A modified TRD (thermoreactive deposition and diffusion) has been investigated in a low-pressure fluidized bed furnace in this study. The specimens were positioned above the surface of the alloying powder. The activator was periodically introduced into the alloying powder to provide the working atmosphere for the required process. Since the pressure of the retort was maintained between 6.7 x 10(2) and 5.3 x 10(4) Pa, the consumption of fluidized gas (argon) was reduced significantly. A modified TRD also significantly increased the working space of the treating zone. The results showed that an 8 mum thick chromium carbide layer could be obtained by treating the tool steel AISI D2 at 1030 degreesC for 4 h. Furthermore, the kinetics of reaction K = K-0 exp(-Q/RT) was empirically determined in this study. lt is shown that the activator, NH4Cl, can effectively lower the activation energy Q, as well as enhance the formation rate of the carbide layer. (C) 2004 Elsevier B.V. All rights reserved.