Precipitation Behavior of TaC in Austenite of Ferritic Heat Resisting Steel with High Chromium Contents

Precipitation behavior of TaC in austenite of 8Cr2WV steel has been studied. Ta contents in the extracted residues were measured chemically for the specimens both usually normalized and step-normalized (directly cooled from a homogenized temperature to a normalizing temperature). In the step-normalized specimen precipitation of TaC is very slow due to lack of precipitation cites. On the contrary, in the usual normalized specimen precipitation of TaC approaches to an equilibrium value within a practical normalizing time, e.g. 0.5 h. The similar phenomenon is observed on NbC in 9CrlMoV steel. The reason for this phenomenon is that a lot of residual dislocations supply the precipitation cites and the induced strain by the inverse transformation assists the precipitation. The average particle size of TaC after the usual normalization increases as (time)1/13 which is much differ from the theoretical prediction of (time)1/5 for the dislocation coarsening. The TaC particles after the usual normalization are spherical or round angular and about 10 nm in diameter. Based on the above investigation solubility product of TaC in austenite of 8Cr2WV steel has been determined.