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1GPa급 DP강 전기저항점용접부의 경도분포와 미세조직의 상관관계
나혜성,공종판,한태교,진광근,강정윤,Na, Hye-Sung,Kong, Jong-Pan,Han, Tae-Kyo,Chin, Kwang-Geun,Kang, Chung-Yun 대한용접접합학회 2012 대한용접·접합학회지 Vol.30 No.2
In this study, the effect of the welding current on the hardness characteristics and microstructure in the resistance spot welding of 1GPa grade cold-rolled DP steel was investigated, Also, correlation between the hardness and microstructure was discussed. In spite of the change in the welding current, the hardness distributions near weld was similar. the hardness in the HAZ and the fusion zone was higher than that of the base metal and the hardness in the fusion zone was variated with the location. Especially, the hardness of HAZ adjacent to the base metal showed maximum value, and softening zone in the base metal adjacent to HAZ was found. With the increasing of welding current, there were no difference in maximum hardness and average hardness in the fusion zone were, but the hardness of the softening zone reduced. The difference in the hardness in each location of weld due to grain size of prior austenite. The softening of the base metal occurred by tempering of the martensite.
슈퍼 듀플렉스 스테인리스강 다층용접부의 미세조직 및 공식(Pitting Corrosion)에 미치는 용접열사이클의 영향
남성길,박세진,나혜성,강정윤,Nam, Seong-Kil,Park, Se-Jin,Na, Hye-Sung,Kang, Chung-Yun 대한용접접합학회 2010 대한용접·접합학회지 Vol.28 No.4
Super-duplex stainless steels (SDSS) have a good balance of mechanical property and corrosion resistance when they consist of approximately equal amount of austenite and ferrite. The SDSS needs to avoid the detrimental phases such as sigma(${\sigma}$), chi(${\chi}$), secondary austenite(${\gamma}2$), chromium carbide & nitride and to maintain the ratio of ferrite & austenite phase as well known. However, the effects of the subsequent weld thermal cycle were seldom experimentally studied on the micro-structural variation of weldment & pitting corrosion property. Therefore, the present study investigated the effect of the subsequent thermal cycle on the change of weld microstructure and pitting corrosion property at $40^{\circ}C$. The thermal history of root side was measured experimentally and the change of microstructure of weld root & the weight loss by pitting corrosion test were observed as a function of the thermal cycle of each weld layer. The ferrite contents of root weld were reduced with the subsequent weld thermal cycles. The pitting corrosion was occurred in the weld root region in case of the all pitted specimen & in the middle weld layer in some cases. And the weight loss by pitting corrosion was increased in proportional to the time exposed at high temperature of the root weld and also by the decrease of ferrite content. The subsequent weld thermal cycles destroy the phase balance of ferrite & austenite at the root weld. Conclusively, It is thought that as the more subsequent welds were added, the more the phase balance of ferrite & austenite was deviated from equality, therefore the pitting corrosion property was deteriorated by galvanic effect of the two phases and the increase of 2nd phases & grain boundary energy.
Al-Si 용융도금된 11%Cr 페라이트 스테인리스강, STS409L GTA 용접부의 미세조직과 경도
박태준,공종판,나혜성,강정윤,엄상호,김정길,우인수,이종섭,Park, Tae-Jun,Kong, Jong-Pan,Na, Hye-Sung,Kang, Chung-Yun,Uhm, Sang-Ho,Kim, Jeong-Kil,Woo, In-Su,Lee, Jong-Sub 대한용접접합학회 2010 대한용접·접합학회지 Vol.28 No.3
Ferritic stainless steels, which have relatively small thermal expansion coefficient and excellent corrosion resistance, are increasingly being used in vehicle manufacturing, in order to increase the lifetime of exhaust manifold parts. But, there are limits on use because of the problem related to cosmetic resistance, corrosions of condensation and high temperature salt etc. So, Aluminum-coated stainless steel instead of ferritic stainless steel are utilized in these parts due to the improved properties. In this investigation, Al-8wt% Si alloy coated 409L ferritic stainless steel was used as the base metal during Gas Tungsten Arc(GTA) welding. The effects of coated layer on the microstructure and hardness were investigated. Full penetration was obtained, when the welding current was higher than 90A and the welding speed was lower than 0.52m/min. Grain size was the largest in fusion zone and decreased from near HAZ to base metal. As welding speed increased, grain size of fusion zone decreased, and there was no big change in HAZ. Hardness had a peak value in the fusion zone and decreased from the bond line to the base metal. The highest hardness in the fusion zone resulted from the fine re-precipitation of the coarse TiN and Ti(C, N) existed in the base metal during melting and solidification process and the presence of fine $Al_2O_3$ and $SiO_2$ formed by the migration of the elements, Al and Si, from the melted coating layer into the fusion zone.
Cr-Mo-V-Ti 저합금강에서 노멀라이징 열처리조건에 따른 석출물의 거동
김홍기 ( Hong-ki Kim ),나혜성 ( Hye-sung Na ),이상훈 ( Sang-hoon Lee ),강정윤 ( Chung-yun Kang ) 한국열처리공학회 2017 熱處理工學會誌 Vol.30 No.2
Heat treatment condition for dissolution of the M23C6 carbides in 2.25Cr-1Mo-V-Ti material for thermal power plant tube was investigated using a dilatometer method. 2.25Cr-1Mo-V-Ti material was heat-treated at 900~1,100oC for 0, 10, 30 min to find the proper dissolution condition of M23C6 carbides. The phase identification and volume fraction of the carbide were measured by using OM, SEM, EBSD and TEM analysis. Optimal heat treatment condition of M23C6 carbide dissolution was selected by predicting dissolution temperature of carbide using Bs points appeared at dilatometer curve. Experimental results showed that the conditions of carbide dissolution was 900, 1,000, 1,100oC for 30 min. Eventually, the optimal heat treatment condition for dissolution was 30 min at 1,000oC considering the minimum coarsening of Austenite grain size. (Received February 24, 2017; Revised March 8, 2017; Accepted March 14, 2017)
409L페라이트계스테인리스강CO2레이저용접부의미세조직과경도
공종판 ( Jong Pan Kong ),박태준 ( Tae Jun Park ),나혜성 ( Hye Sung Na ),엄상호 ( Sang Ho Uhm ),김정길 ( Jeong Kim Kim ),우인수 ( In Su Woo ),이종섭 ( Jong Sub Lee ),강정윤 ( Chung Yun Kang ) 대한금속재료학회 ( 구 대한금속학회 ) 2010 대한금속·재료학회지 Vol.48 No.4
The microstructure and hardness of CO2 laser welds were investigated in the Ti-stabilized ferritic stainless steel 409L. The observed specimen was welded in a fully penetrated condition in which the power was 5 kW and the welding speed 5 m/min. The grain structure near the bond line of the laser welds was produced by epitaxial growth. The grain size was the largest in the fusion zone, and HAZ showed nearly the same grain size as that of the base metal. The HAZ microstructure consisted of subgrains and precipitates that were less than l00 nm in size and that were located along the subgrain boundaries. On the other hand, the hardness was the highest in the fusion zone due to the large amount of small precipitates present. These were composed of TiN, Ti(C,N) and TiO2+Ti(C,N). The hardness decreased continuously from the fusion zone of the base metal. The HAZ hardness was slightly greater than that of the base metal due to the existence of subgrains and precipitates in the subgrain boundary. (Received January 13, 2010)
Cr-Mo-V강(T24)의 재현 HAZ의 결정립 성장에 미치는 피크온도의 영향
이경운(Kyong-woon Lee),이성형(Seong-hyeong Lee),나혜성(Hye-sung Na),강정윤(Chung-Yun Kang) 대한용접·접합학회 2016 대한용접·접합학회지 Vol.34 No.6
Recently developed ferritic heat resistance steel, T24 was used to evaluate microstructure characteristics of simulated heat affected zone. Also, correlation between the prior austenite grain size and amount of M23C6 carbide dissolution was discussed. With the increasing of peak temperature, Grain size steadily increased up to 1050℃ and then rapidly increased at 1150℃. Of the peak temperature 950~1050℃, amounts of M23C6 carbide dissolution are low. But Most of M23C6 carbide that is inhibited grain growth were dissolved above 1050℃ and decreased volume fraction of carbide. This indicates that grain growth may be achieved through dissolution of carbide in the base material. As of welding, due to very rapid heating rate, M23C6 carbide exists above equilibrium solution temperature that is 800℃, even at 1050℃. So, It was confirmed that close correlation between carbide dissolution in the base material and grain growth. Calculated grain size has a linear relationship with peak temperature, on the other hand, measured grain size discontinuously increased between 950~1050℃ and above 1050℃. Grain size of heat affected zone at 1350℃ peak temperature showed maximum 67um and minimum 4um. Also, The number of side showed 3 to 10.