Microanalysis of a stabilised austenitic stainless steel after long term ageing

A detailed knowledge of microstructural changes that occur in austenitic stainless steels during heat treatment and creep is of great interest, since the ductility and the strength of the material may change drastically after a long time. It is of practical importance to be able to predict when and in what amount different phases precipitate in the material. This study is part of a larger project aiming at constructing thermodynamic models for the development of the microstructure during isothermal heat treatment and creep. This work concerns the microstructural evolution during isothermal heat treatment of an austenitic stainless steel using atom probe field ion microscopy (APFIM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The investigated material was niobium-stabilised steel of type AISI 347 with the major chemical composition of 0.05C-18Cr-l lNi-0.8Nb-1.6Mn (wt%). The steel was solution annealed at for 1100°C 3 min to obtain a homogeneous and well defined starting condition and test samples were heat treated at 500–700°C for up to 70000 h. The solution annealed material and the material isothermally aged at 500, 600 and 700°C have been studied using the APFIM for detailed chemical analysis of the matrix. The results have been compared with SEM and TEM investigations of Nb(C,N) precipitates and σ-phase in the material. Nb-N and Mo-B clustering (which probably affects the strength considerably) was found in material aged at 500°C.