Microstructure and precipitation effects in inconel alloy 600.
The main objectives of the project were to investigate the interactions of carbon, titanium and aluminium contents, grain size and cold working, with precipitation reactions which occur during heat treatments similar to those experienced as a consequence of commercial PWR Steam Generator (SG) tube processing. In order to carry out this investigation commercial material was supplemented by a range of experimental casts. The selected casts allowed the investigation of the following compositional variables: (i) the effect of C in alloys free from Ti or Al, (ii) the effect of C in alloys containing both Ti and Al, and (iii) the effect of T! in alloys containing constant C and Al contents. As a result of the experimental programme considerable progress has been made in clarifying the complex structure-property relationships which occur in Alloy 600. The microstructures observed have been characterised and understood in terms of thermal and mechanical treatments. Studies involving analysis of the mechanical properties, have led to a clear understanding of the effects of grain size, precipitate type and distribution and residual cold work on mechanical behaviour. It has been shown that the activation energy for normal grain growth increases with increasing carbon and titanium contents. Explanations of the mechanical properties have been discussed in terms of grain size, dislocation hardening and solid solution hardening and it has been possible to understand the overall material properties with respect to these. The kinetics of precipitation have been studied in depth, since it is the precipitation of chromium carbide which ultimately results in the principle mode of material degradation, namely, intergranular corrosion. A range of carbide precipitation 'C-curves' have been established and related to thermal and mechanical processing. Accelerated corrosion testing has provided an insight into the relationship between precipitation and structure on the stability of material in potentially corrosive environments.
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