Interphase precipitation of nanometre-sized carbides in Ti bearing steels

Abstract

The objective of this work was to study the interphase precipitation of carbides during isothermal transformation at 750°C after being deformed at 900°C in Ti bearing steels. The effects of isothermal time on the microstructures and precipitation behaviours have been investigated using optical microscopy, transmission electron microscopy and microhardness measurement. It was found that the volume fraction and grain size of ferrite increased with the extension of isothermal time. Based on the EDX (Energy Dispersive X-ray) analysis result, the composition of precipitates was TiC. The size of carbide precipitates taken from specimens isothermal at 750°C for 5, 30 and 60 min was 5·8 ± 2·4, 8·4 ± 3·6 and 14·0 ± 3·8 nm, respectively. The morphology of TiC particles is strongly affected by transformation velocity. At the initial stage of isothermal transformation, at 750°C for 5 min, the transformation velocity was too fast to precipitate TiC particles at γ/α interface, therefore only supersaturated random precipitates were observed on the ferrite matrix. With the increase of isothermal time, the transformation velocity gradually reduced so that the particles precipitated at γ/α interface. Both planar interphase precipitation and curved interphase precipitation were observed in the specimens at 750°C isothermal for 30 and 60 min. The selected area electron diffraction patters show that the interphase precipitation adopts the Baker-Nutting (B-N) orientation with ferrite matrix. The existence of interphase precipitation compensates for the microhardness reduction with increase in isothermal time, just from 173HV to 164HV.