1

Cav03-OS-2-3-002 Fifth International Symposium on Cavitation (CAV2003)

Osaka, Japan, November 1-4, 2003

IMPROVEMENT OF FATIGUE STRENGTH ON STAINLESS STEEL BY CAVITATING JET IN AIR

Hitoshi Soyama Department of Mechanical Engineering Tohoku University Aoba-yama 01, Aoba-ku, Sendai, 980-8579, JapanEmail: soyama@mm.mech.tohoku.ac.jp

Dan Macodiyo Department of Mechanical Engineering Tohoku University Aoba-yama 01, Aoba-ku, Sendai, 980-8579, JapanEmail: dan@mm.mech.tohoku.ac.jp

ABSTRACT Beneficial residual stresses can be introduced on the surface of materials thereby impeding the initiation and/or development of surface cracks and subsequently prolonging the fatigue life of components. We have developed a new method of peening technology, cavitation shotless peening by using a cavitating jet in air. This process makes use of pressurized water passed through concentric nozzle. The high-pressure water jet is surrounded by the low-pressure water jet. The cavitating jet occurs in the air. When the cavitation bubbles collapse impacts are produced. This in turn imparts the specimen surface and thereby introducing compressive residual stress. In this study, the potentials of cavitating jet in air as a means of improving fatigue strength has been explored by analyzing stainless steel (JIS SUS316L) in the non-peened and peened conditions with respect to processing times, residual stress and cyclic-stress curves. The residual stress was measured by X-ray diffraction method. The surface residual stress of non-peened and peened specimen was −68 MPa and −350 MPa, respectively. Experimental results confirmed that a rotating beam fatigue strength of the peened specimen increased by 20% in comparison to the non-peened specimen. INTRODUCTION Residual stress is that which remains in a body that is stationery and at equilibrium with its environment. It can be detrimental to the performance of components or beneficial when effectively introduced. In this study beneficial residual stress was introduced by cavitation shotless peening (CSP). CSP is surface enhancement technology for metallic materials. The process is similar to the convectional shot peening except that shot peening uses shots to impact residual stress. In CSP, cavitation impact is induced by a high-speed water jet with cavitation, herein referred to as cavitating jet. The eventual size of fluid particles, consists of bubbles and drops, is determined by the nature of the process leading to breakup and coalescence where significant [1]. A bubble is a mass of gas in external medium and a drop is mass of liquid in a liquid or gaseous medium. Bubbles and drops may break in

quiescent media due to interfacial instabilities caused by density differences, rupture when subjected to large velocity gradients, deform and disintegrate due to turbulent flow conditions, elongate and strip due to strong electrical force, or disintegrate or shatter due to rapid acceleration by shock waves [2]. In this study, the mechanism of bubble collapse was shock wave and microjets before or on the specimen surface. The collapse of a single cavity starts from the outwards and proceeds inwards. Much of the energy is transferred to the internal part which leads to higher collapsing intensity. The collapsing cavitation bubble impinges the specimen causing work hardening to take place. The work hardening, which is dominated by dislocation of forest interaction type, causes plastic deformation on the surface thereby creating a compressive layer which impedes the initiation of fatigue cracks and/or crack propagation. The authors have already investigated the introduction of compressive residual stress and the improvement of fatigue strength in the submerged condition [3-6]. The cavitating jet intensity and the occurring region are controlled with parameters such as upstream pressure and nozzle size. Soyama et al. [7-9] have proved that the surface finish of CSP specimen are known to be of better quality finish than shot peened specimen due to the fact that there is no solid body collision. Soyama and Saito developed and tested the cavitating jet in air for large components in aerospace and automobile industry [10]. A high-speed water jet was injected into a low-speed water jet. Thus, cavitating jet in air was obtained. In this paper, CSP in air is referred to as CSPair. The well known mechanisms of plastic deformation are slip, twinning, displacive transformations and diffusional creep [11]. Martensitic transformation, which is a mode of plastic deformation, occurs as a result decrease in austenitic ratio. Stainless steels undergo deformation induced martensitic transformation forming locally concentrated stress at obstacles such as grain boundaries. Subsequently the plastic deformation of austenite becomes equal to the critical stress. Thus, the austenitic ratio was measured to deduce the relation between it and the compressive residual stress.

2

Metallic components are known to increase their dislocation density when cold worked. Subsequently, the size of perfectly crystalline regions are reduced thereby increasing the average microstrain in the crystal lattice [12,13]. The reduced crystallite size and increased microstrain both produce broadening of diffraction peaks which can be used to attest the degree of cold work. In this study, we investigated the half-width values of the X-ray diffraction profiles to characterize the depth distribution of compressive residual stress in a JIS SUS316L treated by CSPair. It is worth noting that this is the first paper to report the residual stress of stainless steel based on the reducing half width technique. To determine the improvement of fatigue strength, the specimen was tested using the rotating bending test with a stress ratio, R = −1.

PRINCIPLE OF CAVITATING JET IN AIR We have developed two methods of cavitating shotless peening to suit various applications in aerospace and automotive industry. Figure 1 illustrates the difference between the cavitating jet in air (CSPair) and the cavitating jet in water (CSPwater). The principle of the cavitating jet in air lies on the fact that cavitation is done in the atmosphere without a water-filled chamber.

It has also two concentric nozzle for the low- and high- pressure water. The high pressure water jet is surrounded by the low-pressure water jet which acts as a moving water-filled boundary therein creating wall effect as the cavitation takes place [14]. EXPERIMENTAL TECHNIQUES Experimental facility CSP was performed in laboratory facility. Figure 2 shows the schematic diagram for cavitating jet apparatus. The cylindrical test specimen was held onto a special screw attachment in tank A to allow for both rotational and lateral movements. The nozzle has concentric diameters d1 and d2 for high-and low-pressure water, respectively (see Fig. 1). Low-pressure water is pumped to the nozzle using the turbine pump at 0.33MPa and a flow rate of 0.36 m3/min, whilst the high-pressure water is pumped by the plunger pump at 35MPa and flow at rate of 0.03 m3/min. The standoff distance is defined as the distance between the upstream corner of the nozzle of high-pressure water and the surface of the test specimen. The optimum standoff distance sopt was determined qualitatively by an erosion test in which the standoff distance was varied. The high- and low-pressure water jet coalescence into cavitation bubbles. When the bubbles collapse at the specimen surface, they induce compressive residual stress. Water from tank A flows into tank B gravitationally through the valve, Va. The main parameter of the cavitating jet is the cavitation number, which is defined as the measure of the resistance of the flow to cavitation [15,16]. In nozzles and orifices, the flow velocity depends on the pressure difference between the upstream and downstream pressures. Hence the cavitating number σ is given by:

LH

vL

pppp

−−

=σ (1)

where pL, pH, and pv are the low pressure water, high pressure water and vapour pressures, respectively. Since pH > pL > pv, Eq.1 can be rewritten as:

H

L

pp

=σ (2)

The residual stress was measured at different exposure per unit length t to determine the optimum scanning speed vopt. Thus, the exposure time per unit length is expressed as the ratio of the number of scans n to the scanning speed v. (3) Material The cylindrical test specimen, stainless steel (JIS SUS316L) diameter 15x150mm long, was made to conform to JIS Z2274 specifications and had a gauge dimensions as shown in Fig. 3. The chemical composition by weight percentage of the test

Water-filledchamber

High pressurewater

Cavitatingjet

Nozzle

High pressurewaterLow pressure

water

Cavitatingjet

Nozzle

Specimen

1cm 1cm

Water-filledchamber

High pressurewater

Cavitatingjet

Nozzle

High pressurewaterLow pressure

water

Cavitatingjet

Nozzle

Specimen

High pressurewaterLow pressure

water

Cavitatingjet

Nozzle

Specimen

1cm1cm1cm 1cm1cm1cm

Fig. 1 Principle of cavitation shotless peening

Bypass valve

Bypass valve

Plunger pump

Pressure gauge

Tank B

Tank A

Pressure gauge

Turbine Pump

Valve Va

Nozzle

Bypass valve

Bypass valve

Plunger pump

Pressure gauge

Tank B

Tank A

Pressure gauge

Turbine Pump

Fig. 2 Cavitating jet apparatus

vn

t =

3

specimen is shown in Table 1. The specimen was heat treated at 1080oC and then water-quenched. The mechanical properties were designated under JIS 4 specifications. The yield strength and tensile strength were 337 N/mm2 and 552 N/mm2, respectively. The material had an elongation of 53% and hardness of 156HB. Table 2 shows the peening conditions for the JIS SUS316L. X-ray diffraction characterization The residual stress was measured by X-ray diffraction method. In order to evaluate the peening effect, the residual stress σR in the surface was measured by a side inclination method using X-ray diffraction. The stress was measured parallel to the longitudinal direction of the specimen, since the direction of the applied maximum bending stress during the rotating bending fatigue test was in this direction. A Cr tube operated at 30kV and 8mA was used for producing Kβ X-rays. The angle of the solar slit was 1 degree and the slit of 3mm x 5mm. X-rays were counted for 4s at each step using a scintillation counter at angles of ψ = 10, 20.8, 28.1, 34.3, and 40 degrees. Here, ψ is the angle between the normal to the specimen surface and the normal to the diffractive face. The lattice plane hkl was (311). The diffractive angle 2θ without strain was 148.5 degrees and the stress factor of the X-ray diffraction method was –369.5 MPa/deg. The diffractive angle was determined by the half value method. Residual stress was calculated using the sin2θ - sin2ψ method, i.e., the gradient of the line from six points on the diagram using the least square method. Table 1. Composition of JIS SUS316L by weight %.

C Si

Mn P S Ni Cr Mo

0.017 0.31 1.72 0.03 0.009 12.14 16.75 2.00 Table 2. Testing conditions for JIS SUS316L

Nozzle diameter (mm)

Standoff distance (mm)

Upstream pressure (MPa)

Downstream pressure (MPa)

d1 d2 s1 s2 20 0.21 0.8 30 45 52.5

RESULTS Figure 4 shows the plots of the residual stress against the processing time per unit length. The compressive residual stress increased gradually with increase in processing time. The saturation level, i.e., the point at which there is no significant increase in residual stress despite increase in processing time was considered to be at 100 s/mm. At a processing time of t = 0 and t = 100 s/mm, the residual stress was −68 and −350 MPa, respectively. The plots of the residual austenitic ratio against the processing time per unit length have been illustrated in Fig. 5. There was a near-constant austenitic ratio of 90% up to a processing time of 40 s/mm. The austenitic ratio gradually decreased after the saturation level (see Fig. 4). This illustrates further that the austenitic ratio of JIS SUS316L is affected after the sub-surface attains maximum plastic deformation. Figure 6 shows the mean plots of normalized half value width versus the processing time per unit length. For every processing time, the half width was measured twice and the deviation from the mean is shown as a bar. The mean curve of fit was then plotted. It can be seen that the half width value decreased with increased in processing time. Most steels are known to have a positive gradient for the half-width value plot against the processing time. However, the test specimen had a negative slope.

Ø 1

0 ±

0.5

R 50

Ø 1

5

50

Fig. 3 Geometry of test specimen.

-600

-400

-200

0

200

0 20 40 60 80 100Processing time per unit length t mm/s

Res

idua

l stre

ss σ

R M

Pa

Fig. 4 Residual stress changing with processing time

Processing time per unit length t mm/s

Res

idua

l aus

teni

tic ra

tio R

A %

Fig.5 Residual austenitic ratio changing with processing time

60

70

80

90

100

0 20 40 60 80 100

4

The half width value at a processing time of t = 0 and t = 100 s/mm was 1 and 0.94, respectively. From the non-peened surface which is the base material of the CSP specimen of stainless steel, the experimental results in this study indicate that compressive residual leads to plastic deformation which increases the yield strengths while it decreases the half-width values. Figure 7 shows the plots of stress amplitude versus number of cycles to failure. The fatigue limit was considered to be at 107 cycles. Applying Little’s method [17], the non-peened specimen had a fatigue limit of 325 MPa while the CSP had 386 MPa. Thus, the fatigue limit of CSP treated specimen increased by 20% compared with the non-peened specimen. CONCLUSIONS In the present study, cavitation bubble impacts induced by cavitating jet in air was used to introduce compressive residual stress on stainless steel (JIS SUS316L). The specimen was characterized using X-ray diffraction method with a side inclination technique and the fatigue limit determined using a rotating beam fatigue test. The following conclusions are drawn from this study:

1. There was an increase in compressive residual stress until the saturation limit, at a processing time of t = 100 s/mm.

2. Cavitating jet in air can cause transformation plasticity in stainless thereby decreasing their austenitic ratio after the saturation. Consequently, there was a corresponding decrease in half width value. The half width values of the CSP surface and surface layer gradually decrease.

3. There was an increase in fatigue strength by 20% on CSP treated specimen compared to the non-peened specimen. This shows that cavitating jet in air can increase the fatigue life of metallic materials by retarding the initiation and propagation of cracks.

ACKNOWLEDGEMENTS This work was partly supported by the Japan Society for the Promotion of Science under the Grant-in-aid for Scientific Research (B)(2) 13555022 and 14350049. REFERENCES 1. N. Ashgriz and J.Y. Poo, J. of Fluid Mechanics, 221(1990)

p.183. 2. S.S. Sadhal, P.S. Ayyaswamy and J.N. Chung, Transport

Phenomena with drops and bubbles, (1997), p.311. 3. D. Odhiambo and H. Soyama, Cavitation Shotless Peening

for Improvement of Fatigue Strength of Carbonised Steel, Int. J. of Fatigue, 25 (2003) (in press).

4. H. Soyama, J.D. Park and M. Saka, J. Manuf. Science snd Engineering, Trans. ASME, 122 (2000), p.83.

5. H. Soyama, T. Kusaka and M. Saka, Proc. 6th Inter. Conf. on Residual Stresses, (2000), p.674.

6. Y. Yamauchi, H. Soyama, Y. Adachi, K. Sato, T. Shindo, R. Oba, R. Oshima and M. Yamabe, JSME Int. J., Ser. B, 38 (1995), p.31.

7. H. Soyama, K. Saito and M. Saka, J. Engineering Materials and Technology, Trans. ASME, 124 (2002), p.135.

8. H. Soyama, H. Kumano, K. Saito and M. Saka, Proc. APCFS & ATEM’01, JSME, 2 (2001), p.1047.

9. H. Soyama, J. Soc. Mater. Sci. Japan, 47 – 4 (1998), p. 381 (in Japanese).

10. H. Soyama and K. Saito, Proc. Fluids Eng. Conf. 2001, (2001), p.26.

11. R.W. Cahn and P. Haasen, Physical metallurgy, Vol. 2., 4th Revised and Enhanced Edition, Elsevier Science B.V., Netherlands, p.1508.

12. D.O. Macodiyo, H. Soyama and M. Saka, J. Materials Science Letters, 22 (2003), p.115.

13. P.J. Withers and H.K.D.H. Bhadeshia, J. Mater. Sci. and Tech., 17 (2001), p.355.

14. R. Clift, J.R. Grace and M.E. Weber, Bubble Drops and Particles, Academic Press, California (1978), p.321.

15. Brenen, C.E., Cavitation and Bubble Dynamics, (1995), p.20, Oxford University Press.

16. Young, F.R., Cavitation, (1999), p.189, Imperial College Press

17. R.E. Little, Probabilistic Aspects of Fatigue, ASTM STP 511(1972), p.29.

300

350

400

450

500

CSP

Non-peened

104 105 106 107 108

Number of cycles to failure N

Stre

ss a

mpl

itude

σa

MPa

386 MPa

325 MPa

Fig. 7 S-N diagram for SUS316L

Nor

mal

ized

hal

f val

ue w

idth

Fig. 6 Half value width changing with processing time

Processing time per unit length t mm/s

0.8

0.9

1.0

1.1

0 20 40 60 80 100