Journal of Mechanical Science and Technology 31 (11) (2017) 5279~5283

www.springerlink.com/content/1738-494x(Print)/1976-3824(Online) DOI 10.1007/s12206-017-1021-4

Peening narrow nozzles of reactor pressure vessels using ultrasonic cavitation†

Sunghwan Jung1,*, Murugesan Prabhu2 and Hyungyil Lee2 1Department of Mechanical Engineering, Dankook University, Yongin-si, Gyeonggi-do 16890, Korea

2Department of Mechanical Engineering, Sogang University, Seoul 04107, Korea

(Manuscript Received June 6, 2017; Revised July 7, 2017; Accepted July 24, 2017)

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Abstract A peening approach using ultrasonic cavitation was proposed to peen the inner surfaces of narrow nozzles of reactor pressure vessels.

The working theory for the present approach was described and numerically demonstrated. A proof-of-concept experiment using a nar-row nozzle and ultrasonic loading with a frequency of 20 kHz was conducted, and a noticeable improvement in residual stress was achieved at the inner surface of the nozzle. The successful demonstration in this study indicates that the present technique using ultra-sonic cavitation has the potential to effectively peen the inner surfaces of long and narrow nozzles or holes at a low cost using existing conventional peening techniques.

Keywords: Cavitation; Inner surface; Nozzle; Peening; Reactor pressure vessel; Ultrasonic ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction

Tensile residual stresses were observed in deep areas of nozzles near welding joints, which integrate nozzles with the nuclear reactor pressure vessel. The tensile residual stresses initiate crack in the area, thereby severely degrading structural reliability (Fig. 1). Thus, the tensile stresses should be miti-gated by peening to prevent crack initiation caused by the stresses. Shot peening is a cold working process used to pro-duce a compressive residual stress layer by impacting the metal with shot streams, which have been used in the past several decades to treat metal surfaces to improve surface fatigue properties. However, peening the inner surfaces of substantially long and narrow nozzles, such as Bottom-mounted instrument (BMI) nozzles, remain challenging be-cause these surfaces are fully shadowed, thus limiting the ac-cess of the shot streams. In addition, shot peening conducted with metal shots are inapplicable for the nozzles of nuclear reactors because foreign materials are not allowed inside the reactors. The challenges and limitations to peening of the in-ner surface of the nozzles of nuclear reactor pressure vessels prevent the use of shot-peening-based conventional methods. Thus, works using shot peening for the inner surface of noz-zles have not been reported. As an alternative to the conven-tional shot peening method, a water jet peening method was previously proposed [1, 2]. This method uses cavitation in-

duced by water jets supplied to the nozzles. However, the availability of this method is limited because injecting pressur-ized water jets into the nozzles requires a significant cost. Thus, a new peening method is necessary to effectively ad-dress the challenges of peening the inner surfaces of narrow nozzles at a low cost.

For the first time, we propose the application of ultrasonic cavitation to peen the inner surfaces of narrow nozzles. Cavi-ties form, grow, and collapse in water excited by ultrasound.

When cavities collapse, they generate shock waves, which have been reported to impart a compressive residual stress layer on the metals [3, 4]. The present approach uses the shock waves from the cavitation in producing a compressive residual stress layer at the inner surface of the nozzles. The present

*Corresponding author. Tel.: +82 31 8005 3506, Fax.: +82 31 8021 7215 E-mail address: shjung@dankook.ac.kr

† Recommended by Editor Chongdu Cho © KSME & Springer 2017

Fig. 1. Location of cracks at the inner surface of a nozzle. The cracks initiate due to tensile residual stresses near the welding joint.

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approach satisfies the condition for nuclear reactors that no material other than water can be used for the process because it only employs water as the peening material. In addition, with the use of water, the operating cost of the present ap-proach can be significantly reduced in comparison with that of the water jet peening method, which requires significant cost to supply continued water jets into the nozzles.

This work presents a working theory to enable peening of the inner surfaces of narrow nozzles using ultrasonic cavita-tion. A finite element analysis and a proof-of-concept experi-ment using ultrasonic loading and a narrow nozzle were con-ducted to verify the working theory. The residual stresses achieved at the inner surface from the nozzle peening experi-ment were also measured and discussed.

2. Working theory

The schematic of the proposed nozzle peening using ultra-sonic cavitation is shown in Fig. 2. Ultrasound is applied to the nozzle using a vibration probe. Provided that the steel nozzle is fully rigid due to the substantive difference between acoustic impedances of water and steel, the applied ultrasound is confined to propagate only in the axial direction of the noz-zle. The confined ultrasound forms a 1D standing wave be-cause the reflector is located at the other end.

In the 1D standing wave, intensive cavitations occur in the area near the pressure antinodes where the maximum alternat-ing pressure is produced. Owing to the intensive cavitations periodically occurring according to the pressure field, the in-ner surface of the nozzle is peened at the locations of the pres-sure antinodes.

The reflector should be incrementally displaced to peen the areas of the pressure antinodes, thereby expanding the peening coverage. Along with the incremental movement of the reflec-

tor, the pressure antinodes continue to shift (Fig. 3) to expand the peening areas. The reflector only needs to move through the distance between pressure antinodes, which is equal to a half-wave length of the applied ultrasound, to entirely cover the inner surface of the nozzle. Notably, the operation using the reflector allows peening in deep areas without actually inserting the ultrasonic loading source into the nozzle. Thus, the easy loading operation feature of the proposed peening method minimizes the necessary extensive elaboration to drive the loading source into the narrow nozzle.

In addition, the 1D standing wave, which enables peening inside the nozzle, forms regardless of the width and length of the nozzle. Thus, the present working theory for long and narrow nozzles can be equally applied without compromising the peening effect.

3. Acoustic finite element simulation

The present finite element simulation aims to confirm that the differences between acoustic impedances of water and steel (Table 1) can sufficiently confine the applied ultrasound leading to the 1D standing wave, as described in the working theory.

The nozzle for the present study has an inner diameter of 10 mm, an outer diameter of 23 mm, and a length of 245 mm

Fig. 2. Working theory of nozzle peening using ultrasonic cavitation. A 1D standing wave forms in the water-filled nozzle. The inset shows cavitation peening at the pressure antinodes, where cavitation is maxi-mized.

Table 1. Material constants for finite element analysis.

Material constants Steel Water

Density (kg m−3) 7860 1000

Young’s modulus (GPa) 210 –

Bulk modulus (GPa) – 2.2

Acoustic impedance (kg m−2 s−1) 47.2 × 106 1.45 × 106

Fig. 3. Peening coverage expansion by displacing the reflector. The pressure antinodes (indicated by the dash lines) shift when the reflector moves, leading to changes in the maximum cavitation locations, which, in turn, change the most intensive peening zones.

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(the length is set by the position of the reflector). The selected nozzle geometry reflects the dimensions of the BMI nozzle, which is the narrowest and the most challenging to access among the nozzles of the pressure vessels of nuclear reactors. The properties of elements comprising the steel part, including the reflector, as well as the water part, are provided in Table 1. The water part of the simulation model uses four-node linear axisymmetric acoustic elements from Abaqus, and the steel part uses the four-node bilinear solid continuum elements [5]. At the interfaces between water and steel, the two-node linear axisymmetric acoustic interface elements are used to couple the water and steel parts. Cyclic displacement is applied at the location 5 mm below the nozzle inlet with an amplitude of 90 µm and a frequency of 20 kHz to simulate vibration loading. The simulation is run on steady-state dynamic analysis.

The alternating pressure field in the nozzle well represents the expected 1D standing wave, as shown in Fig. 4.

All of the peak pressures are equal in magnitude, and the separation between these peak pressures is equal to the half-wave length, that is, 37.5 mm for the ultrasound in water with a frequency of 20 kHz. This result clearly indicates that the applied ultrasound is fully confined by the side wall, leading to 1D standing wave formation. With respect to the resulting standing wave field, the tip of the vibrator is located 15 mm away from the next pressure antinode. If the tip of the vibrator (carrying the displacement loading) is shifted closer to the next pressure antinode, then the loading frequency of 20 kHz would become the resonance frequency, thereby further in-creasing the pressure amplitude.

4. Experimental procedure and results

A proof-of-concept experiment was conducted to experi-

mentally prove the theory and feasibility of the present peen-ing approach. Fig. 5 shows the experimental setup to measure the stresses at the inner surface of the narrow nozzle induced by vibration loading. A narrow pipe sample made of SPCC steel with a length of 245 mm, an inner diameter of 10 mm, and a thickness of 6.75 mm was selected, matching the dimen-sions in the simulation model. The nozzle was closed at the bottom end with an SPCC steel plate, which served to reflect the incident ultrasound. A steel flat strip with 1 mm thickness was vertically arranged on the side of the nozzle and fixed with a double-sided tape (Fig. 5) to measure the built up stresses at the inner surface by the cavitation using the X-ray diffraction (XRD) method. The strip was used to collect the stresses produced by the cavitation, which was to be removed from the nozzle later after the experiment for the XRD stress measurement.

The vibration probe for loading connected with an ultra-sonic processor (Vibra-Cell VCX130, Sonics) was aligned with the axis of the nozzle. The tip of the vibrator with a di-ameter of 6 mm was fixed 5 mm below the inlet of the nozzle. The nozzle pipe was immersed in water at a temperature of 20 °C. The ultrasonic tip vibrated at a frequency of 20 kHz. The vibration amplitude of the tip was set to 90 µm. The vi-bration was applied for 3 min.

After the vibration was completed, the strip was removed from the nozzle, and the stresses generated by vibration load-ing were measured using the XRD method (Rigaku MiniFlex X-ray diffractometer). Each XRD measurement involved the area of 10 mm by 2 mm. Fig. 6 shows the XRD patterns from unpeened and peened specimens. The operation of the present

Fig. 5. Experimental setup. The insertion of the vibrator’s tip is indi-cated by the yellow lines. The container filled with water is not shown to demonstrate a clear image of the setup. The water level in the con-tainer, which is set during the process, was symbolically represented. The image on the right illustrates the side strip for the X-ray diffraction (XRD) measurement.

Fig. 4. Alternating pressure amplitude.

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X-ray diffractometer was designed to collect only the X-ray signals diffracted from the lattice planes parallel to the surface of the specimen. For the computation of the residual stresses built on the surface of the peened specimen, the peak angles associated with the lattice plane of (200) of the peened and unpeened specimens were selected because the peaks were higher than those of the others. From the peak angles of the lattice plane of (200), the respective interspacing of the lattice plane of (200) for unpeened and peened specimens were com-puted using Bragg’s law, as follows:

sin /d q l= , (1)

where d is the interspacing of diffraction planes, l is the wave length of the X-ray (1.545 A& for the present X-ray), and q is the peak angle of the lattice plane of (200) from the XRD patterns. With the interspacing values from Eq. (1) for peened and unpeened specimens, the strain between the plane of (200) normal to the surface was calculated as follows:

peened unpeened

unpeenedz

d dd

e-

= , (2)

where z is the direction normal to the surface, and dpeened and dunpeened are the interspacing values of the plane of (200) for the peened and unpeened specimens, respectively. In addition, the

residual stresses in the direction parallel to the surface were related to the strain normal to the surface because the surface was stress-free, as follows:

12 ( )z x ySe s s= + , (3)

where S12 = 1.5 × 10−12 Pa−1. The corresponding compliance for the orientation of the plane of (200) was used because the stress computation was conducted based on the cubic structure of steel. Based on the assumption that the stress state on the surface biaxial, the residual stress can be computed as follows:

residual122z

Ses = . (4)

Fig. 7 shows the residual stresses processed from the meas-

urements at the locations of the pressure antinodes. Apprecia-ble compressive (negative) residual stresses were observed in the areas located at the pressure antinodes, as indicated in the working theory. The stresses appeared with nearly equal am-plitude. No stresses were observed in the areas located be-tween the pressure antinodes. The zero values of the stress data from the measurement are not presented in this study. The cyclic variation of the compressive residual stresses shown in the stress data well reflects the alternating pressure field of the 1D standing wave shown in Fig. 4.

Rather surprisingly, at the pressure antinode next to the vi-bration tip, no appreciable residual stress was observed. This finding can be attributed to the acoustic streaming flows in-

Fig. 6. XRD patterns for peened and unpeened specimens made of SPCC steel. Separation between the peaks of peened and unpeened specimens is illustrated in the upper figure.

Fig. 7. Measured residual stresses at the pressure antinodes.

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duced near the tip of the vibrator, which move the cavities away from the surface prior to collapse and impact on the surface.

5. Cavitation near the tip of the vibrator

Cavitation near the tip of the vibrator was inspected using a nozzle sample made of transparent acryl. The images of the cavitation were recorded with a frame rate of 20 kHz. No su-percavities attached to the tip of the vibrator were detected in the recorded images. In Ref. [6], supercavities covered the small tip of the vibrator, interrupting the tip–liquid contact. However, the nozzle geometry constrained the acoustic streaming flows near the tip of the vibrator, thus preventing the formation of supercavities. Thus, in the present case, the tip of the vibrator in the nozzle steadily comes in contact with water, providing a stable loading to achieve cavitation peening on the inner surface.

6. Conclusions

The inner surface of the narrow nozzle was successfully peened using ultrasonic cavitation. Appreciable compressive residual stresses produced at the inner surface near the pres-sure antinodes confirmed the feasibility of the present ap-proach. Formation of the 1D standing wave inside the nozzle ensures that insertion of the loading source into the nozzle is avoided, which significantly simplifies the loading operation. Given the easy loading feature, the present approach will be readily applied at a low cost to peen the inner surfaces of nar-row nozzles or holes. In addition, the efficacy of the present method remains uncompromised to peen nozzles with small diameters because no loading source needs to be inserted. Thus, the present method can be utilized to peen nozzles and holes with narrow openings and prevent the use of existing peening methods employing shot streams.

In the future, in addition to residual stress generation, strain hardening and roughening after peening will be investigated. The operating conditions, including the location of the vibra-tor’s tip, the loading amplitude, the ultrasonic tip size, and the duration time, will be optimized to further improve the resid-ual stress state. Numerical models of acoustic streaming and cavitation will be integrated into the numerical model to im-prove the accuracy of the simulation results.

Acknowledgment

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016 M2B2A9A02945208) and the Ministry of Education (NRF-2016 R1D1A1A029 37009).

References

[1] H. Soyama, J. D. Park, M. Saka and H. Abe, Improvement of residual stress in stainless steel by cavitation jet, Journal of the Society of Material Science Japan, 47 (8) (1998) 808-812.

[2] Y. Nakamura, T. Ohya, K. Okimura, M. Nayama, Y. Nagura and T. Ohta, Residual stress improved by water jet peening using cavitation for small-diameter pipe inner surfaces, In-ternational Conference on Nuclear Engineering, Nice, Acropolis, France (2001).

[3] M. Mathias, A. Gocke and M. Pohl, The residual stress, texture and surface changes in steel induced by cavitation, Wear, 150 (1991) 11-20.

[4] M. R. Sriraman and R. Vasudevan, Influence of ultrasonic cavitation on surface residual stresses in AISI stainless steel, Journal of Material Science, 33 (1998) 2899-2904.

[5] Dassault Systems Simulia Corp., Abaqus User’s Manual, Version 6.14, Providence, RI, USA (2014).

[6] A. Žnidarčič, R. Mettin, C. Cairos and M. Dular, Attached cavitation at a small diameter ultrasonic horn tip, Physics of Fluids, 26 (2014) 023304.

Sunghwan Jung received his B.S. de-gree in mechanical engineering from the University of Iowa, Iowa City, IA, in 1993. He received his M.S. and Ph.D. degrees in mechanical engineering from the Massachusetts Institute of Technol-ogy, Cambridge, MA, in 1995 and 2007, respectively. He is currently an Associ-

ate Professor in Dankook University, Korea. His research interests include ultrasonic peening/cleaning, fracture mechan-ics, wafer spalling, and microfluidics.