Isothermal Decomposition of Ferrite in a High-Nitrogen, Nickel-Free Duplex Stainless Steel

The formation and crystallography of second phases during isothermal decomposition of ferrite (alpha) in a high-nitrogen, nickel-free duplex stainless steel was examined by means of transmission electron microscopy (TEM). At an early stage of aging, the decomposition of alpha started along the alpha/gamma phase boundaries where sigma (sigma) phase and secondary austenite (gamma(2)) precipitated in the form of an alternating lamellar structure. The combined analyses based on the simulation of diffraction patterns and stereographic projection have shown that most of the sigma phase was related to the gamma(2) by the following relation: (111)(gamma) parallel to (001)(sigma) and [10 (1) over bar](gamma) parallel to [110](sigma). The intergranular and intragranular precipitation of Cr2N with trigonal structure were identified, and the orientation relationships (ORs) with alpha and gamma matrix could be expressed as (110)(alpha)parallel to (0001)(Cr2N), [(1) over bar 11](alpha)parallel to[(1) over bar 100](Cr2N); (111)(gamma) parallel to (0001)(Cr2N), and [(1) over bar 10](gamma)parallel to[(1) over bar 100](Cr2N), respectively. The precipitation of intermetallic chi phase was also observed inside the alpha matrix, and they obeyed the cube-on-cube OR with the alpha matrix. Prolonged aging changed both the structure of matrix and the distribution of second phases. The gamma(2), formed by decomposition of alpha, became unstable because of the depletion of mainly N accompanied by the formation of Cr2N, and it transformed into martensite after subsequent cooling. As a result, the microstructure of the decomposed a region was composed of three kinds of precipitates (intermetallic sigma,chi, and Cr2N) embedded in lath martensite.