Deformation and fracture analysis of nitrided steels by acoustic emission measurement

Abstract

The purpose of this work is to study the processes of static tension deformation and fracture of nitrided sheet samples of 08Kh17T (type AISI 439) and 15Kh25T (type AISI 446) steels using the acoustic emission method. The nitriding was performed with pure nitrogen, at T = 1000-1100°C. Nitriding followed by annealing at 675oC leads to the formation of the nitrogen ferrite structure with the Cr2N type precipitation and decreasing of the grain size across the full cross-section of both steel specimens. In spite of the significant strengthening, the nitrided specimens fail mostly by the ductile or quasi-ductile mechanism. The number of the brittle fracture elements in the fractures of the 15Kh25T steel is higher than in the 08Kh17T steel. Crack nucleation on large particles is a cause of the plastic flow stability loss in 15Kh25T steel in the nitrided state.