Jongbong Lee

Effects of Inclusions and Microstructures on Impact Energy of High Heat-Input Submerged-Arc-Weld Metals

Submerged-arc-weld metal made with variations of heat-input and consumables were examined to study the inclusions and their effects on the Charpy impact energy of the weld metal as a measure of the toughness. From the analysis, inclusions, of which diameters were smaller than 2 μm, seemed to be effective for acicular ferrite nucleation and the fraction of acicular ferrite was in proportion to the number density of inclusions <2 μm. However, as the heat input increased, the average size of inclusions increased but their number density decreased. It was also observed that the grain sizes of polygonal ferrite with increased heat input. Accordingly, the decrease of the Charpy impact energy with the heat input is inevitable due to these microstructural changes. But, this does not mean that the acicular ferrite cannot be nucleated.

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

A Study on the Improvement of Numerical Thermal Analysis for Steel Welds

This paper is the first part of the study on the accuracy improvement of numerical analysis of steel welds. The aim of this paper is to raise the accuracy of thermal analysis results, such as the shape and size of the weld cross section and the hardness distribution in HAZ(Heat-Affected Zone). It is known that the factors affecting on the accuracy are thermal properties, metallurgical properties and welding heat source model. It was found that the arbitrary distributed heat source model should be used to predict practical weld cross section shape and size. Also, in order to improve the prediction accuracy of HAZ hardness distribution, it was essential to consider 2 CCT(Continuous Cooling Transformation) diagrams in calculating volume fraction of transformed phases. One is the peak temperature being around melting temperature. The other is the peak temperature being around metallurgical transformation temperature.

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.