원자력발전소용 니켈기 초내열합금 (Alloy 718)의 마찰용접 최적화와 그 기계적 특성에 관한 연구

Abstract

Valve stem material of nuclear power plants is generally made of heat-resistant steels. Because it is operated under the environmental conditions with high temperature and high pressure, its life is limited. When it is replaced with super-alloy, the high temperature properties of the supper-alloy can be utilized and the rapid improvement of its tensile strength can be expected. However, Ni-based Alloy 718 has higher production cost, because it is more expensive than general stainless steels. To solve this problem of parts of valve stem, head part that is directly exposed to the environmental conditions with high temperature and high pressure is made of Ni-basedAlloy 718, and stem part and others are made of general stainless steels. This leads to the improvement in durability by super-alloy and the development of components with low cost. In this paper, friction welding was performed to investigate mechanical properties of Ni-based Alloy 718 with 15mm diameter solid bar. The main friction welding parameters were selected to endure good quality welds on the basis of visual examination, tensile tests, impact energy test, Vickers hardness survey of the bond of area and heat affected zone. And then, the nondestructive technique to evaluate the weld quality was carried out by acoustic emission (AE) and ultrasonic attenuation coefficient. An experimental study was conducted not only to optimize the friction welding (FRW) conditions, but also to investigate the quantitative relationship between the weld quality of strength and toughness and the total number of acoustic emission counts. A quantitative correlation between the total number of AE counts and the weld quality appears to exist, and the optimum weld quality more than 100% of joint efficiency can be detected in real time during processing by the use of AE techniques. It seems likely that extension of this study could lead to the development of an in-process quality monitoring system the friction welding based on the acoustic emission events. The tensile strength of the friction welded joint was shown up to 90% of the Alloy 718 base metal under the condition of the heating time over 5 sec. The optimal welding conditions were n = 2,000rpm, P1 = 200MPa, P2 = 200MPa, t1 = 8sec. and t2 = 5sec. when the total upset length was 4.4 mm. And second one, the creep rupture behaviors of valve stem material (Alloy 718) for nuclear power plant were investigated for the elevated temperatures of 550 to 700°C. Constant stress creep tests were carried out for the base metal of Ni-based Alloy 718 and the friction welded Ni-based Alloy 718. The creep characteristics such as creep stress, creep rupture time, steady state creep rate and so on were evaluated. It seems clear that the creep stress has a quantitative correlation between creep rupture time and steady state creep rate. The stress exponents (n, m) of experimental data for Ni-based Alloy 718 steels at 550, 600, 650 and 700°C were derived as (33.5, -24.9), (26.1, -21.2), (16.8, -12.8) and (10, -8.2) respectively. And, the stress exponents (n, m) of experimental data for friction welded Ni-based Alloy 718 steels at 550, 600, 650 and 700°C were derived as (31.5, -22.4), (22.5, -18.5), (17.4, -14.3) and (6.9, -8.1) respectively. By increasing with the temperature, the stress exponent was decreased. The creep life prediction by Larson-Miller parameter (LMP) method was derived, the result equations for Ni-based Alloy 718 steels and friction welded Ni-based Alloy 718 steels were obtained as follows, respectively