Several 10ppm of boron is added in high Cr ferritic heat resistant steels such as P92 and P122, which have been developed in recent years, for the purpose to improve creep characteristics. Boron has been said to increase the creep strength of high Cr ferritic heat resistant steel, but the role and behavior of boron have not entirely been clear. This study was carried out for the purpose to clear the formation and re-dissolution behavior of boron nitrides in boron bearing high Cr fenitic heat resistant steels during heat treatment at high temperatures Rod samples of 3.6mm diameter with a 2.0mm knife-edge were bent and fractured at room temperature. Inclusions at the bottom of dimples in ductile fracture surface were observed by SEM with EDS. A large number of coarse size BN type inclusions grown to 2 to 5μm and alumina type inclusions under 2μm were distributed at the bottom of dimples. Both BN type and alumina type inclusions were identified by EDS analysis. In order to search the re-dissolution behavior of coarse size BN inclusions, several samples were water-quenched after annealing at various temperatures. During annealing at 1150 °C, coarse size BN type inclusions were not disappeared. At 1200 °C, coarse size BN type inclusions had a tendency to disappear with time. All coarse size BN inclusions were completely disappeared in a short time at 1250 °C. Next, the samples were cooled from 1250 °C at a cooling rate of 100 °C /h, and water-quenched every fifty degrees from 1250 °C to room temperature. A large number of coarse size BN type inclusions were observed in the samples quenched from 1150 °C or below. In these results, it was considered that the coarse size BN inclusions observed were formed at cooling process of the ingot after casting, or at forging process below 1150°C. The critical concentrations of boron and nitrogen for the formation of coarse size BN type inclusions were also inspected by chemical analyses and SEM observations. In the concentration range under 0.001%B and under 0.015%N, BN type inclusions could not be formed, and the equation related [%B] and [%N] is represented as log[%B]= -2.451og[%N] -6.81.
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