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Acta Metall Sin  2017, Vol. 53 Issue (5): 531-538    DOI: 10.11900/0412.1961.2016.00321
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Effects of Hot Working on the Microstructure and Thermal Ageing Impact Fracture Behaviors of Z3CN20-09MDuplex Stainless Steel
Hai ZHANG,Shilei LI,Gang LIU,Yanli WANG()
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
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Abstract  

Duplex stainless steels are widely used in nuclear industry for their excellent mechanical behavior, good weldability and superior ressistance to corrosion, the fracture toughness of which will be deteriorated with ageing time, as they are exposed to a certain temperature (204~538 ℃). In the present work, hot forging will be employed to induce the change of ferrite grain orientation and refinement of austenite grains; it is expected to improve the impact toughness after long-term thermal ageing. The microstructure and impact surface morphology of Z3CN20-09M duplex stainless steel were investigated by using SEM and EBSD. The micro-mechanical properties and impact properties of Z3CN20-09M duplex stainless steel at different thermal ageing time were tested by a nano-indenter and an instrumented impact tester. The results show that the crystal orientation of ferrite changes obviously and the austenite is changed from the original coarse columnar grains to the fine equiaxed grains after hot working. The im pact toughness of cast materials and forged materials decreases greatly with ageing time. The charpy impact energy of both aged and unaged forged-materials is higher than that of cast material. Cast material and forged material exhibit microvoid coalescence fracture in the early of thermal ageing; after 3000 h thermal ageing, the impact fracture features changes from ductile dimples to brittle cleavages in ferrites and tearings or dimples in austenites. However, cleavage features in forged material are significantly less than those in cast material due to the difference in ferrite crystal orientation.

Key words:  duplex stainless steel      hot working      thermal ageing      fracture      grain orientation     
Received:  20 July 2016     
Fund: Supported by National Natural Science Foundation of China (No.51601013) and State Key Lab for Advanced Metals and Materials (No.2015-ZD09)

Cite this article: 

Hai ZHANG,Shilei LI,Gang LIU,Yanli WANG. Effects of Hot Working on the Microstructure and Thermal Ageing Impact Fracture Behaviors of Z3CN20-09MDuplex Stainless Steel. Acta Metall Sin, 2017, 53(5): 531-538.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00321     OR     https://www.ams.org.cn/EN/Y2017/V53/I5/531

Fig.1  SEM-BSE images of Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b)
Fig.2  EBSD images of Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction (b)
Fig.3  EBSD images of Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h(a) phase distribution map(b) inverse pole figure(c) image of grain boundary character distribu- tion
Fig.4  Change of nano-hardness with ageing time for austenite and ferrite in Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b)
Fig.5  Load- displacement curves of ferrite in Z3CN20-09M duplex stainless steel before (a) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b) at different ageing time
Fig.6  Change of Charpy-impact energy with ageing time for Z3CN20-09M duplex stainless steel before and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h
Fig.7  Fracture surface morphologies of Z3CN20-09M duplex stainless steel before (a, c) and after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (b, d) aged for 1000 h (a, b) and 3000 h (c, d)
Fig.8  EBSD image of ferrite in cast Z3CN20-09M duplex stainless steel
Fig.9  Illustration of the impact fracture process of cast Z3CN20-09M duplex stainless steel after thermal ageing (Arrows mark ferrite cleavage along (001) plane, and white dotted lines mark tearing in austenite)
Fig.10  Cleavage facets formed by fracturing of ferrites with the same orientation (as circled by yollow lines) in Z3CN20-09M duplex stainless steel
Fig.11  EBSD image of ferrite in Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h
Fig.12  Illustration of the impact fracture process of thermal aged Z3CN20-09M duplex stainless steel after 20% hot forged reduction+solution treatment at 1050 ℃ for 1 h (Arrows mark ferrite cleavage along (001) plane, and white dotted lines mark microvoid coalescence in austenite)
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