Improved creep behavior of ferritic-martensitic alloy T91 by subgrain boundary density enhancement

The objective of this Study was to increase the creep strength of the ferritic-martensitic (F-M) alloy T91 by enhancing the subgrain boundary density. A thermomechanical treatment Involving a 5 pet compression treatment, followed by an annealing treatment at 1050 degrees C for I hour, then air cooling, and a tempering treatment at 800 degrees C for 0.66 hours, then another air cooling. resulted in ail increase in the sub-rain boundary density of similar to 39 pet, without altering and of the other microstructural features. Creep tests were conducted on both as-received (AR) and subgrain-boundary-enhanced (SGBE) conditions of the F-M alloy T91, over a temperature range of 500 degrees C to 600 degrees C and in the stress range of 150 to 255 MPa in argon. The T91-AR exhibited a higher creep rate than the T91-SGBE by a factor of similar to 2.0 to 8.0. The ratio of time to rupture for the T91-SGBE compared to the T91-AR varied from 1.0 to 5.0. In general, higher ratios were seen at higher stresses. Creep behavior was analyzed on the basis of the Orowan equation, according to which creep rate is controlled by the mobile dislocation density and dislocation velocity. Internal stress calculations performed on both conditions showed a higher internal stress in the T91-SGBE by similar to 10 MPa. Analysis of the sources of internal stress suggest that the higher value for the SGBE condition is due to subgrain boundary density enhancement. The SGBE condition exhibited a temperature-increment benefit of between 8 degrees C and 26 degrees C, such that the creep strength realized for the T91-SGBE was similar to that realized for the T91-AR, but at a higher temperature. The temperature-increment benefit increased exponentially with applied stress but less so with temperature.