• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2003년도 춘계학술발표대회 개요집
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• pp.43-45
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• 2003

The empirical model for predicting the prior austenite grain size in low-alloy steel weld HAZ was developed through examining the effect of alloying element. The test alloys were made by vacuum induction melting. Grain growth behaviors were observed and analyzed by isothermal grain growth test and subsequent metallography. As a result, it was found that the grain growth might be controlled by grain boundary diffusion and the empirical model for grain growth was presented.

• 소성∙가공
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• 제21권7호
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• pp.412-419
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• 2012

The ingot-breakdown scheme of a tower flange material (low-alloy steel) for offshore wind turbine was investigated using finite element (FE) simulations and experimental analyses. Based on compression test results of the low-alloy steel, a deformation processing map was generated using the superposition approach between the dynamic materials model (DMM) and Ziegler's instability criterion. The deformation processing map allowed determination of the optimum process conditions for the tower flange material. Within the FE simulations of the ingot breakdown process, the Cockcroft-Latham criterion, which considers ductile fracture, was used to predict the possibility of forming defects during the hot working process. In general, the critical value for the ductile fracture of steel is 0.74. During the ingot-breakdown under optimum process conditions, the actual tower flange forgings exhibited a relatively uniform shape without any forming defects.

• Tribology and Lubricants
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• 제36권4호
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• pp.207-214
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• 2020

In this study, we investigated the effects of copper and copper-alloy on the frictional and wear properties of low-steel friction material. The proportions of copper and copper-alloy in the brake friction materials used in passenger cars are very high (approximately 5-20% weight), and these materials have significant effects on friction and wear characteristics. In this study, the effects of cupric ingredients, such as the copper fiber and brass fiber, are investigated using the friction materials based on commercial formulations. After the copper and brass fibers from the same formulation were removed, the frictional and wear characteristics were evaluated to determine the influence of the copper and copper-alloy. We evaluated the frictional and wear characteristics by simulating various braking conditions using a 1/5 scale dynamometer. The results show that the friction material containing copper and brass fibers have excellent frictional stability and a low wear rate compared to the friction material that does not contain copper and brass fibers. These results are attributed to the excellent ductility, moderate melting point, high strength, and excellent thermal conductivity of copper and copper-alloy. We analyzed the surfaces of the friction materials before and after the performing the friction tests using a scanning electron microscope-energy dispersive X-ray spectroscope, confocal microscope, and roughness tester to verify the frictional behavior of copper and copper-alloy. In future studies, it will be applied to the development of copper-free friction materials based on the results of this study.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2009년도 춘계학술대회 논문집
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• pp.40-43
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• 2009

This study is for investigating the effect of induction heating with various alloy elements to manufacture the pre-heat treated steels of 100kgf/$mm^2$ for cold heading. For four kinds of steels, the condition of induction heating (especially, induction tempering) were observed, and their microstructure and tensile and compressive properties were investigated. The middle carbon steel and the low carbon Cr-Mo steel are needed the higher Grange-Baughman tempering parameter than that of the low carbon Cr steel to obtain 100kgf/$mm^2$ tensile strength. For accomplishing the pre-heat treated steel of 100kgf/$mm^2$ tensile strength having advanced cold heading. It is needed that the pre-heat treated steel is manufactured by induction quenching and tempering with the low carbon alloy steel to have the high ratio of ferrite and the fine globular cementite simultaneously.

• Nuclear Engineering and Technology
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• 제44권6호
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• pp.673-682
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• 2012

Three dimensional atom probe tomography (3D APT) is applied to characterize the dissimilar metal joint which was welded between the Ni-based alloy, Alloy 690 and the low alloy steel, A533 Gr. B, with Alloy 152 filler metal. While there is some difficulty in preparing the specimen for the analysis, the 3D APT has a truly quantitative analytical capability to characterize nanometer scale particles in metallic materials, thus its application to the microstructural analysis in multi-component metallic materials provides critical information on the mechanism of nanoscale microstructural evolution. In this study, the procedure for 3D APT specimen preparation was established, and those for dissimilar metal weld interface were prepared near the fusion boundary by a focused ion beam. The result of the analysis in this study showed the precipitation of chromium carbides near the fusion boundary between A533 Gr. B and Alloy 152.

• 소성∙가공
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• 제14권3호
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• pp.215-223
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• 2005

A numerical analysis program is developed by FDM scheme for the prediction of microstructural transformation during heat treatment of steels. In this study, multi-phase model was used fur description of diffusional austenite transformations in low-alloy hypoeutectoid steels during cooling after austenitization. A fundamental property of the model consisting of coupled differential equations is that by taking into account the rate of austenite grain growth, it permits the prediction of the progress of ferrite, pearlite, and bainite transformations simultaneously during quenching and estimate the amount of martensite also by using K-M eq. In order to simulate the microstructural evolution during tempering process, another Avrami-type eq. was adopted and method for vickers hardness prediction was also proposed. To verify the developed program, the calculated results are compared with experimental ones of casting product. Based on these results, newly designed heat treatment process is proposed and it was proved to be effective for industry.

• 열처리공학회지
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• 제32권1호
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• pp.17-28
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• 2019

Jominy hardenability curves of low alloy steel containing less than 5 wt.% of alloying elements in total were calculated by applying Scheil's rule of additivity to pre-calculated isothermal transformation curve. Isothermal transformation curve for each phase in steel was approximated as a simple mathematical equation by using Kirkaldy's approach and all coefficients in the equation were estimated from experimental temperature-time-transformation (TTT) and/or continuous cooling transformation (CCT) data in the literature. Then jominy test with simple boundary conditions was performed in computer by applying the finite difference scheme. The resultant cooling curves at each location along a longitudinal direction of Jominy bar were applied to calculate phase fractions as well as mechanical properties such as micro Vickers hardness. The simulated results were compared with experimental CCT data and Jominy curves in the literature.

• 한국재료학회지
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• 제11권10호
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• pp.813-819
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• 2001

The microstructure and the distribution of hardness of Co and A1 alloy powder cladding layer in high carbon 9CrSi alloy steel for roll materials cladded by laser surface cladding were investigated. And, for the evaluation of soundness as the roll materials, we examined the wear resistance of the cladding materials with the wear appratus of pin on disc type. The experimental results showed that the microstructure of laser cladding layer was constituted with the clad surface layer, the alloy layer, the heat treatment layer with base metal. The wear resistance of Ni alloy Powder cladding material was superior to that of Co alloy powder cladding material both at the low speed (0.46m/s) and the high speed(0.92m/s). It seemed that the behavior of wear showed the abrasive wear at the early stage and the adhesive wear at the late stage.

• 대한금속재료학회지
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• 제47권3호
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• pp.139-146
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• 2009

The heat-affected zone (HAZ) of SA508 Gr.4N Ni-Mo-Cr low alloy steel, which has higher Ni and Cr contents than SA508 Gr.3 Mn-Mo-Ni low alloy steel, was investigated on the microstructure and mechanical properties. The HAZ was categorized into seven characteristic zones (CGCG, FGCG, ICCG, SCCG, FGFG, ICIC and SCSC-HAZ) according to the peak temperature from the thermal cycle experienced during multi-pass welding. Post Weld Heat Treatment (PWHT) was conducted in the temperature range of $550{\sim}610^{\circ}C$ for 30 hours to evaluate the effect of PWHT conditions on the microstructure and mechanical properties. Before PWHT, CGHAZ and FGFGHAZ showed high yield strength (YS) ranging from 1000 to 1250 MPa, while YS of SCSCHAZ decreased from 607 MPa (observed for base metal) to 501 MPa. The Charpy impact energies of sub-HAZs fell below 100J at $-29^{\circ}C$, except in the SCSCHAZ. By applying PWHT to sub-HAZ specimens, YS decreased as the PWHT temperature increased. In the case of CGHAZs and FGFGHAZ heat-treated at $610^{\circ}C$, YS dropped drastically to the range of 654~686 MPa. From the Charpy impact test, the upper-shelf energy (USE) increased to approximately 250J and Index temperature ($T_{68J}$) decreased below $-50^{\circ}C$. Specifically, in FGFG, ICIC and SCSC-HAZ, $T_{68J}$ was below -110, which was lower than the case of base metal.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2004년도 제5회 압연심포지엄 신 시장 개척을 위한 압연기술
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• pp.27-36
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• 2004

Contemporary objectives for steel rod rolling processing are increasingly complex and often contrasting i.e. obtaining a desired product with optimum combination of properties such as strength, toughness and formability at lower cost. Low-alloy steel rods have been produced with several heat treatments for drawing and forging processes at room temperature. In order to reduce these heat treatments much of the researches concerning of high temperature mechanical behavior of steel rods have been conducted at wire rod mill of POSCO. In this present work, optimizations of rolling temperature and cooling rate for JS-SCM435 are performed to eliminate softening heat treatment(Low Temperature Annealing) for drawing process. The results from the optimization changed the microstructure of rods after rod rolling from Bainite with high tensile strength of 1000Mpa to Pearlite and Ferrite with appropriate strength of 750Mpa that is equivalent tensile strength after softening heat treatment.