Characterization of the inhomogeneous distribution of light elements in ferritic heat-resistant steels by secondary ion mass spectrometry

The addition of a small amount of boron and nitrogen is known to improve the creep strength and life of ferritic heat-resistant steels. These light elements are thought to be distributed in the microstructures in a complicated manner during the heat treatment and creep of these steels. In order to understand the influence of the light elements on the mechanical properties of ferritic heat-resistant steels, a microscopic distribution analysis of the light elements is of importance. In this study, two types of secondary ion mass spectrometry (SIMS) methods were used to investigate the measurement conditions for analyzing secondary ions derived from boron and nitrogen in steels. In dynamic SIMS with a quadrupole analyzer, boron in the samples was effectively detected as BO₂^- ions under irradiation with primary O₂⁺ ions. From the timeof-flight (ToF) SIMS using a focused primary Bi₃²⁺ ion beam coupled with the exposure of the sample to a low partial pressure of oxygen, it was suggested that boron is enriched in M₂₃C₆ carbides in the steels and it may be, more or less, segregated at prior austenite grain boundaries with a moderate amount of nitrogen during normalizing. It was also demonstrated that nitrogen is precipitated as boron nitride in steels containing an excess amount of boron and nitrogen. The characteristics of the distribution of boron and nitrogen in the steels are discussed on the basis of the thermodynamic properties of these elements in the steels.