중탄소강 SAW 용접금속의 응고 균열 발생에 미치는 용접재료의 영향

방국수,송우현,정홍철 대한금속·재료학회 2018 대한금속·재료학회지 Vol.56 No.2

The effects of submerged-arc welding consumables, wire and flux, on solidification cracking in the weld metal of medium-carbon steel were investigated. It showed that weld metal with a greater amount of C, S, and P had a wider brittleness temperature range, BTR, which resulted in longer total and maximum crack lengths in Varestraint tests. To determine the effect of flux on the amount of C, S, and P in the weld metal, transfers of the elements between the slag and the weld metal were evaluated. While C and S were removed from the weld metal to the slag, P was added from the slag to the weld metal. The amount of each element removed or added, however, was dependent on the basicity of the flux used. As the flux became more basic, more S was removed and less P was added, resulting in less S and P in the weld metal. In contrast, with increasing flux basicity, less C was removed, leaving a larger amount of C in the weld metal. Therefore, to minimize the amounts of these elements in the weld metal, in addition to the wire used, the flux basicity should also be carefully considered.

Evaluation of Weld Metal Hot Cracking Susceptibility in Superaustenitic Stainless Steel

방국수,Seong-hyun Pak,Sang-Kon Ahn 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.6

Solidification cracking susceptibilities of two types of superaustenitic stainless steel, 254SMO and SR50A, were evaluated by transverse Varestraint tests. The susceptibilities were compared with those of conventional austenitic stainless steel 316L, and factors influencing the difference of susceptibility were discussed. The comparison showed that 254SMO and SR50A are more sensitive to solidification cracking than 316L. In the transverse Varestraint tests, both total and maximum crack lengths are longer in the superaustenitic stainless steel. Because of the longer maximum crack length, the superaustenitic stainless steel also has a wider brittleness temperature range of cracking than 316L: about 178 °C for the superaustenitic stainless steel and 43 °C for 316L. It is believed that straight subgrain boundaries owing to the cellular dendritic solidification and segregations of sulfur and phosphorus in the subgrain boundaries of superaustenitic stainless steel make it more sensitive to solidification cracking. In addition to the solidification cracking, reheat cracking is also observed within the previous weld bead in the superaustenitic stainless steel because of fully austenitic solidification with significant segregations. This suggests that caution should be given to the occurrence of reheat cracking when superaustenitic stainless steel is multi pass welded.

대입열용접 열영향부의 조직과 인성

방국수,이종봉,장래웅 대한용접접합학회 1992 대한용접·접합학회지 Vol.10 No.1

용접능률의 향상을 위한 대입열용접법의 적용은 과도한 입열량으로 인하여 용접부의 인성이 저 하한다는 점에서 그 적용에 주의를 요한다. 본 보에서는 대입열용접시 열영향부의 인성 저하의 원인과 그 대책을 강재의 측면에서 검토하였다. 고장력강을 용접하면 입열량이 증가함에 따라 오스테나이트 결정립이 조대화되고 상부 베이나이트와 도상 마르텐사이트가 생성되어 인성이 저하한다. 그 대책으로서는 용접 열싸이클과정중 안정한 질화물, 산화물등을 모재에 미세분산시켜 오스테나이트 결정립 성장을 억제하고, 페라이트, 펄라이트 변태를 촉진시킨다. 이러한 석출물의 형성을 위해서는 주로 Ti, Ca, REM, B등의 합금원소가 이용된다. 소입성이 높은 주질고장력 강에서는 석출물의 분산에 의한 페라이트의 변태 촉진 보다는 Mn, Ni, Cr, Mo, V등의 합금원 소를 첨가하여 소입성을 높여 인성이 우수한 하부 베이나이트 조직을 형성하든가, 탄소량을 저 감시켜 도상 마르텐사이트의 생성을 억제하므로서 인성을 확보한다. 현재 국내에서 제조되고 있는 대입열용접용강중 인장강도 50kgf/mm$^{2}$급강은 기본적으로 용접부 인성이 우수한 TMCP법으로 제조되며, Ti등을 첨가하여 석출물의 효과를 이용하고 N을 억제하여 기지의 인 성을 향상시키는 등의 방법을 병용하고 있다. 인장강도 60kgf/mm$^{2}$ 급강은 조질처리에 의하여 제조되며, 50kgf/mm$^{2}$급강과 같이 Ti, B등의 첨가에 의한 석출물의 효과를 이용 하고 있다.

셀프실드용접 와이어의 Al 첨가량에 따른 용접금속 인성 및 기공형성 변화

방국수,박찬,길웅,장웅성,Bang, Kook-Soo,Park, Chan,Woong, Kil,Chang, Woong-Seong 대한용접접합학회 2011 대한용접·접합학회지 Vol.29 No.1

Three different welding wires were used to study the effects of Al content on weld metal toughness and porosity formation in self-shielded arc welding. Weld metal microstructure showed that while wire with 1.3% Al content contains coarse $\delta$-ferrite, wires with less than 0.5% Al content showed no such phase. In addition to the microstructural differences, cleanliness in weld metal was also different among wires. It showed that weld metal toughness was influenced by the $\delta$-ferrite formation, cleanliness and Ni addition. Even though wires with less than 0.5% Al content showed higher weld metal toughness, they showed relatively poor workability, forming porosities in weld bead in lower arc voltages.

Ti 첨가 가속냉각형 고장력강의 용접열영향부 연화에 미치는 질소함량의 영향

방국수,정호신,박찬 대한금속·재료학회 2017 대한금속·재료학회지 Vol.55 No.3

The effects of nitrogen content on the HAZ softening of Ti-containing high strength steels manufactured by accelerating cooling were investigated and interpreted in terms of the microstructures in the softening zone. Regardless of their content, all of the steels investigated showed a softened zone 9-10 mm wide. The minimum hardness in the zone, however, was different, with lower hardness in the higher nitrogen content steel. Microstructural observations of the steel showed that the amount of soft ferrite was increased in the zone with an increase of nitrogen content of the steel, suggesting that microstructural evolution in the HAZ is influenced by the nitrogen content. Measurements of TiN particles showed that the degree of particles coarsening in the HAZ was lower in the higher nitrogen content steel. Therefore, it is believed that finer TiN particles in the HAZ inhibit austenite grain growth more effectively, and lead to an accelerated ferrite transformation in higher nitrogen content steel, resulting in a higher amount of soft ferrite microstructure in the softened zone.

Effects of Chemical Composition on the Microstructure and Mechanical Properties of FCAW-S Weld Metal Containing 2% Ni

방국수,Woong Kil,Woong-Seong Chang 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.2

In this study the effects of alloying elements on the microstructure and mechanical properties of 600MPa grade FCAW-S weld metals containing 2% Ni were examined. Carbon, manganese and aluminum contents were varied in the ranges of 0.075%-0.101%, 1.19%-1.69%, and 0.66%-1.49% respectively. Regardless of the Al content, all of the weld metals showed a bainite dominant microstructure with no δ-ferrite. This indicates that when a weld metal contains 2% Ni, the Al content can be increased up to around 1.5% without concern about the deterioration of impact toughness due to the presence of δ-ferrite. The tensile strength of the weld metals varied from 595 MPa to 702 MPa dependent upon the chemical composition. Multiple regression analysis showed that while C and Mn have strong influences on the tensile strength, Al has little influence. Therefore, the Pcm index of weld metals could be used as an indication of their tensile strength. Impact toughness of the weld metals was influenced most by tensile strength and showed that the 50J transition temperature increased by 36 °C when the tensile strength was increased by around 100 MPa. Therefore, an excessive increase of the tensile strength should be avoided to attain higher impact toughness. Even when inclusion mean diameters were increased from 0.588 μm to 0.708 μm with an increase of the Al content from 0.66% to 1.49%, the size difference showed little influence on the impact toughness of the weld metals in this experiment.

티타늄 산화물강 열영향부 조직변태에 미치는 망간 및 탄소의 영향

방국수 대한용접접합학회 2004 대한용접·접합학회지 Vol.22 No.2

Effects of manganese and carbon on the HAZ microstructural evolution in 500㎫ grade titanium oxide steels were investigated. Microstructural evolution primarily depends on supercooling. When cooled at 3$^{\circ}C$/s in 0.15%C-1.5%Mn steel, grain boundary and Widmanst tten ferrite formed at 640 and 62$0^{\circ}C$, respectively, followed by competitive formation of acicular ferrite and upper bainite inside of grain at 58$0^{\circ}C$. With an increase of manganese, degree of supercooling increased while critical cooling rate for the formation of gain boundary ferrite decreased. Consequently, the amount of acicular ferrite in HAZ was decreased in 2.0%Mn after initial increase in 1.0 and 1.5%Mn. Therefore, optimum supercooling should be maintained to accelerate acicular ferrite formation in titanium oxide steels. Low carbon steel, 0.11%C-1.5%Mn, showed larger amount of acicular ferrite than higher carbon steel because of effectiveness of diffusionless transformation in low carbon steel.

Comparison of the Effects of Fluorides in Rutile-Type Flux Cored Wire

방국수,정홍철,한일욱 대한금속·재료학회 2010 METALS AND MATERIALS International Vol.16 No.3

Various fluorides, CaF2, Na3AlF6, K2SiF6, MnF3, and MgF2, were added to rutile-type flux cored wires at concentrations of 1.8-2.3% and their effects on hydrogen reduction in weld metals were studied. All the fluorides reduced the hydrogen content but there were differences in the levels of reduction among the wires;CaF2 showed the greatest reduction and MnF3 showed the least. The hydrogen content in the weld metals was not influenced by the fluorine formed in the arc but by the slag basicity due to the small amount of fluorides added. The weld metal with higher slag basicities had a lower hydrogen content. The effects of fluorides on the arc stability, weld metal hardness, and microstructure were also examined. Because of the higher ionization potential of Mn, the wire containing MnF3 had the most unstable arc during welding. The wire containing MnF3 also produced a lower weld metal hardness than the other wires owing to its lower weld metal hardenability due to the greater oxidation loss of the C, Si, and Mn elements during welding.

Implant試驗에 의한 溶接低溫龜製의 評價

방국수,이종봉 대한용접접합학회 1986 대한용접·접합학회지 Vol.4 No.3

(1) Implant 용접저온귀열시험은 용접 열cycle에 따른 금속조직의 경향과 구속상태의 경향을 분 리하여 평가할 수 있는 사용한 시험법이다. (2) 한계파단응력(.sfgma.$_{cr}$ .)은 재료의 탄소다량( $C_{eq}$ .) 및 융접부 광산성수소량 ( $H_{D}$)과 밀접한 관계를 가지며 $C_{eq}$ . 및 $H_{D}$가 작을수록 .sfgma.$_{cr}$ .가 높다. (3) 시험편의 notch형상이 circular이든 spiral이든 한계파단응력은 동일하다. (4) 시험편의 파단은 notch로 부터 발생하기 시작하여 그 부분에서는 수소포화 의벽개파면은 나타낸다.

가속냉각강 GMAW 용접이음부의 강도 변화

방국수 부경대학교 산업과학기술연구소 2001 産業科學技術硏究論文集 Vol.3 No.-