Precipitation in Advanced Stainless Steels

Advanced stainless steels often suffer from complex precipitation behaviour due to the high levels of alloying elements needed to obtain their attractive properties. This demands a correct handling of the material to avoid formation of harmful phases during production, manufacturing and service. An understanding of the precipitation behaviour is thus essential in order to maintain the properties. This thesis has focused on two critical aspects for the duplex stainless steels, namely the precipitation of chromium nitrides and the phase separation in ferrite at low temperatures. Phase separation can compromise the structural stability of duplex stainless steels if they are used in certain temperature ranges for a prolonged period of time. The degradation of mechanical properties can occur slowly and its relation to the evolving nanostructure is of interest for lifetime predictions. The nanostructure evolution and corresponding mechanical property changes during aging has therefore been studied for the duplex stainless steel 2507 by advanced characterisation, property testing and fractography. The loss of toughness and transition in fracture behaviour can be related to a reduced dislocation mobility in the decomposed ferrite as evident by the increased tendency for cleavage failure and deformation twinning. A simple relation between the chromium fluctuations, hardness and impact toughness could be identified. Nitrides are formed isothermally if the material is held at a critical temperature for sufficient time, but the duplex stainless steels are also sensitive to non-equilibrium precipitation during cooling from high temperatures, for example, as in welding. A series of microscopy techniques and heat treatments in a dilatometer have been applied to study the precipitation behaviour of 2507 and its dependence on cooling rate and microstructure. The CrN, in addition to the expected Cr2N, was found to form upon rapid cooling. Electron diffraction studies revealed that CrN is most likely a forerunner to Cr2N which nucleates at the former. The Cr2N in turn acts as nucleation sites for intragranular secondary austenite if a second short-term heat treatment is made, a situation that might occur during multipass welding. The non-equilibrium nitride formation is closely related to the austenite stability and transformation during cooling, and the ferrite grain size. The high-temperature phase equilibria were therefore studied for four duplex stainless steels using in situ neutron scattering. The existence of a fully ferritic state was confirmed and the austenite stability measured. The data can be used for improving the precision of CALPHAD databases for the duplex stainless steels.QC 20180509