Jeong Hun Lee

Enhanced low-cycle fatigue life by pre-straining in an Fe-17Mn-0.8C twinning induced plasticity steel

Enhancement of low-cycle fatigue (LCF) life was achieved by imposing pre-straining in an Fe-17Mn-0.8C twinning induced plasticity steel. Two different straining methods (uni-axial tension and drawing) were utilized with strain levels of 0, 0.2, and 0.4. The beneficial effect of pre-straining for LCF life was elucidated by microstructural observation using scanning and transmission electron microscopy. In both pre-strain modes, the fraction and variant of mechanical twins (MTs) are increased with the amount of pre-strain. Particularly, pre-drawing generated more multi-variant MTs and resulted in better LCF performance than those of uni-axially tensioned specimen. The role of MTs or stacking faults as a barrier of dislocation glide was clarified by transmission electron microscopy analysis of fatigued specimens. Fractographic analysis also showed that crack deflection occurred predominantly in the pre-strained specimens, which was effective in retarding crack propagation.

Effect of tungsten addition on the mechanical properties and corrosion resistance of S355NL forging steel

A study was made to investigate the effect of tungsten (W) addition on the microstructure, tensile properties, Vickers hardness, and corrosion resistance of S355NL forging steel. Mechanical properties were evaluated and considered in the context of the interlamellar spacing of pearlite. Microstructural analysis revealed that the addition of W favors the formation of intragranular acicular ferrite and leads to a decrease in the interlamellar spacing of pearlite. After W addition, the corrosion rust film was intact. The steel containing W showed fewer microcracks distributed in the corroded surface compared with that without W. It was concluded that the addition of W is beneficial for improvement of both the mechanical properties and corrosion resistance of S355NL forging steel.