Assessment of Tungsten Content on Tertiary Creep Deformation Behavior of Reduced Activation Ferritic–Martensitic Steel

Abstract

Tertiary creep deformation behavior of reduced activation ferritic–martensitic (RAFM) steels having different tungsten contents has been assessed. Creep tests were carried out at 823 K (550 °C) over a stress range of 180 to 260 MPa on three heats of the RAFM steel (9Cr-W-0.06Ta-0.22V) with tungsten content of 1, 1.4, and 2.0 wt pct. With creep exposure, the steels exhibited minimum in creep rate followed by progressive increase in creep rate until fracture. The minimum creep rate decreased, rupture life increased, and the onset of tertiary stage of creep deformation delayed with the increase in tungsten content. The tertiary creep behavior has been assessed based on the relationship, $\varepsilon = \varepsilon_{\text{o}} + \dot{\varepsilon }_{\text{m}} t + \varepsilon_3{\exp }\left[ {p\left( {t - t_{\text{t}} } \right)} \right]$, considering minimum creep rate ($\dot{\varepsilon }_{\text{m}}$) instead of steady-state creep rate. The increase in tungsten content was found to decrease the rate of acceleration of tertiary parameter ‘p.’ The relationships between (1) tertiary parameter ‘p’ with minimum creep rate and time spent in tertiary creep deformation and (2) the final creep rate $\dot{\varepsilon }_{\text{f}}$ with minimum creep rate revealed that the same first-order reaction rate theory prevailed in the minimum creep rate as well as throughout the tertiary creep deformation behavior of the steel. A master tertiary creep curve of the steels has been developed. Scanning electron microscopic investigation revealed enhanced coarsening resistance of carbides in the steel on creep exposure with increase in tungsten content. The decrease in tertiary parameter ‘p’ with tungsten content with the consequent decrease in minimum creep rate and increase in rupture life has been attributed to the enhanced microstructural stability of the steel.