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 M2B-type borides with the chemical formulas of Fe(1.35–1.36)Cr(0.92–1.05)B0.96 and Fe0.73Cr0.45Mo0.78B, respectively. The cluster-like boride possesses a possible orientation relationship between body-centred orthorhombic Cr-rich M2B and martensite with 〈11¯0〉M2B//〈1 1 0〉α growth direction. After destabilization, M23(C, B)6 secondary borocarbide with a specific orientation relationship precipitates first and thereafter coarsens following the appearance of M7(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 M6(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 (Fe18.26Cr4.74)(B, C)6 and (Fe3.86Cr3.14)(B, C)3 respectively. The high-resolution TEM results indicate that the nucleation and precipitation of M23(C, B)6 occur at the grain/subgrain boundaries as well as partial within martensite, and a subsequent transformation from M23(C, B)6 to M7(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.