Microstructure and crystallography of borides and secondary precipitation in 18 wt.% Cr-4 wt.% Ni-1 wt.% Mo-3.5 wt.% B-0.27 wt.% C steel

The microstructure and crystallography of eutectic borides and secondary precipitations in 18 wt.% Cr-4 wt.% Ni-1 wt.% Mo-3.5 wt.% B-0.27 wt.% C steel have been investigated extensively. The results show that the as-cast microstructure of Cr-Ni-Mo-containing Fe-B steel is composed of a dendritic martensite with large interdendritic eutectic borides. Transmission electron microscopy (TEM) results confirm that the borides are indexed to Cr- and Mo-rich M ₂B-type borides with the chemical formulas of Fe _(1.35-1.36)Cr _(0.92-1.05)B _0.96 and Fe _0.73Cr _0.45Mo _0.78B, respectively. The cluster-like boride possesses a possible orientation relationship between body-centred orthorhombic Cr-rich M ₂B and martensite with 〈11̄0〉 _M2B//〈1 1 0〉 _α growth direction. After destabilization, M ₂₃(C, B) ₆ secondary borocarbide with a specific orientation relationship precipitates first and thereafter coarsens following the appearance of M ₇(C, B) ₃ precipitation with the increasing destabilization temperature at the same soaking time, thus leading to a large decrease of Cr content in the martensite. However, no M ₆(C, B) secondary borocarbide is found in as-destabilized Fe-B steel. Destabilization treatment has no effect on the morphology of eutectic borides. The secondary borocarbides have the stoichiometry of (Fe _18.26Cr _4.74)(B, C) ₆ and (Fe _3.86Cr _3.14)(B, C) ₃ respectively. The high-resolution TEM results indicate that the nucleation and precipitation of M ₂₃(C, B) ₆ occur at the grain/subgrain boundaries as well as partial within martensite, and a subsequent transformation from M ₂₃(C, B) ₆ to M ₇(C, B) ₃ takes place in situ, which is probably owing to the crystalline defects of dislocations and stacking faults in the structures caused by lattice distortion.