The purpose of this study is to clarify the effect of increase in the grain boundary strength on the torsional fatigue strength of induction hardened steels. The effect of lowering phosphorus content and addition of boron, molybdenum or silicon , as metallurgical factors of increasing the grain boundary strength, on torsional fatigue strength was investigated. Torsional fatigue fracture life in the test condition of high cycles fatigue is prolonged by lowering phosphorus content, or by adding boron, molybdenum or silicon, respectively. In these fatigue test, the crack propagation life against total fracture life is prolonged particularly. The fraction of intergranular fracture in the propagation region of mode I fracture is decreased by lowering phosphorus content or addition of boron etc. This is caused by increase in the grain boundary strength due to lowering phosphorus content or addition of boron etc. Consequently, improvement of torsional fatigue fracture life by lowering phosphorus content or addition of boron etc. is because that static final fracture in the process of fatigue crack propagation is disturbed due to improvement of fatigue fracture toughness caused by increase in the grain boundary strength.
Shot peening is used for the material which are high carbon steel such as carburized steel and spring steel as a surface enhance method to increase fatigue strength. The compressive residual stress and work hardening induced by shot peening are important shot peening characteristics to increase fatigue strength. The compressive residual stress and work hardening induced by shot peening are strongly influenced by material property such as mechanical property and retained austenite, and to investigate influence of material property on shot peening characteristics is very important. In this study, we carried out shot peening to chrome steel and chrome molybdenum steel which changed retained austenite volume and mechanical property, and we investigated relationship between shot peening characteristics and material property. As a result, we gained the following conclusions. 1) It was found that materials with a higher rate of change of retained austenite content are more easily hardened by shot-peening because of a larger amount of deformation-induced martensite and work hardening. 2) When there was almost no change in retained austenite content, the maximum compressive residual stress was about 60% of the 0.2% proof stress. A higher rate of change of retained austenite content tended to result in increased maximum compressive residual stress compared to the 0.2% proof stress. This is probably due to the increased 0.2% proof stress of the shot-peened material.
In recent years, there has been a growing need for weight reduction of automotive suspension springs for the purpose of global environment protection, and with this, high-strength springs have been increasingly required. However, delayed fracture is induced when springs become highly-strengthened. Therefore development of a design method considering delayed fracture and of prevention techniques is an urgent issue. In order to realize the design method for springs that prevents delayed fracture, this report provides the database of fracture mechanics parameters, i.e., threshold stress intensity factors of hydrogen embrittlement for spring steel (hereafter referred to as KIHE). To be specific, the relationship between KIHE and hardness of SUP9A, SUP10, SUP11 and 51CrMoV4 steels, which are frequently used for leaf springs, was examined by the Wedge Opening Loading (WOL) testing. To confirm the usefulness of the relationship, the delayed fracture testing by four-point bending was conducted on test specimens with a semicircular notch. Moreover, the effectiveness of shot peening to prevent delayed fracture, as a prevention technique, was also demonstrated, and the design method considering residual streess to prevent delayed fracture was studied as well. The acquired parameters for the prevention of delayed fracture can be applied not only to leaf spring steels, but also to spring steels in general.
High fatigue strength stainless steel was developed for cylinder head gasket. Bead is press formed on the stainless steel gasket to prevent the leak of combustion gases and cooling water. However, fatigue failure is initiated from micro crack on the bead foot induced by press forming. On the developed steel, the micro crack was suppressed by refining the grain size to 1.5∼2.0μm. In addition, suitable fatigue test method for gasket was invented. It was difficult to properly evaluate the fatigue strength of stainless steel for gasket by traditional fatigue test because the test piece had neither bead nor the micro crack on the bead foot. On this invented test, cyclic compressive load is applied to test piece with bead. It was clarified that the developed steel exhibited higher fatigue strength than the conventional steel.
Stabilizer bars for automotive suspension systems are generally manufactured by bending either a rod with constant wire diameter (WD) or a pipe with constant outside diameter and constant thickness. This type of stabilizer bar contains portions that do not contribute to roll stiffness, or portions whose stress is much lower than the peak. Such portions may be redundant, and therefore unnecessarily increase stabilizer bar weight. Ideas for varying wire diameter distribution and reducing weight already exist, and tubular stabilizer bars have been commercialized with varied WD distribution. However, to the author’s knowledge, “optimizing” WD distribution for weight reduction or stress reduction research could not be found. In this study, the WD distribution for solid stabilizer bars is optimized such that 1) weight is minimized, 2) maximum principal stress is minimized, and 3) maximum shear stress is minimized. By optimizing the WD distribution for a simple U-shape solid stabilizer bar, weight is reduced by as much as 14%, and maximum principal stress and maximum shear stress are reduced by approximately 30% and 13%, respectively. It has been determined that weight reduction is achieved by flattening the “contribution to eye deflection” for each small portion throughout the stabilizer bar. On the other hand, stress reduction can be achieved by reducing the WD for areas of low stress and increasing the WD for areas of high stress. By combining the weight optimization method and the stress optimization method, it is possible to design “a lightest stabilizer bar given a certain stress limit.”
This report provides the survey of Japanese spring technological heritage in the 20th century which was conducted by a JSSE designated committee from 2009 to 2012. The 115 proposed items include coiling machines, spring testing machines, books and papers on spring design & spring manufacturing. Their whereabouts and technical significance are listed in this report. The existing historical coiling machines are listed in chronological manner. The development of spring manufacturing technology is also explained in this table.
X-ray stress measurement is a technique which has been widely applied to residual stress. However, a satisfactory method for measuring residual stress in springs has not been appropriately developed yet. Thus, a research committee in Japan Society of Spring Engineers was established to research the X-ray based stress measurement of springs in 2008. The first item accomplished by the committee was an exploratory investigation to understand the measuring conditions of X-ray stress measurement being used in spring industries. By referring to these results obtained, four subjects were investigated as follows: a) development of a method of residual stress measurement in shot-peened plate, b) development of a method of residual stress measurement in shot-peened round-bar, c) evaluation method for 2θ—sin2ψ diagrams, d) characterization of an X-ray stress constant in ferrous spring steels. The following results were obtained: (1) The errors for 68.3% confidence limits in the residual stress of the shot-peened plate specimen, which were observed by X-ray stress measurement under routine measurement conditions in participating companies, were within 6% of the average residual stress values obtained. Therefore, X-ray stress measurement conditions based on a 68.3% confidence limit were proposed. (2) In a sample with a curved surface, such as a coil spring, the diameter of an X-ray collimator should be less than 20% of the diameter of the spring wire so that the error of the residual stress can be less than 10%. It turned out that the nonlinearity of the 2θ—sin2ψ diagrams and wave-like fluctuation in diffraction intensity—sin2ψ diagrams were observed in the surface of fine-shot-peened round-bars. (3) An evaluation method for 2θ—sin2ψ diagrams showing the pattern of bowing or waving using two statistical approaches and simulations in tri-axial stress states has been investigated. Two parameters CB and Cw representing degrees of the bowing and waving in 2θ—sin2ψ diagrams have been suggested. At least seven diffraction angles in the 2θ—sin2ψ diagram are necessary to evaluate waving. A simulation of the 2θ—sin2ψ diagram enables us to confirm effectiveness of the evaluation technique with CB and CW. (4) A round robin test (RRT) for characterization of an X-ray stress constant in ferrous spring steels such as SUP9A and SUP12 was carried out. As a result, it appears that the average X-ray stress constant K in the two spring steels was -338 ± 13.1 MPa/deg.