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 (a) 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 a started along the a/c phase boundaries where sigma (r) phase and secondary austenite (c 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 r phase was related to the c 2 by the following relation: ð111Þ c k ð001Þ r and ½10 " 1 c k ½110 r :The intergranular and intragranular precipitation of Cr 2 N with trigonal structure were identified, and the orientation relationships (ORs) with a and c matrix could be expressed as 110 ð Þ a k 0001 ð Þ Cr 2 N , " 111 Â Ã a k ½ " 1100 Cr 2 N ; ð111Þ c k ð0001Þ Cr 2 N , and " 110 Â Ã c k " 1100 Â Ã Cr 2 N ; respectively. The precipitation of intermetallic v phase was also observed inside the a matrix, and they obeyed the cube-on-cube OR with the a matrix. Prolonged aging changed both the structure of matrix and the distribution of second phases. The c 2 , formed by decomposition of a, became unstable because of the depletion of mainly N accompanied by the formation of Cr 2 N, 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 r,v, and Cr 2 N) embedded in lath martensite.

Formation of Chromium Nitride and Intragranular Austenite in a Super Duplex Stainless Steel

“…The morphology of the CrN-Cr 2 N clusters was further analyzed with respect to the orientations within individual clusters measured by TKD (see Figure 5). An OR was identified between CrN and Cr 2 N matching the fcc-Cr 2 N OR reported for high-nitrogen austenitic stainless steels, [24] in this case ð111Þ CrN jjð0001Þ Cr 2 N and ½ 110 CrN jj½ 1100 Cr 2 N (see Figure 8(b)). This is reasonable due to the close resemblance of cubic halite-type crystal structure of CrN and the fcc structure of austenite.…”

Effect of Secondary-Phase Precipitation on Mechanical Properties and Corrosion Resistance of 00Cr27Ni7Mo5N Hyper-Duplex Stainless Steel during Solution Treatment

“…It has been shown that the mechanism of σ-phase precipitation is a co-precipitation transition of α phase: α→σ + γ 2 [ 15 , 23 , 24 , 25 ]. The diffusion rate of alloy elements in the α phase is relatively high.…”

Pitting Corrosion Resistance and Repassivation Behavior of C-Bearing Duplex Stainless Steel

“…Alloying N is frequently adopted for DSSs because of its advantages in performance improvement [25,[30][31][32][33][34]. Thus, commercial DSSs including UNS S32101, UNS S32304, UNS S32205, and UNS S32750, contain 0.1-0.3 wt% N. However, alloying C to DSSs has rarely been attempted, although C is found to be beneficial to performances of γ-SSs, similarly to N. Generally, C content was carefully controlled to be less than 0.03 wt% for commercial DSSs, because DSSs can provide more sites to Cr 23 C 6 precipitations such as phase boundaries than the γ-SS [19,35,36].…”