Cyclic deformation and near surface microstructures of shot peened or deeprolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling press ures, respectively. The resulting near surface properties were characterize d by cross sectioning transmission electron microscopy (TEM), residual stre ss and phase analysis as well as interference line half-width and microhard ness measurements. Cyclic deformation curves were obtained by hysteresis me asurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peenin g and deep rolling lead to a complex near surface microstructure, consistin g of nanocrystalline regions, deformation bands and strain induced martensi tic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation be haviour: Plastic strain amplitudes and cyclic creep were drastically decrea sed by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remar kably, the initial residual stress profile and surface strain hardening wer e not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell struct ures were only formed in greater depths whereas the nanocrystalline layer r emained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation. (C) 1999 Elsevi er Science S.A. All rights reserved.