• Journal of Welding and Joining
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• 제30권6호
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• pp.10-14
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• 2012

Recently the low alloy steel plate made with manganese-molybdenum is used widely in steam drum and separator of the new coal-fired power plant boiler. This material is suitable for the vapor storage of high pressure and high temperature. The high temperature creep strength of Mn-Mo alloy is higher than the carbon plate(SA516) that used in the subcritical pressure boiler. It reduces the thickness of the pressure vessel and makes the lightweight possible. Recently in the power plant boiler operation and production process, the damage has happened frequently in the heat affected zone and base material according to the hydrogen crack and delayed crack. This paper describes the research result about the damage case experienced in the boiler steam drum production process and present the optimum manufacture method for the similar damage prevention of recurrence.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.528-533
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• 2002

The mechanical and metallurgical properties of friction welded joints type 5052 Al alloy/A36 steel have been studied in this paper. The joint strength increased with increasing upset pressure and friction time till it reached the critical value. The joint strength was fixed at low strength compare to that of base metal in the case of increasing friction time. Microstructure of 5052 Al alloy was greatly deformed near the weld interface. The very fine and equaxied grain structure was observed at the near interface. The elongated grain was formed outside dynamic recrystallizatoin region at the peripheral part, while the A36 steel' side was not deformed. The hardness of the near interface was slightly softer than that of 5052 Al alloy base metal. The maximum softening width was about 8mm from the interface. In the present work, the friction welding condition, t$_1$=0.5sec, P$_2$=137.5MPa, showed a maximum joint strength (202MPa) when friction pressure, upset time and rotation speed were fixed at 75MPa, 5sec, 2000rev/min and these were the optimum friction welding condition of 5052Al/A36 steel joints.

• 복합신소재구조학회 논문집
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• 제5권1호
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• pp.28-36
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• 2014

This study describes the development of innovative connections between steel beams and concrete-filled tube columns that utilize a combination of low-carbon steel and super-elastic shape memory alloy components. The intent is to combine the recentering behavior provided by the shape memory alloys to reduce building damage and residual drift after a major earthquake with the excellent energy dissipation of the low-carbon steel. The analysis and design of structures requires that simple yet accurate models for the connection behavior be developed. The development of a simplified 2D spring connection model for cyclic loads from advanced 3D FE monotonic studies is described. The implementation of those models into non-linear frame analyses indicates hat the recentering systems will provide substantial benefits for smaller earthquakes and superior performance to all-welded moment frames for large earthquakes.

• 열처리공학회지
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• 제13권5호
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• pp.346-354
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• 2000

Effect of Nb addition on the phase transformation behavior was studied through continuous cooling transformation tests after reheating(reheating CCT) and deforming(deforming CCT) the 0.07%C-1.3%Mn-0.015%Ti-(0~0.08)% Nb steels. Transformation temperatures for deforming CCT were lower than those for reheating CCT, and the critical cooling rate for bainite transformation during deforming CCT was lower than that during reheating CCT. These enhanced hardenability for deforming CCT was considered to come from the sufficient solid solution of Nb in austenite during high temperature reheating before deformation. With Nb addition, the phase transformation temperature decreased, the bainite formation was enhanced, and the hardness of steel increased. Furthermore, these phenomena were more remarkable for deforming CCT than for reheating CCT. From the results, Nb-Ti bearing low carbon steel was considered to be a very favorable alloy system with good strength/toughness balance by recrystallization control rolling process.

• Journal of Welding and Joining
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• 제35권3호
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• pp.68-74
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• 2017

Characteristics of dissimilar metal welds between alloy steel ASTM A387 Gr. 91 and carbon steel ASTM A516 Gr.70 made with Flux cored arc welding(FCAW) have been evaluated in terms of microstructure, mechanical strength, chemical analysis by EDS as well as corrosion test. Three heat inputs of 15.0, 22.5, 30.0kJ/cm were employed to make joints of dissimilar metals with E71T-1C wire. Post-weld heat treatment was carried out at $750^{\circ}C$ for 2.5 h. Based on microstructural examination, Intragranular polygonal ferrite and grainboundary ferrite were formed only in first layer of weld metal. Another layers consisted of acicular ferrite and $Widmannst{\ddot{a}}tten$ ferrite. The amount of acicular ferrite was increased with decreasing heat input and layer. Heat affected zone of alloy steel showed the highest hardness due to the formation of tempered martensite and lower bainite. Lower and upper bainite were formed in heat affected zone of carbon steel. Tensile strengths of dissimilar metal welds decreased with increasing heat inputs. Dissimilar metal welds showed a good hot cracking resistance due to the low HCS index below 4. The salt spray test of dissimilar metals welds showed that the weight loss rate by corrosion below 170 hours was decreased with increasing heat inputs due to the increase of the amount of acicular ferrite.

• 열처리공학회지
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• 제5권2호
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• pp.122-132
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• 1992

This study has been performed to investigate into some effects of the power density and traverse speed of laser beam on the optical microstructure, hardness and wear characteristics of medium carbon low alloy steel treated by laser surface hardening technique. The results obtained from the experiment are summarized as follows : (1) Optical micrograph has shown that finer lath martensite is formed and the amount of undissolved complex carbides increases as the traverse speed increases under the condition of a given power density, whereas the coarsening of lath martensite and the reduction of undissolved complex carbides occur with increasing the power density at a given traverse speed. (2) Hardness measurements have revealed that as the traverse speed increases, hardness values of outermost surface layer more of less decrease under low power densities, but are uniformly distributed under high power densities, also showing that they are uniformly distributed at low traverse speeds and more or less decrease at high traverse speeds with increasing the power density. (3) The effective case depth has been found to decrease from 0.26 mm to 0.17 mm with increasing the traverse speed from 1.5 m/min to 3.0 m/min at a given power density of $25.48{\times}10^3w/cm^2$ and to increase from 0.20 mm to 0.36 mm with increasing the power density from $19.11{\times}10^3w/cm^2$ to $38.22{\times}10^3w/cm^2$ at a given traverse speed of 2.0 m/min. (4) Wear test has exhibited that the amount of weight loss of laser surface hardened specimen with respect to sliding distance at a given load increases with increasing traverse speed at a given power density and decreses with increasing power density at a given traverse speed.

• 한국분무공학회지
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• 제13권4호
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• pp.173-179
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• 2008

The current machinery and tools of secondary channel of the nuclear power plants were produced in the carbon-steel and low-alloy steel. What produced with the carbon-steel occurs wall thinning effect from flow accelerated corrosion by the fluid flow at high temperature, high pressure. Several nuclear power plants in Korea have experienced wall thinning damage in the area around the impingement baffle-installed. Wall thinning by flow accelerated corrosion occurs piping system, the heat exchanger, steam condenser and feedwater heaters etc,. Feedwater heaters of many nuclear power plants have recently experienced sever wall thinning damage, which will increase as operating time progress. This study describes the comparisons between the numerical results using the FLUENT code and experimental data of down scale model.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2005년도 추계학술대회 논문집
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• pp.456-459
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• 2005

Continuous cooling transformation behaviors were studied fur low carbon HSLA steels containing three different level $(1\~3\;wt\%)$ of Ni addition. Thermo-mechanical processing (TMP) simulations to construct continuous cooling (CCT) diagram were conducted by using Gleeble system. As cooling rate increased, pearlite, granular bainite, acicular ferrite, bainitic ferrite and lath martensite were transformed from deformed austenite. Fully bainitic microstructure were developed at all cooling rate condition in high Ni containing steel due to hardenability increasing effects of Ni. Ni also influenced the transformation kinetics. At the slowest cooling rate of $0.3^{\circ}C/s$, transformation delayed with decreasing Ni contents because of the diffusion of substitutional alloy elements. However, cooling rate slightly increased to $1^{\circ}C/s$, transformation kinetics accelerated with decreasing Ni contents because nucleation of bainite was sluggish due to hardening of residual austenite.

• 대한금속재료학회지
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• 제50권4호
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• pp.300-309
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• 2012

Low-carbon steels and a stainless steel were oxidized isothermally and cyclically between $1050^{\circ}C$ and $1200^{\circ}C$ for up to 100 min in air to find the effect of alloying elements of Si, Mn, Ni, and Cr on their oxidation. The most active alloying element of Si was scattered inside the oxide scale, at the scale-alloy interface and as internal oxide precipitates beneath the oxide scale. Manganese, which could not effectively improve the oxidation resistance, was rather uniformly distributed in the oxide scale. Nickel and chromium tended to present at the lower part of the oxide scale. Excessively thick porous scales formed on the low-carbon steels, whereas thin but non-adherent scales containing $Cr_2O_3$ formed on the stainless steel.

• 열처리공학회지
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• 제13권4호
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• pp.253-258
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• 2000

The present study aimed to develop the TRIP(transformation induced plasticity) aided high strength low carbon steel sheets using reverse transformation process. The cold-rolled 0.14C-6.5Mn steel was reverse-transformed by slow heating to intercritical temperature region and air cooling to room temperature. An excellant combination of tensile strength and elongation of $98.3kgf/mm^2$ and 44.4% appears. This combination comes from TRIP phenomena of retained austenite during deformation. The stability of retained austenite Is very Important for the good ductility and it depends on diffusion of carbon and manganese during reverse transformation. The air cooling after holding at intercritical temperature retards the formation of pearlite and provides the carbon enrichment in retained austenite, resulting the increase of elongation in cold-roiled TRIP steel.