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Korean Journal of Metals and Materials > Volume 56(3); 2018 > Article
Korean Journal of Metals and Materials 2018;56(3): 221-226. doi: https://doi.org/10.3365/KJMM.2018.56.3.221
Cr강의 미세조직과 파괴인성에 미치는 열간단조 및 열처리 공정의 영향
이동준1, 권용남1, 김민석1, 구가은1, 허상현2, 김남용2, 이진모2
1재료연구소 소재성형연구실
2(주)태웅 기업부설연구소
Effect of Hot Forging and Heat Treatment on the Microstructure and Mechanical Properties of Cr Steel
Dong Jun Lee1, Yong-Nam kwon1, Min Suk Kim1, Ga Eun Ku1, Sang Hyun Heo2, Nam Yong Kim2, Jin-Mo Lee2
1Materials Deformation Department, Korea Institute of Materials Science (KIMS), Changwon 51508, Republic of Korea
2Research Institute Attached, Taewoong, Busan 46751, Republic of Korea
Correspondence  Dong Jun Lee ,Tel: +82-55-280-3839, Email: djlee@kims.re.kr
Received: 11 September 2017;  Accepted: 27 December 2017.  Published online: 6 March 2018.
ABSTRACT
In this study, the effects of hot forging and heat treatment (quenching and tempering) of cast Cr alloy steel on the microstructures and mechanical properties were investigated. The hot forging was performed at a compressive ratio 0.5 at 1,250 oC. The heat treatment process was quenching (860 oC for 2 hours and water quenching) and tempering (655 oC for 2 hours and air cooling). The microstructures of the hot forged specimen showed bainite, pearlite and ferrite mixed phases with high tensile strength, but showed low fracture toughness. The heat treated specimens after hot forging showed tempered martensite microstructure and high fracture toughness but relatively low yield and tensile strengths. After tensile and fracture toughness tests, the cast and the hot forged specimens both showed cleavage fracture surfaces, which occurred between lamellar structures. However, the heat treated specimen had a ductile fracture surface with dimple shaped fractures. From these results, we could conclude that the high fracture toughness was caused not by the cleavage fracture mode in the pearlite and bainite phases, but delayed fracture due to a ductile fracture mode in the tempered martensite phase.
Keywords: Cr steel, heat treatment, fracture toughness, microstructure, tempered martensite
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