Effect of ultrasonic impact treatment assisted with high energy electropulsing on microstructure of D36 carbon steel
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trasonic impact treatment (UIT) combined with high energy electropulsing (EP) was applied to low carbon steel to introduce severe plastic deformation on the surface. The investigation indicated that a strengthened layer with a maximum hardness of approximately 330 HV on cross section was obtained, in comparison with the hardness value of 260 HV resulted from UIT solely. Alongside with high hardness, the enhanced structure layer was extended to a distinguishing depth of 2 mm. Microstructure in the cross section revealed a crack-free superficial layer by EP-UIT and pearlite colonies here experienced morphology variations by redistribution and spheroidization of cementite. A 3 lm oxide layer consisting of amorphous oxide and nitride as well as MnFe2O4 and hematite crystalline was formed on the treated surface. Thermal and athermal effect of EP was the key factor in these phenomena and it is assumed that acoustic softening, electro plasticity, and thermal softening were engaged simultaneously.
I. INTRODUCTION
It is well known that mechanical properties of metal products are significantly determined by the condition of the surface and its microstructure. Although an overall improvement in the base material can influence the properties remarkably, such changes, in most case, are not economic and sometimes even not accomplishable. On the other hand, the surface modification upon metal can be quite efficient in improving the fatigue, wear, and corrosion resistance of metal by changing the grain structure, hardness, and residual stresses. Ultrasonic impact treatment (UIT), which was first invented in former Soviet Union in 1950s,1,2 has attracted considerable attention over decades as a promising residual stress based post-weld surface treatment. This treatment involved a metallic striker or indenter oscillating at an ultrasonic frequency in an order of 18,000–27,000 Hz, and working piece will undergo treatment by direct contact with it. Enormous experimental studies have been reported to certificate the fascinating advantages of UIT,3–9 including: (i) relaxation in stress concentration by smoothing geometrical profile, (ii) introduction of compressive residual stress, (iii) formation of a working hardening layer on surface, (iv) crystalline refinement to nanometer size, (v) reduction roughness as well as surface area, etc. Moreover, UIT Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.437
devices are neither expensive nor complex and, in most case, portable. The treatment itself is convenient to carry out and environment friendly, which makes UIT a promising approach than traditional treatment. However UIT is not a panacea inasmuch as it is a coldwork-based treatment which means the increase of working hardening also induce exacerbated ductility and plasticity. Surface damages caused by UIT have been indicated recently in literature.5 Yekta et al.6 applied UIT upon steel welded attachments at proper, under- and over-treatment. Even the over-t
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