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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 550~610℃ 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℃, 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℃, 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 (T68J) decreased below .50℃. Specifically, in FGFG, ICIC and SCSC-HAZ, T68J was below .110, which was lower than the case of base metal.

keyword : heat-affected zone, SA508 Gr.4N, Ni-Mo-Cr Low alloy steel, post weld heat treatment

In this study, microstructural investigation was conducted on the separation phenomenon occurring during Charpy impact tests of API X80 linepipe steels. Particular emphasis was placed on the role of microstructural phases present in the API X80 steels such as acicular ferrite, bainite, and hard secondary phases. Detailed microstructural analysis of fractured impact specimens showed that highly elongated bainite worked as prior initiation sites for separations, and that the number and length of separations increased with increasing volume fraction of bainite. In the steels having high work hardenability, tearing-shaped separations were found because the hammer-impacted region was seriously hardened during the impact test, which led to the reduction in the impact toughness. As the test temperature decreased, the tendency of separations increased, but separations were not observed when the cleavage fracture prevailed at very low temperatures. Thus, the minimization of the formation of bainite and secondary phases in the steels would be beneficial for preventing or minimizing separations because separations deteriorated low-temperature impact toughness.

keyword : API X80 Linepipe steel, charpy impact test, separation

The effect of sub zero treatment on the damping capacity in austempered ductile cast iron investigated. Austenite transformed in to martensite by subzero treatment, and with the decrease of subzero treatment temperature, volume fraction of martensite increased. Damping capacity of austempered ductile cast iron was highly increased by subzero treatment, with the decrease of subzero treatment temperature, damping capacity was slowly increased. With the decrease of subzero treatment time, damping capacity was rapidly increased to 30 min. and then slowly increased. With the increase of volume fraction of martensite, damping capacity rapidly increasing to 5% and then slowly increased.

keyword : subzero treatment, damping capacity, retained austenite, austempered ductile cast iron, ε martensite

Mechanical properties and precipitation behavior of backward extruded 6061 Al alloy for pressure vessel were investigated using tensile test, transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). In this study, solution heat treatment (SHT) was performed at 535℃ for 30~90 min and aging treatment was conducted at 177 and 190℃ for 1~7 h. Maximum tensile strength of 36.6 kgf/mm2 and yield strength of 33.29 kgf/mm2 were achieved at the aging time of 5 h at 190℃. TEM observation showed that fine needle-like β'' phase which has 35~45 nm of length was uniformly distributed in the aged 6061 Al alloy specimen. From tensile test, TEM and DSC analysis, it is expected that aging time of 2~5 h at 190℃ is suitable for the extruded A6061 used as pressure vessels.

keyword : A6061 alloy, precipitation, aging, needle-Like β″ phase

The molten salt electrolysis is applied to reduce titanium dioxide to titanium metal using calcium chloride as an electrolyte and the reaction mechanism of the reduction process is examined by analyzing the reaction products. The process conditions to obtain titanium metal for 900℃ correspond to 2.9~3.2 V and 24 hours. The reaction products for 2.9 V at 900℃ include irregular-shaped titanium oxides such as Ti4O7, Ti3O5 and Ti2O3 and polyhedral CaTiO3. Using these microstructure analysis, the sequential reaction mechanism for the electrochemical reduction of titanium dioxide to titanium is proposed.

keyword : titanium, titanium dioxide, molten salt electrolysis, calcium chloride, reaction mechanism

Recently, new extraction technology for manganese nodule has been developed as alternative noble metallic resources. It is important to understand thermodynamic behaviors of phosphorus in low basic slag system from the viewpoint of the refining processing optimization. Thermodynamic behaviors of phosphorus in the FeO-MnO-CaO-SiO2-MgO(satd). slag system were investigated at 1723 K with various oxygen potential and slag composition of low basicity. The experimental results for dependence of phosphorus on oxygen potential and slag basicity indicated that the dissolution mechanism of phosphorus into slag of low basicity could be derived as follows; [P]+5/4O2+(O2-)=(PO3.5(2-)) Present experimental results implied that stability of phosphorus in slag would be depended on both of O2- (basicity) and content of Ca2+ in molten slag. The thermodynamic effect of FeO, MnO and Na2O on low basicity on phosphate capacity was discussed.

keyword : manganese nodule, thermodynamic, basicity, stability of phosphate ion, phosphate capacity

This paper is concerned with effects of Mn and heat-input on the mechanical properties of EGW welds. Four different kinds of welding consumables were fabricated by varying Mn contents such as 1.3, 1.5, 1.7, 2.0%Mn and each consumable was welded for EGW on four heat-input conditions between 190 and 340 KJ/Cm. Mn contents were decreased as heat-input increases and alloy elements (C, Si, Ti, B, Al) to deoxidize easily also revealed similar tendency to Mn. Their microstructure, Charpy impact property and strength were investigated, and it is found that Charpy impact property and strength exhibit a strong dependence on change of microstructure by Mn contents and heat-input. The increase of Mn contents or the decrease of heat-input made the microstructure fine and increase volume fraction of acicular ferrite, thereby leading to the great improvement of Charpy impact property and strength. In case of single EGW, optimum Mn contents are over 1.7% for the toughness and strength.

keyword : electro gas welding, flux cored wire, toughness, strength, manganese, heat input

The effects of aging time on the microstructure and shear strength of the Low Temperature Cofired Ceramic (LTCC)/Ag pad/Electroless Nickel Immersion Gold (ENIG)/BGA solder joints were investigated through isothermal aging at 150℃ for 1000 h with conventional Sn-37Pb and Sn-3Ag-0.5Cu. Ni3Sn4 intermetallic compound (IMC) layers was formed at the interface between Sn-37Pb solder and LTCC substrate as-reflowed state, while (Ni,Cu)3Sn4 IMC layer was formed between Sn-3Ag-0.5Cu solder and LTCC substrate. Additional (Cu,Ni)6Sn5 layer was found at the interface between the (Ni,Cu)3Sn4 layer and Sn-3Ag-0.5Cu solder after aging at 150℃ for 500 h. Thickness of the IMC layers increased and coarsened with increasing aging time. Shear strength of both solder joints increased with increasing aging time. Failure mode of BGA solder joints with LTCC substrate after shear testing revealed that shear strength of the joints depended on the adhesion between Ag metallization and LTCC. Fracture mechanism of Sn-37Pb solder joint was a mixture of ductile and pad lift, while that of Sn-3Ag-0.5Cu solder joint was a mixture of ductile and brittle (Ni,Cu)3Sn4 IMC fracture morphology. Failure mechanisms of LTCC/Ag pad/ENIG/BGA solder joints were also interpreted by finite element analyses.

keyword : Low temperature co-fired ceramic, LTCC, interfacial reaction, intermetallic compound, IMC, shear strength, finite element modeling, FEM