Temperature History and Metallographic Structure of 0.45%C Steel Processed by Frictional Stir Burnishing

概要

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Significant enhancement of hardness characteristics was achieved through the Frictional Stir Burnishing (FSB) process. The enhanced region was generated by the FSB process where the thickness and hardness were approximately 200 µm and 900 HV, respectively, under the conditions of spindle speed of 10000 min-1, a feed rate of 200 mm/min and a processing load of 750 N. The processing temperature was approximately 600∼750 °C under compressive stress condition. Although this temperature is lower than A3 transform point (normally 780 °C), it is known that A3 transform point decreases under the compressive stress condition. And after the tool passed, the temperature dropped rapidly. This process can be assumed to be the same as a quenching process. However, the obtained hardness value is harder than that of the regular quenched material. When the processing load was 500 N, extremely hard regions over 1000 HV were obtained. In this case, it is thought that the material was not transformed into martensite. It can be concluded that the severe stirring process of FSB caused the grain refinement.