It has been noticed that a metal bellows made of high strength SUS631 reveals low fatigue life. On the other hand the author has been successful improving fatigue strength of SUS304 metal bellows by reflected shot peening method and improved tempering process to decrease the grain diameter. In this study, the experiment to improve fatigue strength of SUS631 bellows was studied. Grain diameter was adjusted by changing BA temperature after cold working. The hardness was increased by ageing treatment after bellows forming. Lastly shot peening was carried out using reflection plate. Grain diameter tends to decrease in proportion with decreasing BA temperature, and minimum grain diameter was 12μm. The optimum shot peening condition was found where the pressure is 0.3MPa and the glass beads size is 97μm. Fatigue limit of 107 cycles of SUS63l bellows was improved by 17% from SUS304 bellows when the optimum condition is applied.
High durability is required for automotive engine valve springs. To ensure the performance of spring, it is necessary to evaluate and measure the residual stress distribution of spring surface and inward direction. Generally, it has been assumed that the residual stress of a material surface treated by the shot peening is uniform. However, the round and spiral shape of a coil spring brings various peening angles corresponding to the surface location, then the uneven peening angles may leads to a non-uniform residual stress on the coil spring surface. In the meanwhile, an analysis method of non-liner sin2 φ diagram, or φ-split analysis, is well known to directionally deformed material. In this study, residual stress distribution of spring material deformed by the shot peening from the different angles was measured and, non-linearity of sin2 φ diagram was studied. In addition, the relationship between microstructure and residual stress was studied by using field emission type scanning electron microscope (FE-SEM). The result reveals that the non-linearity in the sin2 φ diagram for the shot peening samples. The extent of φ-split became larger with the increasing shot peening angle and was dependent not only on the mass fraction of carbide particles but also on the carbide particles size distribution.
This subject focuses on the simulated nitriding analysis of the compound layer depth, diffusion zone depth and Vickers hardness distribution on springs. The authors studied the effect of material element for nitriding by multiple linear regression analysis and found the relationship between nitrogen concentration and Vickers hardness. There have been no report that describes an analysis of the simulated hardness distribution on spring material. In this paper, nitriding temperature, nitriding time and ammonia concentration are considered. The authors further studied the way to deepen nitrogen diffusion zone while making the compound layer as shallow as possible.
The recent development of lightweight yet high output automotive engines has increased demand for valve springs with high fatigue resistance. This has led to the development of valve spring wire with high fatigue strength. The Cr-Si steel oil tempered wire (SAE 9254) has been commercially used for the valve spring wire, and our company developed C and Si-enriched oil tempered wire that has both high-heat resistance and high fatigue strength. In this research, the authors focused on evaluating the effect of prior γ grain sizes on fatigue strength. It was found that finer structure is effective in improving fatigue strength.
So-called “super-elastic materials” like TiNi alloys having large elastic deformability have widely been used for the frame of glasses, the antenna of mobile phones and so on. On the other hand, the unique Ti alloys which have low elastic modulus, high strength and large elastic deformability are developed by Toyota Central R&D., Inc.. These new developed alloys named “GUMMETAL” are put into practical application for the frame of glasses. The alloy belongs to β(bcc)type Ti alloy and the Young's modulus in room temperature is as small as about 40GPa with the 2.5% of elastic deformability to be classified as a super-elasticity. Furthermore, they have nonlinear and nonhysteresis properties without following Hook's law. The authers found them very attractive for applying to new springs having unique characteristics. So, a workshop is established to study the optimum heat treatment condition for spring application observing the accompanying metal structure. Then three kind of spring (compression-, tension- and torsion-type springs) were examined for spring characteristics and discussed their merits in comparison with conventional steel springs.
The demand on more accurate Wahl's stress correction factor for a suspension coil spring is growing stronger as the initial pitch angle becames larger with the strengthened spring material in recent years. This research committee performed the following activities under such circumstances. 1) Clarification of the precondition of the conventional coil spring characteristic formula, and comprehension of an application limit; a) Numerical analysis comparison was executed on three patterns of the derivation process of correction factor formulas after clarifying the derivation precondition of the each formula. A. M. Wahl, A. Röver, J. K. Wood, E. Honegger, O. Göhner, P. Henrici, C. J. Ancker & J. N. Goodier, M. Bergsträsser, D. G. Sopwith. b) The formula of Bergsträsser which is adopted in the DIN standard and the formula of Wahl were compared. c) The correction factor diagram was summarized on the formula which takes the effect of the initial pitch angle into account. 2) Calculation of the stress and deflection correction factor by FEA; a) Linear FEA for a 1/4 circle beam was executed using the solid element, and the accuracy of a stress correction factor formula was verified. b) Nonlinear FEA for one turn of active coil with different pitch angles were executed using the solid element, and the influence of a pitch angle was investigated. c) The presumed correction factor formula of stress and deflection was derived using Design of Experiments and FEA, and the correction factor diagram was created. 3) Suggestion of better FEA modeling; a) FEA by the beam element was executed on an actual coil spring, and the essential points of the beam element FEA were proposed by comparing with experimental results. b) The tendency of the stress correction factor derived from the beam element FEA was studied. Moreover, end turn contact conditions with a coil seat was examined.