Investigations on the nanocrystallization of 40Cr using ultrasonic surface rolling processing

doi:10.1016/j.apsusc.2008.06.034

Applied Surface Science

Volume 255, Issue 5, Part 1, 30 December 2008, Pages 1824-1829

https://doi.org/10.1016/j.apsusc.2008.06.034 Get rights and content

Abstract

Ultrasonic surface rolling processing (USRP) was applied on quenched and tempered 40Cr. Microstructure observations of USRP specimen surface indicate that the processing can both get nano-structured layers, with grain size of 3–7 nm, and reduce the surface roughness to 0.05 μm. Life of the rolling processing tip is about 800 times longer than the fixed one, which makes the processing practical and economical. Tests of mechanical properties show that microhardness of USRP specimen surface was increased by 52.6%; residual compressive stress can reach −846 MPa. It has been determined by contrast wear test that USRP can reduce friction coefficient and improve the wear-resistant property.

Introduction

Surface severe plastic deformation (S²PD) is an important surface nanocrystallization process that has been recently developed to produce engineering components with a surface nanocrystalline (nc) layer and a coarse-grained interior [1], [2], [3], [4], [5], [6]. S²PD-based processes have been shown to improve the tensile strength, microhardness, wear resistance, and fatigue strength [7], [8], [9], [10], [11] of materials.

Many S²PD-based processes have been put forward recently, among which, there are several methods involve repeated impacts on the work-piece surface by high-speed balls, shots or peening, including surface mechanical attrition treatment (SMAT), ultrasonic shot peening (USSP) and ultrasonic peening (UP) [12], [13], [14], [15], [16]. Some other methods add static load, which pushes ultrasonic vibratory device with constant pressure, to the dynamic energy occurred through the ultrasonic vibration, including ultrasonic cold forging technology (UCFT) and ultrasonic surface metallic nanocrystallization technology (USMNT) [4], [17].

Compared with pure dynamic loading mode, combination of dynamic and static load can bring satisfying processed surface with smaller and more uniformly distributed grains, which is very important for practical application.

Both UCFT and USMNT are using fixed working tip, which is easily worn-out because of the slide friction during processing even though it is made of sintered-carbide material. Frequent replacements of the tip cause waste and inconsistent operation. In ultrasonic surface rolling processing (USRP), rolling working tip is used to solve the problem. Tests show that under the assurance of the final processed surface roughness R_a ≤ 0.4, being of sintered-carbide material, the cumulative life of rolling tip was 400 h, while that of fixed working tip was 30 min, which makes USRP more practical and economical.

Microstructure of nano-layer is observed and variations of mechanical properties of USRP surface are examined in this paper. In order to show the effectiveness and reliability of USRP, contrast wear and friction test are carried out. The principles and device of USRP are also described briefly.

Section snippets

Ultrasonic surface rolling processing device

Fig. 1 shows configuration of USRP device. It consists of two parts: ultrasonic wave generator and USRP operator; the latter part includes piezoelectric ceramic energy transducer, amplitude changing rod and working tip. Static pressure between USRP operator and the work-piece can be applied by spring fixed at the bottom or compressed air which can also cool the transducer. Center frequency of output ultrasonic vibration is 20 kHz and the amplitude range is 0–25 μm.

Under a certain feeding rate,

Microstructure variation of specimen surface along depth

Fig. 2, Fig. 3 indicate that plastic flow is formed on the surface with thickness about 150 μm. Plastic deformation amount decreases gradually along depth from top surface, which is similar to the tendency of low carbon steel after 60 min SMAT [19]. Because grain refinement is mainly decided by plastic deformation amount, material structure presented graded distribution along depth as well.

The original metallographic structure of specimen is bcc (body centered cubic) tempered sorbite and ferrite.

Contrast friction and wear test

Weights of USRP and polished specimens before and after friction were measured and listed in Table 1. The wear weight of USRP disk is only 1/6 of that of polished one.

Curves of friction coefficient μ of two specimens were drawn in Fig. 11. A possible explanation of the results is that: at the beginning of test, oil protective film between friction pairs was not formed yet, so it was pure solid friction at this time and μ was relatively big; as the protective film formed, μ became smaller; after

Conclusions

(1)
USRP can get nano-structured layers, with grain size of 3–7 nm, on quenched and tempered 40Cr. Plastic flow can reach to 150 μm deep with graded distribution along depth.
(2)
After USRP, surface roughness was reduced to 0.06 μm, surface hardness was increased by 52.6% and the residual compressive stress can reach −846 MPa with effective depth of about 1.4 mm.
(3)
Rolling working tip can reduce wear and prolong the tip's life by 800 times compared with the fixed one, which makes the processing practical and

Acknowledgement

The authors acknowledge the financial support by National Science Foundation through grant no. 50405019.

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Investigations on the nanocrystallization of 40Cr using ultrasonic surface rolling processing

Abstract

Introduction

Section snippets

Ultrasonic surface rolling processing device

Microstructure variation of specimen surface along depth

Contrast friction and wear test

Conclusions

Acknowledgement

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Sci. Metall. Mater.

Acta Mater.

Metal Phys. Adv. Technol.