Effect of Tungsten on Mechanical Properties of Reduced Activation Ferritic-Martensitic Steel Subjected to Intercritical Heat Treatment

Fusion welded joint of RAFM steel as in other ferritic steels possesses lower creep rupture strength than the base st el. The failure in the joint is associated with the soft intercritical region of heat affected zone, which is subjected to peak temperatures in between Ac1 and Ac3 during weld thermal cycle. In this investigation effort has been initiated to understand the effect of tungsten in softening the 9Cr-W RAFM steels on intercritical temperature exposure. Metallographic investigations were carried out on the heat-treated steels by optical and transmission electron microscopes. The steels at all heat treated conditions had tempered martensitic structure. Refinement of prior austenitic grain of the steels was noticed on soaking in the intercritical temperature range. The grain size of the steels increased with soaking at temperatures well above the Ac3 transformation temperature. TEM investigation revealed that the martensitic laths were decorated with M23C6 type of carbides and the presence of MX type of (Ta,V)C carbides inside the laths. Hardness, tensile and creep tests were carried on the steels to elucidate the effects of soaking at temperatures in and around Ac1 and Ac3 transformation temperatures. All the steels suffered reduction in mechanical strength after soaking at temperatures in the intercritical temperature range. Different softening tendency of the RAFM steel having different tungsten content on intercritical annealing has been explained based on the microstructural investigation.