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Vol. 7
November Issue
Year 2006
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Science Update


in Vol. 7 - November Issue - Year 2006
Macro and Micro Strain in Polycrystalline Metal Controlled by Cavitation Shotless Peening



Author: Hitoshi Soyama (Ph. D.)








Fig. 1: Aspects of surface








Fig. 2: Debye ring detected by 2D PSPC


Fig. 3: Distribution of X-ray diffraction of NP, CSP and SP


Fig. 4: Introduction of compressive residual stress and decrease of FWHM by CSP


Fig. 5: Distribution of X-ray of CSP changing with processing time

Introduction

These days, peening methods using cavitation impacts have been developed. The peening mehethod was called "cavitation shotless peening (CSP)" [1]-[4], as shots were not required. The great advantage of CSP is that the increase of surface roughness of CSP was considerably small compared with that of a shot peened one, since there was no solid collision in CSP.   

Normally, the cavitation in CSP is produced by injecting a high-speed water jet into a water-filled chamber [1]-[3]. Recently the cavitating jet in air without a water-filled chamber was created [5],[6]. It was found that the ability of optimized cavitating jet in air was more powerful compared with that of a cavitating jet in water [5],[6]. It was also revealed that full width at half maximum (FWHM) of the diffractive X-ray distribution from the surface treated by CSP was decreased in polycrystalline metals, even though the compressive residual stress was introduced by CSP.  In case of shot peening (SP), FWHM was certainly increased. The FWHM was closely related to micro strain, i.e., strain in grains. On the other hand, the compressive residual stress was macro strain. For the case of mono-crystalline material, FWHM was increased [7]. It might be that the micro strain induced by defects such as dislocation in polycrystalline metals was released by CSP.  

In the present paper, the FWHM assumed as micro strain and the residual stress was investigated by using X-ray diffraction. The tool steel alloy for forging die was chosen as a tested material, as the life time of the forging die was improved by CSP [8]. 

Tested Specimen
 
In order to investigate the peening effect of CSP compared with SP, tool steel alloy Japanese Industrial Standards JIS SKD61 was used. The specimens were given the normal heat treatment used for hot forging die.  The specimens were heat treated at 873K for 1 hour and then quenched, followed by a further heat-treatment at 1123K for 1 hour and 1295K for 1.5 hours, respectively. Then they were tempered at 833K for 5 hours.  Four specimens were prepared for the test as shown in Table 1. Specimen B was treated by CSP using a cavitating jet in air whose injection pressure was 20 MPa.  In order to compare the results of CSP and SP, the specimen C was treated by SP. The injection pressure of the shot was 0.1 MPa, the peening time was 5 seconds and casting steel shot 0.3 mm in diameter  was used.  The arc height of the Almen strip under these conditions was 0.323 mm using N gage. The specimen D was treated by the cavitating jet in air whose injection pressure was 30 MPa changing with processing time per unit length. The processing time was defined by the scanning speed and number of scans. 

X-Ray Measurement

The residual stress and FWHM in the surface was measured by X-ray diffraction system which had two dimensional position sensitive particles counter (2D PSPC). The tube used was a Cr tube operated at 35 kV and 40 mA. The diameter of used collimator was 0.5 mm. X-rays were counted for 5 minutes at each step using 2D PSPC at condition of  deg and ? = 0 deg at ? = 90; ? = 0, 45, 90, 135, 180 deg at ? = 75 deg; ? = 0, 45, 90, 135, 180 deg at ? = 60 deg; ? = 0, 45, 90, 135, 180 deg at ? = 45 deg; ? = 0, 45, 90, 135, 180 deg at ? = 30 deg, i.e., total 21 frames. The diffractive plane was the (200) plane and (211) of ?-Fe, the diffractive angle 2? was 106 and 156 deg.  Sample orientation ? was set as 50 deg at 2? = 106 deg, and as 102 deg at 2? = 156 deg. The used Young’s modulus and Poisson ratio were 210 GPa and 0.28 for calculation of residual stress using 2D-method [9].  6 components of residual stress were obtained by one-set of measurements, i.e., 21 frames, using the 2D PSPC.   

Surface Aspects
 
Figure 1 illustrates the aspects of specimen surface. In case of CSP, the polished lines were observed and plastic deformation pits induced by the cavitation impacts were marked by blue arrows. The surface roughness Ra of NP and CSP were 0.06 m and 0.10 µm, respectively.  On the other hand, that of SP was 1.01 µm. Namely, the peened surface by CSP was very smooth. 

Macro and Micro Strain
 
Figure 2 shows a part of Debye ring of ?-Fe in (200) plane, i.e., 2? = 106 deg detected by 2D PSPC for NP, CSP and SP. Figure 3 reveals the distribution of X-ray diffraction as a function of diffraction angle 2?, integrating the results of Fig. 2 along ?-axis. Clearly, FWHM of CSP 0.86 deg is smaller than that of SP 1.94 deg, even though NP case was 1.33 deg. Actually, FWHM of NP, CSP and SP of ?-Fe in (211) plane, i.e., 2? = 156 deg were 3.24 deg, 2.55 deg and 3.96 deg, respectively. The residual stress obtained from the data of 2? = 106 deg using 2D method were -590 ± 20 MPa for NP, -910 ± 20 MPa for CSP and -970 ± 20 MPa for SP. Those of 2? = 156 deg using 2D method were -330 ± 20 MPa for NP, -740 ± 10 MPa for CSP and -630 ± 20 MPa for SP.  The difference in each case depends on the penetration depth of X-ray. When conventional sin2? method was applied, the residual stress using 2? = 156 deg were -390 ± 50 MPa for NP, -830 ± 10 MPa for CSP and -780 ± 10 MPa for SP. Although CSP can introduce the compressive residual stress, FWHM of CSP is getting smaller than that of NP and it does not depend on the diffractive plane. 

Figure 4 reveals the distribution of X-ray diffraction as a function of processing time per unit length. Specimen D was used. FWHM of specimen A (NP), B (CSP) and C (SP) were also shown in Fig. 4.  Figure 5 illustrates the distribution of X-ray diffraction as a function of diffraction angle 2?, changing with the processing time as a results of specimen D. As shown in Figs. 4 and 5, when the compressive residual stress was introduced by SP as mentioned above, FWHM of SP was increased.  However, compressive residual stress was increasing with the processing time of CSP and FWHM was decreasing with the time. This means that CSP releases micro strain which was introduced by heat treatment or machine finishing, although CSP can introduce the macro strain into the material surface.  Namely, macro and micro strain in polycrystalline metal can be controlled by CSP, individually. 

Acknowledgement

This work was partly supported by the Japan Society for the Promotion of Science under the Grant-in-aid for Scientific Research (B) 17360047. 

References

1. 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.
2. Odhiambo, D. and Soyama, H., 2003, “Cavitation Shotless Peening for Improvement of Fatigue Strength of Carbonaized Steel,” International Journal of Fatigue, Vol. 25, Nos. 9 – 11, pp. 1217 – 1222.
3. 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.
4. Soyama, H., 2006, “Improvement of Fatigue Strength of Metallic Materials by Cavitation Shotless Peening,” Metal Finishing News, Vol. 7, March Issue, pp. 48 – 50.
5. 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.
6. Soyama, H., 2005, “High-Speed Observation of a Cavitating Jet in Air,” Journal of Fluids Engineering, Trans. ASME, Vol. 127, No. 4, pp. 1095 – 1101. 
7. Soyama, H., Saito, S., Macodiyo, D.O. and Koyanagi, M., 2005, “C-V Characteristics of Backside Damage Gettering Introduced by a Cavitating Jet in Silicon Wafer,” Proceedings of 2nd International Symposium on Mechanical Science Based on Nanotechnology, pp. 85 – 88.
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. 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.

Specimen No.

Surface grinding

Peening

Processing time

A

#1500 emery paper

None

-

B

#1500 emery paper

CSP

100 s/mm

C

#1500 emery paper

SP

5 s

D

#2000 emery paper

CSP

0 – 8 s/mm

Table 1 Specimen preparation


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