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Acta Metall Sin  2015, Vol. 51 Issue (11): 1333-1340    DOI: 10.11900/0412.1961.2015.00121
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MECHANISM OF CRACK NUCLEATION AND PROPA- GATION OF FERRITE DUCTILE IRON DURING IMPACT FRACTURE UNDER LOW TEMPERATURES
Xinning ZHANG,Yingdong QU,Rongde LI,Junhua YOU
School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870
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Abstract  

Due to its excellent ductility and moderate strength, QT400-18L ferrite ductile iron has been widely used in producing core components of wind power equipment such as the hub of a wind turbine. Most of the researches have focused on the exploration of mechanical properties at low temperature, but none of them give the explanation on microcosmic mechanism of ductile iron during low temperature impact and the mechanism of crack nucleation and propagation of ferrite ductile iron during impact fracture has not been analyzed. In this work, the impact toughness of QT400-18L ferrite ductile iron was measured by V-notch Charpy impact test at different temperatures, the influence of low temperature impact toughness and the fracture behavior of ferrite ductile iron were discussed. The results show that the cleavage fracture resistance of QT400-18L ferrite ductile iron is reduced with the decrease of impact temperatures. Above ductile-brittle transition temperature (DBTT), most of the total fracture energies are expended during the crack propagation process. Below DBTT, both crack initiation energy and crack propagation energy decrease obviously. By using in situ fracture metallographic observation method, crack initiation and propagation of QT400-18L ferrite ductile iron under different temperatures were analyzed. Above DBTT, graphite nodules play the role of crack blunting and reducing crack propagation rate; in DBTT range, the fracture morphology shows mixed fracture with cleavage and dimples, which are related to graphite nodules; below DBTT, deformation twins lead to the nucleation of microcrack and result in cleavage fracture, the deformation twinning could possibly play a significant role in the ductile to brittle transition of QT400-18L ferrite ductile iron.

Key words:  graphite nodule      ductile-brittle transition      cleavage fracture      deformation twin     
Fund: Supported by National Natural Science Foundation of China (No.51274142), Natural Science Foundation of Liaoning Province (No.2014028015) and Program of Bureau of Shenyang Science and Technology (No.F15-199-1-15)

Cite this article: 

Xinning ZHANG,Yingdong QU,Rongde LI,Junhua YOU. MECHANISM OF CRACK NUCLEATION AND PROPA- GATION OF FERRITE DUCTILE IRON DURING IMPACT FRACTURE UNDER LOW TEMPERATURES. Acta Metall Sin, 2015, 51(11): 1333-1340.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00121     OR     https://www.ams.org.cn/EN/Y2015/V51/I11/1333

Fig.1  Variations of impact load (a) and absorbed energy (b) of ferrite ductile iron with deflection during impact at different temperatures (Fm—maximal dynamic load, Fy—yield load)
Fig.2  Fracture paths of ferrite ductile iron after impact at

-20 ℃ (a), -45 ℃ (b) and -80 ℃ (c)

Fig.3  SEM image of fracture below V-notch of ferrite ductile iron after impact at -20 ℃
Fig.4  Morphologies near fracture of ferrite ductile iron before (a) and after (b) impact at -20 ℃
Fig.5  Deformation morphology of matrix of ferrite ductile iron after impact at -20 ℃
Fig.6  Morphology of micro-void around graphite of ferrite ductile iron after impact at -20 ℃
Fig.7  SEM image of longitudinal section of fracture in ferrite ductile iron after impact at -20 ℃
Fig.8  SEM image of fracture below V-notch of ferrite ductile iron after impact at -45 ℃
Fig.9  SEM image near fracture of ferrite ductile iron after impact at -45 ℃
Fig.10  Graphite morphologies near fracture of ferrite ductile iron before (a) and after (b) impact at -45 ℃
Fig.11  Deformation morphologies of slip line (a) and cleavage (b) of matrix around graphite of ferrite ductile iron after impact at -45 ℃
Fig.12  SEM image of fracture of ferrite ductile iron after impact at -80 ℃
Fig.13  Schematic of cleavage fracture of ferrite ductile iron after impact at -80 ℃ (Arrow indicates crack propagation direction )
Fig.14  Morphologies near fracture of ferrite ductile iron after impact at -80 ℃

(a) cleavage step (b) river pattern

Fig.15  SEM image of cleavage tongue around graphite of ferrite ductile iron after impact at -80 ℃
Fig.16  Schematic of cleavage tongue model
Fig.17  SEM image of deformation twins on fracture of ferrite ductile iron after impact at -80 ℃
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