High fatigue strength of coil springs for automobiles is intensely required for better fuel economy and weight reduction. For this subject, springs have been developed in terms of both material and processing sides. The coil springs developed with combination of the designed material, the advanced nitriding and shot peening processes achieved distinction in fatigue strength. In the present paper, following topics for this development are introduced: 1) Spring steels with high resistance to temper softening at nitriding temperature were designed. 2) A shot peening technique was developed to increase residual stress on the nitrided surface of springs. 3) It was verified that the developed springs had similar notch sensitivity to a conventional spring discussing fatigue strength of the steels using both test wires and spring with artificial defects. 4) Fatigue strength of the developed springs was improved approximately 35% as compared with that of the conventional spring. 5) Improvement of the fatigue strength was considered to be caused by the nitriding and shot peening which shifted the initial point of fracture to inner side of the material.
To understand the influences of corrosion test conditions on the corrosion behavior of suspension spring steels, we investigated the corrosion loss, shape of pits and structure of rusts at the conditions of wet-ratio 8%, 33%, 100% for SUP7 steel and UHS2000, the τmax=1300MPa-grade suspension spring steel. It was revealed that the corrosion was the most accelerated on wet-ratio 33%. Corrosion resistance was superior in UHS2000 which contains alloy elements such as Ni, Cr, Mo. Formation of amorphous rust was thought to give a big effect on the corrosion resistance, that is, corrosion resistance increased with increasing the fraction of amorphous rust. The amorphous rust was thought to act as a barrier against corrosive species. The reason that corrosion was accelerated on wet-ratio 33% can be explained that the amount of amorphous rust was minimum on that condition. And the reason that UHS2000 is superior in corrosion resistance can be explained by the fact that ratio of amorphous rust was much higher in UHS2000 than in SUP7.
Ordinary we measure mechanical properties, for instance tensile strength, reduction of area, bending strength, to inspect steel wires for spring. But it takes much time to measure these properties, and can not grasp the whole of the wire because these measurements are destructive methods. It is also difficult to judge the wire from each property because various factors are superimposed in it. For these reasons, we need to know relative properties of steel wires as a whole, for which we aimed at magnetic properties to be measured it according to the theory of electromagnetic induction. This method is to acquire magnetic properties through the output voltage using a couple of coils. We thought this method is useful to relate mechanical properties with micro-structural properties of oil tempered wires annealed at low temperatures.
We have investigated the characteristics and formation mechanism of tarnish on tin plated copper and copper alloy strips for electrical and electronic parts. Further more, we have also researched the elimination method of tarnish. The main results obtained are summarized as follows: 1) It was found that tarnish was a kind of tin oxide formed through the action of oxygen and humidity in the air. It was also revealed that the oxygen bearing layer of tarnished surface was thicker than that of normal surface. Thus, it is supposed that the “yellow” appearance of tarnish is due to the thickening of oxygen bearing layer of tarnished surface. 2) The contact resistivity and surface roughness of tarnished surface were not inferior to that of normal surface. 3) The solderability of tarnished surface was not so good as that of normal surface when nonactive flux was used. But differences in solderability were not observed with active flux. 4) To eliminate tarnish, chemical methods, especially the pickling method, were found much effective.
Elevated temperature and room temperature tension properties of phosphor bronzes containing 4%, 5%, 6%, 7% and 8% Sn, with thickness of 0.3mm are investigated. Results are as follows; 1) Serration appears at 473-573K on Load-Elongation diagrams for elevated temperature tension test. And the relation has nothing to do with Sn content. Serration does not appear in case of room temperature tension test. 2) Tensile strength does not depend on Sn content above 623K for elevated temperature tension test. Tensile strength depends on Sn content above 723K for room temperature tension test. 3) Reduction of area of Sn 4%, 5% reaches to the minimum at 373K and reaches to the maximum at 750K, and that Sn 7%, 8% reaches to the minimum at 623K and reaches to the maximum at 740K for elevated temperature tension test. Shortness temperature area does appear in case of elevated temperature tension test. Shortness temperature area does not appear in case of room temperature tension test. 4) There is an inflection point at around 280MPa, HV180 on the curve of relation between elevated temperature tensile strength and Vickers hardness. This relation seems to be represented by two straight lines, and the relation has nothing to do with Sn content.
In order to obtain technical information concerning effects of surface nature on spring strength behavior of thin sheet spring, surface finishing and heat treatment were undertaken for SK5, GIN6, SUS304, maraging steel and SUS631. Thickness of sheet, surface roughness, Vickers hardness, spring limit and Young's modulus of these steels were measured. The results of the present works are summarised as follows: 1) Thickness scatter was considerably reduced by lapping. 2) Surface roughness was improved by barrel finishing. 3) Spring limits of steels tested, except for maraging steel, were decreased by lapping. Young's modulus showed no change. 4) Spring limits of steels tested, except for SK5, were partially recovered by barrel finishing. Young's modulus indicated no change. 5) Spring limits were recovered and Young's modulus were somewhat increased by heat treatment after barrel finishing.
In this paper effects of intial curvature and torsion of wires and feeding conditions on the shape of formed coils are studied theoretically and experimentally. The main results are as follows; Curvature and torsion of the formed coils varies proportionally to the trigonometoric function of the inclination angle of the wire bundle plane from horizontal position. Especially, the vertical setting of the plane influences the curvature directly and the horizontal setting has big influence on the torsion of coils. When the intial torsional direction of wires coincide with that of formed coil, combined effect will be expected. The effect of feeding conditions on shape of formed coils seems to be less when the initial curvature and torsion become minimal. As the compressive force of feed rollers increase, the wire in front end of coiling pin become straighter, which reduces the effects of intial curvature and torsion of wires on coils.
Coil springs made of silicon nitride (Si3N4), which has higher strength and heat resistance than other ceramics, have recently developed. By examining the range of strengths and the heat resistance of the coil springs, it is shown that the coil springs can be used at temperatures up to 1000°C. Using Weibull failure theory, we derive an equation to calculate effective volumes of coil springs and examine the influence of effective volume on the strength of coil springs. The results show that the mean strength of coil springs decreases with increasing effective volume. Therefore the strength of coil springs can be estimated from an equation of the effective volume, and this equation can be used in design of coil springs. Furthermore, by conducting proof tests, coil springs that have relatively large defects can be eliminated and so only highly reliable coil springs can be placed in service.
Fatigue life of ceramic materials scatter in a widerange depending on an inherent flaw size. Therefore, it is important to conduct proof test as the procedure to exclude components with flaws, whose size is larger than a certainsize. In this paper, a numerical analysis of sintered siliconnitride were conducted to calculate minimum cyclic fatigue life after proof testing. Its reliability was predicted by the procedure based on 2 parameter weibull distribution of the fracture stress of a smooth specimen and plane-strain fracture toughness value. Cyclic fatigue tests of Si3N4 was conducted after proof testing to compare with a numerical simulation. The numerical simulation coincided with the experimental life distribution.