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니켈계 상용 용가재 적용을 통한 난용접성 247LC 초내열합금 용접부의 응고균열민감도 저감방안
천은준 대한용접접합학회 2023 대한용접·접합학회지 Vol.41 No.1
The solidification cracking susceptibility of 247LC superalloy dissimilar welds with ERNiCrCoMo-1, ERNiCrMo- 3, and ERNiFeCr-2 commercial fillers was quantitatively evaluated using Varestraint testing for the sound weld geo- metries of gas turbine blades. We confirmed that the solidification brittle temperature range (BTR) of the 247LC/ ERNiCrMo-3 dissimilar weld was 217 K, which was measured using a thermo-vision camera during the Varestraint testing, and the BTR increased to 260, and 485 K with the ERNiCrCoMo-1 and ERNiFeCr-2 fillers, respectively, Based on theoretical calculations of the weld solidification path (i.e., using Scheil’s model; performed via the soft- ware Thermo-Calc) for each dissimilar weld, the variation in the BTR can be considered to be highly dependent on the solid-liquid coexistence temperature range of the dissimilar welds, and it is also correlated with the low-temper- ature formation of topologically closed packed phases during the terminal stage. Based on the experimental and the- oretical results, the commercial Ni-based filler ERNiCrMo-3 is expected to be an effective welding material for 247LC dissimilar welding and the successful manufacturing of high-soundness blades.
천은준,이주승,도형협,김성주,박용호,강남현,Chun, Eun-Joon,Lee, Ju-Seung,Do, Hyeong-Hyeop,Kim, Seong-Ju,Park, Yong-Ho,Kang, Nam-Hyun 한국재료학회 2012 한국재료학회지 Vol.22 No.1
Research into the development of high strength (1 GPa) and superior formability, such as total elongation (10%), and stretch-flangeability (50%) in hot-rolled steel was conducted with a thermomechanically controlled hot-rolling process. To improve the overall mechanical properties simultaneously, low-carbon steel using precipitation hardening of Ti-Nb-V multimicroalloying elements was employed. And, ideal microstructural characteristics for the realization of balanced mechanical properties were determined using SEM, EBSD, and TEM analyses. The developed steel, 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V), consisted of ferrite as the matrix phase and second phase of granular bainite with fine carbides (20-50 nm) in both phases. The significant factor of the microstructural characteristics that affect stretch-flangeability was found to be the microstructural homogeneity. The microstructural homogeneity, manifest in such characteristics as low localization of plastic strain and internally stored energy, was identified by grain average misorientation method, analyzed by electron backscattered diffraction (EBSD) and hardness deviation between the phases. In summar, a hot-rolled steel having a composition 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V) demonstrated a tensile strength of 998 MPa, a total elongation of 19%, and a hole expansion ratio of 65%. The most important factors to satisfy the mechanical property were the presence of fine carbides and the microstructural homogeneity, which provided low hardness deviation between the phases.
온도 제어형 레이저 열처리에 따른 NiCrBSi 및 CoCrWC 용사 코팅층의 나노구조 형성 거동
천은준 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.4
For surface hardening of a continuous casting mold component, a thermal spray coating of NiCrBSi (Metco-16C) and CoCrWC (Stellite-1) was performed followed by laser heat treatment of the coatings. To support selective modification of the thermal spray coating, a metallurgically determined surface temperature was maintained during the laser heat treatment, by real-time control of the laser power. In other words, nonhomogeneities in the macrosegregation of certain alloying elements, and voids in the as-sprayed state, could be improved. The main microstructural features of the Metco-16C coating laser-heat-treated at 1423 K were nanosized (100–150 nm) Cr5B3, M7C3, and M23C6 precipitates with a lamellar structure of Ni (FCC) and Ni3Si as the matrix phase. Those of the laser heat-treated Stellite- 1 coating at 1473 K were fine (30–250 nm) precipitates of WC, M7C3, and M23C6 based on a Co (FCC) matrix. The results show that laser heat treatment at 1423 K increased the hardness of the Mecto-16C coating to 1115 HV from the as-sprayed state (754 HV), while treatment at 1473 K increased the hardness of the Stellite-1 coating from 680 HV to 860 HV.
레이저 가공 기반 사출 금형강의 선택적 표면 개질 연구: 레이저 나이트라이딩에 따른 AlN 형성 및 경화 거동
천은준,박창규,박원아 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.1
Laser surface heat treatment and laser nitriding processes were applied using selective surface modification techniques to investigate phase transformation, microstructural evolution, and surface hardening behaviors for two types of plastic injection mold steels, AISI 1045 and P21. During laser surface heat treatment, a 245% hardness increase compared to that of the base metal (290 HV) was achieved due to martensite transformation of the AISI 1045 steel. However, for the AISI P21 steel, hardness within the heat-treated zone was largely unchanged from that of the base metal (410 HV) despite being accompanied by martensite transformation. Compared to that of the base metal, this static hardness behavior of the heat treated P21 steel was due to coarsening of Cu particles induced by the laser irradiation. To overcome the static hardness behavior of P21 steel, laser nitriding was used. The laser-nitrided specimen (at 4500 J/mm heat input) was approximately 40% (577 HV) harder than the base metal (410 HV) and was highly correlated with nitride formation. Nitrogen successfully penetrated the surface of the specimen during laser irradiation and formed a nitrided layer mainly composed of an AlN phase. Thus, the surface hardening behavior of AISI P21 steel after laser nitriding could be largely attributed to the AlN phase development.
오스테나이트계 스테인리스강 레이저 용접부의 응고균열 거동 (Part 2)
천은준(Eun-Joon Chun),이수진(Su-Jin Lee),서정(Jeong Suh),강남현(Namhyun Kang),Kazuyoshi Saida 대한용접·접합학회 2016 대한용접·접합학회지 Vol.34 No.5
A numerical simulation of the solid/liquid coexistence temperature range, using solidification segregation model linked with the Kurz-Giovanola-Trivedi model, explained the mechanism of the BTR shrinkage (with an increase in welding speed) in type 310 stainless steel welds by reduction of the solid/liquid coexistence temperature range of the weld metal due to the inhibited solidification segregation of solute elements and promoted dendrite tip supercooling attributed to rapid solidification of laser beam welding. The reason why the BTR enlarged in type 316 series stainless welds could be clarified by the enhanced solidification segregation of impurity elements (S and P), corresponding to the decrement in δ-ferrite crystallization amount at the solidification completion stage in the laser welds. Furthermore, the greater increase in BTR with type 316-B steel was determined to be due to a larger decrease in δ-ferrite amount during welding solidification than with type 316-A steel. This, in turn, greatly increases the segregation of impurities, which is responsible for the greater temperature range of solid/liquid coexistence when using type 316-B steel.
980 MPa급 열연강의 권취온도와 탄화물 거동에 따른 신장플랜지성
천은준 ( Eun Joon Chun ),이주승 ( Ju Seung Lee ),도형협 ( Hyeong Hyeop Do ),김성주 ( Seong Ju Kim ),최윤석 ( Yoon Suk Choi ),박용호 ( Yong Ho Park ),강남현 ( Nam Hyun Kang ) 대한금속재료학회 ( 구 대한금속학회 ) 2012 대한금속·재료학회지 Vol.50 No.7
To analyze the factors on stretch-flangeability for 980 MPa-grade hot-rolled steels, two types of steels (Fe-Cr and Fe-Mo) were manufactured by hot-rolling. Manufactured steels at the low coiling temperature, such as 400 and 500℃, had poor stretch-flangeability due to un-uniformly distributed carbides and a large deviation of interphase hardness. However, when the coiling temperature was set at 650℃ with Fe-Cr steel, 998 MPa of ultimate tensile strength, 19% of total elongation and 65% of the hole expanding ratio were achieved by microstructural constituents of polygonal ferrite (PF) and granular ferrite (GF) dispersed with fine carbides (<50 nm). Therefore, the material to attain 980 MPa with superior formability was the Fe- Cr steel that was precipitation-hardened in polygonal ferrite and granular ferrite at the coiling temperature 650℃.