Understanding the Role of Molybdenum on the Precipitation in PH Stainless Steels

Doctoral thesis, 2007

The main part of the present work concerns studies of the influence of molybdenum on the microstructure of martensitic PH stainless steels, during ageing at 475°C. A short investigation of the silicon impact has also been conducted. The microstructural investigations have been performed using atom probe technique and energy filtering transmission electron microscopy. Hardness measurements were also carried out in order to correlate the microstructure of the materials with their mechanical properties. Nanoflex® has served as a reference material and three additional Nanoflex-based test alloys with different molybdenum (Mo-poor and Mo-free) and silicon (Si-rich) content has been produced and investigated. The precipitation process of Nanoflex during ageing was investigated in details. It was proved that clustering of solute elements take place already after 5 min of ageing and gives large contribution to the material hardness. Later on, it was discovered that there is a continuous nucleation of six different phases in this material (one Cu-rich, two Ni-rich, one Cr- rich and two Mo-rich). These phases nucleate in contact with each other and form multiphase particles in the matrix. This results in increased precipitate volume fraction and decreased interparticle distance, explaining the continuous hardness increase for up to 100 h. Furthermore, there is also segregation of molybdenum, silicon and phosphorus to the lath boundaries. This film hinders the lath boundary precipitates from growing during ageing, but it also has an embrittling effect. Present work shows that a high nominal molybdenum content in the material promotes the nucleation of clusters in the early stages of ageing. Silicon, on the other hand, has no effect on the nucleation of clusters. During further ageing, molybdenum and silicon slows down the growth and coarsening of existing precipitates and promotes the formation of new phases. This results in an early softening of the Mo-poor (40 h) and Mo-free materials (4 h), while the Si-rich material increases in hardness for up to 100 h of ageing. Furthermore, the segregation of molybdenum, silicon and phosphorus to the lath boundaries decreases as the molybdenum content decreases in the materials. The lath boundary precipitates therefore grows faster, which also contributes to the early softening of these materials. In the Si-rich material, both molybdenum and silicon precipitate primarily in the matrix and the lath boundaries are therefore weakly enriched in these elements.