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스테인레스계 자유단조품의 공냉 중 크랙발생에 관한 유한요소해석
강성훈(S. H. Kang),김현수(H. S. Kim) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5
In this work, heat treatment simulation was carried out to investigate the reason for crack generation during air cooling of stainless seal housing and housing cover manufactured by open-die forging. These two forged parts were sequently made by upsetting, cogging and piercing processes. After forging, two forged products were heat-treated at the temperature of 1040℃ for 12 hrs and then were cooled at the rate of 30℃/hr for 31 hrs within the furnace. Successive cooling was carried out in the air just after the surface temperature of the forged part reached 100 ℃ in the furnace. However, the large cracks were observed in the longitudinal and hoop directions of the forged seal housing during air cooling. Thus, numerical analysis of air cooling process was carried out in order to find the reason for crack generation. From the simulation results, it was found that the stress state is strongly dependent on the temperature variation and phase transformation. Especially, the tensile and compressive stresses were repeatedly generated within the forged products during cooling process. It was also found that temperature decrease near the surface is strongly related with generation of tensile stress, on the contrary, compressive stress near the surface is resulted from the phase transformation. The predicted tensile stresses in both the longitudinal and hoop directions of seal housing were more than two times than those of housing cover. Thus, it can be concluded that the higher tensile stress led to the crack generation in case of seal housing due to volume effect related with temperature difference between inside and outside of the forged product.
강성훈(S. H. Kang),윤여웅(Y. W. Yun),이영선(Y. S. Lee) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.5
With the recent increase in the demand for the net-shape forming, numerical simulations are being commonly adopted to increase the efficiency and effectiveness of design of bulk metal forming processes. Proper consideration of tribological problems at the contact interface between the tool and workpiece is crucial in such simulations. In other words, lubrication and friction play important roles in meta forming by influencing the metal flow, forming load and die wear, In order to quantitatively estimate such friction condition or lubricant characteristic, the constant shear friction model is widely used for bulk deformation analyses. For this new friction testing method base on the forward or backward extrusion process is proposed to predict the shear friction factor in this work. In this method, the tube-shaped punch pressurizes the workpiece so that the heights at the center and outer of punch(or mandrel) become different according to the friction condition. That is, the height at the center of punch is higher than that at the outer of the punch when the friction condition at the contact interface is severe. From this founding, the proposed friction testing method can be applied to effectively evaluate the friction condition in bulk meta forming processes.