# 퀜칭한 SCM 440 강에서 초음파 전파특성에 미치는 템퍼링온도의 영향

• Lee, K.W. (Dept. of Metallurgical Engineering, Hanyang University) ;
• Kim, M.I. (Dept. of Metallurgical Engineering, Yonsei University) ;
• Park, U.S. (Dept. of Material Engineering, Seoul National Polytechnic University)
• 이계완 (한양대학교 금속공학과) ;
• 김문일 (연세대학교 금속공학과) ;
• 박은수 (서울산업대학 재료공학과)
• Published : 1991.09.30

#### Abstract

The effect of tempering temperature on the ultrasonic propagation velocity at SCM 440 steel quenched from $870^{\circ}C$ and $1000^{\circ}C$ has been studied by metallurgical and crystallographical observation. The measurements of ultrasonic velocity were made on the specimen by appling an immersion ultrasonic pulse-echo technique with a constant frequency of 10 MHz. The quenched microstructure of this steel was a lath martensite. As the tempering temperature was increased, the martensite was transformed into the tempered martensite composed of cementite and carbide. The ultrasonic velocity increased with increasing the tempering temperature. It was thought that these were resulted from the microstructural transformation. The change of ultrasonic propagation velocity with quenching and tempering heat treatment was resulted from microstrain due to the change of internal stress. Considering these results concerning to the change of ultrasonic propagation velocity. the phenomena of microstructural transformation were estimated. Consequently, it was thought that the degree of quenching and tempered heat treatment of steel could be nondestructively evaluated with the change of ultrasonic propagation velocity.