Grain refinement of T91 alloy by equal channel angular pressing

We report on the grain refinement of modified 9Cr–1Mo ferritic-martensitic steel (T91) by equal channel angular pressing, a severe plastic deformation method. Microstructural refinement depends on processing temperature (300–700 °C) and extrusion strain (1.2–2.3). The average grain size has been refined by over an order of magnitude down to 300 nm, accompanied by a hardness increase of up to 70%. The refined microstructure undergoes little grain growth or softening up to 500 °C. At an annealing temperature of 700 °C or higher, significant softening occurs as a result of grain growth.