EFFECT OF SEVERE PLASTIC DEFORMATION AT HIGH TEMPERATURE ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF FERRITIC-MARTENSITIC STEEL T91

The microstructure and mechanical properties of ferritic-martensitic steel T91 after severe plastic deformation (SPD) and subsequent annealing (tempering) have been studied. The SPD of steel T91 was carried out by the method of multiple “upsetting-extrusion” at a temperature of 875 °C, subsequent annealing was carried out in the temperature range of 550...650 °C for 1…100 h. It has been found that the multiple “upsetting-extrusion” process allows the formation of an ultra-fine grained structure with an average grain size of 110 nm, which remains stable at the annealing temperatures of 550 and 600 °C for up to 100 h of exposure. After annealing at these temperatures, the microhardness remains at the level 3100 and 2780 MPa, respectively, which is noticeably higher than with standard N&T treatment of this steel (2480 MPa). The refining of grain and MX-type carbide precipitates, due to severe plastic deformation, also leads to an increase in strength characteristics during tensile tests at temperatures of 20 and 600 °C which are significantly higher than after standard N&T treatment.