Role of a thermo-mechanical treatment on the subgrain boundary density and on creep behavior of ferritic-martensitic alloy T91

The objective of this thesis is twofold; to determine a thermo-mechanical treatment to enhance the subgrain boundary density in ferritic-martensitic (F-M) alloy T91, and to quantify the increased creep strength due to the higher subgrain boundary density. It was observed that a thermo-mechanical treatment involving 5% compression treatment, followed by an annealing treatment at 1050°C for one h: air cool and a tempering treatment at 800°C for 40 min: air cool, resulted in an increase in the subgrain boundary density by ∼39% without changing any of the other microstructural features.; Creep tests were conducted on both the as-received (AR) and subgrain boundary enhanced (SGBE) conditions of F-M alloy T91 in a stress range of 150-255MPa and over a temperature range of 500-600°C in argon. Creep behavior was analyzed on the basis of the Orowan equation, according to which creep rate is controlled by dislocation velocity and mobile dislocation density. Under all test conditions T91-SGBE exhibited a lower minimum creep rate by a factor of ∼2-3.5 and a longer time to rupture as compared to T91-AR by a factor of ∼1.03-5. Dislocation density evolution was studied after a short-term creep test conducted at 600°C: 155MPa to a strain of 0.017. Dislocation density decreased during creep by ∼20% for both conditions, though the T91-AR took 7 h to reach that strain, while T91-SGBE required 21 h, indicating a higher rate of dislocation annihilation in T91-AR as compared to T91-SGBE. Internal stress calculations performed on both conditions revealed a higher internal stress in the SGBE condition as compared to the AR condition; by ∼10MPa. A higher internal stress in the SGBE condition reduced the effective stress and increased the creep strength. The additional internal stress in the SGBE condition is likely due to the enhanced subgrain boundary density. The temperature increment benefit due to subgrain boundary density enhancement increases exponentially with applied stress and the increase is slightly higher at 600°C as compared to 550°C, which is again slightly higher as compared to 500°C. The activation energy was similar for the two conditions, confirming that the operating creep mechanism was similar for both conditions.