Differences and similarities in various aspects of the two types of axles.
(1) Differences in chemical composition. Japanese axles mainly use high-quality carbon steel, its steel cleanliness is relatively low, the number of internal inclusions and large size; while European axles are mostly used in high-quality low-alloy steel and medium-alloy steel, the purity of steel is higher.
(2) Differences in heat treatment process routes. Japanese axles will be rolled, forged and normalized in the process, followed by rough machining. After that, surface treatment by high-frequency induction surface quenching and low-temperature tempering will be used to enhance the surface hardness of specific locations. These enhanced surface treatments are the key locations where the axle needs to be resistant to wear and fatigue fracture. In contrast, European axles use a tempering heat treatment process for the entire axle billet.
(3) Differences in surface hardness and tensile strength. After high-frequency induction surface quenching and low-temperature tempering treatment of Japanese axles, the surface of the formation of a hardened layer of 3 to 4 mm thickness, accompanied by a transition zone of about 4 mm thick, making the average hardness of up to 550 HV, the surface tensile strength of about 1,900 MPa. In contrast, after the tempering heat-treated European axles, the hardness and tensile strength of their surfaces are relatively low.
(4) Differences in internal microstructure and properties. The microstructure and mechanical properties of the Japanese axle in the middle of the inner and outer surfaces as well as on the surface of the bore are poor. Although the tensile and impact properties meet the standard JISE4502-1, the hardness of the matrix organization is only 180 HV. In contrast, the microstructures of European axles (except for EA200 axles) at these locations are mainly tempered martensite and bainite, and the mechanical properties gradually weaken from the outside to the inside, but the gradient of the change remains relatively small.
(5) Difference in residual compressive stress. Residual compressive stress plays a significant role in inhibiting crack expansion and reducing its expansion rate. Japanese axles, by carefully regulating the parameters of the high-frequency quenching process, are able to obtain an ideal residual compressive stress distribution within a certain depth of the surface. European axles, on the other hand, need to use the appropriate heat treatment and machining technology to effectively manage the residual stress state.
(6) Complexity of heat treatment process. Compared with European axles, the surface heat treatment process of Japanese axles is more complex and difficult to control. The machining of different types of axles, different parts of axles, and complex cross-section shapes have a significant impact on the process regime and the design of inductors (coils), and this impact is largely dependent on experience. In the early or short term of production, this process complexity may lead to inconsistent process and quality of the axle, which requires a long period of adaptation. Nevertheless, China has advanced equipment in overall tempering heat treatment, and therefore has a significant advantage over the overall tempering heat treatment process route for European axle shaft blanks.
(7) Weight difference. Subject to the limitations of the axle material and surface heat treatment process, the same grade of Japanese axles tend to be heavier than the use of alloy steel European axles.
