Materials Letters
Volume 62, Issue 20, 31 July 2008, Pages 3564-3566
Relieving micro-strain by introducing macro-strain in a polycrystalline metal surface by cavitation shotless peening
H. Soyamaa, , and N. Yamadab
aDepartment of Nanomechanics, Tohoku University, Aoba 6-6-01, Aramaki, Aoba-ku, Sendai 980-8579, Japan
bBruker AXS K.K., 3-9-A Moriya-cho, Kanagawa-ku, Yokohama 221-0022, Japan
Received 7 March 2007; accepted 27 March 2008. Available online 4 April 2008.
Abstract
Peening using cavitation impact is called “cavitation shotless peening CSP”, since there is no requirement for shot in the process. Micro- and macro-strain of polycrystalline metal peened by CSP were evaluated using X-ray diffraction methods, as the full width at half maximum (FWHM) of the X-ray diffraction profile from the peened surface was decreased, although compressive residual stress was introduced. It was found that CSP reduced the micro-strain in the surface, but simultaneously introduced compressive residual stress, i.e., a macro-strain. The results demonstrate that the micro-strain is relieved by CSP without the need for heat treatment, and is, therefore, a sort of annealing. Thus, CSP can renew the metallic material while the shape itself is maintained.
Keyword: X-ray techniques; Microstructure; Surface modification; Residual stress; Cavitation
Article Outline
1. Introduction
2. Experimental details
3. Results
4. Conclusions
Acknowledgements
References
1. Introduction
The collapse of cavitation bubbles gives rise to cavitation impact from shock waves [1]. This can cause severe damage in hydraulic machinery such as pumps and ships' propellers. However, the impact can be utilized to enhance the fatigue strength of polycrystalline metallic materials by introducing compressive residual stress and work hardening the surface in the same way as shot peening [2], [3], [4] and [5]. Peening method using cavitation impacts is called “cavitation shotless peening CSP” [3], [4] and [5]. It has been found that the full width at half maximum (FWHM) of the X-ray diffraction peak from the surface of peened tool steel alloy was decreased by CSP, even though compressive residual stress had been introduced into the surface by CSP [6]. The introduction of compressive residual stress into a metallic material means that macro-strain is introduced. It is commonly known that different types of residual strain exist in polycrystalline metals, i.e., macro- and micro-strain. In the present paper, strain which is homogenous on a large scale involving many grains is called macro-strain. On the other hand, strain caused by adjacent grains and random strain in grains is called micro-strain. Polycrystalline metal after quenching or mechanical finishing has large micro-strain, i.e., a high density of dislocations. The decrease of the FWHM and the introduction of compressive residual stress by CSP suggest that CSP, by introducing macro-strain into the surface, relieves the micro-strain near the surface of polycrystalline metals with initial large micro-strain. Of course, in the case of single crystalline material such as silicon, the FWHM is increased by CSP [7].
The FWHM is related to the micro-strain, grain size, the randomness of the macro-strain and the geometry of the X-ray diffractometer itself. It has already been reported that the FWHM is closely related to the micro-strain and is changed during fatigue tests [8] and [9]. It was verified by transmission electron microscopy (TEM) that the dislocation density was reduced by cyclic loading in a fatigue test by sweeping dislocations into the cell boundaries [9]. Ultrasonic vibration can also move dislocations in metals [10]. As vibration with high frequency components is induced by the shock wave as a cavitation bubble collapses, the vibration can move dislocations and some dislocations may disappear by moving to the grain boundary or by joining together in a grain. In order to evaluate the lattice strain and lattice size of nano-size powders, a fundamental parameter approach [11] and [12] was applied in the analysis of the X-ray diffraction profiles. Since the diameters of the plastic deformation pits induced by CSP are of the order of several tens of micrometers to several millimeters, using statistical analysis, such as the X-ray diffraction method, which can be used for analysis on this scale, is a better option than observation using TEM.
In the present paper, in order to prove that micro-strain is relieved by CSP through the introduction of a compressive macro-strain, the treated metal was investigated using X-ray diffraction. Note that this is the first report describing the relief of micro-strain by CSP induced macro-strain.
2. Experimental details
The material chosen for the tests was tool steel alloy (Japanese Industrial Standard JIS SKD61) which is used for forging die, and the life time of which can be improved by CSP [13]. This was heat-treated in the same way as hot forging die, that is, by heating at 873 K for 1 h, then quenching, and followed by further heat-treatments at 1123 K for 1 h and 1295 K for 1.5 h. The material was then tempered at 833 K for 5 h. The surface of the specimen was polished using #1500 grade and #2000 grade emery paper and finished with polishing powder. The size of the specimen was 45 mm long, 15 mm wide, and 18 mm thick. CSP was carried out using a cavitating jet in air [6] at an injection pressure of 30 MPa with a nozzle size of 1 mm. The processing time per unit length, t, depends on the scanning speed, v, and number of scans, n, as follows:
(1)
The micro-strain was evaluated using X-ray diffraction employing a fundamental parameter approach. The X-ray diffraction data were collected using equipment employing Bragg Brentano geometry and an X-ray tube operated at 40 kV and 40 mA using Cu Kα X-rays. Both the angle of the primary and secondary soller slits were set to 2.5°, the divergence slit and anti-scatter slit were set to 0.5°, and the receiving slit was 0.1 mm. A solid state detector was used to detect the diffracted X-rays in a 2θ range from 40° to 140° in 0.02° steps with 20 s exposure for each step. The specimen was rotated in the plane, and the area evaluated was 23 mm in diameter.
The residual stress in the surface was measured by X-ray diffraction using a 2D method [14]. An X-ray tube operated at 35 kV and 40 mA with Cr Kα peak was used. X-ray diffraction patterns were measured using a two-dimensional position sensitive proportional counter (2D PSPC) with the specimen rotated at angles, , of 0, 45, 90, 135, and 180° for 4 different values of the axial tilt angle, ψ, equal to 15, 30, 45, 60°, and also at = 0° and ψ = 0°. The 2θ diffraction angle of 106° for the α-Fe (200) reflection was used to evaluate the residual stress. The incident angle, ω, was set to 40.25° and the exposure time was 10 min for each frame.
3. Results
Fig. 1 shows the residual stresses σ1 and σ2 of the specimen as a function of processing time per unit length. σ1 is the longitudinal stress and σ2 the lateral residual stress. Before CSP, the residual stresses in both directions after polishing were compressive and were about − 600 MPa. After CSP, the compressive stresses increased and saturated at about − 1000 MPa. Clearly, the CSP has introduced compressive residual stress, i.e., macro-strain into the surface of the specimen.
Full-size image (10K)
Fig. 1. Introduction of compressive residual stress by cavitation shotless peening.
View Within Article
Fig. 2 illustrates the variation with CSP processing time of the X-ray diffraction peak profiles from the (110), (200) and (211) reflections. In all cases, the peak profiles get sharper with increasing CSP. The peak intensity in each case increased until t = 8 s/mm and then decreased slightly at t = 10 s/mm. Fig. 3 shows the variation in the FWHM of the peaks with processing time. The FWHM is normalized to the value of the FWHM before CSP, i.e., at t = 0 s/mm. It can be concluded from Fig. 2 and Fig. 3 that the peak intensities increase and the FWHMs decrease with CSP. The increase in peak intensity and the decrease in FWHM signify that CSP enhances the crystalline perfection of the specimens. That is, micro-strain introduced by the heat treatment and mechanical polishing is relieved by CSP.
--------------------------------------------------------------------------------
Full-size image (43K)
Fig. 2. X-ray diffraction peak profiles for micro-strain analysis.
View Within Article
--------------------------------------------------------------------------------
Full-size image (13K)
Fig. 3. Decrease of the FWHM by cavitation shotless peening.
View Within Article
Fig. 4 shows the variation of micro-strain with processing time calculated using the fundamental parameter method. The micro-strains were obtained from each of the peak profiles shown in Fig. 2. The micro-strain determined from the (110) reflection was 0.064 ± 0.003 before CSP, and reduced by nearly one half, i.e., 0.034 ± 0.003 at t = 2 s/mm. At t = 8 s/mm it was reduced to 0.003 ± 0.002, and finally was 0.0007 ± 0.0022 at t = 10 s/mm. The micro-strain determined from the (200) reflection was 0.106 ± 0.009 before CSP, and became 0.001 ± 0.002 at t = 10 s/mm and that from the (211) reflection was 0.042 ± 0.005 before CSP, and 0.0007 ± 0.0043 at t = 10 s/mm. In all three cases, the micro-strain was decreased by CSP. That is, the CSP relieved the micro-strain by introducing compressive residual stress into the material surface. Cyclic loading and ultrasonic vibration move dislocations [8], [9] and [10]. Thus, the plastic deformation and/or high frequency components in the vibration induced by the collapsing cavitation bubbles move the dislocations, relieving the micro-strain.
--------------------------------------------------------------------------------
Full-size image (12K)
Fig. 4. Relief of the micro-strain by cavitation shotless peening.
4. Conclusions
In order to investigate the micro- and macro-strain before and after CSP, the surface of tool steel alloy was treated by CSP and analyzed using X-ray diffraction methods. It was found that CSP can relieve the micro-strain introduced by heat treatment and mechanical finishing by introducing a macro-strain, such as a compressive residual stress, in the material. Note that the fundamental parameter method can be used to evaluate the micro-strain in bulk polycrystalline metals.
Acknowledgements
This work was partly supported by Japan Society for the Promotion of Science under Grant-in-Aid for Scientific Research (B) 17360047.
References
[1] C.E. Brennen, Cavitation and Bubble Dynamics, Oxford Univ. Press (1995).
[2] H. Soyama, T. Kusaka and M. Saka, Journal of Materials Science Letters 20 (2001), pp. 1263–1265. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (32)
[3] H. Soyama, K. Saito and M. Saka, Journal of Engineering Materials and Technology, Trans. ASME 124 (2002), pp. 135–139. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (34)
[4] D. Odhiambo and H. Soyama, International Journal of Fatigue 25 (2003), pp. 1217–1222. Article | PDF (461 K) | View Record in Scopus | Cited By in Scopus (36)
[5] H. Soyama and D.O. Macodiyo, Tribology Letters 18 (2005), pp. 181–184. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (9)
[6] H. Soyama, Journal of Engineering Materials and Technology, Trans. ASME 126 (2004), pp. 123–128. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (17)
[7] H. Soyama, S. Saito, D.O. Macodiyo and M. Koyanagi, Proceedings of 2nd International Symposium on Mechanical Science Based on Nanotechnology (2005), pp. 85–88.
[8] S. Taira, Experimental Mechanics 13 (1972), pp. 449–463.
[9] D.J. Quesnel, M. Meshii and J.B. Cohen, Materials Science and Engineering 36 (1978), pp. 207–215. Abstract | PDF (788 K) | View Record in Scopus | Cited By in Scopus (4)
[10] I. Ostrovskii, N. Ostrovskaya, O. Korotchenkov and J. Reidy, IEEE Transactions on Nuclear Science 52 (2005), pp. 3068–3073. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (4)
[11] H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures (2nd edition), J. Wiley and Sons Inc., New York (1974), p. 996.
[12] A.A. Kern and A.A. Coelho, A New Fundamental Parameters Approach in Profile Analysis of Powder Data, Allied Publishers Ltd. (1998), pp. 144–151.
[13] H. Soyama, Y. Takano and M. Ishimoto, Technical Review of Forging Technology 25 (2000), pp. 53–57.
[14] B.B. He and K.L. Smith, Proceedings of ICRS-5, Linkoping, Sweden (1997), pp. 634–639.
Re: Shotpeening advice
Reply With Quote
Originally Posted by oldcorollas
i just know some things that engine builders do, don't have any practical method, but they hear someone else did it, so they had to keep up or lose customers..)
Bookmarks