MAGNETIC METHODS FOR CHARACTERISATION OF COLD ROLLED AISI TYPE 304 STAINLESS STEEL

Abstract Magnetic hysteresis loop and magnetic and acoustic Barkhausen noise measurements have been made to characterise strain induced a′-martensite in cold worked AISI type 304 stainless steel. Both magnetic hysteresis and MBN measurements are found to be more sensitive in detection of a′-martensite as compared to equivalent δ-ferrite and X-ray diffraction measurements. ABN signals could be observed only in the case of 50% cold worked specimens containing high amount of a′-martensite. The extent of the a′-martensite formed could be related to the area and slope of the hysteresis loop and the intensity of the MBN signal. Coercivity increases with increase in the degree of cold work. The ABN signal detected around the field value of 450Acm−1 in the 50% cold worked specimen suggests the operation of single domain rotational processes. Coercivity in cold worked AISI type 304 stainless steel is found to depend on the degree of cold work as against weak or non-dependence on strain found for other steels by Mamayev et al. The present study established the superiority of magnetic methods over other methods in detection of very small amounts of magnetic phase in a paramagnetic phase. It is therefore possible to apply Barkhausen noise measurements for early detection of deformation related damage like fatigue damage taking place at room temperature (300K) and below in components made of AISI 304 stainless steel or any other steel having metastable austenite by detecting the a′-martensite formed during the damage process.