고온 재료의 무시할 수 있는 크리프 및 크리프-피로 특성에 관한 연구

Abstract

Recently, the operating temperatures of thermal and nuclear power systems, fuel cell systems, etc. has been increased to increase the efficiency of the system. Therefore, creep and creep-fatigue (CF) behaviors at high- temperatures is a major concern. Type 316L(N) austenitic stainless steel and Alloy 800H become typical high-temperature materials, which are being widely applied to these systems because the two steels are superior to other materials in high-temperature strength and economy. Especially, Type 316L(N) stainless steel is expected as one of candidate materials which can be used for major structural components of sodium-cooled fast reactor (SFR). Also, the steel will be chosen to use for Molten Carbonate Fuel Cell (MCFC) and Solid Oxide Fuel Cell (SOFC) which can be expected to achieve high-power efficiency. This ‘LN’ steel (carbon 0.03% max. and nitrogen 0.1%) was developed as a new version to improve the high-temperature creep strength and intergranular corrosion resistance. Since the SFR structures are designed to be used for 30∼40 years at a high temperature up to 550℃, one of the most important properties on creep behavior. Especially, negligible temperature curve (TNEC), which is set as accumulated creep time and temperature, should be provided to designer and engineer to judge whether creep for simplified design of components operating at high-temperatures should be considered or not. In addition, the heat resistance steel Alloy 800H is widely used as a new version strictly controlled with a carbon content (0.05∼0.1%) in Alloy 800, and it is one of major candidate materials to be used for hot gas duct, core barrel, core support, control rod system, and intermediate heat exchanger (IHX) in a very high-temperature reactor (VHTR). The VHTR components will be often subjected to the thermal stress due to start-up and shut-down or due to variations in operating conditions. In between start-up and shut-down or transients the components will be subject to fatigue and creep-fatigue (CF) effects. The CF is known as a major damage mechanism governing total life in high-temperature structures. Therefore, to do a safe design of reactor and assessment of structural integrity et al., it is significantly important to understand the CF characteristics and to achieve experimental test data for Alloy 800H. This study was carried out and considered carefully for negligible creep characteristics for Type 316L(N) steel and the CF behavior for Alloy 800H. Firstly, regarding the negligible creep for Type 316L(N) steel, a basic concept for TNEC curve is preliminarily defined, and then methodology to generate the TNEC curve is proposed. To do this, long-term creep life prediction for Larson-Miller Parameter (LMP) and Wilshire Equation (WE) Model was carried out and assessed to find out a suitable method in creep-life prediction of Type 316L(N) steel. Then, dependency of a reference stress affecting to TNEC curve is investigated using three reference stresses: minimum yield strength at 0.2% offset (Rp02(min)), average yield strength at 0.2% offset (Rp02(moy)), and maximum allowable stress (Sm), and the results are compared with the TNEC curve in RCC-MRx in France. Secondly, regarding the CF behavior for Alloy 800H, the CF tests have been carried out through a series of fully reverse strain-controls (strain ratio, Rε=-1) under an identical condition of a total strain range of 0.6% at three temperatures of 700, 750 and 800℃ in an air environment, and holding time in the CF tests was applied for 60 seconds. In this study, to well understand the CF behavior was compared and assessed with the tested results of continuous Low Cycle Fatigue (LCF) preliminarily tested at SMARE Lab. (Strength of Material And Reliability Evaluation laboratory) in Pukyong National University.

The results obtained for negligible creep of Type 316L(N) steel and the CF behavior of Alloy 800H can be summarized as follows.

1) Long-term creep life prediction for generating the TNEC curve for Type 316L(N) steel was carried out, and it was identified that WE model was superior to LMP method. Using the WE model, the TNEC curve for Type 316L(N) steel was drawn successfully.

2) For the results applied for three reference stresses of minimum yield strength at 0.2% offset (Rp02(min)), average yield strength at 0.2% offset (Rp02(moy)), and maximum allowable stress (Sm) affecting to TNEC curve, the Sm curve showed a large difference when compared with RCC-MRx curve, but the Rp02(moy) curve showed good agreement with RCC-MRx curve.

3) When the TNEC curves for 304, 316L and 316L(N) stainless steels were compared together, it followed well the order in creep strength as 316L(N)>316L>304. Type 304 and Type 316 steels showed a large difference in TNEC curve, but Type 316L(N) and 316L steels was almost similar in TNEC curve. It is thus identified that Type 316-series steel can be used during longer time at higher temperature without creep damage.

4) Repeated stress response behavior for Alloy 800H showed initial repeated hardening phenomenon in the CF and LCF tests, the degree of hardening rate in the CF was lower than that of continuous LCF, and the degree of hardening rate was reduced linearly with increasing in temperature.

5) In the CF case, compressive valley stress was higher than tensile-peak stress at all temperature conditions. The difference between compressive valley stress and tensile-peak stress was observed to be lower with increasing in temperature. And in the CF and continuous LCF, the stress in half life is reduced with increasing in temperature, and plastic strain was increased slowly with increasing in temperature. In addition, crack initiation life was decreased with increasing in temperature.

6) In the CF tests, it was observed for intergranular fracture mode in crack initiation region, and in crack propagation region, it was observed for a mixed mode with transgranular fracture and intergranular fracture.

These findings provide engineering insights into creep and creep-fatigue behavior of Type 316L(N) and Alloy 800H.