Effect of electropulsing on surface mechanical properties and microstructure of AISI 304 stainless steel during ultrasonic surface rolling process

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Abstract

The present work integrates 3D digital optical microscopy (OM), nano-indentation, X-ray diffraction (XRD), scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) to systematically investigate the effect of electropulsing on the surface mechanical properties and microstructure of AISI 304 stainless steel during the ultrasonic surface rolling process (USRP). Compared with the original USRP, the introduction of electropulsing with optimal parameters can effectively facilitate surface crack healing and improve surface hardness and wear resistance dramatically, and the residual compressive stress is further enhanced. Meanwhile, more martensite phase and fewer deformation twins can be found in the strengthened layer. Rapid improvement of the surface mechanical properties should be attributed to the ultra-refined grains, accelerated martensitic phase transformation and suppressed deformation twining induced by the coupling effect of USRP and electropulsing. The high strain rate given by USRP, increased stacking fault energy and accelerated dislocation mobility caused by electropulsing are likely the primary intrinsic reasons for the observed phenomena.

Introduction

AISI 304 stainless steel is characterized by its excellent combination of strength, ductility and response to high corrosion resistance and is thus extensively used in automotive [1], machinery [2], [3], [4], nuclear [4], [5], [6], [7], petrochemical engineering [4], [8] and other civilian fields. However, the lower surface hardness and the weaker wear resistance of AISI 304 stainless steel restricts its much broader applications [4], [6], [7]. Therefore, how to develop an effective surface strengthening technology with low energy and cost consumption and high efficiency becomes an urgent and imperative issue to be resolved.

Surface nanocrystallization technology is an effective and feasible method to solve the problem, which can enhance the surface mechanical property drastically owing to the refined surface grains [5]. The ultrasonic surface rolling process (USRP) is a novel surface nanocrystallization technology based on severe plastic deformation (SPD), which utilizes the combination of dynamic impact superimposed on static extrusion to induce a strengthened layer at and below the material surface to a certain depth with high hardness and wear resistance, finally facilitating high service performance for the whole component [5], [9], [10]. Currently, USRP is used with increasing frequency to improve the fatigue, corrosion, and wear resistance performance of mechanical components [11], [12], [13]. However, the remarkable hardening in the surface-strengthened layer—especially for AISI 304 stainless steel, which has high work hardening capacity—induced by SPD shows an extreme obstruction to the further deformation and strengthening inside the metal. Moreover, the severe deformation limitation caused by the introduction of large amounts of immobile dislocations restricts the ultimate top surface hardness [9], [14]. Therefore, there is no significant improvement to be achieved in surface hardness in the subsequent process [9].

The electropulsing-assisted process is a recently developed advanced technique implemented by an instantaneous high-energy pulse current input, which can significantly influence the plasticity [15], [16], [17], recrystallization [18], [19], [20], [21], [22], phase transformation [23], [24], [25], [26], structure evolution [27], [28], [29], [30], [31], [32], [33], casting microstructure [34], [35] and fatigue life [26], [35], [36], [37], [38] of metallic materials. The electropulsing-assisted process has been studied extensively and shows a good application foreground in many industrial fields. A. Rahnama and Rongshan Qin [32] investigated the effect of electropulsing on the interlamellar spacing and mechanical properties of hot-rolled 0.14% carbon steel. They found that the interlamellar spacing increased with increasing number of pulses, and softening occurred during the treatment, which was attributed to the formation of a precipitation-free zone, increase in the value of interlamellar spacing and the spheroidization of the lamellar structure. Hui Song et al. [37] studied the microcrack healing and local recrystallization in pre-deformed titanium alloy sheet by high-density electropulsing, and they confirmed that electropulsing had a positive effect on surface crack healing. S.V. Konovalov et al. [36] researched the evolution of dislocation substructures in fatigue-loaded and failed stainless steel with intermediate electropulsing treatment, and they reported that considerable evolution of dislocation substructure types conducive to fatigue resource increase 1.75 times as large. Xiaoxin Ye et al. [31] reported the influence of electropulsing globularization on the microstructure and mechanical properties of Ti–6Al–4V alloy strip with lamellar microstructure, and the electropulsing-induced microstructural change resulted in remarkably increasing elongation-to-failure whereas the tensile strength remained unchanged. However, in-depth investigations of how electropulsing affects material microstructure and how this affects mechanical behavior during USRP are still lacking.

In the present work, a method called the electropulsing-assisted ultrasonic surface rolling process (EP-USRP) is proposed to target the shortcoming in USRP for AISI 304 stainless steel. As a result, a strengthened layer with higher surface hardness, greater wear resistance and higher residual compressive stress is obtained. Meanwhile, the coupling effect of USRP and electropulsing is investigated and discussed based on experiments.

Section snippets

Experimental procedure

Commercial annealed AISI 304 stainless steel rods were cut into pole-shaped samples and then processed by turning to the specimens with a dimension of Ф16.8 mm×150 mm to maximize the uniformity of surface morphology characteristics. The chemical composition of the as-received AISI 304 stainless steel is presented in Table 1.

The USRP and EP-USRP experiments were carried out on a self-built platform based on a conventional lathe. A schematic diagram for setup is shown in Fig. 1. The valid clamping

Evolution of axial surface roughness and surface micromorphology

Variations of the axial surface roughness and modified profile of the specimens before and after treatment are shown in Fig. 2. It can be seen from the result by a Surtronic S25 contact-type surface roughness tester (Fig. 2A) that the axial surface roughness on specimens after USRP and EP-USRP reach approximately Ra 0.2 µm, showing a remarkable decline compared with Ra 1.2 µm of the turning surface. For these specimens treated by EP-USRP, the axial surface roughness slightly decreases with

Ultra-refined grain in strengthened layer induced by the coupling effect of USRP and electropulsing

In metallic materials with relatively low stacking fault energy (SFE), deformation twinning plays an important role in the plastic deformation process [45], [46], [47], [48]. As depicted in Fig. 8, many deformation twins are found after USRP (Fig. 8B) and EP-USRP (Fig. 8C), but there are fewer annealing twins in the annealed specimen (Fig. 8A). The difference is that relatively fewer deformation twins are formed in EP-USRP. Moreover, the impact depth of plastic deformation induced by EP-USRP is

Conclusion

Compared with USRP, EP-USRP can further effectively facilitate surface crack healing, receive better surface roughness, enhance surface hardness, improve surface wear resistance and strengthen the residual compressive stress at a suitable frequency (600 Hz) for AISI 304 stainless steel. The introduced electropulsing in EP-USRP changes the mechanism of plastic deformation—i.e., enhanced dislocation cross-slip and accelerated martensite phase transformation but suppressed deformation twinning. The

Acknowledgments

The authors wish to acknowledge the financial support from Science & Technology Research Funding Project of Guangdong Province (No. 2014B090901029) and Key Enterprises & Innovation Institutes Supporting Funding Project of Nanshan District of Shenzhen City (No. KC2015ZDYF0021A).

References (60)

  • H. Wang et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2016)
  • Y. Dong et al.

    J. Nucl. Mater.

    (2015)
  • A. Bahri et al.

    Wear

    (2015)
  • X.-L. Lu et al.

    Appl. Surf. Sci.

    (2015)
  • J. Xu et al.

    Corros. Sci.

    (2013)
  • M.N. Gussev et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2013)
  • H. Wang et al.

    Surf. Coat. Technol.

    (2015)
  • Y. Liu et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2014)
  • I. Nikitin et al.

    Scr. Mater.

    (2004)
  • L. Waltz et al.

    Scr. Mater.

    (2009)
  • X. Ye et al.

    J. Mech. Behav. Biomed. Mater.

    (2014)
  • V.E. Gromov et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2010)
  • Y. Zhao et al.

    Mater. Des.

    (2013)
  • X. Ye et al.

    J. Alloy. Compd.

    (2014)
  • Y. Jiang et al.

    J. Alloy. Compd.

    (2016)
  • Q. Xu et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2011)
  • A. Rahnama et al.

    Scr. Mater.

    (2015)
  • H. Liao et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2011)
  • A. Rahnama et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2015)
  • X.F. Zhang et al.

    Scr. Mater.

    (2013)
  • S.V. Konovalov et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2010)
  • H. Song et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2008)
  • H.W. Zhang et al.

    Acta Mater.

    (2003)
  • J.Z. Lu et al.

    Acta Mater.

    (2010)
  • K. Wang et al.

    Acta Mater.

    (2006)
  • K. Lu et al.

    Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process.

    (2004)
  • J.W. Christian et al.

    Prog. Mater. Sci.

    (1995)
  • A.Y. Chen et al.

    Acta Mater.

    (2011)
  • W. Zhuang et al.

    Appl. Surf. Sci.

    (2014)
  • X.J. Cao et al.

    Appl. Surf. Sci.

    (2010)
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