오스테나이트계 스테인리스강에서 장시간 열처리가 미세조직과 기계적 성질에 미치는 영향

Abstract

Austenitic stainless steels are widely used for structural components in chemical, petrochemical and nuclear industries due to their good combination of mechanical properties, fabrication and corrosion resistance. Most power plant facilities such as nuclear power, thermal power and petrochemical plants are serviced for a long period of time. Especially, they are used in high temperature and high pressure environment, where deterioration of materials frequently occurs and the materials are degraded during their operations. Major factors that cause material deterioration are equipment corrosion at high temperature, operating conditions, excessive operation time and so on. The deterioration of equipment is promoted due to various problems in most cases. Particularly, in the case of thermal power plants, equipment is deteriorated due to material degradation in steam boiler tubes, pipes, headers, turbine blades, turbine casings, steam turbine rotors. Therefore, a technology capable of quantitatively evaluating the degree of deterioration of facilities is indispensable for safety evaluation and life assessment. There is a need to secure a standardized technique for evaluating the remaining life of the product. The purpose of this study is to investigate the microstructures and mechanical properties evolution of austenitic stainless steels AISI 304 and AISI 316L subjected to long-term heat treatment at elevated temperature. Microstructure analysis was carried out through OM(Optical Microscope), SEM(Scanning Electron Microscope) and MFM(Magnetic Force Microscope), X-ray diffraction analysis was performed for structural analysis. The ferrite fraction was measured using a ferrite scope. EPMA(Electron Probe Micro Analyzer) and WDS(Wavelength Dispersive X-ray Spectroscopy) were used for the element characterization. Hardness and Tensile tests were performed to evaluate mechanical properties. Both AISI 304 and AISI 316L had annealing twins within grain interior and δ-ferrite along the grain boundaries. In AISI 304, annealing twins remained even after increasing the aging time, whereas in AISI 316L annealing twins were disappearing. Also, grains were spheroidized with increasing aging time. The decomposition of δ-ferrite is decomposed into Cr23C6 and sigma phases. As the aging time increased, many particles were formed around the δ-ferrite, and the particles were found to be Cr carbide. The δ-ferrite will be decomposed into Cr23C6 and σ phases in both austenitic stainless steels. But in the X-ray diffraction analysis, only AISI 304 shows increase in ferrite peaks. This is closed relation with chromium depletion due to the long-term heat treatment and martensite transformation(sensitization induced martensite) near the grain boundary. The strength of AISI 304 is affected by the precipitates and decomposition of δ-ferrite. In addition, the strength is also increased due to martensite transformation near grain boundary. In the case of AISI 316L, the strength at 1,000 hours aging time decreased a little caused by the widening of incoherent interface between sigma phase and austenite matrix. For further aging time, the increase in strength was due to the deposition of M23C6, which had been primarily deposited only at grain boundaries for up to 100 hours. Both materials became brittle by the material deteriorated resulting in the decrease in elongation and the toughness.