• 한국해양공학회지
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• 제10권3호
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• pp.24-33
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• 1996

Up to now, most of studies on mechanical properties in friction welded components are about tensile and bending strength. However the fatigue studies on the friction-welded components subjected to repeated stress are not available. The purposes of this study are the development of fundamental design and the development of in-process real-time weld quality evaluation technique by acoustic emission for the bar-to-bar dissimilar friction welding of hydraulic valve spool steels.

• Journal of Welding and Joining
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• 제28권5호
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• pp.51-57
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• 2010

Brittle microstructure created in a heat affected zone (HAZ) during the welding of low alloy steel can be eliminated by post-weld heat treatment (PWHT). If the PWHT is not possible during a repair welding, the controlled bead depositions of multi-pass welding should be applied to obtain tempering effect on the HAZ without PWHT. In order to anticipate and control the tempering effect during the temper bead welding, the definition of temperature curve obtained from the analytical solution was suggested in this research. Because the analytical solution for heat flow is expressed as a mathematical equation of weld parameters, it may be effective in anticipating the effect of each weld parameter on the tempering in HAZ during the successive bead depositions. The reheating effect by the successive bead layer on the brittle coarse grained HAZ formed by earlier bead deposition was estimated by comparing the overlapped distance between the temperature curves in the HAZ. Three layered weld specimens of SA508 base metal with A52 filler were prepared by controlling heat input ratio between layers. The tempering effect anticipated by using the overlapped distance between the temperature curves was verified by measuring the micro-hardness distribution in the HAZ of prepared specimens. The temperature curve obtained from analytical solution was expected as a good tool to find optimal temper bead welding conditions.

• 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.

• 한국정밀공학회지
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• 제25권7호
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• pp.116-123
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• 2008

In the fossil power plant, the reliability of the components which consist of the many welded parts depends on the quality of welding. The residual stress is occurred by the heat flux of high temperature during weld process. This decreases the mechanical properties as the strength of fatigue and fracture or causes the stress corrosion cracking and fatigue fracture. Especially, the accidents due to the residual stress occurred at the weld parts of high-temperature and high-pressure pipes and steam headers. Also, the residual stress of the welded part in the recently constructed power plants has been brought into relief as the cause of various accidents. The aim of this study is the measurement of the residual stress using the x-ray diffraction method. The merits of this are more accurate and applicable than other methods. The materials used for the study is P92 steel for the use of high temperature pipe on super critical condition. The variables of tests are the post-weld heat treatment, the surface roughness and the depth from the original surface. The test results were analyzed by the distributed characteristics of the full width at half maximum intensity (FWHM) in x-ray diffraction intensity curve and by the relation of hardness with FWHM.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2006년도 춘계 학술대회 개요집
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• pp.121-123
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• 2006

The aim of present study is to investigate the effect of welding parameters and heat treatment conditions on the formability of the laser welded silicon steel sheet. It was found that there is optimum range of the heat input ($0.6{\sim}0.7kJ/cm$) and gap distance ($0.125{\sim}0.150mm$) for the high tensile strength and the avoidance of the fracture in weld metal, Also, it was essential for the improvement of formability to perform pre- and post-welding heat treatment which cause the uniform mixture of base metal and welding consumable.

• 대한공업교육학회지
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• 제31권1호
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• pp.200-210
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• 2006

초기 응력부식균열에 대한 감수성 평가 시험법인 C링 시험법(C-ring Test)은 용접부의 후열처리 균열감수성을 평가하는 시험법으로 적합하다고 알려져 있다. C링 시험법은 실제 용접부 열영향부에 노치를 제작하여 잔류응력이 열영향부에 미치는 영향을 평가할 수 있다는 장점을 가지고 있다. 따라서 본 연구에는 C링 시험법을 이용하여 원자력 발전소 압력용기에 사용되는 Co-Mo강의 용접부 결함 발생 현상을 검토 하고자 Co-Mo강에 대하여 후열처리를 실시하였으며, C링 시험법을 통하여 용접 열영향부의 후열처리 균열을 평가하고자한다. 결론은 다음과 같다. 바나듐 모디파이 합금(AMAX 3Cr-1Mo­-0.1V, JSW 3Cr-­1Mo-­V-­Ti-­B 및 JSW $2\frac{1}{4}Cr$-1Mo-­V-­Ti-­B강)은 Berkeley $3Cr-1\frac{1}{2}Mo-\frac{1}{2}Ti$ 및 $2\frac{1}{4}Cr$-­1Mo합금강보다 후열처리 균열에 대한 감수성 보다 높게 나타났다. C-ring 시험에 있어서 바나듐 모디파이 합금은 2종의 후열처리조건과 3종의 응력조건에서 모두 균열을 나타냈으나 Berkeley $3Cr-1\frac{1}{2}Mo-\frac{1}{2}Ni$강과 Luken $2\frac{1}{4}Cr-1Mo0.1C$재료는 690MPa와 더 이상의 응력조건에서 2종의 후열처리를 실시하는 조건에서도 균열이 발생하지 않았다. C-ring 시험결과, 균열깊이는 바나듐 모디파이 재료인 경우, 낮은 후열처리온도와 비교해서 높은 후열처리온도인 경우가 감소하고 있다. C-ring 시험에 대한 SEM분석결과 균열은 오스테나이트 결정 입계를 따라 파괴되는 파괴양상을 나타내고 있으며 연성파면은 나타나지 않았다.

• Journal of Welding and Joining
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• 제34권3호
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• pp.61-68
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• 2016

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 E91T1-B9C wire. Post-weld heat treatment was carried out at $750^{\circ}C$ for 2.5 h. Based on microstructural examination, tempered martensite and lower bainite were formed in first layer of weld metal. The amount of tempered martensite was decreased and the amount of lower bainite was increased with increasing heat input and layer. Heat affected zone of alloy steel showed the highest hardness due to the formation of tempered Martensite and lower Bainite. Tensile strengths of dissimilar welds decreased with increasing heat inputs. Dissimilar welds seemed to have a good hot cracking resistance due to the low HCS index below 4. The salt spray test of dissimilar metals showed that the corrosion rate increased with increasing heat inputs due to the increase of the amount of lower Bainite.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2006년 추계학술발표대회 개요집
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• pp.18-19
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• 2006

The purpose of this study is to evaluate the behavior of residual stress at the weldment of pressure vessel by local Post Weld Heat Treatment(PWHT). In order to do it, residual stress were measured before and after local PWHT by XRD on the test piece first. And then, the results of finite element(FE) analysis based on thermal-elasto-plastic-creep theories were verified by comparing with the measured results.

• Journal of Welding and Joining
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• 제32권3호
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• pp.95-101
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• 2014

Joining of grey cast iron by fusion welding has much difficulties for its extremely low ductility and low toughness because of the flake form of the graphite. And the brittle microstructure, i.e. ledeburite may be formed during fusion welding by its rapid cooling rates. By these kinds of welding problem, preheat and post heat treatment temperature must be increased to avoid weld crack or welding problems. In order to avoid these fusion welding problem, friction welding of cast iron was carried out for improving joint soundness, establishing friction welding variables. There is no factor for evaluating friction weldability in continuous drive type friction welding. In this point of view, this study proposed the parameters for calculating friction weld heat input. The results obtained are as follows ; 1. There was a close relationship between tensile strength and flash appearance of friction welded joint. 2. Tensile strength was decreased and flash was severely oxidized as increasing frictional heating time. 3. As increased forging pressure $P_2$, flash had a large crack and tensile strength was decreased. 4. As powdered graphite by rotational frictional force induced flat surface and hindered plastic flow of metal, tensile strength of welded joint was decreased. 5. Heat input for continuous drive type friction welding could be calculated by the factors of $P_1$, $P_2$ and upset distance(${\delta}$).

• Corrosion Science and Technology
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• 제20권6호
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• pp.426-434
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• 2021

The effect of two different annealing temperatures on the level of the second phase precipitated in the microstructure and the corrosion behaviors of super austenitic stainless steel were examined. The sample annealed at a higher temperature had a significantly lower fraction of the sigma phase enriched with Cr and Mo elements, showing more stable passivity behavior during the potentiodynamic polarization measurement. However, after the welding process with Inconel-type welding material, severe corrosion damage along the interface between the base metal and the weld metal was observed regardless of the annealing temperature. This was closely associated with the precipitation of the fine sigma phase with a high Mo concentration in the unmixed zone (UMZ) during the welding process, leading to the local depletion of Mo concentrations around the sigma phase. On the other hand, the fraction of the newly precipitated fine sigma phase in the UMZ was greatly reduced by post-weld heat treatment (PWHT), and the corrosion resistance was greatly improved. Based on the results, it is proposed that the alloy composition of welding materials and PWHT conditions should be further optimized to ensure the superior corrosion resistance of welded super austenitic stainless steel.