Influence of plastic deformation upon the half-width of engineering metallic materials in hard state
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I.
INTRODUCTION
SHOT peening can raise the fatigue strength of a metallic material to a great extent.[1–4] It is well known that the compressive residual stresses induced in the surface layers, where the fatigue failure commonly initiates, and the microstructural improvement of such layers are the two important strengthening mechanisms.[5,6] But, on the other hand, almost all of the experimental results have illustrated that shot peening decreases the half-width values of the materials in hard state,[7–13] the shot-peening strengthening effect of which is usually the most pronounced.[5,14] According to this fact, many researchers claimed that the peening-affected layers of hard materials were work softened, and some of them proposed that the mechanism was similar to that of cyclic softening occurring in fatigue testing.[11,12,13,15–20] In order to resolve the preceding contradiction, in this article, the effect of plastic deformation on the half-width is investigated, and the reliability of using this parameter to judge whether the peening-affected layer of a hard material is work hardened or work softened is inspected. II.
EXPERIMENTAL PROCEDURE
The specimens were taken out of a cold-rolled alloy steel sheet 4-mm thick with a composition of (wt pct) carbon 0.37, silicon 1.43, manganese 0.94, chromium 1.02, nickel 1.93, molybdenum 0.53, vanadium 0.10, sulfur 0.0025, and phosphorus 0.011. Longitudinal direction was parallel to the rolling direction. No decarburized layer was observed on the surface. After having been shaped, the specimens were annealed at 700 7C for 5 hours in argon, heated at 830 7C for 10 J.B. LI, Associate Professor, H.B. XU and R. CHEN, Graduate Students, and Z.G. WANG, Professor, are with the State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China. Manuscript submitted October 12, 1995. 3662—VOLUME 27A, NOVEMBER 1996
minutes in a salt bath, and quenched in oil followed by a tempering treatment at 300 7C for 2 hours. The Young’s modulus of the material is 2.1 3 105 MPa and the hardness is HRC 52. In order to fully remove the decarburized and machining affected layers, the fine processing methods, such as finish grinding, chemical milling, and electrolytical polishing, were adopted. The final dimensions of the specimens are shown in Figure 1. Some of the specimens were shot peened in an air peener. The peening parameters of the detected surfaces were as follows: the distance between the nozzle and the specimens was 150 mm, the shot diameter was 0.6 mm, the jet pressure was 0.3 MPa, the surfaces shifted three times at a speed of 10.6 cm/min under the shot flow, the coverage was over 100 pct, and the Almen peening intensity was 0.38 mm A. In order to flatten the specimens, their back surfaces were also peened one time before and two times after the detected surface was peened and the used jet pressures were 0.25 and 0.3 MPa, respectively. Incremental step tensile or compressive loading was applied in th
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