Heat-resistant stainless steel with excellent spring properties at high temperature was developed for exhaust gasket. Recently, the temperature of the automotive exhaust system parts has been rising. The press-formed bead for sealing on the gasket undergoes elastic deformation while pressed, and the repulsive force of the bead prevents the exhaust gas leak. At high temperature the strength of the steel decreases, and the repulsive force of the bead becomes weak. Therefore, it is difficult to prevent the exhaust gas leak. The developed steel is austenitic stainless steel which does not transform at high temperature, and the strength is improved through cold rolling and nitrogen solution. In addition, the drastic hardness decrease is prevented as a result of the retardation of the austenite recovery and recrystallization by the nitride particles which are formed at high temperature. Furthermore, the particles contribute precipitation hardening. The developed steel shows the hardness exceeding 400HV after aging at 600℃ for 400 hours.
Influence of stress ratio on bending fatigue limit of high strength steel (SUP9A) containing a semi-elliptical surface slit subjected to shot peening (SP) was investigated. SP was conducted on the specimens containing a semi-circular surface slit with the crack depth (a=0.1, 0.2 and 0.3 mm). Bending fatigue tests were carried out under stress ratio, R = －1. These results showed that the fatigue limit of the shot-peened specimens having the slit under 0.2 mm in depth was almost same as that of the shot-peened smooth specimens. Meanwhile, some of the specimens fractured from the surface other than the slit. Thus, the maximum depth of slit which could be rendered harmless by SP was 0.2 mm under R=－1. The compressive residual stress induced by SP was reduced due to relaxation. Compared to the results of R=0 and 0.4, the size of semi-circular slit that could be rendered harmless by SP was same. The maximum depth of crack which can be rendered harmless by SP was predicted based on fracture mechanics. The estimated values were in good agreement with experimental values.
In application of flexible thin materials, it is very important to evaluate mechanical properties of these materials in both analytical and technological interests. In this study, a new and convenient mechanical testing method (Own-weight Circular Ring Method) is developed for measuring Young’s modulus in a flexible material (thin plate, or wire), especially, a curved/curled material. The method is based on a nonlinear theory that takes into account large deformation behaviors of flexible materials. By means of measuring maximum horizontal displacement or maximun vertical displacement in flexible rings, Young’s modulus can be easily obtained for various thin materials (plate and wire). Measurements were carried out on a thin wire (piano wire). The results reveal that the new method is suitable for thin flexible materials. In the meantime, the new "Own- weight Circular Ring Method" proposed in this paper is quite a promising method and can be extended to measure Young’s modulus of every thin layer in a flexible multi-layered material formed by PVD, CVD, Electrodeposition, Coating, Paint, Cladding, Lamination, and others.
In general, sin2 ψ method is applied for measuring residual stresses of coil springs. There is no difficulity to perform the measuring in case of Spring steels, Ferritic or Martensitic steels, but in case of Austenitic stainless steels, measurement in high accuracy can’t be carried out for reason of stress induced transformation (Austenite⇒Martensite) and fiber structure strongly oriented through rolling and drawing processes. On the other hand, two-dimensional detector has been developed in the field of X-ray diffraction recently. This detector can also be applied to multiaxial stress measurement which is called 2D method. In this research, we tried to compare 2D method with sin2 ψ method to improve the accuracy in measuring residual stress of stainless spring. In this study, it was clarified that the 2D method is suitable for measuring residual stresses in high accuracy for Austenitic stainless steels compared with general sin2 ψ method.
In MacPherson strut applications for automotive suspension systems, the desired coil spring reaction force vector that minimizes damper friction and king pin moment is typically determined by Statics/Kinematics calculations. There is not a single device available on the open market today which can simulate the coil spring reaction force vector within the suspension system. Such a programmable coil spring reaction force generator was developed in 2003, and was then improved in 2013 from the standpoint of accuracy, durability and reliability. Using this modified device, the relationship between the spring reaction force vector and damper friction, as well as spring reaction force vector and king pin moment, can be experimentally studied to confirm vehicle characteristics without actually making any prototype coil springs. The validity of this device was proven by comparing to actual coil spring based testing data. Depending upon the desired characteristics of a particular vehicle, the requirement for the coil spring reaction force vector can be experimentally determined with this device. To aid in this effort the device was designed in a universal manner for any strut application by mere replacement of strut-dependent seat adapters. This paper describes how beneficial this device is to determine the ideal requirement for coil spring reaction force vector via actual measured data.
Spiral spring has been used in various mechanical applications because of its simple shape. In automotive reclining seat device, when the seat back is tilted backward, spiral spring is tightened elastically. This provides elastic energy that retracts the seat back to its original position. Therefore, it is important to predict spring characteristics such as torque performance. However, in classical theory, it was impossible to predict them with sufficient accuracy. In actual design, it is necessary to make some prototype springs and measure the characteristics for re-design. Furthermore, accurate shape measurement technology is indispensable for quality assurance for volume production. However, classical method has not been sufficient in accuracy and speed. In this report, we provide actual examples of the product quality improvement by the design prediction accuracy improvement with CAE and non-contact measurement technology.
Hydrogen embrittlement sensitivity of SAE9254 (tensile strength of 1.7 – 2.0 GPa class) and vanadium added spring steel (9254V) with a tensile strength of 2.0 GPa class was evaluated by conventional strain rate tensile tests (CSRT) and torsion strength tests, using smooth specimens respectively. In the CSRT evaluation, the maximum tensile stress of the both steels decreased with the increment of diffusible hydrogen content. On the other hand, in the torsion tests, the maximum shear stress hardly exhibited any decrease until the hydrogen content reached 6mass ppm, where the cracking trace changed from shear plane (transverse direction of the specimen) to a resolved tensile stress plane (45°against the shear plane); fractgraphically, from micro-void coalescence (MVC) to intergranular fracture, and the torsional strength began to decrease. The resistance to hydrogen embrittlement of the CSRT properties of 9254V was superior to that of vanadium-free SAE9254 in the equivalent tensile strength level. Although the superior performance for the latter steel attributes partially to the reduction of phosphorous and sulfur contents, it should be noted that the addition of vanadium contributes the refining of prior austenite grains followed by the reduced intergranular segregation of phosphorus and sulfur. The hydrogen trapping effect at the vanadium carbide interface may contribute too. However, there was no difference between SAE9254 and 9254V in hydrogen embrittlement sensitivity when the torsion test is conducted.
In order to investigate the validity of the current JIS fatigue design diagrams for round wire helical compression springs, fatigue tests were conducted for non-shot-peened and shot-peened springs made of piano wires and valve-spring quality oil-tempered Chromium Silicon steel wires. All the spring wires supplied for the testing were commercial ones and were processed in commercial spring manufacturing lines to coil springs. Fatigue tests were conducted at room temperature until fracture or 107 cycles, under various constant stress ratios from 0.1 to 0.7. Also, load loss of each non-fractured spring at fatigue test was measured. Using each load loss thus obtained, residual shear strain γ (Shear stress relaxation divided by shear modulus) was calculated. It was found that the fatigue strengths obtained were all above the current JIS fatigue design diagrams and that the current JIS maximum design stresses were lower than those determined by the criterion that residual shear strain γ <= 0.05 % is met. Comparison of current JIS standard with the corresponding SAE and EN standards were also made.