Abstract
ABSTRACT Effects of microalloying the grade 91 steel with boron along with control on nitrogen content (P91BN) on its microstructure, tensile and creep properties have been investigated and compared with those of the standard grade 91 steel (P91). The P91BN steel possesses finer martensite lath structure and M23C6 precipitates in comparison with the P91 steel. Tensile strengths of the P91BN steel were comparable with the P91 steel up to 823 K. The P91BN steel has possessed higher tensile strengths than the P91 steel at higher temperatures (above 823 K). Microalloying the steel with boron decreases the creep deformation rate with significant increase in creep rupture life. Finer lath width, higher number density of fine M23C6 precipitates in pre-creep condition along with sluggish coarsening of M23C6 precipitates resulting in better stabilisation of martensite structural features in the P91 steel on boron micro-alloying, leads to the significant improvement in high temperature tensile and creep strengths.
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