Boron distribution in 9–12% chromium steels

Martensitic 9–12% chromium steels are used in steam turbine power plants for large components such as turbines and pipes. The key property of these materials is long-term creep resistance at elevated temperatures (up to 600–650°C). Extensive alloy development has taken place worldwide over the last 20 years to improve the high-temperature properties of the 9–12% chromium steels. An alloying strategy that has proven to be very effective for increased creep strength is the addition of small amounts of boron. The mechanism for improved high temperature stability in this case is not yet fully understood. In this work a number of recently developed 9–12% chromium steels have been investigated with the atom probe technique to clarify the distribution of boron in the microstructure. It was found that only very small amounts of boron remain dissolved in the matrix after completed heat treatment (solution treatment and tempering). Instead almost all of the boron ends up in M23C6 carbides, possibly decreasing the coarsening rate of these precipitates.