Investigation of Ta-MX/Z-Phase and Laves Phase as Precipitation Hardening Particles in a 12 Pct Cr Heat-Resistant Steel

Abstract

A 12%Cr martensitic/ferritic steel was designed and produced to study Laves and Z-phase as precipitation hardening particles under creep conditions at 650ºC. To ensure the precipitation of Laves after tempering, additions of W and Cu were selected according to thermodynamic calculations. It is known that Z-phase formation does not follows the classical nucleation theory. Indeed, MX particles are transformed into Z-phase by Cr diffusion from the matrix to the precipitate. Therefore, to promote fast Z-phase formation, Ta, Co and N additions were used to produce Ta-MX which will be transformed into Z-phase. As main results, Laves precipitation was successfully achieved after tempering with a particle size of 196nm. Concerning Z-phase, the transformation of Ta-MX into Z-phase after tempering was confirmed by the formation of hybrid nanoparticles of 30nm. Although, W and Ta have a low diffusion in martensitic/ferritic matrix, characterization of the precipitates after isothermal aging revealed that Laves and Z-phase have a fast growth kinetic, reaching 400nm and 143nm respectively at 8760h. As consequence, creep test at 650ºC showed prematurely fails after few thousand hours. Therefore, investigations focused on the growth and coarsening behavior of Laves and Z-phase, seem to be the next researcher field of martensitic/ferritic steels.