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Effect of Shot Peening on Microstructural Evolution of 500-7 Ductile Cast Iron
Applied Microscopy 2018;48:73-80
Published online September 9, 2018
© 2018 Korean Society of Microscopy.

Yubing Zhang, Keesam Shin*

School of Advanced Materials and Engineering, Changwon National University, Changwon 51140, Korea
Correspondence to: *Shin K, http://orcid.org/0000-0002-9507-2087, Tel: +82-55-213-3696, Fax: +82-55-261-7017, E-mail: keesamgg@gmail.com
Received July 19, 2018; Revised August 27, 2018; Accepted August 28, 2018.
This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract

Ductile cast iron is widely used for many automotive components due to its high wear resistance and fatigue resistance in addition to the low cost of fabrication. The improvement of wear resistance and fatigue properties is key to the life time extension and performance increase of the automobile parts. Surface nanocrystallization is a very efficient way of improving the performance of materials including the wear- and fatigue-resistance. Shot peening treatment, as one of the popular and economic surface modification methods, has been widely applied to various materials. In this study, ductile cast iron specimens were ultrasonic shot peening (USP) treated for 5 to 30 min using different ball size. The microstructures were then microscopically analyzed for determination of the microstructural evolution. After the USP treatment, the hardness of pearlite and ferrite increased, in which ball size is more effective than treatment time. With USP treatment, the graphite nodule count near the surface was decreased with grain refinement. The lager balls resulted in an increased deformation, whereas the smaller balls induced more homogenously refined grains in the deformation layer. In addition, formation of nanoparticles was formed in the surface layer upon USP.

Keywords : Ductile cast iron, Shot peening, Grain refinement, Graphite, Pearlite
Figures
Fig. 1. Microhardness depth profile of ferrite and pearlite with ball size. USP, ultrasonic shot peening.
Fig. 2. Surface SEM micrographs. (A) Before; ball size 1.0 mm, (B) Ultrasonic shot peening (USP) 10, (C) USP 20, (D) USP 30; ball size 1.5 mm, (E) USP 10, (F) USP 20, (G) USP 30.
Fig. 3. Graphite area fraction of 500-7 near surface. USP, ultrasonic shot peening.
Fig. 4. Cross-sectional graphite area fraction with shot peening time and ball size. (A) 1.0 mm, (B) 1.5 mm. USP, ultrasonic shot peening.
Fig. 5. Cross-sectional EBSD analysis. (A) Before, (B) ultrasonic shot peening (USP) -1.0-5, (C) USP-1.0-10, (D) USP-1.0-15, (E) USP-1.0-20, (F) USP-1.0-25, (G) USP-1.0-30.
Fig. 6. Cross-sectional EBSD analysis. (A) Before, (B) ultrasonic shot peening (USP) -1.5-5, (C) USP-1.5-10, (D) USP-1.5-15, (E) USP-1.5-20, (F) USP-1.5-25, (G) USP-1.5-30.
Fig. 7. (A) Grain size in surface and (B) in a specific depth with ultrasonic shot peening treatment time and ball size.
Fig. 8. Surface TEM analysis. (A) Before ultrasonic shot peening (USP), USP-1.5, (B) 5 min, (C) 10 min, (D) 15 min, (E) 20 min, (F) 25 min.
Fig. 9. Surface TEM analysis. Ultrasonic shot peening (USP)-1.5 (A) 15 min, (B) 20 min, (C) 25 min, (D) 30 min.
Fig. 10. Surface TEM analysis. Ultrasonic shot peening (USP)-1.5 (A) 30 min BF, (B) 30 min DF, (C) 30 min BF, (D) 30 min DF. The circled spots in the insets are for the respective DF.
Fig. 11. Specific depth TEM analysis. Ultrasonic shot peening (USP)-1.5-30 min (A) BF, (B) DF, and 200 μm deep (C) BF.
Fig. 12. Cross-sectional TEM analysis. Ultrasonic shot peening (USP)-1.5-30 min.
Tables

Original specimen composition and mechanical property

ISOComposition (wt. %)Mechanical property


CSiMnPSNiBal.R (MPa)ɛ (%)Hardness (HV)
1083 500-7~3.75~2.25~0.450.06<0.06<0.02Fe5007~200

Ultrasonic shot peening (USP) treatment parameters (amplitude, 70 μm; frequency, 20 kHz)

SpecimenTreatment time (min)
Ball size: 1.0 mm
 500-70
 USP-1.0-55
 USP-1.0-1010
 USP-1.0-1515
 USP-1.0-2020
 USP-1.0-2525
 USP-1.0-3030
Ball size: 1.5 mm
 USP-1.5-55
 USP-1.5-1010
 USP-1.5-1515
 USP-1.5-2020
 USP-1.5-2525
 USP-1.5-3030

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