Hong-Chul Jeong

Mechanical and oxidation properties of Ti–xFe–ySi alloys

Abstract New Ti-based alloys having reasonable mechanical properties and good oxidation resistance were designed by alloying with low cost elements of iron and silicon. From tensile tests at room temperature and 400 °C, Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys were found to have the optimum combination of strength and ductility, which are comparable to the Ti–6Al–4V alloy. The precipitation of fine Ti-silicides and the reduction in α/β colony sizes are primarily responsible for the obtained mechanical behavior. The air oxidation resistance of Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys between 700 and 1000 °C is far superior to that of the Ti–6Al–4V alloy, and even better than that of TiAl intermetallics. The formation of amorphous silica within the TiO2-rich oxide layers increased the oxidation resistance significantly.

Mechanical Properties and Microstructures of High Heat Input Welded Tandem EGW Joint in EH36-TM Steel

In the coarse grained HAZ of conventional TiN steel, most TiN particles are dissolved and austenite grain growth easily occurrs during high heat input welding. To avoid this difficulty, thermal stability of TiN particles is improved by increasing nitrogen content in EH36-TM steel. Increased thermal stability of TiN particle is helpful for preventing austenite grain growth by the pinning effect. In this study, the mechanical properties and microstructures of high heat input welded Tandem EGW joint in EH36-TM steel with high nitrogen content were investigated. The austenite grain size in simulated HAZ of the steel at was much smaller than that of conventional TiN steel. Even for high heat input welding, the microstructure of coarse grained HAZ consisted of fine ferrite and pearlite and the mechanical properties of the joint were sufficient to meet all the requirements specified in classification rule.

Quantitative Analysis on the Effects of Welding Parameters on Diffusible Hydrogen Contents in Weld Metal Produced by FCAW Process

The effects of welding parameters such as contact tip-to-work distance (CTWD), voltage, and current on the weld metal diffusible hydrogen contents (HD) were investigated and rationalized by the calculation of heating time and amount of heat generated in the extension length of flux cored wire. As CTWD increased from 15 to 25mm, HD decreased from 8.46 to 5.45mL/100g deposited metal. Calculations showed that, with an increase of CTWD, the amount of heat generated increased from 46 to 92J in addition to an increase of heating time. Increase of current from 250 to 320A, however, gave little variation of HD. It showed that no significant change in the amount of heat generated was found, and heating time was decreased with an increase of current. It also showed that CTWD is more influential than voltage in relatively lower heat input ranges, while voltage is more in higher input ranges

Prediction Model for the Microstructure and Properties in Weld Heat Affected Zone: V. Prediction Model for the Phase Transformation Considering the Influence of Prior Austenite Grain Size and Cooling Rate in Weld HAZ of Low Alloyed Steel

Abstract In this study, to predict the microstructure in weld HAZ of low alloyed steel, prediction model for the phase transformation considering the influence of prior austenite grain size and cooling rate was developed. For this study, six low alloyed steels were designed and the effect of alloying elements was also investigated.In order to develop the prediction model for ferrite transformation, isothermal ferrite transformation behaviors were analyzed by dilatometer system and ‘Avrami equation’ which was modified to consider the effect of prior austenite grain size. After that, model for ferrite phase transformation during continuous cooling was proposed based on the isothermal ferrite transformation model through applying the ‘Additivityrule’. Also, start temperatures of ferrite transformation were predicted by A r3 considering the cooling rate. CCT diagram was calculated through this model, these results were in good agreement with the experimental results. After ferrite transformation, bainite transformation was predicted using Esaka model which corresponded most closely to the experimental results among various models. The start temperatures of bainite transformation were determined using K. J. Lee model. Phase fraction of martensite was obtained according to phase fractions of ferrite and bainite.Key Words : Phase transformation, Avrami equation, Prior austenite grain size, Heat affected zone, Modelling

Effect of Restraint Stress on the Precipitation Behavior and Thermal Fatigue Properties of Simulated Weld Heat Affected Zone in Ferritic Stainless Steel

Abstract Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of 200to 900due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, M 6 C with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.Key Words : Thermal fatigue, Ferritic stainless steel, Heat affected zone, Precipitation

Prediction Model for the Microstructure and Properties in Weld Heat Affected Zone : IV. Critical Particle Size for the Particle Coarsening Kinetics in Weld HAZ of Ti Added Low Alloyed Seel

A kinetic model fur the particle coarsening behavior was developed. The proposed model considered the critical particle size which can be derived from Gibbs-Thomson equation unlike the conventional approach. In this study, the proposed particle coarsening model was applied to study the coarsening behavior of titanium nitride (TiN particle) in microalloyed steel weld HAZ. Particle size distributions and mean particle size by the proposed model were in agreement with the experimental results. Meanwhile, using additivity rule, the isothermal model was extended to predict particle coarsening behavior during continuous thermal cycle.