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Acta Metall Sin  2016, Vol. 52 Issue (7): 778-786    DOI: 10.11900/0412.1961.2015.00625
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Junjun CUI1,Liqing CHEN1(),Haizhi LI2,Weiping TONG2
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China.
2 Key Laboratory for Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China.
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Austempered bainitic ductile iron has been widely used in machinery components and parts due to its low fabrication cost, excellent mechanical properties, and abrasive wear resistance. In order to get a fine bainitic matrix, austempering process is usually adopted which consists of austenitizing temperature, austempering temperature and time. For quenched ductile cast iron, tempering plays an important role in subsequent heat treatment process. However, less attention has been paid on the microstructural evolution and mechanical properties of the austempered bainitic ductile iron after tempering treatment. Thus, in this work, 3.55C-1.95Si-0.36Mn-3.58Ni-0.708Cu-0.92Mo-0.65Cr (mass fraction, %) bainitic ductile iron was subjected to austempering and subsequent tempering treatment, and the effect of tempering on microstructures and properties has been investigated by using OM, EP MA, SEM, TEM and XRD. The microstructural evolution during tempering has been investigated, and mechanical properties and wear resistance have also been measured and analyzed. The results show that microstructural evolution of the bainitic ductile iron during tempering contains recovery and recrystallization softening processes of twin martensite and dislocation substructure, decomposition of retained austenite, dissolution of supersaturated carbon and phase transformation in martensite and transformation in eutectic cementite. With increasing tempering temperature, there is a gradual decrease in micro- and macro-hardness of substrate microstructure and compressive strength of austempered low alloyed bainitic ductile iron. When the bainitic ductile iron was tempered at 450 ℃, the eutectic cementite has the lowest micro-hardness value due to the precipitation of α phase in its slice layer and the compressive ratio is thus higher. The mechanical properties of the austempered low alloyed bainitic ductile iron was even worse when tempered at 600 ℃. Under the wear condition of dry sand/rubber wheel, the austempered low alloyed bainitic ductile iron possesses the best wear resistance when tempered at 450 ℃. The worn morphology observation by SEM indicates that the worn surfaces were caused by plastic deformation and micro-cutting. The plastic deformation plays an important role in wear process, while the precipitated and finely distributed Mo2C contributes a lot to the improvement of wear resistance when tempered at 450 ℃.

Key words:  bainitic ductile iron      austempering      tempering treatment      microstructural evolution      mechanical property      wear mechanism     
Received:  03 December 2015     

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Fig.1  Schematic of isothermal quenching (IQ) and subsequent tempering (T) treatments for low alloyed bainitic ductile iron
Fig.2  OM image of low alloyed bainitic ductile iron with IQ heat treatment (A—retained austenite, M—martensite, B—bainite, Gr—graphite)
Fig.3  Low (a) and high magnified (b) TEM images of martensite of low alloyed austempered bainite ductile iron (Inset in Fig.3a show the selected area electron diffraction (SAED) pattern along [110] zone axis, T—twin crystal)
Fig.4  OM images of austempered low alloyed bainitic ductile iron after tempering at 300 ℃ (a), 450 ℃ (b) and 600 ℃ (c)
Fig.5  XRD spectra of the low alloyed bainitic ductile iron after IQ and IQ-T treatments
Fig.6  TEM images and SAED patterns (insets) of austempered bainitic ductile iron after tempering at 300 ℃ (a), 450 ℃ (b) and 600 ℃ (c)
Process Macro-hardness of substrate / HV Macro-hardness of cementite / HV Hardness
Compressive strength / MPa Compressive ratio / %
IQ 571.3 1045.5 56.8 2320 18.0
IQ-T-300 619.6 906.5 54.9 2390 21.1
IQ-T-450 540.0 746.9 51.3 2300 26.7
IQ-T-600 452.4 954.7 47.4 1890 18.5
Table 1  Mechanical properties of the low alloyed bainitic ductile iron with IQ and IQ-T treatments
Process Weight loss / g Wear resistance / g-1
IQ 0.8212 1.2200
IQ-T-300 1.0944 0.9137
IQ-T-450 0.5408 1.8500
IQ-T-600 1.2406 0.8060
Table 2  Wear resistance of low alloyed bainitic ductile iron with IQ and IQ-T treatments
Fig.7  TEM image of austempered low alloyed bainitic ductile iron after tempering at 450 ℃ (F—acicular ferrite)
Fig.8  SEM image of cementite in the low alloyed bainite ductile iron after tempering at 450 ℃
Fig.9  SEM worn surface images of the low alloyed bainitic ductile iron after IQ (a) and tempering at 300 ℃ (b), 450 ℃ (c) and 600 ℃ (d)
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