It can be said that austenitic stainless steel SUS304 has the relationship of Hall-Petch in which proof stress increases with the decrease of a grain diameter. Fine grain improves both strength and toughness. In order to increase the fatigue strength of automobile parts, the author proposed the new improvement method on fatigue strength, through the clarification of the process of fatigue fracture and defensive factors. By decreasing grain diameter, it can be expected (1)proof stress (HV) increase (2)greater compressive residual stress (3)shorter amount of fatigue crack of stage I (4)surface roughness become smooth. In this study, in order to investigate relationship between grain diameter and fatigue strength in SUS304 steel, the experiment was carried out by using the metal bellows of SUS304 steel where grain diameter was adjusted by changing BA temperature after cold working. It could be realized that fatigue strength increases in proportion with decreasing grain diameter. The main reason why the fatigue strength is improved is attributed to the experimental result that the surface roughness becomes smooth by decreasing grain diameter in metal bellows.
Weight reduction and high capacity of corrosion fatigue strength have been demanded for suspension springs from hot-formed coiling process. We suggested SUP190 for the hot-formed coil springs for this purpose. On the other hand, shift from the hot-formed coil springs to cold-formed coil springs has been proceeding recently, because the cold-formed coil springs have advantage of high formability and high shape freedom. The cold-formed coil spring materials are generally obtained by the quick heat treatment. The quick heat treatment have the advantage of fine structure, fine austenitic grain and minimum decarburization. We focused on the advantage of the cold-formed coil springs made of material processed by the quick heat treatment. In the meanwhile, we have further developed a means of applying the quick heat treatment to the hot-formed coil spring materials of SUP190. However since SUP190 is alloy steel, it is necessary to consider the change of transformation temperature and solid solution of carbides. Therefore, we concentrated on the basic evaluation for heat treatment by using the hot-working simulator. In addition, the evaluation of the heat treatment in production furnace and the examination of spring characteristic were conducted.
Delayed fracture is one of the most important problems for strengthening spring steels and reducing the weight of coil springs. It has already been reported that the compressive residual stress by shot peening lengthens the delayed fracture life. However, whether the compressive residual stress suppresses the crack initiation or the crack propagation has not been clarified. Therefore, we researched the effect of compressive residual stress on the occurrence of crack and its propagation by using the newly developed AE (Acoustic Emission) system. First, it was clarified that the shot peened spring had a long life for delayed fracture, even if increasing the diffusible hydrogen. It was also clarified that the crack occurred as soon as the experiment started, regardless of shot peening. From the observation of delayed fracture surface, it was also clarified that the QC (Quasi-Cleavage) region became deeper as the surface magnitude and the depth of compressive residual stress became larger. The mean growth rate of crack was estimated by dividing the depth of QC region by the total time to delayed fracture. The mean growth rate of crack became smaller for larger surface compressive residual stress and its depth. It can be concluded that the compressive residual stress caused by the shot peening suppresses the propagation of crack, and contributes to enhance the delayed fracture strength.
This subject focuses on the simulated shot peening analysis of the impact energy and simulated impact energy distribution on coil spring. The authors studied hit condition by changing peening angle, and checked relationship between impact energy and residual stress. There have been no report that describes an analysis on the simulated shot peening effect on helical material whereas reports on flat material exist. In this paper, shot size, peening angle, and spring wire overlapping are considered. The effectiveness of this analysis is verified in comparison with the previous test result where light was used to check the shadow of shot shower.
Simplified theoretical formulae based on shot peening parameters and materials properties have been developed to predict the maximum of the compressive residual stress produced by shot peening for hardened steel. These formulae are based on the motion equation for a shot and the analytical model proposed by Li et al., who utilized Hertz theory of elastic contact and a simplified elasto-plastic theory. Two cases which deformation of shots was elastic or elasto-plastic at impact were treated. Experimental verifications were carried out under various shot peening conditions for a carburized specimen. Shots were made of cast steel (HV800, HV590 and HV450), fused zirconia and glass, each having three levels in diameter. Actual residual stress distributions were measured by X-ray diffraction method. The predicted maximum of the compressive residual stress by the proposed formulae agreed well with the experimental values in wide variety of shot peening conditions.
The mechanism of single-degree-of-freedom and six-degree-of-freedom vibration isolation is proposed. For the single-degrere-of-freedom vibration isolator, positive and negative magnet-springs were created by regulating the magnetic fields; a low-damping, passive vibration controller with a blind sector was then created by combining them with metal springs. Overall vibration is absorbed by the blind sector and magnet-spring, which automatically adjust the phases; the magnet-spring's resonance energy and shock vibration are absorbed by an oil damper. These passive vibration controllers were verified by experiments to have even greater vibration-absorbing properties than air-suspension or liquid-mount devices. Furthermore the six-degrere-of-freedom vibration isolator was constructed using magnet-spring and magnetic support device, and indicated the good characteristics of the transmissibility.