10  TEM images of tensile sample fracture of Mn18Cr18N0.7 alloy with solution treated (a, b) and aged at 750 ℃ for 24 h (c, d) (Inset shows SAED pattern of deformation twins)
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