Some metallurgical aspects of Dynamic Strain Aging effect on the Low Cycle Fatigue behavior of C–Mn steels

Abstract

Carbon–Manganese steels and associated welds are commonly used, and sometimes to sustain loads in the Low Cycle Fatigue domain. Nevertheless, the metallurgy of these C–Mn steels is rather complex, due to the interaction of solute atoms (carbon and nitrogen) with dislocations during deformation which leads to metallurgical instabilities: Lüders strain, Static Strain Aging (SSA) and Dynamic Strain Aging (DSA). The DSA phenomenon is an interaction during the test between solute atoms and dislocations which are submitted to an supplementary anchorage if the temperature is sufficient to allow the diffusion of solute atoms leading to a discontinuous plastic deformation localized in bands associated with serrations on the stress–strain curve. In C–Mn, the temperature domain where the phenomenon is present is from 150°C to 300°C. If these metallurgical instabilities induce an increase in hardness, unfortunately they produce a decrease of ductility detrimental to components safety. The results of the DSA effect on LCF behavior in C–Mn and Low Alloyed steels reported in the literature are very confused and contradictories. In this study, two C–Mn steels with a different sensitivity to DSA are investigated in the Low Cycle fatigue domain. As reported from some authors, the fatigue life seems enhance or reduce in the temperature domain where the DSA is maximum, but the decrease of the strain rate always decreases the number of cycles to failure.