High carbon steel wire containing approximately 1 mass% Si was developed for spring wire. Mechanical properties of this wire were evaluated and high tensile strength after annealing, high sag resistance at 150°C to 250°C and a high limit of fatigue strength were confirmed. Mechanisms of these high performances were investigated by morphologies using TEM. The observations of the developed steel after annealing at 400°C revealed the presence of a high amount of Si in lamella ferrite, significantly fine lamella cementite with suppression of spheroidizing and lowered lattice distortion in lamella pearlite. These results lead to the conclusion as follows, 1) Tensile strength and sag resistance are enhanced by solid solution hardening of Si in lamella ferrite. 2) A high limit of fatigue strength is caused by reduction of lattice distortion, which is considered as the origin of fatigue fracture, in lamella pearlite due to high temperature annealing such as 400°C.
There are very few theoretical studies about a constant-force spring. Considering the strain energy necessary for flattening a coiled spring, Votta proposed an analytical theory of such a spring. This theory, however, could not be applicable for all stages of displacements, especially for small displacements, because the uncoiled length of the spring can not be flattened. In this report, we perform a new theoretical analysis of the constant-force spring based on the theory of Elastica. Then, the extension mechanism and the deformed shape of the spring, which could not be clarified in the formulas given by Votta, are analyzed. As a result, it is found that the force rises rapidly and approaches a constant value as the spring is extended infinitely. Furthermore, the experimental verification of this analysis is carried out using a commercially available constant-force spring. The theoretical values are in good agreement with the experimental ones. Consequently, the new theory is proved to be of practical use.
Warm Shot Peening is well-known as the shot peening process within warm temperature range. however, the effect of warm shot peening to fatigue strength was not certain and no substantial study has been made. The requirements of coil spring with higher fatigue strength and sag resistance have been increasing to obtain the mass saving of vehicle. While several new spring materials have been developed in recent years, the essence of those developments were to make the strength of spring higher, so-called high strengthed spring. As for high strengthened spring, the shot peening process becomes further essential to relieve the increased notch sensitivity due to higher strength, However, for the shot peening process to high strengthened spring, the shot with higher hardness is also required. This can decrease the life of both shot and shot peening equipments. Warm shot peening under the tempering temperature may become one solution to those problems, because the hardness of spring can be temporarily reduced only during the shot peeningprocess. In this paper, the following experimental results concerned with the effect of warm shot peening, are summarized. 1) The effect of warm shot peening on fatigue life is larger for high strengthened spring. It is especially more effective when the spring hardness is over the shot hardness. 2) The appropriate temperature of warm shot peening is from 473 to 598k. 3) The main reasons why the warm shot peening is effective to the improvement of fatigue life, are both the increase of compressive residual stress distribution and higher coverage, which can be caused by more effective deformation under warm temperature. The increase of surface hardness by warm shot peening could be an another reason for the improvement of fatigue life.
With the miniaturization of switches, relays, connectors and so on, more severe bend formability is required for copper alloy strips, which are main comprising materials of these electronic components. Therefore, it is inevitably necessary to find a quantitative evaluation method for the bend formability of copper alloy strips besides development of these materials. In this research, the authors have deveroped a newmethod to estimate the bend formability of copper alloys using their three basic mechanical propertiesobtained from tensile test, and made an attempt to improve the bend formability of copper alloys for spring applications with the help of thismethod. The results are summarized as follows: 1) To evaluate the bend formability of copper alloy strips, strain distribution coefficient (f), which is a new representation of basic mechanical property, is proposed in addition to uniform strain (εu) and maximum strain due to necking (εm). Derivation method of f from tensile test results is shown. Furthermore we present quantitative relation between the bend formability and the three basic mechanical properties. 2) To improve the bend formability, it is necessary to control the three basic mechanical properties, especially, increasing εu and f, which suppress wrinkle occurrence and development during bend process, is effective. 3) For improving the bend formability of NB-109 alloy for spring use, it is effective to maintain large εu and f through decreasing cold rolling ratio after recrystalization treatment. In the case of high cold rolling ratio, it is effective to increase εu and f through rising final annealing temperature.
Stainless steel strip is widely used for flat spring which requires corrosion resistance. SUS301 steel is especially used for the material of a spring as it is given high strength by cold rolling. However, scattering of the angle of bent product due to the large springback of the material appears to be a problem in a certain production. In this study, the effect of slight differences in thickness and hardness on the amount of springback in two lots of SUS301 is investigated. The material specification of each is within the tolerance of JIS G 4313, however, the lots are from different steel makers. L-shape bending test which is similar to a multiforming mechanism is adopted in this investigation. Results obtained show that the springback angle decreases with the increment of thickness and the decrement of hardness, but the change itself is very small within the same material lot. In addition, the relationship between the springback angle and the hardness does not apply through the different material lots, and it even appears to have a reversed tendency. Therefore, hardness is not appropreate to estimate the springback. Within the present experiment, the springback angle correlates with the stress corresponding to the strain at the surface of bent strip regardless of the difference in not only material lots but also bending direction against rolling direction.
Evaluations of material strength characteristics based on experimental data are necessary for the development of the high-strength spring steels and design of the springs. We constructed the material strength database for coil springs on the network in order to be utilized for a material development and a design. This database includes fatigue properties and other mechanical properties of the cold forming wire spring steel, and is composed of three different types of database, that is, test results, analyzed data and related standards. This database system would permit a search, analysis and graphic presentation of the experimental data. It has approximately fifty commands developed originally, and can make a new relationship among several material strength properties using a statistical regression such as a multiple regression analysis. This system can also retrieve and display material strength characteristics in arranged manner, such a form of the diagram, as S-N diagram and endurance limit diagram. The WWW browser is used as the interface of this database system.