No abstract

The addition of small amounts of boron to 9 to 12 pct chromium steels has been found to decrease their creep rate at 823 K to 923 K (550°C to 650°C). In this article, the behavior of boron during austenitizing, tempering, and isothermal heat treatment at 873 K (600°C) is studied using high-resolution microscopy and microanalysis as well as using atomistic modeling. It was found that increasing the boron content from 9 to 40 ppm decreased the coarsening constant of M 23 C 6 by a factor of almost 2. Most of the added boron was incorporated in M 23 C 6 . Atomistic modeling showed that boron diffusion in ferrite is dominated by an interstitial mechanism at 873 K (600°C). However, the generation of vacancies when carbide precipitates dissolve may promote a distribution with substitutional boron atoms. The absence of a fast mechanism for the transition from substitutional to interstitial occupancy will make the slow substitutional boron diffusion in the matrix rate controlling for the coarsening process.

No abstract

The microstructure of a trial martensitic chromium steel containing a high content of boron (250 ppm) was characterized in detail in the as-tempered and aged conditions. This steel has a similar composition and heat treatment as the TAF steel that still is unsurpassed in creep strength among all 9 to 12 pct chromium steels. Characterization was performed by using scanning electron microscopy, energy-filtered transmission electron microscopy, secondary ion mass spectroscopy, and atom probe tomography. Focus was placed on investigating different types of precipitates that play a key role in improving the creep resistance of these steels. The low tempering temperature of 963 K (690°C) is enough for the precipitation of the full volume fraction of both MX and M 23 C 6 . A high boron content, more than 1 at. pct, was found in M 23 C 6 precipitates and they grow slowly during aging. The high boron level in the steel results in metal borides rather than BN with the approximate formula (Mo 0.66 Cr 0.34 ) 2 (Fe 0.75 V 0.25 )B 2 . Two families of MX precipitates were found, one at lath boundaries about 35 nm in size and one dense inside the laths, only 5 to 15 nm in size.