• Journal of Welding and Joining
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• 제2권1호
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• pp.4-17
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• 1984

Overlaid weld metals of austenitic stainless steel in a pressure vessel of power reactor are usually post-weld heated for a long period of time after welding. The PWHT is considered as a kind of sensitizing and it is important to check the soundness of the weld metal after PWHT, especially about the precipitation of carbides. The purpose of this report is to obtain information on the relation between the change of microstructure and Post-Weld Heat Treatment in the overlaid weld metals. Metallurgical aspects of the problem on austenitic stainless steel heated at $625^{\circ}C$, $670^{\circ}C$, $720^{\circ}C$ and $760^{\circ}C$ for 3, 10, 30, 100 and 300 hours have been investigated by means of optical-micrography, micro-hardness measurement, scanning electron microscope and electron-probe micro analysis. From the results obtained, the following conclusions are drawn; 1) The PWHT above $625^{\circ}C$ for a long time causes a diffusion of carbon atoms from low alloy steel into stainless steel, and consequently carbon is highly concentrated at the boundary layer of stainless steel. 2) C in ferritic steel migrated to austenitic steel and carbides precipitated in austenitic steel along fusion line. At higher temperatures, the ferrite grains coarsened in the decarburized zone. 3) In the change of microstructure of stainless steel overlaid weld metal, the width of carbides precipitated zone and decarburized zone increased with increase of PWHT temperature and time. 4) At about $625^{\circ}C$ to $760^{\circ}C$, chromium carbides, mainly $M_{23} C_6$, precipitate very closely in the carburized layer with remarkable hardening. 5) Precipitation of delta ferrite from molten weld metal depends on solidification phenomenon. There was a small of ferrite near the bond in which the local solidification time was short, comparing with after parts of weld metal. Shape and amount of ferrite were not changed by Post-Weld Heat Treatment after solidification.

• 자원리싸이클링
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• 제13권5호
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• pp.45-50
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• 2004

탄화철을 합성하는 공정을 수소($H_2$) 환원과 $CO-H_2$ 혼합가스에 의한 탄화의 2단계 과정으로 나누어서 수행했다. 환원종료 후 미량의 암모니아 가스를 첨가하여 환원철의 표면을 개질한 후, 탄화시간 경과별 탄화상태를 C/S 분석기(Low C/S determinator), 뫼스바우어 분광 분석기(Mossbauer spectroscopy), XRD(X-ray diffraction patterns), SEM(Scanning electron microscopy), TEM(Transmission electron microscopy), XPS(Photoelectron spectroscopy), 및 라만분광기 (Raman spectroscopy)를 사용하여 조사하였다. 연구결과, 미량의 암모니아가스로 환원철 표면을 개질함으로써 탄화철의 분해 및 유리탄소의 석출을 방지할 수 있을 뿐만아니라 6.68wt% 이상 10wt% 까지 탄소가 과고용된 상태에서도 분해되거나 유리탄소를 석출하지 않고 안정상태를 유지하였다. 이러한 결과로부터 철(Fe)과 세멘타이트(cementite, $Fe_3C$)가 혼합되지 않고 고탄화철(SIC, super iron carbide)인 Fe5C2 상태의 안정한 단일상을 얻는데 성공하였다.

• 한국주조공학회지
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• 제38권5호
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• pp.103-110
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• 2018

코발트기 주조용 초내열합금 X45를 이용하여 다양한 형태의 열처리에 따른 미세조직과 기계적 특성의 변화에 대하여 고찰한 결과를 요약하면 다음과 같다. 1) 합금의 응고 시 결정립계와 수지상간 경계를 따라 Cr이 다량 함유된 조대한 $M_{23}C_6$ 탄화물과 W과 Co의 함량이 높은 $M_6C$ 탄화물이 형성되어 있었고, $1274^{\circ}C$에서 용체화 처리하면 대부분의 탄화물이 용해되었다. $1150^{\circ}C$에서의 용체화 처리 동안 일부 결정립계 탄화물이 용해되지만 공정탄화물 근처에서 새로운 탄화물의 석출이 일어났다. $927^{\circ}C$와 $982^{\circ}C$에서 시효처리만 했을 때 용체화 처리 후 시효처리 한 시편 보다 공정 탄화물 근처에서 석출되는 탄화물의 양이 많았고, 크기가 작았다. 2) $1150^{\circ}C$에서 용체화 처리한 후 시효처리 하면 경도의 증가가 뚜렷하지 않으며, $927^{\circ}C$와 $982^{\circ}C$에서 시효처리만 했을 때 다량의 매우 미세한 탄화물의 석출에 의하여 경도의 증가 폭이 더 컸다. 상온 항복강도는 $927^{\circ}C$에서 시효처리 했을 때 가장 컸지만 인장강도와 연신율은 $982^{\circ}C$에서 12시간 시효처리 했을 때 가장 컸다. 고온 크리프 특성은 $982^{\circ}C$에서 12시간 시효처리 했을 때 가장 우수하였다. 3) 장시간 고온 노출 되는 동안 탄화물의 석출과 성장이 일어났고, 8000시간 고온에서 노출시킨 시편에서는 수지상 중심부에도 탄화물이 석출되어 크리프 파단 수명이 증가하였다.

• 한국분말재료학회지
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• 제3권3호
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• pp.181-187
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• 1996

Cobalt and VC powders were ball milled with M2 grade high speed steel powders under various ball to powder ratios. The powders milled under higher ball to powder ratio become finer, more irregular and have a broader size distribution, and thus possess a lower compressibility and a better sinterability regarding densification. Increasing the ball to powder ratio lowered the sintering temperature to obtain the density level necessary to isolate all the pores. Lowering the sintering temperature is very critical to maintain fine microstructure since grain and carbide coarsening are accelerated by higher sintering temperature due to more liquid phase formation. The powders obtained by ball milling at 20 to 1 ratio has the lowest compressibility but has the best sinterability, almost compatible to unmilled pure M2 powders. A sintered body over 97% theoretical density with fine microstructures having average grain size of ~10 microns was obtained from the powder by sintering at 1260 $^{\circ}C$ for 1 hour in vacuum. XRD results indicate that two types of carbides are mainly present in the sintered structure, MC and $M_{6}C$ type. The MC type carbides are more or less round shaped and mainly located at the grain boundaries whereas the $M_{6}C$ type are angular shaped and mainly located inside the grains.

• 한국표면공학회지
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• 제47권2호
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• pp.93-98
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• 2014

The $Cr_2AlC$ carbides oxidized at 700, 850 and $1000^{\circ}C$ in air from 70 hours up to 360 days. They oxidized according to the reaction; $Cr_2AlC+O_2{\rightarrow}{\alpha}-Al_2O_3+CO(g)$. The scales consisted primarily of the thin, essentially pure $Al_2O_3$ layer and the underlying Al-dissolved $Cr_7C_3$ layer. They grew via the outward diffusion of Al and carbon, and the inward diffusion of oxygen. The oxidation resistance of $Cr_2AlC$ was excellent due to the formation of the protective $Al_2O_3$ layer. Even when $Cr_2AlC$ oxidized at $1000^{\circ}C$ for 360 days, the ${\alpha}-Al_2O_3$ layer was only about 4 ${\mu}m$-thick.

• 한국표면공학회지
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• 제35권4호
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• pp.241-248
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• 2002

Hardfacing layers of Stellite 6 alloy with different molybdenum content are deposited on AISI 1045 carbon steel using plasma transferred arc welding (PTAW). The properties of the hardfacing layer are investigated in order to clarify the effect of molybdenum addition to the cobalt-base alloy. With an increase in molybdenum contents, the size of Cr-rich carbides in the interdendritic region is abruptly refined, but volume fraction of the carbide slightly increases. Also, with an increase of Mo, $M_{6}$ C type carbides are formed instead of Cr-rich $M_{7}$ $C_{3}$ type carbided, and this microstructural change enhanced the mechanical properties of Stellite 6 alloy.

• 한국분말재료학회지
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• 제4권3호
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• pp.170-178
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• 1997

An investigation was performed to apply the M3/2 grade high speed steel for metal injection molding using both prealloyed and elementally blended powders. The injected samples were subjected to a debinding step in $H_2/N_2$ gas atmosphere at a ratio that affected the carbon content of the material. The carbon content ranged from 1.4wt.% to 1.43wt%. with increasing $H_2$ content up to 80% $H_2$ in $H_2/N_2$ atmosphere for the prealloyed powders. The carbon contents of the elementally blended powders exhibited 1.44wt.% and 1.62wt.% at 10% $H_2/N_2$ and 20% $H_2/N_2$ gas, respectively. This level decreased to 0.17wt.% upon increasing the $H_2$ content. The sintered density of both powders increased rapidly as the temperature reached the liquid phase forming temperature. After forming the liquid phase, the density rapidly increased to the optimum sintering temperature for the prealloyed powders, whereas the density of mixed elemental powders goes up slowly to the optimum sintering temperature. The optimum sintering temperature and density are 126$0^{\circ}C$ and 97.3% for the prealloyed powders and 128$0^{\circ}C$ and 96.9% for the elementally blended powders, respectively. The microstructure of the specimen at the optimum sintering temperature consisted of fine grains with primary carbides of MC and $M_6C$ type for the prealloyed powders. The elementally blended powders exhibited coarse grains with eutectic carbides of MC, $M_2C$ and $M_6C$ type.

• 한국분말재료학회지
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• 제1권2호
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• pp.190-197
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• 1994

The densification behavior during a sintering of M2 and T15 grade high speed steel powder compacts was reported. Sintered densities over 98% theoretical were achieved by a liquid phase sintering in vacuum for both grades. The optimum sintering temperature range where full densification could be achieved without excessive carbide coarsening and incipient melting was much narrower in M2 than in T15 grade. The sintering response was mainly affected by the type of carbides present. The primary carbides in M2 were identified as $M_6C$ type whereas those in T15 were MC type which provides wider sintering range. The addition of elemental carbon up to 0.3% lowered the optimum sintering temperature for both grades, but had little effect on expanding the sintering range and sintered structure.

• 열처리공학회지
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• 제13권4호
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• pp.246-252
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• 2000

Stellite 12 alloy-powder was overlaid on 410 stainless steel valve seat using plasma transferred arc(PTA) process. Variation of characteristic of microstructure and hardness of deposit with current(90~150 A) and preheat temperature(R.T.~$400^{\circ}C$) was investigated. Important conclusion obtained are as follows; All welding conditions used produced a sound deposit layer with no defect in single pass welding. The maximum deposit had 4.0~4.8 mm in thickness and its bead width was increased with increase of current and preheat temperature. The deposit showed hypoeutectic microstruture, which was consisting of primary cobalt dendrite and networked $M_7C_3$ type eutectic carbides. The amount of eutectic carbides was decreased and its dendritic secondary arm spacing was increased with increase of current. Hardness of the deposit was decreased with increase of current. Preheat temperature up to $400^{\circ}C$, however, showed little influence on the hardness and microstructure. The hardness was also influenced by diluted Fe content near the interface in addition to microstructure and dendritic secondary arm spacing. Hot hardness at $500^{\circ}C$ showed higher than 300 HV.

• 한국표면공학회지
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• 제17권1호
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• pp.7-13
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• 1984

Carburizing of a 12%Cr steel containing 0.6%Si was performed at 950$^{\circ}C$ for various times, and the microstructure, hardness and the water characteristics of the carburized chromium steel were examined. The results obtained in this study are as follows: 1. Carbide-dispersed layer (CD layer) with fine dispersion of $Cr_7C_3$ in martensite matrix was formed by carburizing. The radius and amount of the carbides in the surface region of CD layer were about 0.3${\mu}m$ and 35% by volume, respectively. 2. Chromium steel carburized and quench-tempered showed better wear resistance and hardness than ordinary high chromium tool steel. It is concluded from these results that fine dispered carbides are very effective in improving wear resistance and hardness.