Correlation between Thermoelectric Power (TEP) and Martensite Start temperature (Ms) measurements of 9Cr-W-V-(Ta) martensitic steels

Low Activation Martensitic (9Cr-W . V,Ta) steels are studied for Fusion reactor applications. This paper deals with the experimental results obtained by using Thermoelectric Power (TEP) technique and Martensite Start (Ms) temperature measurements by dilatometry performed after partial austenite double right arrow ferrite phase transformations of different LAM steels. Six 9-11Cr/0.7-3W/V,Ta base steels have been studied with different nominal concentrations of interstitial (C+N) elements ranging from 0.1 to 0.2 weight percent. TEP and Ms values have been correlated with the residual interstitial (C+N) concentration in solid solution of the non transformed austenite, which generates martensite during cooling to the ambient temperature. These experimental results seem to evidence that there is a decrease of the C and N concentration within the non transformed austenite during both isothermal and anisothermal (i.e. during continuous cooling from the austenite phase field) austenite to ferrite transformations. Some complementary microstructural examinations show that this decrease is associated with a heavy precipitation of chromium rich carbides and/or nitrides during the early stages of the ferrite growth.