Observation of molybdenum-nitrogen clustering in highly alloyed martensite

A new creep-resistant chromium steel, designed for steam turbine applications at up to 600°C, has been studied using atom-probe field-ion microscopy (APFIM). The steel was of the type 10.6Cr0.1C1Mo1WVNbBN with a CN rati 2 : 1. APFIM analyses were made after solution treatment at 1150°C, using two different cooling rates, and after interrupted tempering at 750°C for different times. Before tempering, it was found that up to 60% of the accessible nitrogen was detected as MoN2+ molecular ions, even at a low cooling rate. This is interpreted as a clustering effect between N and Mo in austenite, which is preserved in the untempered martensite, thereby lowering the activity of N. The detection of MoN2+ ions decreased with tempering time: upon tempering, nitride precipitation quickly reduced the number of clusters, which was also reflected in a simultaneous reduction of V in the martensite matrix.