Mats Hättestrand

Coarsening of precipitates in an advanced creep resistant 9% chromium steel: quantitative microscopy and simulations

Abstract Coarsening of MX and M 23 C 6 particles is studied experimentally using energy filtered transmission electron microscopy (EFTEM), and with simulations obtained with the dictra software. dictra takes into account the multicomponent aspects of thermodynamics and diffusion. Both the EFTEM measurements as well as the simulations indicate very low coarsening rates for both kinds of precipitate. The influence of a change in composition on the coarsening rate is investigated by simulations. For MX the coarsening rate is almost independent of the V/Nb ratio, but highly dependent on the N-content. For M 23 C 6 the coarsening rate decreased slightly if Mo was exchanged to some extent with W.

Microanalysis of two creep resistant 9–12% chromium steels

Abstract The ferritic chromium steels P122 and P92 have been investigated using APFIM and TEM. The two steels are similar in composition with the exception of an addition of copper to P122, allowing a higher chromium content. The investigated materials were tempered at 770°C and isothermally aged at 600°C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M 23 C 6 , MX and Laves phase have been analysed with APFIM. Steel P122 contains 0.9% copper and it was found that the matrix concentration of copper drops during ageing from 0.4% to an equilibrium level at 0.1%, which is in good agreement with previous thermodynamical calculations. No copper was found in M 23 C 6 , MX or Laves phase. Copper instead forms a separate phase. This phase was identified by TEM. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in steel P122 compared with steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M 23 C 6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface.

Influence of strain on precipitation reactions during creep of an advanced 9% chromium steel

Abstract Creep tested samples of the 9% chromium steel P92 were investigated using energy filtered transmission electron microscopy (EFTEM) and atom probe field ion microscopy (APFIM). The mean size and the volume fraction of precipitates of type M 23 C 6 , VN and Laves phase as a function of creep time and temperature were determined. A comparison with isothermally aged material showed that coarsening of M 23 C 6 carbides is accelerated by the strain, while the effect of strain on VN precipitates is insignificant. It was also found that the number density of Laves phase particles is higher in creep tested material compared to isothermally aged material. Possible mechanisms to explain these observations are discussed.

Boron distribution in 9–12% chromium steels

Abstract 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.

Microstructural development during ageing of an 11% chromium steel alloyed with copper

Precipitation reactions occurring during ageing at 600 or 650°C of the 11% chromium steel P122 have been investigated. Atom probe field ion microscopy (APFIM) and energy filtered transmission electron microscopy (EFTEM) were used to measure the volume fraction and size of different types of precipitates present in the structure. It was found that the addition of 0.87 wt.% copper to the steel gives rise to formation of copper precipitates during tempering and ageing. The influence of this copper precipitation on the growth and coarsening of other phases present and possible consequences for the creep strength of the material are discussed.

Mo precipitation in a 12Cr–9Ni–4Mo–2Cu maraging steel

Abstract A new maraging steel 1RK91, with the composition 12Cr–9Ni–4Mo–2Cu (wt.%) was investigated after heat treatments at 475°C up to 400 h using atom probe field ion microscopy (APFIM) and tomographic atom probe (TAP). According to the analytical microscopy investigations preceding this study, the high strength of the material was attributed to a new Mo-rich quasicrystalline R′ phase. Therefore the present study concerned evolution of Mo-rich precipitates. The investigation revealed that nucleation of a Mo-rich phase starts between 1 and 2 h of aging at the matrix near the Ni-rich precipitates that form earlier and at the martensite lath boundaries. Compositional changes of these two types of precipitates with aging time were followed. After 400 h of aging only a few small matrix precipitates were observed. Their structure could not be easily determined using electron diffraction because of their small size. The influence of Mo precipitates on the mechanical properties of the material is discussed.

Nanoscale Precipitation in a Maraging Steel Studied by APFIM

This article summarizes findings from our previous investigations and recent studies concerning precipitation in a maraging steel of type 13Cr-9Ni-2Mo-2Cu (at.%) with small additions of Ti (1 at.%) and Al (0.7 at.%). The material was investigated after aging at 475 degrees C up to 400 h using both conventional and three-dimensional atom-probe analyses. The process of phase decomposition in the steel proved to be complicated. It consisted of precipitation of several phases with different chemistry. A Cu-rich phase was first to precipitate and Mo was last in the precipitation sequence. The influence of the complex precipitation path on the material properties is discussed. The investigation clearly demonstrated the usefulness of the applied techniques for investigation of nanoscale precipitation. It is also shown that, complementary methods (such as TEM and EFTEM) giving structural and chemical information on a larger scale must be applied to explain the good properties of the steel after prolonged aging.