열간금형 수명향상을 위한 열간금형 표면 하드페이싱 기술 개발에 관한 연구

Abstract

Thermal softening phenomenon of the hot forming tool steel leads to the premature wear of the hot forming tools. Hence, several researchers have paid attention to the development of wear resistance coating technologies to enhance the service life of the hot forming tool. The aim of this thesis is to develop the hardfacing technology of the hot forming tool surfaces for the improvement of the operation life of the hot forming tool. In order to obtain proper thickness of the hardfacing specimen for the wear experiment, transient heat transfer and thermal stress analyses have been performed using the ABAQUS V6.11 Temp-displacement module. Using the results of the FE analyses, the temperature, effective stress and principal strain distributions in the specimen have been investigated. From the results of the investigation, it has been shown that an appropriate thickness of the hardfacing layer is 2 mm. Hardness and wear experiments in the elevated temperature have been performed to investigate wear characteristics of the hardfaced specimen. The specimen of the experiments have been fabricated via the deposition of the Stellite 6, Stellite 21 and NOREM 02 superalloys on the STD 61 hot working tool steel using laser-aided direct metal rapid tooling (DMT) process. The results of the hardness experiments in the elevated temperature have been revealed that the hardness of the hardfaced specimen is conspicuously greater than that of STD61 specimen when the ambient temperature is larger than 700 oC. The results of the wear experiment in the elevated temperature have been shown that the weight loss, the worn volume, the worn width, and the worn depth of the hardfaced specimen is remarkably smaller than those of the STD 61 specimen. In addition, it has been revealed the friction, lubrication and friction induced work harding characteristics are highly improved when the Stellite 21 is deposited on the STD 61. From these results, it have been noted that the Stellite 21 is a proper hardfacing material of the STD 61 hot working tool steel. Wear experiments in the eleveated temperature have been carried out to investigate the influence of thickness of the hardfaced layer on the wear characteristics of the Stellite 21 hardfaced specimen. The results of the experiments have been shown that the weight loss, the worn volume, the worn width, the worn depth and the friction coefficient of the hardfaced layer is minimized when the thickness of the hardfaced layer is 2 mm. Based on the above result, proper material and thickness of the hardfaced layer for the STD 61 hot working tool have been obtained to develop the hardfacing technology of the hot forming tool surfaces for the improvement of the operation life of the hot forming tool.