On the non-equilibrium segregation of Boron in 9-12%Cr steels

The addition of only 20-150 ppm of boron to martensitic 9-12%Cr steels has a strong enhancing effect on their creep strength, and boron-added martensitic creep strength enhanced steels have been introduced in power plant constructions. Boron is found to redistribute in the martensitic steels during cooling after normalizing in a non-equilibrium segregation process. In this work we attempt to quantify boron segregation during solution treatment in a martensitic 9%Cr steel containing 108 ppm of boron by atom probe tomography and transmission electron microscopy. The investigated material was a solution treated at 1100C for 1 h and cooled in air. Plate-like boride of Cr2B was found on a prior austenite grain boundary (PAGB). The distribution of B, Cr, Fe, Co, Mo, C and other elements in boride and matrix was quantified. Boron is considered to segregate onto PAGBs by non-equilibrium mechanism

Fine (Cr,Fe)₂B borides on grain boundaries in a 10Cr–0.01B martensitic steel

A 10Cr creep resistant martensitic steel with 108 ppm B was normalized at 1100 \ub0C for 1 h and air cooled. Fine (Cr,Fe)2B borides were observed on the majority of prior austenite grain boundaries, all of which were high angle boundaries, as revealed by EBSD-based reconstruction of parent austenite grains. Some high angle boundaries including twin boundaries were boride-free. Segregation of boron to austenite grain boundaries during slow cooling from 1100 \ub0C led to boride nucleation and growth. Their size increased with decreasing cooling rate. Borides were verified by atom probe tomography, auger spectroscopy, transmission and scanning electron microscopy

Analyzing Boron in 9-12% Chromium Steels Using Atom Probe Tomography

Microscopy Society of America 2019. Small additions of boron can remarkably improve the long-term creep resistance of 9-12% Cr steels. The improvement has been attributed to boron segregation to grain boundaries during quenching, and subsequent boron incorporation into certain families of precipitates during tempering. However, the detailed mechanisms are not yet fully understood. Atom probe tomography (APT) is an excellent technique for gaining insights into boron distribution, however, in order to acquire accurate analysis of boron in 9-12% Cr steels using APT, there are several key challenges. In order to better understand and address these challenges, we developed a novel method for site-specific APT specimen preparation, which enables convenient preparation of specimens containing specifically selected grain boundaries positioned approximately perpendicular to the axis of the APT tip. Additionally, when analyzing boron at boundaries and in carbides (as diluted solute) and borides, a widening of the profile of boron distribution compared to other elements was repeatedly observed. This phenomenon is particularly analyzed and discussed in light of the evaporation field of different elements. Finally, the possible effects of detector dead-time on quantitative analysis of boron in metal borides are discussed. A simple method using 10B correction was used to obtain good quantification