Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
AN X-RAY STUDY OF SHOT PEENING MATERIAL DURING FATIGUE Shigenobu TAKAHASHI Graduate student, KANAZAWA UNmRSITY Kakuma-machi, KANAZA WA 920-1192, JAPAN
Munetoh HASHIMOTO SUMITOMO HEAVYINDUSTRIES LTD Asahi-cho, OHBU 474-0024, JAPAN
Yukio HIROSE Dept., of Computer Science, KANAZAWA UNIVERSITY Kakuma-machi, KANAZA WA 920-1192, JAPAN
ABSTRACT A two-phase (austenite - ferrite) stainless steel sample was shot peened and then studied to measure changes in fatigue strength caused the shot peening. Other mechanical properties of surface were also measured; i.e., microstructure, residual stress, work hardening, etc.
KEY WORDS Shot peening, Stainless steel, Work hardening, Residual stress, Fatigue limit
INTRODUCTION Improvement in fatigue strength and residual stress in the surface of certain stainless steels such as spring steels and cogwheels is necessary for many industrial applications and is found to occur as a result of shot peening. Residual stress and a work hardening layer of the surface occur from shot peening. Martensite is found on the pole surface layer of austenite steel systems. A two-phase stainless steel (SUS329Jl-JIS) system was studied to show the effect of shot peening on the microstructure of the surface layer, the residual stress, and Vickers hardness, etc. Rotating and bending fatigue measurements were made in order to identify factors that influential fatigue strength. An austenite stainless steel (SUS304-JIS) specimen was examined to learn if work hardening occurred during shot peening.
EXPERIMENTAL 2.1. Test specimen A two-phase stainless steel SUS329Jl (hereafter written as X material) and an austenite ’ system stainless steel SUS304 (hereafter written as Y) were used. The mechanical property of each sample is listed in Tablel. After annealing at 923K for OShr, the austenite 50% and the ferrite 50%, X, sample was ground. Y material was assumed to have an austenite microstructure that was given a solid solution heat treatment (cooled quickly at 1423K for lhr) and ground. Shot peening processing conditions of X and Y materials are shown in Table2. The shot peening of two conditions (A, B) was given to three conditions and Y materials in X material. Moreover, it is shown that the surface is rough in Table3.
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Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
Table 1
Tensile strength 0.2% Proof Strength Elongation Reduction of area Vickers hardness
Mechanical property. SUS329Jl [X material] ou MPa 778 00.~ MPa 598 El% 39 cp% 71 HV 331
sus304 [Y material] 706 245 71 19 210
Table 2
Conditions of shot peening processes. A B C Air type Peening machine 0.3MPa Pressure 40 set Peening time Nozzle 9=9 mm Stainless 1 Steel 1 Steel Material Hardness (HV) 250 450 600 Arc height (mmA) 0.059 0.138 0.155 Distance 110 nun
.2. Test conditions 2.2.1. Hardness measurement Vickers hardness was measured (load of 49N for 30sec.) before and after shot peening. 2.2.2. X-ray diffraction experiment A parallel beam X-ray stress measurement unit (Rigaku MSF-2M) was used to measure the residual stress and the volume fractions of austenite, ya. The X-ray diffraction exposure conditions are shown in Table4. The analysis of the surface layer was with this unit. Table 4
X-Ray conditions. Ferrite (a) IAustenite (y) Characteristic X-ray CrKa Diffraction reflection y-Fe 220 a-Fe 211 Diffraction angle 28 155.0 128.5 Sin29 0.0-0.6 Tube voltage 30kV Tube current 1omA Filter Vanadium
2.2.3. Test of fatigue strength Frequency fatigue test and rotating bending fatigue tests were performed. Cyclic loading speed of the load was measured at room temperature at a frequency of f=30Hz and a stress ratio of R=-1. The test specimen had a diameter of @=8mm, 30mm in length; and grip part diameter @=15mm of the parallel part.
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Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
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RESULTS 3.1. Relationship of hardening and shot peening Fig.1 contains values of Vickers hardness (HV) distribution of sample X and Y. Vickers hardness increased going from a depth of lOO-150ym toward the surface of the material. 700 0 600 -
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(b) SUS304 [Y]. (a) SUS329Jl [Xl. A plot of Vickers hardness versus depth from the shot peened surface.
3.2. X-ray results X-ray data shows the shot peening condition A, (as shown in Table2) produced a comparatively small change in surface roughness. Fig.2 shows the X-ray diffraction patterns of the surface of X and Y material after shot peening. The change in volume fraction of austenite, ya on the surface of X material changed from (Fig.2) 20 to 50 percents. The peak height of the y phase lowers, and the peak height of the a phase increased after shot peening. Shot peening produced martensite on the surface of sample. 10000 5000 t
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----
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(a) SUS329Jl [Xl. Fig.2
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(b) SUS304 [Y]. X-ray diffraction data.
Fig.3 shows the 28-sin2v diagrams of X and Y at a depth of 25pm after shot peening. Similar straight-line data was observed at other depths. The residual stress distribution after shot peening according to condition A is shown in Fig.4. The residual stress of each phase was shown about an aspect and y phase in X material. X material, Y material, and reducing the residual stress was produced by shot peening the surface, and X material was returned to the residual stress value of the original material with Y material at a depth of about 100p.m and for a two phase at about 125-15Opm. Fig.5 shows the volume fraction of austenite distribution after shot peening of both materials. The volume fraction of austenite was about 38% on the surface in SUS304 and about 20% in SUS329Jl. X material was returned to the volume fraction of austenite value of the original material at a depth about 130pm and Y material was returned to it about 1OOpm. Moreover, it almost corresponds to the settling point of the residual stress in X material.
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
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130.0 - Cr Ka y-Fe 220
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(a) SUS329Jl (b) SUS304 A plot of residual stress versus depth from the shot peened surface. I
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Depth from Shot-peened surface y pm
(b) SUS304. (a) SUS329Jl. A plot of volume fraction of austenite versus depth from the shot peened surface.
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
3.3 Result of fatigue strength test The result of the fatigue strength test of X and Y material is shown in Fig.6. The fatigue strength was improved by shot peening. The fatigue limit after (rw=212MPa and shot peening became aw=25 1MPa. Improvement in the strength increased with shot peening time. An increase in fatigue strength is thought to be the result of a reduction in residual stress and increase in surface hardness.
105 106 107 108 Number of cycles to rupture N, cycles
Fig.6
S-N diagram.
DISSCUSSION 4.1. Residual stress The average residual stress can be calculated by equation (1): equation (1) o~fta,(l-f)=a’ Here, residual stress of the ferrite phase and each austenite phase and f show the volume fraction of the ferrite phase in ou and cry, and cr’ is on the average comprehensively residual stress. Fig.8 shows the result. The residual stress was converged to OMPa about 250p.m in depth. Moreover, it is thought that it depends upon distribution shape of the on the average comprehensively residual stress to the residual stress of a phase. It is thought about the residual stress value of the a phase in the experiment and the influence of the effect of the processing of a phase by the shot peening produced a high compression residual stress compared with to the y phase. We hope to study the effect of shot peening and work hardening on the a phase in the fatigue. 200 ------
0 -200 -400 -600 -800 -1000
Fig.8
0
100 150 200 50 Depth from shot-peened surface y pm
A plot of residual stress versus depth from the shot peened surface of SUS329Jl steel.
4 ,I‘.l,,’ 121 “9
Copyright(c)JCPDS-International Centre for Diffraction Data 2000,Advances in X-ray Analysis,Vol.43
4.2. Hardness The change in the volume fraction of transformed martensite, a’ and Vickers hardness, HV obtained from the change in the volume fraction of austenite on the surface obtained is shown in Table5. X material was excellent in fatigue strength though Y material had risen than X material as for the volume fraction of transformed martensite as a result. As for fatigue strength, it is understood that an increase in hardness results from an increase in the volume fraction of the martensite after shot peening process.
X Y
Table5 Results of material test. HV a’ % After After Before Before 0 30 310 470 0 74 265 408
% 51 54
CONCLUSION Shot peening was applied to a two-phase stainless steel (SUS329Jl-JIS) and an austenite stainless steel (SUS304-JIS). We concerned (after analyzing the surfaces of the samples and after fatigue tests were measured on the samples.) that: (1) It was steeled the two-phase stainless steel, the austenite system stainless steel, and the processing martensitic transformation was formed in the following surface layer neighborhood of the shot peening both. Moreover, the influence depth and the austenite distribution to the residual stress distribution were almost corresponding in the two-phase stainless steel. (2) It is thought that the residual stress of the dual phase stainless steel produced by shot peening contributes to the work hardening of alpha phase. (3) As a result of the fatigue strength test, fatigue strength of stainless steel was found to improve by shot peening. It is believed that the influence of work hardening is larger than the influence of residual stress as a factor in improving of the fatigue strength of the dual phase stainless steel.
REFFERENCE 1. K. Asami and M. Hironaga, Journal of the Materials Science, Japan, 43, (1994) 12-18. 2. M. Hayashi and K. Enomoto, Journal of the Materials Science, Japan, 45, (1996) 1107-1113. 3. H. Nakamura and T. Amakazu, Transactions of the Japan Society of Mechanical Engineering, 30, (1964) 494-500. 4. K. Hayashi and Y. Natsume, Journal of the Materials Science, Japan, 21, (1972) 11181124. 5. K. Saruki, S. Horita, H. Fujita and T. Arai, Journal of the Materials Science, Japan, 37, (1988) 770-776. 6. H. Ouchida, A. Nishioka and T. Hayama, Journal of the Materials Science, Japan, 21, (1972) 733-739.
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