Boron effect on the microstructure of 9% Cr ferritic–martensitic steels

Highlights: • Detailed TEM characterization of BN, M₂₃C₆, VN and TaC precipitates in B-alloyed EUROFER97. • Determination of B content influence on density and composition of M₂₃C₆ and MX precipitates and herewith on microstructure. • α-Al₂O₃–BN–TaC–VN precipitation sequence of different phases during cooling was proposed. • Decreasing of thermal stability of microstructure with boron content was measured. - Abstract: The microstructure of reduces-activation 9Cr–WTaV steel alloyed with 83 and 1160 wt. ppm ¹⁰B was detailed analysed using transmission electron microscopy. The influence of boron content on the precipitation behaviour of M₂₃C₆ and MX (VN and TaC) phases and, hence, on the formation process of steel's grain and lath structure was studied. VN precipitates, which play an important role in the stabilisation of the lath structure, exhibit most sensitive reaction on presence of boron. Their spatial density significantly reduces in the alloy with 83 ppm boron. In the steel with 1160 wt. ppm boron, no formation of VN was detected, whereas TaC particles precipitate at the lath and grain boundaries. These changes in the structure stabilisation mechanism lead to an increasing lath width and a decreasing thermal stability of laths and grains. Analytical investigations of several BN particles reveal their complex multi-phase structure and allow conclusions to be drawn with respect to their precipitation sequence