High fatigue strength of valve springs for automobiles is intensely required for weight reduction and high velocity revolution. For this subject, springs have been developed in terms of both chemicals in the material and wire processing sides. In this paper the high fatigue-proof spring was developed using the high Si oil tempered wires with high heat resistance by increased Si. As results, the developed valve spring exhibits the good performance. The experimental results are as follows; 1) Results of springs with shot speening Between developed spring and control spring iSWOSC-VTS j, there was no difference in the hardness distribution and residual stress distribution. Developed spring durability and sag resistance are improved 10% and 6% respectively, as compared with the control spring. 2) Results of springs with nitriding and shot peening Developed spring with high temperature nitriding and hard shot peening achieved big improvement of 25% in the durability and 6% in the sag resistance compared with the control spring with low temperature nitriding and normal shot peening.
For the weight reduction of suspension coil springs for automobiles, high-strength steel has been developed. Generally, strength and corrosion fatigue shows contradictory behavior, so the technological basis is to improve corrosion fatigue strength in addition to durability and sag resistance. As a solution for the problem, element design which improved the performance against corrosion and optimum condition of shot peening process were studied. Thus the high-strength suspension coil springs in this paper obtained superior corrosion fatigue strength, compared with the spring made of SUP7. Moreover, as a new basic evaluation method, corrosion fatigue test of springs with an artificial pit was proposed.
In this study the effect of newly developed Sn hot dipping process for the surface coating of the copper alloys were examined. The copper alloys tested contain a small amount of Ni and Sn. They have good electrical conductivity and high temperature characteristics and are being used widely for springs in small parts as relays and connecters. Mechanical properties of the hot dipped alloy such as the surface roughness, the spring limit, the tensile strength and Young’s modulus at elevated temperatures were measured. The process resulted in much less degradation of the original Cu alloys than the conventional electroplating.
Low- and high-cycle fatigue properties were investigated for spring steels SUP7, 9A and 12, and tool steels SKD61 and 11 whose tensile strength ranged from 1300 to 2130 MPa. Surface fracture, where cracks initiated at the specimen surface, occurred at the low cycle region below 105 cycles, whereas internal fracture, where cracks initiated at the internal inclusions, occurred at the high cycle region above 105 cycles. An unique curve of σa/σyc vs. Nf was obtained for the surface fracture regardless of steel and strength level. Where σa and σyc are stress amplitude and cyclic yield strength, respectively. Using a crack propagation parameter of σa3ai1/2, it was observed that fatigue life for Al-oxide, Ca-oxide and Cr-carbide inclusions in the descending order. Where ai is inclusion diameter. This suggests that the internal fatigue properties are affected by species of inclusion as well as its size.
Generally, we have very great difficulties to measure residual stress and to investigate the shot-peened effects after shot peening on the inner surface of small bore size bellows made of SUS304 steel. However we obtained a greater compressive residual stress on the inner surface of the bellows using a reflection shot peening method and found the peened effects. In addition we found that this method in comparison with non-shot peening one brought fairly improved fatigue strength. But it is difficult to measure residual stress of SUS304 steel. And then this experimental conditions are decided by measuring residual stress of SUS631 steel bellows which has an identical shape and dimensions of SUS304 steel bellows. We established the best shot peening conditions from the relations among pressure, glass beads size and fatigue strength obtained by the aboved conditions. So it is confirmed by the experiments that the reflection shot peening method is successful for small bore size bellows made of SUS304 steel.
To meet the requirement for higher allowable design stress of the suspension springs, new steel were developed placing emphasis on corrosion fatigue resistance. Corrosion fatigue life proved to be increased by improvement of hydrogen embrittlement resistance as well as corrosion resistance. Corrosion resistance was improved through the control of rust composition by the addition of copper and nickel. Hydrogen embrittlement resistance was improved through the improvement of toughness and the introduction of hydrogen trapping sites by the addition of titanium. Newly developed steel (UHS1900) subjected to a 1200MPa maximum shear stress has the same corrosion fatigue life as that of SUP7 steel subjected to a 1100MPa maximum shear stress. In addition, UHS1900 demonstrated better toughness, ductility, hydrogen embrittlement resistance, fatigue life in the air, fatigue life in the corrosion atmosphere, and sag resistance when it was tempered relatively lower temperature of 200∼300°C. This suggests that application of the lower tempering temperature enables higher stress design for UHS1900 springs.
Austenitic stainless steels such as SUS 304 have been widely used for small sized springs depending on improved strength generated by deformation induced martensitic transformation during wire drawing process. It has the advantage of superior characteristics of corrosion resisting, heat resisting and nonmagnetic properties too. However, actual fatigue endurance of austenitic stainless steels is not superior compared with those of other steels with the same strength characters and the endurance test data has a tendency of scattering. It is pointed out in some occasion that the endurance limit of stainless steel wire is superior for the materials with lower martenisitic transformed phase contents despite having low tensile strength. As controversial comment can be also heard in other occasions, the effect of martensitic transformation upon fatigue endurance of austenitic stainless steel wires has not been commonly understood in general. Therefore, the reasonable methods for controlling the quality of small sized stainless steel springs has not been established yet. For this reason, they have been applied in only a limited cases. In this study, some trials for grasping the governing factors to improve fatigue endurance and the statistical fatigue characteristics of austenitic stainless steel have been conducted. Research Committee on Endurance of Stainless Steel Wires was established in July 1995 to investigate the governing factors in the fatigue endurance of austenitic stainless steel wire form the viewpoints of tensile strength, surface roughness, hardness of surface layer and deformation induced martensitic transformation. A lot of experimental data have been accumulated in this committee until October 1999. As a result, valuable experimental data and some tips for the manufacturing process of wire and spring were obtained as presented in this paper.