Papers by Author: Won Jon Yang

Abstract: A series of fracture tests and elastic-plastic FEM analysis were carried out for three-point bend specimens with different notch root radius in the cleavage temperature region of a RPV low alloy steel. The cleavage initiation distance (CID) from the notch tip was measured by SEM photographs on the fracture surface of each specimen. The local cleavage fracture stress σf * was defined as the stress σyy at the cleavage initiation site. The σf * increased with decreasing notch root radius. The CID’s were larger in specimens with bigger root radius. This implies that a larger volume of material and possibly bigger microcracks should be involved in the cleavage fracture process of those specimens. The σf * is considered to have a certain relationship with the level of stress-strain concentration in different geometry specimens.

Abstract: The influence of surface treatment such as nitriding, TNHT(Ti Nano Heat-treatment, PHILOS TECHNOLOGIES, INC.) and PVD coating on the thermal crack propagation behavior of hot work die steel was investigated. To examine the thermal fatigue resistance, the cyclic thermal shock system consisted of induction heating and water spray quenching unit was constructed and Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Thermal stress depending on test temperature was also simulated by FEM. The TNHT specimen showed lower Lm value than as-heat treated specimen but, in the case of maximum and average crack length, the TNHT specimen exhibited higher value than those of as-heat treated specimen. This means that the small number of large cracks were initiated and propagated selectively in the TNHT specimen. This result can be caused by two contrary effects of diffusion layer, those are introduction of the residual compressive stress good for mitigation of thermal tensile stress and very high surface hardness harmful for crack initiation. However, Lm value of salt-bath nitriding specimen was very high due to the white layer in spite of the existence of diffusion layer.