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

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-Cr 2 N where rod-shaped trigonal Cr 2 N 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 Cr 2 N, 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 Cr 2 N 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.

Effect of Ferrite Proportion and Precipitates on Dual‐Phase Corrosion of S32750 Super Duplex Stainless Steel with Different Annealing Temperatures

“…On the one hand, the nitrogen content in ferrite increased with the increase in temperature, [7,46] which is consistent with the thermodynamic calculation result shown in Figure 4h, leading to the supersaturation of nitrogen in ferrite grains during the rapid cooling process. [42][43][44][45][46] Under this condition, the finely dispersed chromium nitrides, generally called "nonequilibrium nitrides" or "quenched-in nitrides," [4,7,8] precipitated in ferrite grains. On the other hand, there were no nonequilibrium nitrides at δ/γ boundaries and within small ferrite grains since it was easy for nitrogen to diffuse from small ferrite grains to neighboring austenite, but it is difficult for nitrogen to diffuse from large ferrite grains to neighboring austenite during water cooling.…”

“…In addition, many studies showed that the fast cooling rate at high temperatures led to the formation of nonequilibrium nitrides. [7,8,[42][43][44][45][46] Kim [44] clarified that the precipitation of nonequilibrium nitrides in the ferrite grains increased the sensitization degree, thereby decreased the resistance of intergranular corrosion. Whereas, the work by Bettini [8] showed that the finely dispersed nonequilibrium nitrides were not harmful to the DSSs with high contents of Cr, Mo, and Ni.…”

Characteristics of Flakes Stacked Cr2N with Many Domains in Super Duplex Stainless Steel

“…Intermetallic phases (χ and σ) and Cr 2 N precipitate in grains or at phase boundaries during slow cooling in the critical temperature range of 1000 • C (1273 K) to 700 • C (973 K). Another aspect is that a high density of chromium nitrides is generated in the interior of the ferrite grains as a result of super-saturation with nitrogen by rapid cooling from high temperatures by quenching [21]. As the nitrogen solubility in ferrite is quite low, it leads a super-saturation of nitrogen in ferrite [22].…”

Development of duplex stainless steels compositions

“…To obtain the duplex structure of steel containing nearly equal fractions of austenite and ferrite, a balance of stabilizers of these phase components is necessary; therefore, with other things being equal, the content of nickel in DSS is lower than in conventional corrosion resistant austenitic stainless steels. Higher level of alloying of advanced DSS leads to the precipitations of undesirable secondary phases, in particular the σ-phase [7] and chromium nitrides [8]. Other secondary phases such as chromium carbides [9] and/or carbonitrides of microalloying elements [10] can also affect the properties of these steels, but the negative effect of the σ-phase and chromium nitrides is predominant [7][8].…”

“…Higher level of alloying of advanced DSS leads to the precipitations of undesirable secondary phases, in particular the σ-phase [7] and chromium nitrides [8]. Other secondary phases such as chromium carbides [9] and/or carbonitrides of microalloying elements [10] can also affect the properties of these steels, but the negative effect of the σ-phase and chromium nitrides is predominant [7][8].…”