Nanosized MX Precipitates in Ultra-Low-Carbon Ferritic/Martensitic Heat-Resistant Steels

Abstract

Nanosized MX precipitates in ultra-low-carbon ferritic/martensitic heat-resistant 9Cr-W-MoVNbTiN steels were characterized by transmission electron microscope (TEM) using carbon film replicas. The steels were prepared by vacuum induction melting followed by hot forging and rolling into plates. The plates were normalized at 1100°C for 1 hour, cooled in air, and tempered at 700°C for 1 hour. The results show that bimodal nanosized MX precipitates distribute densely and homogeneously in the matrix within martensitic lath after normalizing-and-tempering heat treatment. The larger nanosized MX precipitates with the size of 30 to 50 nm are rich in Nb, while the smaller ones with the size of about 10 nm contain less Nb but more V. Small addition of Ti causes an increase in the number of the larger nanosized MX precipitates. The total number density of the nanosized MX precipitates in the ultra-low-carbon ferritic/ martensitic steels is measured to be over 300/lm 2 , much higher than that in conventional ferritic/martensitic steels. Short-term creep test results show that the ultra-low-carbon ferritic/ martensitic steels with high dense nanosized MX precipitates have much higher creep rupture strength than conventional ASME-P92 steel. The strength degradation of the ultra-low-carbon ferritic/martensitic heat-resistant steels during creep is also discussed in this article.

The Computational Design of W and Co-Containing Creep-Resistant Steels with Barely Coarsening Laves Phase and M23C6 as the Strengthening Precipitates

“…[3], [5,28] and is used in the simulations performed to calculate the coarsening rate for the Laves phase and M 23 C 6 precipitates at a fixed temperature of 923 K (650°C). Prior to the design study optimization it was verified for a number of existing 9 to 12 pct Cr steels that the coarsening rate of Laves phase and M 23 C as predicted by Eq.…”

“…By analyzing the coarsening rate as defined in Eq. [3], it is found that the coarsening rate mainly depends on the diffusion coefficients and concentration gradients of alloying elements at the interface. The diffusion coefficient of W at the set temperature of 923 K (650°C) is very low, close to 10 À20 m 2 /s, while those of other elements (including Mo) are higher than 10 À19 m 2 /s.…”

“…The coarsening rate as calculated by Eq. [3] depends on the sum of the contributions from each element which is determined by the concentration gradient at the interface and diffusion coefficient. The small addition of W introduces a large concentration gradient of W at the interface of Laves phase and matrix.…”

“…Considering the very small diffusion coefficient of W, the coarsening rate is significantly reduced by the addition of W (Eq. [3]). In addition, W atom is bigger than Fe atom and can cause lattice misfit, which can possible decrease the diffusion rate of other Laves phase forming elements.…”

“…They generally have very small initial sizes and are very stable, and hence they are very good precipitationhardening sources even during very long time service. [1][2][3] On the other hand, some precipitates have large initial sizes and are unstable during service at elevated temperatures, and hence are regarded as undesirable components. Laves phase and M 23 C 6 are two such examples.…”