Generally, carbon oil quenched-tempered steel wire is often used for engine valve springs. When the carbon content is greater than 0.7%, it is easy to form a twin martensite structure after quenching, and the toughness is insufficient, and the fatigue life cannot meet the requirements. When the carbon content is less than At 0.6%, the tensile strength after quenching is low. It cannot meet the stress requirements, so the carbon content is generally controlled at 0.65%. That is, 65Mn steel is used.
Compared with oil-quenched-tempered alloy spring steel wire, 65Mn has poor hardenability, low strength, insufficient toughness, and a more significant gap in anti-relaxation performance. For valve springs for high-load engines such as airplanes and automobiles, 50CrVA is more commonly used in China and Europe. 50CrV4), the United States and Japan mostly choose 55CrSiA (SAE9254, SUP12). Compared with the two, 55CrSiA steel wire has high allowable stress, good relaxation resistance, and higher allowable temperature. The allowable relaxation rate is not more than 6% as the standard, 65Mn The maximum operating temperature is 150 degrees. 50CrVA is 200 degrees, 55CrSiA is 250 degrees. The advantages of 50CrVA are low decarburization and crack sensitivity, and the surface quality is easy to control. In recent years, Europe has improved the allowable stress of Cr-V steel wire. Increase the carbon content from 0.5% to 0.67%, that is, 67CrVA is used.