Precipitation Reactions at High Temperatures in 9-12% Chromium Steels
This thesis reports on quantitative investigations of microstructural changes, mainly precipitation reactions, occurring during heat treatment, ageing and creep of 9-12% chromium steels. Several alloys with varying composition have been investigated. The main experimental techniques used have been atom probe field ion microscopy (APFIM) and energy filtered transmission electron microscopy (EFTEM). In some cases experimental results have been compared with thermodynamical simulations performed using the software packages Thermo-Calc and DICTRA.
The 9-12% chromium steels P92 and P122 were investigated after isothermal ageing at 600°C and 650°C for up to 26000 h. APFIM was used to measure the composition of the matrix and precipitates of type M23C6, VN and Laves phase. Size distributions of the different types of precipitates were evaluated using EFTEM and digital image analysis. A Cu-rich phase was found in steel P122. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in P122 than in P92, indicating an accelerating effect of copper particles on the nucleation of Laves phase. In both steels the creep strength decreases at higher temperatures because of faster coarsening of M23C6 carbides and a coarser distribution of Laves phase particles formed during the creep process.
Several boron containing steels were investigated. Enhanced concentrations of boron were found inside M23C6 carbides. Possibly, boron stabilises M23C6 precipitates and reduces the coarsening rate of this phase. In one case, addition of boron was found to give a higher number density of VN nitrides after tempering.
Investigation of creep tested material showed that the coarsening of M23C6 carbides is accelerated by the strain, while no effect on VN nitrides could be detected. Also, it was found that the number density of Laves phase particles was higher in creep tested material compared to isothermally aged material.
Thermodynamical simulations of growth and coarsening of precipitates have been performed and the results were compared with experimental data. The precipitation reactions which have been simulated are (1) growth of M7C3 and M23C6 carbides during tempering at 750°C and (2) coarsening of M23C6 and VN precipitates during ageing at 600°C and 650°C. The results agreed resonably well with experimental observations.
9-12% chromium steel
energy filtered TEM
atom probe field ion microscopy