A method of using backward extrusion of a cylindrical cup to evaluate the anti-galling ability of a zinc phosphate coating is proposed. In this method, galling generation on the inner surface of the cup is judged by the rapid increase in the extrusion load in the extrusion load-stroke diagram. By using a divided punch, the tip of the punch is left at the extrusion end when the main body of the punch is returned after extrusion, thus the galling situation on the inner surface of the cup is perfectly maintained. It is confirmed that the rapid increase in the extrusion load is caused by the generation of galling, and the residual coating thickness measured by FE-AES at the galling generation stroke is smaller than the protruded peak height Rpk of the punch tip.
The purpose of this study is to clarify the optimum approach to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working of medium carbon steel. Increase in warm deformation resistance during warm working by addition of Cr is small. On the other hand, Cr has the effect of suppressing the annihilation of dislocations in the air cooling process after warm working. Thus, by adding Cr, the tensile strength after warm working is effectively improved, without increasing warm deformation resistance during warm working. Increasing of Si is effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working, too. Si has an effect of the solid solution hardening. This effect by increasing Si is due to the large temperature dependence of solid solution hardening effect. The control of microstructural factors, that are pearlite fraction and cementite spacing, is not effective as a method to obtain high tensile strength after warm working, without increasing warm deformation resistance during warm working.
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