발간논문

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This paper described a basic study of the pulse shaping effects on weld dimension and weld discontinuties in the continuous seam welds of STS 310S using Nd:YAG laser. Further. laser weldability tests for STS 310S and Inconel 600 was carried out. Pulse shapes used in this experiment were general type, which has been applied generally in laser material processing, ramped-up type (3 steps) and ramped--down type (3 steps). The pulse energy was constant regardless of types of pulse shape. The penetration of laser welds became deeper as the pulse width was longer and he peak power was increased. The pulse of ramped-up type was most effective for deep penetration compared with others. With regard to the laser weldability and reduction of weld discontinuties such as porosity and hot cracking, the general type pulse was the worst and the ramped-down type pulse was the best among 3 types of pulse shape studied in this investigation.

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The micromechanisms of fracture processes are identified by in situ scanning electron microscopy (SEM) fracture observations using single edge notched specimen. In addition, fracture surface of impact specimen was analyzed to investigate the difference of fracture behavior between plain stress condition and plain strain condition. The observation of in-situ fracture process indicated that as strains are applied, microcracks are formed at the interface between soft proeutectoid ferrite and acicular ferrite. Then. the microcracks are easily propagate along the proeutectoid ferrite phase. leading to final fracture. These findings confirm that proeutectoid ferrite play an important role in reducing the toughness of the weld metal.

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The reduction rate of iron oxide pellets by CO, H₂ and CO-H₂ gas mixture was investigated in the range of 800 to 1000℃ by using a thermogravimetric analysis. At 900℃ the redcution rate of pellet in the range of FeO-Fe reduction stage was found to be 0.35 ㏖/㎡.sec for 100 H₂, 0.30 ㏖/㎡.sec for 40%CO-60%H₂, and 0.17 ㏖/㎡.sec for 100% CO. The value of activation energies for the reduction reactions were 6.1 ㎉/㏖, 7.6 ㎉/㏖ and 8.6 ㎉/㏖, respectively. By applying mixed control plot method, the effective diffusion rate constant De and chemical reaction rate constant kc were estimated and applied to the unreacted core shrinking model for single interface. The proposed redcution curves by present model showed the good agreement with the experimental results.

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The behavior of electromotive force(EMF) between bosh and hearth has been studied during furnace operation. The EMF value turned out to be closely related to the height and the thermal state of Liquid phase in the hearth. The measured EMF was in good agreement with the casting operation. The relationship between the EMF signal and the temperature difference was obtained. Using the EMF measuring technique, it was possible to estimate the liquid level and to determine the proper tapping time.

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