슈퍼 듀플렉스 스테인레스강의 미세조직 및 기계적 특성에 미치는 열처리 후 냉각속도의 영향

Abstract: The aim of this study was to analyze the effect of the cooling rate after heat treatment on the microstructure and mechanical properties of 2507 duplex stainless steels. Heat treatment was carried out at 1050℃ for 1 hr, followed by controlled cooling. The cooling rates were 175.6 × 10-3℃/s, 47.8 × 10-3℃/s, 33.3 × 10-3℃/s, 16.7 × 10-3℃/s, 11.7 × 10-3℃/s, 5.8 × 10-3℃/s and 2.8 × 10-3℃/s, which resulted in variations of the microstructure, such as the fractional change of the ferrite phase and sigma phase formation. Fatigue, hardness, impact and tensile tests were performed on the specimens with different cooling rates. The precipitation of the σ phase caused a hardness increase and a sharp decrease of toughness and tensile elongation. The fatigue limit of the sample with a cooling rate of 5.8 × 10-3℃/s was 26MPa higher than that of the sample with a cooling rate of 175.6 × 10-3℃/s. Our observations of the fracture surface confirmed that the higher fatigue resistance of the specimen with a cooling rate of 5.8 × 10-3℃/s was caused by the delay of the fatigue crack growth, in addition to higher yield strength.