Papers by Author: Eui Pak Yoon

Abstract: The present study is a systematic investigation of the effects of microstructural changes, which have originated from the variations of filler metals and welding processes, on the J-R properties of simulated welds. Two AISI Type 347 weld metals, with different carbon contents, deposited by a GTAW process and two AISI Type 347 weld metals, with different carbon contents, deposited by a SMAW process were used in this study. The J-R tests were conducted at 316oC (600oF). The welds deposited by the GTAW process showed higher fracture resistances when compared to the welds deposited by the SMAW process. The J-R fracture resistance of the Type 347-GTAW processed weld with high carbon content was remarkably low when compared to the weld with low carbon. The J-R fracture resistances were decreased by coarse Nb(C, N) precipitates in the Type 347 weld deposited by the GTAW process. In the case of the SMAW welds, mainly coarse Ti-rich particles which had originated from the shielding of the welding rods deteriorated the fracture resistances.

Abstract: The present work is a further investigation into the effects of the carbon (C), nitrogen (N) and niobium (Nb) contents on then fracture properties of the Type 347 stainless steels at 316oC. 9 heats of systematically designed alloys were examined. Through SEM-EDS, TEM and XRD analyses, two kinds of precipitates, Nb(C,N), CrNbN were identified in the Type 347 steels with a high ratio of wt% N to wt% C, on the other hand only Nb(C,N)s were found in the Type 347 steels with a low ratio of wt% N to wt% C. The tearing moduli were decreased in the range of 52~60% as the carbon content increased from 0.03wt% to 0.05wt%. The tearing moduli were lowered by 52~59% in the alloys with a high nitrogen. It was deduced from the microstructure analysis results that the coarse Nb-rich precipitates control the fracture resistance of the Type 347 as they act as the potential sites for the nucleation of micro-voids.

Abstract: This paper investigates the effect of Bubbling and AlCuP on primary Si size in Al-18%Si alloy. Bubbling process and AlCuP treatment are effective for refining primary Si in hypereutectic Al-Si alloy. Both the bubble and AlCuP can be used as nucleation sites of primary Si. Many nucleation sites can make the size of primary Si decreased. So, this experiment suggests that the mix using of bubbling process and AlCuP treatment are more effective for nucleation site of primary Si. For deciding the optimum treatment order, the experiments are processed 3 kinds of order with varying stirring time and holding temperature; 1)AlCuP treatment and then bubbling process, 2)simultaneously using of bubbling process and AlCuP treatment, 3)bubbling process and then AlCuP treatment.

Abstract: The objective of this investigation was to correlate the chemical composition of welding rods for gas tungsten arc welding with the fracture resistance and tensile properties of type 347 welds through the systematic tests and microstructural analyses. Five weld metals which differed in contents of carbon, nitrogen and niobium each other and a high δ-ferrite containing weld metal were deposited by the six different welding rods. J-R fracture resistance and tensile properties were evaluated for the type 347 welds. The microstructural examinations were performed to relate key microstructural features to mechanical properties. It was found that the contents of Nb(C,N) precipitates in type 347 welds were determined by the mixed function of carbon and nitrogen and niobium contents in welding rods. The strengths of type 347 welds were in direct proportion to the contents of Nb(C,N) and J-R fracture resistances were inversely proportional to the contents of Nb(C,N). It was concluded that the type 347 weld with high fracture resistance and adequate strength was obtainable by controlling the sum of carbon and nitrogen contents near 0.1wt% and a limitation of the carbon content below 0.04 wt% in welding rod.

Abstract: In the casting Al alloys, Inclusions are formed by various melt treatment and the some alloying elements have a bad effect on quality of products. Generally, inclusions deteriorate mechanical properties of casting products as well as provide heterogeneous nucleation sites of hydrogen porosity. But the difficulty of removing it is the fact that the inclusions are measured generally small about 20㎛ in the melt. So, in this study, Experiments were carried out on commercial die casting aluminum alloy A383 (JIS: ADC 12). The separation system including a D.C. electric field with steady D.C. magnetic field was adopted for reduction of inclusion in molten Al alloy. Also the quantitative and qualitative analysis of inclusion in Al alloy was taken by using the PoDFA (Porous Disc Filtration Apparatus) and EPMA.

Abstract: The structural control of A356 alloy, which was not studied among various electromagnetic processing of materials, was considered applying the alternating current and direct current magnetic flux density. The main aim of the present study is to investigate the effects of electromagnetic vibration on the macro and microstructure of A356 alloy in order to develop a new process of structural control in A356 alloy. When the electromagnetic vibration is conducted for changing the shape of primary aluminum, at low frequency (≤60Hz), the shape of dendrite is changed speroidal shape. When the electromagnetic vibration is conducted for changing the shape of eutectic silicon, a morphological change of the eutectic silicon from coarse platelet flakes to fine fiber shape is observed with EMV (Electro Magnetic Vibration) process at high frequency (≥500Hz).

Abstract: The structural control of Al-Si alloy, which was not studied among various electromagnetic processing of materials, was considered applying the alternating current and direct current magnetic flux density. The main aim of the present study is to investigate the effects of electromagnetic vibration on the control of the size of primary Si phase in order to develop a new process of structural control in Al-Si alloy. If the current density conducted for making high frequency electromagnetic vibration (EMV) (≥ 500Hz), the size of primary Si phase goes to small. If it conducted for making low frequency EMV (≤ 200Hz), the size of primary Si phase goes to large. This phenomenon considered to be related the collision, agglomeration and diffusion of silicon atoms.

Abstract: The structural control of Al-Si alloy, which was not studied among various electromagnetic processing of materials, was considered applying the alternating current and direct current magnetic flux density. The main aim of the present study is to investigate the effects of electromagnetic vibration on the macro and microstructure of Al-Si alloy in order to develop a new process of structural control in Al-Si alloy. When the electromagnetic vibration is conducted for changing the shape of primary aluminum, at low frequency (≤ 60Hz), the shape of dendrite is changed speroidal shape. When the electromagnetic vibration is conducted for changing the shape of eutectic silicon, the fact that a morphological change of the eutectic silicon from coarse platelet flakes to fine fiber shape is observed with EMV (Electro Magnetic Vibration) process at high frequency(≥ 500Hz).