Abnormal magnetic behaviour of powder metallurgy austenitic stainless steels sintered in nitrogen

The magnetic response of AISI 304L and AISI 316L obtained through powder metallurgy and sintered in nitrogen were studied. AISI 304L sintered in nitrogen showed a ferromagnetic behaviour in as-sintered state while AISI 316L was paramagnetic. After solution annealing both were paramagnetic. Magnetic behaviour was analysed by using a vibrating sample magnetometer, a magnetic ferritscope and magnetic etching. A microstructural characterization was performed by means of optical metallography, X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive analysis of X-rays (EDS). Some samples when needed were submitted to aged heat treatments at 675 and 875 oC for 90 min, 4, 6, 8 or 48 h. The main microstructural feature found was the presence of a lamellar constituent formed by nitride precipitates and an interlamellar matrix of austenite and/or ferrite. The abnormal magnetic response was explained based on this.