In order to reduce the weight of the suspension springs by increasing the strength level of the steels used, it is necessary that the corrosion fatigue property should be improved. However, the dominant factor governing the corrosion fatigue property of the suspension spring has not been satisfactorily revealed yet. Therefore, the investigation into elucidating the mechanism has been carried out to evaluate the effects of corrosion pits, shot peening, hydrogen embrittlement, prior-austenite grain size, and so forth independently on the corrosion fatigue property. In this work, focus was placed upon the effects of corrosion pit depth and shot peening on the fatigue property. The specimens having the different artificial corrosion pits depth were subjected to fatigue tests. Special attention was paid to exclude the influence of hydrogen. As a result, it was revealed that the fatigue limit decreased with the increase of the corrosion pit depth, presumably due to the decrease in the compressive residual stress at the bottom of corrosion pit. Moreover, the effect of prior-austenite grain size on fatigue properties turned out to be almost negligible.
In order to reduce the weight of the suspension springs by increasing the steel strength, it is necessary to improve the corrosion fatigue property. However, the dominant factor governing the corrosion fatigue property of the suspension spring has not been satisfactorily revealed yet. Therefore, the investigation into elucidating the mechanism has been carried out to independently evaluate the influences of corrosion pits, shot peening, hydrogen, prior-austenite grain size, and so forth on the corrosion fatigue property. In this study, the influence of hydrogen on rotary bending fatigue test was investigated using specimens with artificial corrosion pits after shot peening. The dominant factors governing the corrosion fatigue property of the suspension spring were discussed based on the result of the present study in conjunction with the results of the previous study on the influences of shot peening and corrosion pit depth. The conclusions obtained were as follows: (1) In the case of the specimen without corrosion pit or with only shallow corrosion pits having still large compressive residual stress at the bottom of the pits, the fatigue property was fairly good but declined with the increment of charged hydrogen due to the diffusible hydrogen. On the other hand, in the case of the specimen with deep corrosion pits having only small compressive residual stress, the fatigue property was poor regardless of the charged hydrogen content. We inferred from this result that the deep corrosion pit releases the compressive residual stress at the bottom of pits. (2) The effect of prior-austenite grain size on fatigue properties of hydrogen charged specimens turned out to be almost negligible.
Laser forming is a new process which utilizes the thermal stresses generated during laser scanning. The parameters to control the process are mainly laser power scale, beam diameter, scanning speed, and repetition number of scans. Stainless steel sheet is suitable for laser bending in thermal and mechanical properties of materials. This paper investigated the application of laser bending in stainless steel wire. Temperature gradient of steel wire by laser heating was small due to shape of wire, therefore the shape of the wire is not suitable for laser bending. From these experiments, steel wire could bend in shape of coil by laser beam using proper conditions of laser irradiation.
In recent years, flexible materials with very high performance are widely used and large deformation analyses of these materials have attracted considerable attention. Therefore, it is very important to evaluate mechanical properties, especially, Young's modulus, of these flexible materials. In this study, a new and convenient mechanical testing method (Compression Column Method) is developed for measuring Young's modulus of each layer in a thin flexible multi-layered material (thin plate, rod, or wire). The method is based on a nonlinear theory under the assumptions of the geometrical nonlinearity that takes into account large deformation behavior of multi-layered materials. Exact analytical solutions for large deformation are obtained in terms of elliptic integrals. By means of measuring the horizontal displacement at the fixed end or the vertical displacement at the middle point of the buckled column, Young's modulus of each layer can be easily obtained for various thin and long multi-layered materials. Measurements were carried out on a two-layered material consisting of PVC (a high-polymer material) and SUS (a stainless steel material). As a result, it is made clear that the new method is suitable for thin flexible multi-layered materials. In the meantime, the new method proposed in this paper can be applied widely to measure Young's modulus of every thin layers formed by PVD, CVD, Electrodeposition, Coating, Paint, Cladding, Lamination, and others.
Springs used for a car require a long term performance in severe corrosive environments such as the salt damage. In late years the use of Nano-Indentation method is prosperous in the hard film measurement used for a semiconductor device or electronic parts, but there are few examples applied to a soft film such as the coating film. In this article, focusing on Nano-Indentation method as one of the methods to evaluate coating film quantitatively, I evaluated properties of matter of the coating film for springs.
Influences of tensile strength, hydrogen concentration and applying stress mode (tension or torsion) on delayed fracture properties have been investigated using SAE9254 steel treated in their tensile strength range of 1700-2000MPa. In conventional strain rate techniques (CSRT), the delayed fracture strength tends to decrease with the increment of diffusible hydrogen concentration, especially less than 1 mass ppm. On the other hand, in torsional tests, the hydrogen cracking susceptibility shows less abrupt change than in CSRT, while the fracture torque decreases and intergranular cracks are observed in the notch root of specimen when hydrogen is charged above 10 mass ppm.
In coil spring manufacturing process, the low temperature annealing after coiling is an important process. It is known that the dimension of coil spring varies after the low temperature annealing. We measured systematically this dimensional change. After the low temperature annealing, the coil diameter of SWP-B (SW-C, SWOSC-V) spring becomes smaller, and that of stainless steel wire (SUS304WPB) becomes larger. The low temperature annealing is also important from the stand point of sag resistance. When we do not apply this annealing, we get large sag in a clamping test. The value is more than 10 times compared with normal processing. Don't miss low temperature annealing process, in order to obtain a good spring.