Microanalysis of 9-12% chromium steels P92 and P122

The 9-12% chromium steels P122 and P92 have been investigated using atom probe field ion microscopy (APFIM) and transmission electron microscopy (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 degrees C and isothermally aged at 600 degrees C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M23C6, 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 about 0.1%. No copper was found in M23C6, MX or Laves phase. Instead copper forms a separate phase, that 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 to steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M23C6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface. The role of different alloying elements in the two steels is discussed.