MICROCRACK GROWTH AND FATIGUE BEHAVIOR OF A DUPLEX AND SUPER DUPLEX STAINLESS STEEL

M. BALBI; S. HEREÑÚ; A. F. ARMAS; I. ALVAREZ-ARMAS

Rosario, Argentina

Congreso; 10th Inter-American Congress of Electron Microscopy 2009, CIASEM 2009/1st Congress of the Argentine Society of Microscopy, SAMIC 2009; 2009

Inter-American Committee of Electron Microscopy Societies

The kinetics of microcrack growth during cycling has been studied in a S32205 duplex stainless steel and in a S32507 super duplex stainless steel in the as-received and aged (100hr at 475ºC) conditions. Cylindrical specimens with shallow notch were subjected to constant plastic strain range of 0.3% in both thermal conditions. The characteristic features of the surface damage and crack growth in the duplex stainless steel in both thermal conditions have shown striking differences in micro-crack density, nucleation place and propagation rate even though the fatigue live of the as-received and aged material are comparable. In the as-received material, micro- crack density is low and nucleates mainly at grain and phase boundaries or second phase particles. In the aged condition, slip markings firstly appear in the ferritic phase and they are the preferred site for microcrack nucleation. The surface damage evolution has also been correlated with the internal dislocation structure. In as-received material, the factor that controls nucleation and growth of microcrack are the persistent slips bands (PSB). In aged material the formation of slip marking on the ferritic phase due to deformation and twinning mechanism is observed. The slip marking serve as preferential site for fatigue crack nucleation. In this work we propose that the rate of microcrack growth in aged material is represented by an exponential law.