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

Abstract

The formation and crystallography of second phases during isothermal decomposition of ferrite (α) 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 α started along the α/γ phase boundaries where sigma (σ) phase and secondary austenite (γ2) 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 σ phase was related to the γ2 by the following relation: \( (111)_{\gamma } \parallel (001)_{\sigma } \) and \( [10\bar{1}]_{\gamma } \parallel [110]_{\sigma } . \)The intergranular and intragranular precipitation of Cr2N with trigonal structure were identified, and the orientation relationships (ORs) with α and γ matrix could be expressed as \( \left( {110} \right)_{\alpha } \parallel \left( {0001} \right)_{{{\text{Cr}}_{2} {\text{N}}}} \),\( \left[ {\bar{1}11} \right]_{\alpha } \parallel [\bar{1}100]_{{{\text{Cr}}_{2} {\text{N}}}} \,;\,(111)_{\gamma } \parallel (0001)_{{{\text{Cr}}_{2} {\text{N}}}} \), and \( \left[ {\bar{1}10} \right]_{\gamma } \parallel \left[ {\bar{1}100} \right]_{{{\text{Cr}}_{2} {\text{N}}}} , \) respectively. The precipitation of intermetallic χ phase was also observed inside the α matrix, and they obeyed the cube-on-cube OR with the α matrix. Prolonged aging changed both the structure of matrix and the distribution of second phases. The γ2, formed by decomposition of α, 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 α region was composed of three kinds of precipitates (intermetallic σ,χ, and Cr2N) embedded in lath martensite.