Microstructural evolution in creep aged of directionally solidified heat resistant HP-Nb steel alloyed with tungsten and nitrogen

The effect of tungsten and nitrogen on the as cast and the creep aged microstructures of heat resistant HP-Nb steel was studied. The steel was directionally solidified under two cooling rates of 31.2 and 7.6 K sec−1. Creep rupture tests were performed at temperatures of 1150-1255 K on the specimens prepared from the cast ingots in transverse and longitudinal directions. It is shown that the addition of nitrogen significantly increases the eutectic temperature and thus refines the dendrites and alters the morphology of M7C3 eutectic carbide. Also, it is found that due to short time aging, nitrogen addition decreases the M7C3 carbide fragmentation, increases the secondary M23C6 precipitation density, and impedes the formation of Cr-rich M23C6 carbide on the primary Nb(C, N) carbide/matrix interface. These phenomena enhanced the creep resistance of the steel. The apparent activation energy and creep exponent of the steel containing both tungsten and nitrogen were determined as 210±30 kJ mol−1 and 5.2, respectively, indicating that the grain boundary region plays a significant role in controlling the creep deformation of the steel.