Fatigue strength improvement of age-Hardened 18ni maraging steel by stress-Saser surface treatment and subsequent aging
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Communications Fatigue Strength Improvement of Age-Hardened 18Ni Maraging Steel by Stress-Laser Surface Treatment and Subsequent Aging
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Y. IINO, Y. NAKAJIMA, H. MAKINO, and M. NISHIJIMA Fatigue resistance of maraging steel is not good compared with the superior static strength and toughness. The reduction of the fatigue strength is significant when a sharp stress raiser exists. A possible method to improve the fatigue strength might be to add compressive residual stress in the surface zone. Laser surface heat treatment has been used for improvement of fatigue strength of carbon steels, where the compressive stress takes place in the treated zone by the martensite transformation. The residual stress together with the high hardness is said to increase the fatigue strength. Singh et al. [1] observed that the fatigue limit of 1045 steel is raised by as much as 30 pct. Kikuchi et al. [2] determined that the fatigue limit of 0.45C steel is increased by 98 MPa over untreated specimens. Gnanamuth t3] measured residual compressive stress of 462 MPa in laser treated 1045 steel. Iino and Shimoda t4] reported that laser surface heat treatment of carbon steel under pretensile stress gives higher compressive residual stress than without preload and causes higher fatigue strength of the steel. In this communication it is shown that laser surface heat treatment of age-hardened 18Ni maraging steel under prestress and subsequent aging causes higher hardness than matrix and improves the fatigue strength by as much as 40 pct. A rectangular smooth specimen (10 mm wide x 5 mm thick • 55 mm long) was machined from solution annealed 18Ni maraging steel and then aged at 753 K for 10.8 ks followed by air cooling. The chemical compositions (wt pct) are 0.01C, 0.05Si, 0.01Mn, 18.71Ni, 4.75Mo, 0.57Ti, and 8.54Co. The mechanical properties after the aging are yield strength of 1820 MPa, tensile strength of 1890 MPa, reduction area of 39.8 pct, and elongation of 11.6 pct. One CO2 laser pass (Gaussian mode, argon gas shield, beam diameter of 3 mm) was irradiated on (1) the surface of the specimen which was preloaded using a screw-drive loading apparatus [4] (Figure 1) and (2) the surface of the specimen with no preload. A specimen treated by (1) is termed, in this report, as stress-laser treated specimen, and that by (2) as unstress-laser treated specimen. The treating conditions are shown in Table I. Preload was set so that the maximum surface stress is near the yield strength. Colloidal carbon was sprayed on the surface to enhance laser energy
Y. IINO, Assistant Professor, and H. MAKINO and M. NISHIJIMA, Students, are with Toyota Technological Institute, Tempaku, Nagoya 468, Japan. Y. NAKAJIMA is Researcher, Aichi Steel Works Ltd., Arao, Tokai 476, Japan. Manuscript submitted December 4, 1987. METALLURGICAL TRANSACTIONS A
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I Strain amplifier
Fig. 1 --Schematic of preloading. [4]
absorption. The side surfaces of the treated specimens were machined to 1 mm to avoid the edge effect of the irradiation, and the f
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