Effects of boron modifications on phase nucleation and dissolution temperatures and mechanical properties in 9%Cr steels: sensitivity and stability

Effects of boron modifications in 9%Cr steels were investigated by considering design parameters, i.e., sensitivity and stability with the help of analytical equations as reported previously. The interference of phase temperature(s) for a phase on the rest of the phases was discussed. Overlapping of phase temperature for two distinct phases was observed. Invariability, an invariable composition(s), and susceptibility for phase temperatures were determined. Full stability, partial stability, and no stability for phase/property were analytically discussed. In this respect, boron modification only delayed the formation of detrimental phases. A mixture of positive, negative, and transient sensitivity was observed for some phases. However, a few phases showed both negative and transient sensitivities. Nevertheless, only negative sensitivity and only positive sensitivity found in some phases and properties. At last, the self-changing capability of linear and non-linear functions was found to confirm the nature of boron (i.e., minimization or maximization or point of inflexion). A decrease in a phase/property band was observed for designed boundary condition (DBC) as regards studied boundary condition (SBC). For phases and properties, the band of variation decreased by ~52% and ~32%, respectively, for the DBC as regard SBC and verified upper limit of 350 ppm boron modification as optimized structure-property in 9%Cr steels

Influence of boron on microstructure and mechanical properties of Gleeble simulated heat-affected zone in P91 steel

In the present endeavor, a Gleeble thermo-mechanical simulator has been used to simulate subzones of the heat-affected zone (HAZ) of boron-free P91 and boron-modified P91B steels to investigate the influence of boron on microstructure and mechanical properties. The prior austenite grain (PAG) size remained similar to that of parent metal in the fine-grained heat-affected zone (FGHAZ) and inter-critical heat-affected zone (ICHAZ) of P91B steel. The microhardness value of simulated specimens, including parent metals was observed to be similar. But impression creep resistance of boron-containing steel was significantly higher than that of the boron-free steel. The presence of boron decreased precipitates size and increased fraction of low energy Σ3 coincident-site lattice (CSL) boundaries that attributed to improvement of creep resistance. However, there was a slight reduction in solid solution hardening due to increased area fraction of precipitates in parent metal and simulated specimens of boron-containing steel. Hindering of alloy element partitioning during impression creep of P91B-ICHAZ was observed, which was possibly due to the presence of boron in soluble form in the iron matrix and the segregated form on grain and sub-grain boundaries (SGBs). Further, the addition of boron was observed to retard M23C6 precipitate coarsening in the P91B-ICHAZ in comparison to its P91 version