Cyclic behaviour of a 316L stainless steel hardened by an explosive treatment

A type 316L stainless steel has been treated by a primary explosive which induces a marked hardening in the material, especially in the first two or three hundred micrometers, but which reaches the rear face of the samples (5 mm thick). After treatment, the surface roughness is characterized by a low Ra factor while the microstructure of the surface layers is composed of a recrystallization in fine grains, followed by a profuse twinning from a depth of about 0.5 μm. When cycled with a plastic strain controlled mode, at Δϵp/2 = ± 10−3, the treated material exhibits a high cyclic stress level compared with the reference material all along the test. However, the fatigue life of the treated material is shorter than that of the reference one. When cycled with a stress-controlled mode with a high stress level, the fatigue life is seven times longer for the treated material than for the untreated one. This behaviour is analysed by its relation to the microstructure and the state of the residual stresses measured by X-ray diffraction.