Microstructural evolution of austenite in a microalloyed Fe30% Ni alloy
The study of the physical metallurgy of microalloyed steels has been an important field of research for nearly forty years. During this time the hot working characteristics have been comprehensively investigated, simulated and modelled. Unfortunately, the actual microstructural behaviour during hot working cannot be followed completely due to the unavoidable phase transformation of these steels upon cooling. This transformation prohibits direct study of the deformed austenite, by disordering the dislocation structures developed during hot working. In order to avoid the problem of transformation, a model alloy has been developed. This allows the retention of the austenitic structure to room temperature, while retaining similar thermodynamic and deformational properties to conventional microalloyed steels. The alloy was based on a matrix of iron with 30wt. % nickel, and niobium and carbon additions to the level of 0.1% and 0.09% respectively. The use of such an alloy to simulate the hot working behaviour of traditional microalloyed steels means that the study of softening and precipitation events in the austenite matrix is possible as the phase transformation is avoided. The Fe-30%Ni-Nb alloy has undergone thermomechanical processing. Hot plane strain compression tests have been carried out in order to study the precipitation kinetics. Before testing, the material was solution treated at 1250°C to allow supersaturation of the niobium at lower temperatures. Double-deformation plane strain compression testing has been carried out over a range of temperatures (900-1050°C) at a strain rate of 10s 1 and with delay times between deformations varying from is to 1000s. This testing has allowed the study of both the static precipitation and recrystallisation kinetics from the resulting flow stress behaviour. Precipitation has been evident from the stress-strain curves. Transmission electron microscopy of thin foils of the hot-worked material has been completed to investigate the dislocation structures produced. This shows the presence of a microbanded substructure. The particle populations have been studied using conventional transmission electron microscopy. Direct observation of particles precipitated in the austenite matrix has been achieved by electron spectroscopic imaging studies of the as-deformed material. This is important, as it shows preferential precipitation upon the dislocation structure, and not within the matrix. The overall study shows that the iron-nickel alloy is a good model austenite in many respects. It has similar hot-working characteristics: deformation behaviour, work hardening response, and recrystallisation behaviour. The present work also reports similar NbC precipitation behaviour to that found in conventional C-Mn based microalloyed steels. It appears that the alloy is an excellent model for microalloyed austenite under hot-working conditions, and should prove to be a valuable material for future investigations.