Ji Hui Kim

Design of Erosion Resistant Overlay Welding Material

최근 국제유가의 가파른 상승은 석유자원 확보와 대체에너지 개발 경쟁을 더욱 치열하게 하고 있다. 특히, 중동 등 온난지역의 에너지자원이 고갈되어가고 기후온난화로 인해 북극 등 극한지역의 자원탐사가 가능해지면서 극한지 유정과 오일샌드 개발이 가속화되고 있다. 이에 따라 내마모성, 저온인성 등의 우수한 성능을 갖는 첨단소재에 대한 요구가 증대되고 있다. 오일샌드는 중질유인 비투먼(bitumen)을 함유하는 모래 또는 암석으로, 이들을 모아 Fig. 1과 같이 비투먼을 추출한 후에 정제과정을 거치면 석유를 얻을 수 있다. 1배럴의 석유를 추출하기 위해서는 오일샌드 2톤이 필요한 것으로 알려져 있다. 캐나다 오일샌드의 2008년 일일 생산량은 120만 배럴로 하루에 240만톤의 오일샌드가 처리되고 있다. 추출과정에서 오일샌드와 고온수의 슬러리를 이송하는 hydrotransport 라인의 내벽은 Fig. 1과 같은 마식환경에 노출되어 있다. 마식(erosion)이란 고체입자, 액체 등의 반복적 충돌에 의해 소재의 표면이 손실되는 현상을 말한다. 현재 사용되고 있는 API-X65/X70급 강재의 마식률은 1,000시간에 수 mm 정도이며, 이로 인해 파이프라인 교체수명이 1년 미만인 것으로 알려져 있다. 이러한 오일샌드 이송라인을 비롯하여 해저자원의 양광 파이프라인 등과 같이 극심한 마식환경에 사용하기 위한 소재로는 WC, B

The Effect of Boron on the Sliding Wear Behavior of Iron-Based Hardfacing Alloys for Nuclear Power Plants Valves

A new iron-based wear resistance alloy was developed to replace the Co-containing Stellite 6 alloys in nuclear power industry. The effect of B addition on the wear resistance was investigated. Sliding wear tests of Fe-Cr-C-Si-xB (x = 0.0, 0.3, 0.6, 1.0 and 2.0 wt%) alloys were performed in air at the room temperature under a contact stress of 103 MPa. Low-boron alloys containing less than 0.6 wt% boron showed an excellent wear resistance than any other tested alloys. The improvement was associated with the matrix hardening by promotion of the γ→α′straininduced martensitic transformation occurring during the wear test. However, the alloys containing more than 1.0 wt% boron showed slightly increased wear loss compared to the low-boron alloys because of the absence of the strain-induced martensitic transformation and the presence of the brittle FeB particles, aiding crack initiation.

Sliding Wear Behavior of Steam Generator Tube Materials in Nuclear Power Plants

Wear damage of steam generator tubes of nuclear power plants can cause leakage of radioactive substances. So the evaluation of tubes’ integrity is very important from the viewpoint of nuclear ecocide. In the present study, sliding wear behaviors of Inconel 600 and 690 steam generator tube materials mated with 409 stainless steel commonly used as the support plate were investigated at room temperature in an air environment. For more precise prediction of wear behaviors of steam generator tubes, Archard equation was modified, and the modified wear coefficients were estimated as a function of sliding distance. When using the modified Archard equation, the reliabilities for prediction of wear behavior of Inconel 600 and 690 mated with 409 stainless steel increased from 71.8% to 83.8% and from 60.2% to 85.2%, respectively.

The Effect of Molybdenum on Cavitation Erosion and Corrosion Resistance of Fe-Cr-C-Si Hardfacing Alloys

The cavitation erosion behavior of Fe-Cr-C-Si-xMo (x = 0, 3 and 6 wt.%) hardfacing alloys were investigated for up to 50 hours by using 20 kHz vibratory cavitation erosion test equipment. With increasing Mo contents, M23C6 and M6C carbides were formed instead of Cr-rich M7C3 and M23C6 type carbides observed in the interdendritic region of the Mo-free Fe-Cr-C-Si hardfacing alloy. This microstructural change was responsible for the improvement of mechanical properties such as hardness and cavitation erosion resistance of the Mo-added Fe-Cr-C-Si hardfacing alloy.

Thermotransport of Hydrogen in Zr-1.0Nb-1.0Sn-0.1Fe Alloy

Hydrogen redistribution caused by thermotransport in the Zr-1.0Nb-1.0Sn-0.1Fe alloy under the temperature gradient which is likely to be encountered between nuclear fuel cladding and primary cooling water (300-340) was investigated. The heat of transport (Q*) of hydrogen was determined by using a steady state technique to evaluate the magnitude and direction of thermotransport of hydrogen in the alloy. The values of Q∗ were 23.1, 23.7 and 27.1 KJ/mol for the hydrogen concentration of 73.4, 75.8 and 94.3 ppm by weight respectively. In other words, hydrogen was transported from hot region to cold region and the value of Q∗ increased with increasing overall hydrogen concentration. The Zr-1.0Nb-1.0Sn-0.1Fe alloy had the smaller Q∗ value than that of Zircaloy-4 when compared with same overall hydrogen concentration. Thus, Zr-1.0Nb-1.0Sn-0.1Fe alloy has better resistance to the formation of hydride due to thermotransport than Zircaloy-4 does.

Reciprocating Sliding Wear Behavior of Inconel 600 Mated with SUS 304 Stainless Steel at Elevated Temperatures

The sliding wear behavior of a steam generator in a nuclear power plant (Inconel 600) was investigated at 225, 250 and 300°C. Effects of wear parameters such as sliding distance and contact stress were examined with SUS 304 (austenitic). In the prediction of the wear volume by Archard wear equation, the standard error range was calculated to be ±0.53×10-9 m3 and the reliability to be 71.9 % for the combination of Inconel 600 and SUS 304. The error range was considered to be relatively broad because the wear coefficient in Archard equation was assumed to be constant. However, the wear volume turned out to increase parabolically with the sliding distance owing to the combination of strain hardening of the wear surface and the oxidative wear. Based on the experimental results, the wear coefficient was modified as a function of the rotating sliding distance. The calculation with the modified wear equation showed that, although the error range was not significantly narrowed, the reliability increased from 71.9 to 78.1 %.

The Cavitation Behavior of Fe-Cr-C-Si-Ni Alloys

The cavitation erosion behavior of Fe-Cr-C-Si-xNi (x = 1, 2 and 3 wt.%) alloys were investigated for 50 hours using a 20 kHz vibratory cavitation erosion test equipment. 1 wt.% Ni added Fe-based hardfacing alloy showed excellent cavitation erosion resistance, comparable to the stellite 6. Above 1 wt.% Ni, however, the erosion resistance deteriorated quickly. It is conjectured that Ni addition above 1 wt.%, which has been shown to increase the stacking fault energy (SFE), resulted in reduction of the work hardening rate during the erosion test. Therefore, the enhanced cavitation erosion resistance of the 1 wt.% Ni alloy over the 2 and 3 wt.% Ni alloys could be explained in terms of the SFE, Ms temperature and work hardening.

Effect of Ti Addition on the Fracture Toughness of Al-Ti-B Alloys Synthesized by High-Energy Ball Milling and Spark Plasma Sintering

Al alloys containing boron or its compound are attractive for neutron shielding materials since boron exhibits high thermal neutron absorbing capability. However, poor fracture toughness and low ductility of boron compounds restrict their usage as structural materials. By adding Ti as a third element, it is expected to improve the toughness of Al-B alloys. The present study investigates the effect of the Ti addition on fracture toughness of the (Al+Xat%Ti)2at%B (X = 0.5, 1, 1.5 at%) alloy fabricated by high energy ball milling and spark plasma sintering (SPS). The SPS method was used to consolidate (Al+Xat%Ti)2at%B (X = 0.5, 1, 1.5 at%) alloy with the pressure of 50 MPa. Charpy impact tests showed that the fracture toughness of the 1 at%Ti added-alloy was 4 MPam1/2 , and that it was three times higher than that of Al-B alloys. Further reduction of the Ti addition down to 0.5 at% at 1.9 MPam1/2 or up to 1.5 at% at 2 MPam1/2 slightly decreased the fracture toughness. The microstructures of the present specimens were investigated by FE-SEM and TEM to describe the relationship with fracture toughness.