Ratcheting Behavior of Sensitized Non-Conventional Austenitic Stainless Steel

Abstract

The aim of this investigation is to study the effect of ratcheting behavior on a sensitized non-conventional austenitic stainless steel. The metallurgical investigation of solution annealed as well as sensitized stainless steels were carried out in order to determine the microstructural characteristics, hardness, grain size distribution, tensile properties, ratcheting behavior and post-ratcheting tensile properties of differently heat treated stainless steel specimens. The microstructural examinations have been done with the help of optical microscope and scanning electron microscope. The microstructure of the selected steel in solution annealed condition reveals that the steel owns nearly equiaxed austenite grains with annealing twins throughout the matrix. The phenomenon of plastic strain accumulation due to asymmetric cyclic loading is known as ratcheting. Stress-controlled cyclic loading experiments have been carried out at room temperature for constant mean stress levels with varying stress amplitudes after each 50 cycles. Cyclic loading rate for stress control test is 50 MPa/s.Ratcheting deformation is found to increase with increase in stress amplitude values for constant values of stress mean. However, the rate of strain accumulation increases with increasing stress amplitude. The analyses of stress-strain results indicate that the steel cyclically softens in both solution annealed as well as sensitized conditions. This nature of softening is also seen in the post-fatigue tensile tested samples. The YS and TS value of unratcheted tensile as well as post ratcheted tensile steel samples were found to reduce. The fracture surfaces of the sensitized specimens consists of rock candy type fracture whereas dimple fracture was present in solution annealed sample. The XRD analyses attributes to in-situ martensitic transformation during deformation. It was noticed that the phase transformation plays a significant role on the variation in TS of the steel