As an alloying element to increase the resistance to softening during tempering, silicon content was modified to investigate its effect on the microstructure and properties of 100Cr6 bearing steel. It turned out that increasing the silicon content brought about two difficulties in the manufacturing process, promoting the decarburisation during heating and/or hot rolling and retarding the spheroidisation of cementite. The maximum size of non-metallic inclusions was predicted to reduce evidently with increasing silicon content, although the total oxygen content in steels was nearly the same. There has been no fine carbide precipitation during tempering that can be attributed to the increasing silicon content, leading to the stabilisation of retained austenite due to the diffusion of carbon atoms from martensite. As a result, the new Si modified high carbon chromium bearing steel showed superior rolling contact fatigue characteristics to conventional steel.

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The hydrofonning technology may bring many advantages to automotive applications in terms of better structural integrity of the parts, lower cost from fewer part count, material saving, weight reduction, lower springback, improved strength and durability and design flexibility. In this study, the whole process of front sub-frame parts development by tube hydroforming using steel material having tensile strength of 440 MPa grade is presented. At the part design stage, it requires feasibility study and process design aided by CAE (Computer Aided Design) to confirm hydrofonnability in details. Effects of parameters such as internal pressure, axial feeding and geometry shape in automotive engine cradle by the hydroforming process were carefully investigated. Overall possibility of hydroformable engine cradle parts could be examined by cross sectional analyses. Moreover, it is essential to ensure the formability of tube material on every forming step such as pre-bending, preforming and hydroforming. At the die design stage, all the components of prototyping tool are designed and interference with the press is examined from the point of deformed geometry and local thinning.