Expressions for Solubility Products of Fe3Nb3C carbide and Fe2Nb Laves Phase in Niobium Alloyed Ferritic Stainless Steels

Precipitation sequence in niobium-alloyed ferritic stainless steel

“…M 6 C type carbide may be an equilibrium phase [16,18]; it was characterized mostly during its coarsening stage. An effective radius r was determined assuming that the area of each particle is equal to that of a circle of radius of r. At least 100 particles were measured in each experiment.…”

“…There are thermodynamic data for Nb(C,N), but unfortunately not for the Laves phase and M 6 C type carbide in multi-component systems of the type considered here. However, solubility products have recently been determined [18] and can be used to estimate G: where the concentrations are in wt% and T is the absolute temperature (table 3). M 6 C includes Cr and Nb (figure 3), so the diffusion of both elements needs to be considered.…”

Effect of Si on Precipitation Behavior of Nb-Laves Phase and Amount of Nb in Solid Solution at Elevated Temperature in High Purity 17%Cr-0.5%Nb Steels

“…In both steels, the amounts of Nb in total precipitates and in the Laves phase of the specimens aged at 1173 K were smaller than those of the specimens aged at 1073 K. The amounts of the carbonitrides precipitates aged at 1173 K is larger compared with those in the specimens aged at 1073 K, in spite of the Si contents. Taking into account the Fe 3 Nb 3 C solubility product reported by Fujita et al, 7) the increase in carbonitrides likely to correspond to the increase in the M 6 C. From the results mentioned above, it was clear that the equilibrium amount of Nb in solid solution for 17Cr-0.5Nb steels decreased by adding Si with the temperatures from 1073 K to 1173 K due to increasing of the Laves phase precipitated.…”

“…7,8) At high temperature, precipitation of the Laves phase competes with that of M 6 C. Concerning this competition, Fujita et al 7) reported that M 6 C was a stable precipitate at 973-1123 K in a 19Cr-0.4Nb-0.014C-0.017N (mass%) steel and at 1023-1173 K in a 19Cr-0.8Nb-0.014C-0.016N (mass%) steel. On the other hand, in a 14Cr-0.3Nb-0.15Ti-0.5Mo-0.011C-0.010N (mass%) steel, the Laves phase was a stable precipitate at 973-1173 K when M 6 C precipitation was suppressed by Ti addition, and the solubility product of the Laves phase was obtained experimentally under certain assumptions.…”

“…On the other hand, in a 14Cr-0.3Nb-0.15Ti-0.5Mo-0.011C-0.010N (mass%) steel, the Laves phase was a stable precipitate at 973-1173 K when M 6 C precipitation was suppressed by Ti addition, and the solubility product of the Laves phase was obtained experimentally under certain assumptions. 7) Because Si is a useful alloying element for oxidation property, 9) a moderate amount of Si, 0.3-1 mass%, is added to commercial Nb-bearing ferritic stainless steels. 10) The effect of Si on the precipitation behavior of the Laves phase in a martenstic 9Cr-2Mo steel was reported by Iseda et al…”