Ratcheting behavior of a non-conventional stainless steel and it's associated microstructural variations.

Abstract

The aim of this investigation is to study the influence of asymmetric cyclic loading on accumulation of plastic strain with attendant in-situ microstructural variations that occur during cyclic deformation of a non-conventional stainless steel. 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 300 K for constant mean stress levels with varying stress amplitudes and constant stress amplitudes with varying mean stress. These tests have been carried out by using a 100 KN servo-hydraulic INSTRON (model: 8500R) testing machine. Cyclic loading rate for stress control test is 50 Mpa/s. The stress, nominal strain and the actuator displacement data were continuously recorded during each test to acquire at least 200 data points per cycle. All fatigue tests have been done upto 50 and 100 number of cycles for further analysis. Transverse sections from the gauge portion of the ratcheted samples have been cut after the cyclic loading tests and were subjected to X-ray diffraction (XRD) analyses using Cu Ká radiation. Microstructural examinations have been done with the help of scanning electron microscope (SEM). In addition, ratcheted samples were investigated to assess any variation of their hardness values. The results highlight that the nature of strain accumulation is dependent on the combinations of means stress and stress amplitude. The results of post-ratcheting hardness tests indicate that there is considerable variation in hardness with number cycles. XRD results indicate that considerable amount of austenite transforms to martensite during ratcheting deformation and the amount of martensite varies with the combination of stresses. Increase in ratcheting strain with increase in mean stress or stress amplitude can be explained from increased plastic damage to the material as well as shifting of zone of plastic deformation.