High Resolution Microanalysis of Creep Resistant 9-12% Chromium Steels

Abstract: This thesis deals with detailed studies of the microstructure of 9-12% chromium steels. Three alloys with improved creep resistance have been studied in different stages of heat treatment and after creep testing. Two of them were manufactured using PM/HIP and were of the type 10.5CrMoWVNbN, the main difference being an addition of boron to one of them. The third alloy, of the type 9CrMoVNb, had also a boron addition. Compared to the other two, this alloy contained less nitrogen and no tungsten had been added.

The microstructure after hardening and tempering consisted of tempered lath martensite with intergranular precipitates, mainly carbides of M₂₃C₆-type, and fine intragranular MX precipitates. It was found that a large fraction of the boron enters M₂₃C₆ already during tempering, and was retained there during aging. Phosphorus was found to segregate to the carbide/matrix interface during creep testing and aging. Phosphorus segregation to lath boundaries during creep testing at 600 °C was also observed.

Nitrogen-molybdenum clusters were found in hardened material. After tempering and creep testing, nitrogen was mainly found in fine MX precipitates. This phase was basically vanadium nitride, with some chromium and niobium. The chromium content of these nitrides was higher than predicted by thermodynamic equilibrium calculations.

Laves phase formed early during creep testing of the PM materials, and phosphorus and silicon were strongly enriched in this intermetallic phase. A low concentration of carbon was also found in Laves phase. It is believed that enrichment of phosphorus and silicon at internal interfaces favours the nucleation and growth of Laves phase.

Atom-probe analyses of the matrix in the tempered and creep tested material indicated the presence of very small precipitates, rich in chromium, vanadium and nitrogen. A latent creep resistance of the material is suggested, giving rise to dynamical nucleation of small precipitates on dislocations during creep.

In addition to these studies, two 12% chromium weld metals with different nitrogen content were analysed. It was found that also here intragranular MX precipitates were based on vanadium nitride.