Effect of Nb Addition on Cu Precipitation in Ferritic Stainless Steel

Abstract

Formations of Cu-rich precipitate in 18%Cr-1.5%Cu stainless steels without and with 0.5%Nb have been examined mainly using transmission electron microscopy. The specimens solution-treated at 1250°C were isothermally transformed at various temperatures between 500 and 850°C. Fine spherical Cu-rich solute zones were nucleated in the steels in the beginning of aging. These Cu-rich zones exhibited bcc structure and then transformed into 9R structure with twins during aging. The spherical 9R-Cu particles changed into rod-shaped fcc-Cu particles after prolonged aging. Laves phase of Fe2Nb type started to form in the Nb-added steel almost after finishing nucleation and growth of Cu particles. Addition of Nb in the steel delayed the nucleation of the Cu-rich zones due to slow diffusion of Nb atoms in ferrite.

Effect of Nb Addition on the Growth and Coarsening of Cu-particles in Ferritic Stainless Steel

“…These spherical zones were also detected in the specimen aged for 30 s, but they were not detected in the specimen aged for 10 s. 26) The spherical zones were Cu-rich solute clusters described in a previous paper. 26) Growth of the spherical zones is observed in Figs. 1(b)-1(f).…”

“…These results indicate that the growth of Cu particles was suppressed by the addition of Nb. As the authors previously pointed out, 26) the inclusion of Nb atoms in Cu particles is not preferable from a thermodynamic viewpoint. As a result, outward diffusion of Nb atoms from Cu particles is expected to occur during the growth of Cu particles.…”

“…Our previous study showed that although precipitation of the Laves phase in 18%Cr-0.48%Nb stainless steel during aging at 700°C was observed, age-hardening by the precipitation of the Laves phase was minimal. 26) Since precipitation of the Laves phase consumes solute Nb atoms, an increase in hardness by the precipitation of the Laves phase should accompany a decrease in hardness owing to the reduction of solution hardening by solute Nb atoms. For the 18%Cr-0.48%Nb steel, the effect of precipitation hardening was cancelled out by the solute depletion in the matrix.…”

“…These spherical zones with striped contrasts were Cu-rich particles with a 9R structure containing twins. 26) Although Cu-rich solute zones with a bcc structure should be distinguished from Cu particles with a 9R structure from a crystallographic viewpoint, they are denoted together as "Cu particles" hereafter.…”

Small-angle X-ray Scattering Studies on Aging Precipitation of High-strength Soft-magnetic Stainless Steels

“…The shape of the Fe 2 Mo precipitates with the Laves-phase structure is consistent with the primarily needle-or a plate-shaped form of Fe 2 Mo precipitates in steels in previous studies. [15][16][17] Since the inter-precipitate distance of Fe 2 Mo was approximately a few hundred nanometers, these Fe 2 Mo precipitates were less effective in enhancing the Vickers hardness.…”