Cyclic true stress - true strain curves were obtained using general-purpose shipbuilding steel in the large strain region. Test methods are incremental step test and static tensile test after cyclic loading. The stress-strain curves for static and cyclic loading were compared to investigate the cause of the difference between each other. A simple method for estimating the cyclic stress-strain curves was proposed. The diameter and curvature of the smallest cross-section of the specimen were measured using telecentric measurement device, and the true stress - true strain curve was obtained using the Bridgman correction method. Using the measurement method, cyclic stress-strain diagrams were obtained in the region where the true strain exceeded 1% by performing cyclic tensile tests using the incremental step method and static tensile tests after cyclic loading. The influence of the test method and maximum displacement conditions on the cyclic stress-strain curve was small within the scope of this study. There was little difference in the elongation between the static tensile test and the static tensile test after cyclic loading. The difference in yield stress between static and cyclic loading was discussed in terms of macro-yield mechanisms at intergranular and transgranular. A simple method for estimating cyclic stress-strain curves from a static stress-strain curve was proposed. The specimens used in this study are general-purpose shipbuilding steels, and the results should be applicable to similar steels for welding and structures.

抄録全体を表示

A tailored blank (TWB) is a blank for press forming in which multiple steel sheets are welded together. There is concern that TWB with hydrogen dissolved by laser welding may fracture at the weld bead if formed immediately after welding. The time required for hydrogen to be released from the weld bead tends to be longer for high-strength steel sheets. In order to estimate how long to wait after welding before forming, it is necessary to know the time dependence of the maximum hydrogen concentration in the weld. Therefore, Fick's two-dimensional diffusion equation was applied to the laser weld to obtain an analytical solution and to study the diffusion behavior of hydrogen. As a result, it was found that the analytical solution can accurately represent the hydrogen concentration distribution in the weld, including the high temperature state of the weld immediately after welding, and that the diffusion coefficient can be determined by regressing the logarithm of the measured average hydrogen concentration in the weld. Hydrogen diffuses from the weld metal to the heat-affected zone and partly to the base metal. Therefore, the determined diffusion coefficient is only phenomenological. It is also shown that once the diffusion coefficient is determined, the maximum hydrogen concentration at the center of the weld bead can be determined from the measured average hydrogen concentration. Furthermore, although extrapolating from experimental data, it is possible to estimate the initial amount of hydrogen dissolved during welding, and it is shown that the time evolution of the hydrogen concentration distribution in the laser weld can be completely described.

抄録全体を表示

A growth rate of an intermetallic compound (IMC) composed of Fe and Al was evaluated to predict the distribution of IMC at the Fe/Al dissimilar material interface during laser brazing. A carbon steel crucible was made, then the aluminum alloy of braze material was melted in the crucible, and the IMC thickness formed at the Fe/Al solid-liquid interface was measured at heating temperatures between 750~900 ºC. EDS analysis revealed that the IMC at the interface was FeAl3 or Fe2Al5, the IMC growth follows a parabolic law, and the growth rate constants obtained at each temperature were evaluated using the Arrhenius equation to clarify the temperature dependence of the growth rate. The temperature history at the Fe/Al interface was extrapolated by measuring the temperature history inside the carbon steel plate during laser brazing. The IMC thickness was calculated by combining the extrapolated temperature history and the IMC growth rate equation. The calculated IMC thickness agreed with the actual IMC thickness formed at the Fe/Al interface. Therefore, the validity of the IMC growth rate of carbon steel and aluminum alloy was verified.

抄録全体を表示

Arc efficiencies of gas tungsten arc welding (GTAW) were measured using two different methods, liquid nitrogen calorimetry and water-cooled copper anode calorimetry, across a range of welding currents from 98 A to 302 A. Arc efficiencies determined with water-cooled copper anode calorimetry remained constant throughout the entire current range, while those measured with liquid nitrogen calorimetry exhibited a concave curve. A hypothesis was proposed to explain the changes in arc efficiency with varying welding currents. Accordingly, a decrease in arc efficiency at 149 A welding current was caused by reduced Joule heating due to increased electrical conduction from iron-vapor contamination in an arc. For welding currents above 227 A, a decrease in arc efficiency was attributed to increased penetration depth, leading to more uniform heating in the base metal beneath the arc column, thereby preventing heat conduction within the metal. Thereafter, the relationship between penetration depth and arc efficiency was experimentally confirmed. Comparing with the arc efficiency of gas metal arc welding (GMAW), it was found that the mechanism in which arc efficiency changes with welding current in GTAW was different from that in GMAW.

抄録全体を表示