Microstructure evolution of carbidic austempered ductile iron at different austempering temperatures
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Microstructure evolution of carbidic austempered ductile iron at different austempering temperatures Penghui Yang1,2,3 , Hanguang Fu1,* , Rafik Absi3,* , Rachid Bennacer3 Moumen Darcherif3 , Shengqiang Ma4 , Jian Lin1 , and Xingye Guo1
,
1
Key Laboratory of Advanced Functional Materials, Ministry of Education, School of Materials Science and Engineering, Beijing University of Technology, Number 100, Pingle Garden, Chaoyang District, Beijing 100124, People’s Republic of China 2 CY Cergy Paris University, 33 Boulevard du Port, 95011 Cergy Pontoise Cedex, France 3 Graduate School of Engineering, ECAM-EPMI, 13 Boulevard de l’Hautil, 95092 Cergy-Pontoise Cedex, France 4 School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi Province, People’s Republic of China
Received: 29 June 2020
ABSTRACT
Accepted: 4 November 2020
This work aims to elucidate the microstructure characteristics of each phase in carbidic austempered ductile iron and the nucleation and growth of acicular ferrite in an austempering temperature range of 250–450 C. The research results showed that the carbides (Fe, Cr)3C obtained by the eutectic reaction had high indentation hardness and indentation modulus, and the interface between acicular ferrite and eutectic carbides belonged to incoherent boundary. Since the nucleation driving force of acicular ferrite increased with an increase in temperature difference (DT), the size of acicular ferrite gradually decreased with the decrease in isothermal temperature. When the austempering temperature was above 400 C, acicular ferrite preferentially nucleated around the graphite nodules and inside grains. However, acicular ferrite preferentially nucleated around the graphite nodules and the eutectic carbides at temperature below 400 C. Additionally, a fine twin structure had been found inside some acicular ferrite after austempering at a low temperature (250 C).
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List of symbols D Diffusion coefficient C Concentration t Time x Distance Q Diffusion activation energy
R T D0 uac ucc g uc n
Gas constant Temperature Proportional constant Chemical potential of acicular ferrite Chemical potential of austenite Chemical potential of graphite Quantity of atoms in the embryo
Handling Editor: P. Nash.
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https://doi.org/10.1007/s10853-020-05543-0
J Mater Sci
DGV r e gn2/
Free energy difference of a single atom in the new phases and old phases Surface energy Average strain energy of a single atom in the nucleus Surface area of nucleus
3
DGD n0 GG c acc GcFe acFe xcc DT T0 DGN
Added value of free enthalpy for a single atom in the crystal defect Quantity of atoms provided by the defect to the crystal nucleus Free energy of graphite Activity of carbon in austenite Free energy of Fe in austenite Activity of Fe in austenite Content carbon in austenite Undercooling degree Austenitizing temperature The maximum driving force of nucleatio
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