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Vol. 8
September Issue
Year 2007
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Science Update


in Vol. 8 - September Issue - Year 2007
Shield Cavitation Shotless Peening And Its Application for Welded Parts



Co-Author Akira Kai (Ph. D.)


Co-Author Hitoshi Soyama (Ph. D.)


Fig. 1: Nozzle assembly of shield cavitation shotless peening


Fig. 2: Setting for shield CSP


Fig. 3: Introduction of compressive residual stress by shield CSP





Fig. 4: Welded thin plate specimen for fatigue plate bending test (a) Dimension (b) Weld macrostructure


Fig. 5: Residual stress at weld toe versus processing time


Fig. 6: Improvement of fatigue strength by shield CSP

Introduction

A peening method using cavitation impact has been developed [1]-[3] and it is called “cavitation shotless peening (CSP)”, as shots are not required. It has been reported that CSP improve fatigue strength of various alloy [1]-[6] and life time of components e.g. gear [7], forging die [8]. The one of CSP introduced in the previous report [9], a high-speed water jet is injected into a concentric low-speed water jet, which can be conducted in air without a water-filled chamber. However, this CSP method uses a large amount of water and throws much splay, therefore peening is conducted in a water collection chamber containing a work and the nozzle.

In order to overcome the restriction of apparatus, a new method called “shield cavitation shotless peening method” was developed. The nozzle and work surface are covered by a small enclosure, the CSP can be conducted without the large water collection chamber.

Recently, new peening methods such as CSP, water jet peening [10] and laser peening [11] are expected to apply welded parts in piping and pressure vessels made of stainless steel in nuclear power plants. These welded parts have complex shapes as described in a report e.g. [12] and repair welding parts is no exception. However, in the case of large and continuous body like piping, it is difficult to peen weld parts with each peening method because these peening methods require a large work chamber to collect splashed water or to submerge work in water.

In the present report, the shield CSP method is introduced and proved its usability of application for welded regions of stainless steel.

Shield Cavitation Shotless Peening Method

The shield CSP nozzle assembly consists of a high-speed water jet nozzle, a low-speed water jet nozzle and a shield enclosure as shown in Fig. 1. These elements are aligned concentrically. The cavitating jet is generated at the outlet of the low-speed water jet nozzle and thereafter impinges to the work surface. The shield enclosure is equipped with a seal cushion made of silicon rubber at the bottom end and with the two drain ports at the upper side. After the cavitating jet collides with the work surface inside the shield enclosure, water rises up along the inside wall and then goes back to the water header tank via the drain ports.

Since capacity of the shield enclosure is small, the condition of the shield CSP method is different from former CSP. For instance, the distance between nozzle and work surface was optimized. Because it is a key parameter of cavitation, impact is associated with bubble growth. At the same time, residual cavitation bubbles after collision mitigate cavitation impact as shock absorber, so the distance is adjusted to prevent interference to the generation of the cavitating jet at nozzle. The CSP condition including injection pressure was determined by several kinds of experiment. For example, paint removal testing was performed to evaluate the peening area. The CSP condition was decided by considering the looseness which may happen in the field as well.

Introduction of Compressive Residual Stress

In order to verify introduction of compressive residual stress into welded part, the shield CSP was applied to the region of the repair welding on a seamless pipe of AISI 316L stainless steel (SS). Figure2 indicates a photo of adjustment of the shield CSP, the shield enclosure was lifted off the pipe surface. The high-speed water jet was aligned to both the center of the welding bead and the pipe. The shield enclosure was contacted to the pipe surface after the alignment, and then the shield CSP method was conducted.

Dimension of the pipe specimen was 216.3 mm in diameter, 12.5 mm in thickness and 100mm in length, respectively. Before repair welding, a notch with a maximum depth of 4mm and with an arc length of 50mm was slotted by a cutting whetstone wheel with a  thickness of 3mm. The notch was formed to V-shape using a hand grinder; opening at surface was about 5mm. The groove was filled by hand TIG welding using ER316 filler. The welding bead of 60 mm in length and 10 mm in width was formed by four welding paths.
Band steel was welded at both ends of the pipe specimen to join a motor drive shaft. In order to peen the whole welded area, the pipe specimen was rotated under the shield enclosure. Degree of peening is defined as processing time per unit length which is calculated by peripheral scanning speed and number of scan path.

Residual stress at the heat affected zone (HAZ) was measured with a X-ray diffraction method (XRD) by the parallel method. Measured area was 5x5 mm using a vinyl mask, and the position was close to the center of welding bead in the circumferential direction. X-ray was generated by Cr tube with 30kV and 8mA. Diffraction plane (311) which the diffract angle 2?  is 148.5 deg was used. A pair of solar slits with an angle of 1 deg was used and the incident X-ray was narrowed by an opening slit of 4mm. Diffract X-ray was counted by a scintillation counter with a 0.2 deg step. The used Young’s modulus, Poisson ratio and stress factor were 192 GPa, 0.28 and -369.5 MPa/deg, respectively. 

Residual stress at the surface of HAZ caused by welding reached from 100 MPa to 200 MPa in the axial direction. Compressive residual stress was introduced at the surface by the shield CSP method, and its value increased with processing time per unit length. According to XRD measurement, residual stress at the surface of HAZ was 0, -300 and -400 MPa for processing time of 1 s/mm, 10 and 20 s/mm, respectively.

In order to evaluate the thickness of modified layer by CSP, XRD measurement and removal of surface of measured area by electro polishing were conducted alternatively. Figure 3 shows the distribution of residual stress in depth direction which is obtained from not-peened specimen and CSP specimen peened by 20 s/mm. For the not-peened specimen, tensile residual stress of about 100 MPa was detected all over the measured depth. On the other hand, for the CSP specimen, compressive residual stress of about -400 MPa was detected at the surface. Compressive residual stress was introduced up to 0.3 mm from the surface. According to previous report which 316L SS plate specimen was peened by other CSP in air [3], compressive residual stress at the surface was almost same. It is concluded that the shield CSP method can introduce compressive residual stress as well as former CSP method without a work chamber.

Improvement of Fatigue Strength

In order to verify the improvement of fatigue strength of a welded part, residual stress measurement and fatigue plate bending testing were performed. The appearance and dimension of the specimen are shown in Fig. 4 (a). A rectangular plate of a 180 mm x 90 mm was cut from a rolled plate with a thickness of 3mm, and was slotted a semicircular groove with a radius of 1.5 mm in the longer direction. A hand TIG welding was conducted along the groove by single path. After the welding, five pieces of the fatigue bending specimen were cut from a welded plate. Figure 4 (b) shows the weld macrostructure etched electrically using 10% oxalic acid. Weld metal completely filled the semicircular groove by the single welding path, and there was no weld defect such as undercut. Profile of the welding bead was convex; thereby a concave was formed at the weld toe. The crack of every broken specimen was located at the weld toe due to stress concentration there. The shield CSP method was conducted using a jig which has the recess engraved with the same shape of the specimen. The recess makes a flat surface in order to avoid water leakage from the step between the specimen and the jig when the specimen was installed. The peened area was traversed by moving the jig with feed screw.

Figure 5 shows residual stress of HAZ measured at weld toe versus processing time per unit length. The residual stress at the weld toe was measured with a 2D-XRD method [13]. Since area of the HAZ was small, the collimator of a 0.1 mm in diameter was used to narrow the measured area. Configuration of 2D-XRD was the same as previous report [14] except the diffraction angle which is 130.2 deg associated with the diffraction plane (220) of  ?-Fe. Tensile residual stress about 150 MPa was detected in not-peened specimen. On the other hand, compressive residual stress was detected in all CSP specimens. It can be concluded that the shield CSP introduced compressive stress into the small concave at the weld toe.

Figure 6 illustrates improvement on fatigue strength by shield CSP method. Each S-N curve was obtained from fatigue plate bending test using series of not-peened specimen and CSP specimen peened by 10, 20 and 40 s/mm. Fatigue limit at 107 cycles of each specimen in Fig. 6 is calculated by Little’s method [15]. For not-peened specimen, the fatigue limit is 102 MPa. On the other hand, the fatigue limit of CSP specimen of 20 s/mm reaches 244 MPa. Furthermore, fatigue life of all CSP specimens is larger than that of not-peened specimen in the whole stress range. It is elucidated that the shield CSP method improved fatigue property of the welded plate dramatically. The fatigue strength and fatigue limit of CSP specimens peened by 10 and 40 s/mm is slightly lower than that of 20 s/mm. However, severe decline on fatigue strength due to excess or deficiency of processing time is hardly seen, so it can be concluded that the shield CSP has wide tolerance of deviation in apparatus setting.

Conclusions

It is elucidated that the shield CSP method can introduce compressive residual stress as well as former CSP. The advantage in elimination of a work chamber is expected to realize peening of large and continuous assembly in field. The shield CSP method also improves fatigue strength of a welded part even the small concave formed at the weld toe. The practical usability of the shield CSP method for applying to a welded part is also demonstrated.
This research project has been conducted under the research contract with the Japan Nuclear Energy Safety Organization (JNES).

References

1. Soyama, H., Park, J. D. and Saka, M., 2000, “Use of Cavitating Jet for Introducing Compressicve Residual Stress,” Journal of Manufacturing Science and Engineering, Trans. ASME, Vol. 122, No. 1, pp. 83-89.
2. Soyama. H., 2004, “Introduction of Compressive Residual Stress Using a Cavitating Jet in Air,” Journal of Engineering Materials and Technology, Trans. ASME, Vol. 126, No. 1, pp. 123-128.
3. Soyama, H. and Mikami, M., 2005, “Cavitation Peening by Water Jet with Water Jet in Water,” Shot Peening Technology, Japan, Vol. 18, No. 2, pp.10-11 (in Japanese).
4. Soyama, H., 2000, “Improvement in Fatigue Strength of Silicon Manganese Steel SUP7 by Using Cavitation Jet,” JSME international Journal, Series A, Vol. 43, No. 2, pp. 173-178.
5. Soyama H., Saito, K., and Saka, M., 2002, “Improvement of Fatigue Strength of Aluminum Alloy by Cavitation Shotless Peening,” Journal of Engineering Materials and Technology, Trans. ASME, Vol. 124, No. 2, pp. 135-139.
6. Macodiyo, D. O. and Soyama, H., 2003, “Cavitation Shotless Peening for Improvement of Fatigue Strength of Carbonized Steel,” International Journal of Fatigue, Vol. 25, Nos. 9-11, pp. 1217-1222.
7. Soyama, H, and Macodiyo, D. O., 2005, “Fatigue Strength Improvement of Gears Using Cavitation Shotless Peening,” Tribology Letters, Vol. 18, No. 2, pp. 181-184.
8. Soyama, H., Takano, Y. and Ishimoto, M., 2000, “Peening of Forging Die by Cavitation,” Technical Review of Forging Technology, Vol. 25, No. 82, pp. 53-57 (in Japanese).
9. Soyama, H., 2006, “Improvement of Fatigue Strength of Metallic Materials by Cavitation Shotless Peening,” Metal Finishing News, Vol. 7, March issue, pp. 48-50.
10. Saitou, N. and Morinaka, R., 2005, “Reduction of Residual Stress on Nuclear Reactor Internals by Water Jet Peening,” Journal of the Japan Welding Society, Vol. 74, No. 7, pp. 25-28.
11. Sano, Y., Obata, M. and Yamamoto, T., 2005, “Residual Stress Improvement of Weldment by Laser Peening,” Journal of the Japan Welding Society, Vol. 74, No. 8, pp. 25-28 (in Japanese).
12. Enomoto, K., Hirano, K., Mochizuki, M., Kurosawa, K., Saito, H. and Hayashi, E., 1996, “Improvement of Residual Stress on Material Surface by Water Jet Peening,” Journal of The Society of Materials Science, Japan, Vol. 45, No. 7, pp. 734-739 (in Japanese).
13. He, B. B. and Smith, K. L., 1997, “A New Method for Residual Stress Measurement Using an Area Detector,” Proceedings of International Conference on Residual Stresses, pp. 634-639.
14. Soyama, H., 2006, “Macro and Micro Strain in Polycrystalline Metal Controlled by Cavitation Shotless Peening,” Metal Finishing News, Vol. 7, November issue, pp. 48-50.
15. Little, R. E., 1972, "Estimating the Median Fatigue Limit for Very Small Up-and-Down Quantal Response Tests and for S-N Data with Runouts," Probabilistic Aspects of Fatigue, ASTM STP 511, pp. 29-42.




Author: Akira Kai (Ph. D.), Hitoshi Soyama (Ph. D.)

Co-Author: Akira Kai (Ph. D.)
E-mail: akai@mm.mech.tohoku.ac.jp

Co-Author: Hitoshi Soyama (Ph. D.)
E-mail: soyama@mm.mech.tohoku.ac.jp

Department of Nanomechanics
Tohoku University
6-6-01 Aoba, Aramaki, Aoba-ku
Sendai, 980-8579, Japan