Long-term stability of TaC particles during tempering of 8%Cr–2%W steel

Precipitation behavior of 8%Cr–2%W martensitic steel during tempering at 740 °C was studied. Both M23C6 and TaC were observed at an early stage of tempering by TEM and XRD. M23C6 is coarse, about several 10’s nm. On the other hand, TaC is very fine, about 10 nm, when tempering up to 10 h. Particle size of TaC is gradually increases, however the size remains below 30 nm even after tempering for 1000 h. This suggests that TaC particles grow very slowly under the actual service conditions. After the saturation of precipitation of TaC, the time exponent for coarsening of TaC particles is about 0.1, the value of which is much smaller than the theoretical value, 1/3 or 1/5. This is re-confirmed by the X-ray integral breadth for (1 1 1) TaC peak. The reason for the small time exponent is discussed in terms of the interaction between TaC and dislocations during tempering.