BN type inclusions formed in high Cr ferritic heat resistant steel

Abstract

Scanning electron microscopy observations of fractured steel rod samples were made to clarify the formation and dissolution behaviour of boron nitrides in high Cr ferritic heat resistant steels during heat treatment at high temperature. A large number of coarse size BN type inclusions of 2 to 5 mm were distributed at the bottom of the dimples. They did not dissolve during annealing at 1150uC. However, they had a tendency to begin dissolving and reducing its particle size with time at 1200uC and dissolved completely in a short time at 1250uC. From the chemical analyses of boron and nitrogen in many high Cr heat resistant steels and SEM observations of their fractured samples, critical boron and nitrogen concentrations for the formation of coarse size BN type inclusions of 0 . 001%B and 0 . 015%N were found, and the solubility limit of BN was represented as log [%B]522 . 45log [%N]26 . 81 at 1150uC.

Microstructural Evolution of Boron Nitride Particles in Advanced 9Cr Power Plant Steels

“…al [13] shows that the two results are generally consistent with each other since the predicted B and N composition range which results in BN formation is generally comparable. However, Sakuraya et.…”

“…The estimated area percentage based on image analysis is less than 0.002%, which means over 97% of the BN particles observed in the as-rolled condition were dissolved after the 1448 K (1175°C) / 1 hour heat treatment. The results suggest that austenitising at 1448 K (1175°C) is very effective at dissolving the BN particles formed during the rolling process, which is lower than the required temperature reported by Sakuraya et al [13]. In their work, it was reported that BN started to dissolve at 1473 K (1200°C), and that 1523 K (1250°C) resulted in complete dissolution of BN, however, the N content (480 ppm) in their steel was approximately double that in Steel B (270 ppm).…”

“…However, Sakuraya et. al [13] calculated the BN solubility product in a Fe-Cr-B-N system whilst the thermodynamic calculation approach used in the current work takes all the elements in the 9Cr-3W-3Co-Nb-V steel into account.…”

“…Previous studies carried out by Abe and co-workers [10,13] have shown that BN can be avoided by carefully controlling the B and N levels in the steel making process. However, this approach normally requires the suppression of N to a level of ~100 ppm or lower, which can be difficult for large scale air castings.…”

“…However, this approach normally requires the suppression of N to a level of ~100 ppm or lower, which can be difficult for large scale air castings. Therefore, in the current study, attempts have been made to both prevent the precipitation of BN and also to dissolve the BN present after normalising in two steels which contain higher N than the suggested maximum [13] by altering the heat treatment conditions. Thermodynamic calculations were first employed to assist in the design of the new heat treatment conditions.…”