Abstract
Precipitation of chromium nitrides and formation of intragranular austenite were studied in detail for the super duplex stainless steel grade 2507 (UNS S32750). The situation of multipass welding was simulated by heat treatment at 1623 K (1350 °C) and quenching followed by short heat treatments at 1173 K (900 °C). The microstructural evolution was characterized using transmission and scanning electron microscopy, electron backscatter, and transmission Kikuchi diffraction, and it was observed that the interior of the ferrite grains contained chromium nitrides after quenching. The nitrides were predominantly of CrN with a cubic halite-type structure and clusters of CrN-Cr2N where rod-shaped trigonal Cr2N particles had nucleated on plates of CrN. After heat treatment for 10 seconds at 1173 K (900 °C), the nitride morphology was transformed into predominantly rod-shaped Cr2N, and finely dispersed intragranular secondary austenite idiomorphs had formed in the nitride-containing areas within the ferrite grains. After 60 seconds of heat treatment, both the Cr2N nitrides and the secondary austenite were coarsened. Analysis of electron diffraction data revealed an inherited crystallographic relationship between the metastable CrN and the intragranular austenite. The mechanism of chromium nitride formation and its relation to secondary austenite formation in duplex stainless steels are discussed.
Highlights
DUPLEX stainless steels, consisting of ferrite and austenite, possess a unique combination of high corrosion resistance in combination with high mechanical strength
In higher resolution using STEM, a mixed nitride morphology is distinguished (see Figures 4(a) and (b)), and a fine dispersion of nanosized precipitates are seen within the ferrite (see Figure 4(c))
In agreement with the present case, the CrN has been reported to have a disk-like or plate-like morphology, while the Cr2N appears as rod-shaped particles adjacent to the CrN phase.[11,12,13]
Summary
DUPLEX stainless steels, consisting of ferrite and austenite, possess a unique combination of high corrosion resistance in combination with high mechanical strength. METALLURGICAL AND MATERIALS TRANSACTIONS A austenite grains,[15,16] or by formation of intragranular idiomorphs in the ferrite.[15,17] Ramirez et al.[15,18] suggested that the formation of intragranular austenite could be related to chromium nitrides; this mechanism was never confirmed. This is important to understand since the nitrides and intragranular austenite are prone to form in duplex stainless steel welds, and in particular, the intragranular austenite can lead to a decreased corrosion resistance.[19]. The nitride precipitation and secondary austenite formation mechanisms are discussed
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