Fatigue Strength of Laser-Processed Hot Work Tool Steel and Its Reliability
Satoshi HANAKI, Yahong SUN, Masato YAMASHITA, Hitoshi UCHIDA and Nobuhiro TSUJII
Abstract:A fatigue life of die-casting dies depends on heat checks, which are fine shallow cracks on a surface due to cyclic thermal stress. According to the recent study, the laser-processing is effective to repair cracks and extend the life of dies. Therefore, the purpose of this study is to evaluate the fatigue strength of laser-processed hot work tool steel to validate the reliability of repaired dies. In this study, the rotating bending fatigue test for the laser-processed hot work tool steel specimens has been carried out. Because test results have scatter, S-N curves are decided based on the evaluation of fatigue strength distribution. As a result, the fatigue strength of the laser-processed specimens decrease remarkably in compared with that of the non-processed ones. The notch sensitivity might have risen because the structure became brittle due to rapid cooling. However, it can be recovered by heat-treatment at 773K. This could be due to a high yield strength resulted in a secondary-hardening effect. The fractgraphic results also demonstrate the same tendency. The fracture surface of heat-treated specimen is similar to that of base metal, while that of laser-processed one shows different appearance. The laser-processing method can be carried out at relatively low cost and effective for the repair of heat check. Therefore, we can conclude that this method is available for the extension of the life span of dies. Key Words:Hot work tool steel, Laser-processing, S-N curve, Fatigue strength, Reliability, Fractography