Experimental study on macro- and microstress state, microstructural evolution of austenitic and ferritic steel processed by shot peening

In this study, shot peening (SP) treatment was implemented on a duplex (austenite and ferrite) stainless steel. By using X-ray diffraction residual stress determination and line profile analysis, the residual stress (including macro- and microstress) and microstructure evolutions in the surface layer of the shot peened material were discussed. Results pointed out that the initial thermal stress state were compressive (-80 MPa) in alpha phase and tensile (90 MPa) in gamma phase owing to the different thermal expansion coefficients of the two phases. After SP, the nonreversible plastic deformation of the surface layer and the interior of bulk was the main cause of the macrostresses. SP introduced more compressive residual stress into the gamma phase, the maximum compressive stress in gamma phase was similar to 1.3 times larger than that in alpha phase (-912 vs. -717 MPa) and occurred deeper in the sample interior (similar to 75 vs. 10 mu m). The inhomogeneous partitioning of the plastic strains, which was ascribed to the difference of the relative strength and deformation mechanism between the phases, was the contributing cause of microstresses. The average microstresses in surface layer of shot peened alloy were tensile in alpha phase and compressive in gamma phase, the magnitude of which changed with depth since the phase-specific plastic deformation was also a function of depth. The quantitatively microstructural analysis indicated that more deformation was stored in the gamma phase, the microstrain and dislocation density at the surface in gamma phase were about four and 2.6 times larger than that in alpha phase (7.25 vs. 1.83 x 10(-3), 3.9 vs. 1.5 x 10(15) m(-2)), respectively. Microhardness measurements showed that SP-induced work hardening was more evident in austenite, the hardness of austenitic phase was found significantly higher than that of ferritic phase in near surface region.