Spring weight-saving will be achieved by increased design stress. However, in case of the conventional spring steel, high strength steel reduces corrosion fatigue life and toughness, and it gets down the reliability of springs in actual environment. In this paper, we propose newly developed spring material with emphasis on improvement of shot peening to satisfy higher strength and corrosion fatigue life simultaneously. The chemical elements of developed material were designed under the following philosophies, where Si stays at 1.8% for sag-resistance, C is reduced to 0.4% for the corrosion fatigue life, Ni is added by 0.5% for the controled forming of corrosive pit, and a trace of B and Nb is added for the strength of austenitic grain boundary which be way of corrosion fatigue's crack propagation and fine grain size respectively. Experimental results by straight test piece and coil spring under high stress shows that the developed spring material is effective for improving corrosive fatigue life and delayed fracture strength. Developed spring material can achieve 20% weight-saving under 1200MPa design stress comparing with the conventional spring steel.
High strength spring steel wire is intensely required for weight saving of coil spring. Induction hardening (quenching & tempering) gives higher strength and stable quality to the spring wire effectively. Generally, higher strength steel becomes more sensitive to hydrogen embrittlement. Relationship between austenite grain size and hydrogen embrittlement is not clear down to date. In this paper, a delayed fracture test under torsional load and a measurement of diffusible hydrogen were carried out using ITW® Induction heating quenched and Tempered Wire with various austenite grain size and various tensile strength. It was revealed that delayed fracture properties were improved by refining of austenite grain size. Improvement was even recognized in super high strength ITW® over 2160N/mm2 grade.
In order to respond to the needs for high-stress valve spring designing, we developed a new valve spring steel which has superior fatigue property and sag resistance without sacrificing the cold-coiling performance of wire. This paper describes the following results: 1) The tensile strength of newly developed steel was 200MPa higher than that of conventional SAE9254 after heat treatment which simulated the nitriding process. 2) Austenite grain size number of the developed steel was 2 point higher than that of SAE9254 after oil tempering process. 3) Fracture toughness of the developed steel was 5MPa√m higher than that of SAE9254 compared at the same hardness level. This is thought to be the effect of Ni addition and higher tempering temperature. 4) It was confirmed that conventional cold coiling process being adopted to SAE9254 is applicable to this newly developed steel. 5) The developed steel is less sensitive against inclusions during fatigue test. If the inclusion size is same level, 16% higher cyclic stress can be applied for the developed steel than for SAE9254. 6) We have also developed the inclusion control technique for high Si steel, and high fatigue performance are being stably obtained.
Crack-healing behavior of silicon nitride ceramics and high temperature strength of crack-healing zone have been investigated previously systematically in our last work. In this paper, the cyclic fatigue strength behavior of crack-healed ceramics was investigated in detail. Two kinds of the compositions of Si3N4 and SiC with different sintering additive were tested. One contains the 8wt.% Y2O3 as a sintering additive, and was called SNS-Y8 specimen in this paper, another contains the 5wt.% Y2O3 and 3wt.% Al2O3 as a sintering additive, and was called SNS-Y5A3 specimen in this paper. The semicircular crack (average diameter is 100μm and 200μm) was made in the center of specimen using Vickers hardness indenter. Cracked specimen were heattreated at 1200°C-1300°C for 1 hour in the air. Cyclic fatigue strength test was conducted at room temperature. Crack-healed material has shown the outstanding fatigue strength behavior and attains almost the same strength as smooth specimen do. The surface state of the specimen which did not fail for cyclic fatigue experiments of 2×106 cycles was observed by SEM., any new crack was not found on the surface of crack-healed specimen which survived fatigue test of 2×106 cycles.
A way of improving the fatigue strength of valve springs is to increase the tensile strength of oil tempered wire. However, the more the tensile strength increases, the more coiling operation becomes difficult. Therefore it has been difficult to make a great progress in fatigue strength depending on enhanced tensile strength only. In order that the high fatigue strength coexists with the easiness of coiling, high strength wire with controlled hardness on its surface is developed. Further more, the valve spring is treated by nitriding process at higher temperatures than usual, and the optimized triple hard shot peening is performed. As a result, the developed valve spring seems to exhibit the excellent performance of the world highest level
Valve springs with high fatigue strength corresponding to the incresement of working stresses are required for the higher generating power and the better fuel economy of automobile engines. For this purpose, high strength oil tempered wires are being used. In order to achieve the high strength for the valve spring, modification of manufacturing processes is being applied. In this case, the cause and the effect for the improvement of the fatigue strength has not yet been explained explicitly. Therefore, in this report, the comparison of fatigue life between valve springs made by conventional processes with oil tempered wires and the ones made by a new manufacturing processes was made. As a result of the fatigue test, the fatigue life of the latter reached to maximum 7 times longer than that of the former. It was cleared that the improvement of the fatigue life was materialized by the difference of compressive residual stresses at the depth of 0.2mm below the inner side surface of the valve springs.
This study was intended to connect the modeling of crevice and the electrochemical method of testing in order to examine the crevice corrosion behavior of shot-peened high carbon steel S60C. Experiments were carried out under various potentials on the anodic polarization curve which was obtained in 3% NaCl solution. The effect of shot-peening was compared and examined from corrosion morphology. The main results obtained are summarized as follows; The crevice corrosion of carbon steel in 3% NaCl solution seems to take place when the potential is set at -0.750--0.800V (vs. S. C. E.) corresponding to passive state region on the anodic polarization. In the event when we intend to understand the crevice corrosion behavior in short time, the correlation between the modeling of crevice and the electrochemical methods of testing seems possible. The pH of solution within the crevice shifts to acid side until it becomes 4.6 owing to the hydrolysis of metallic ions. The corrosion morphology was represented by the general corrosion. The shot-peening may be an effective corrosion inhibition method since the corrosion depths in the outside of crevice or the part of crevice are smaller than the untreated specimen.