PRECIPITATION PROCESSES IN A SUPERMARTENSITIC 13Cr6Ni2.5MoTi STEEL DURING QUALITY HEAT TREATMENT

The balance between strength and toughness of supermartensitic steels can be controlled by heat treatment parameters. Quality heat treatment usually consists of quenching and single or double tempering above the A(c1) temperature. Such a treatment results in stabilization of reverted austenite in the tempered martensite. Tempering can also be accompanied by intensive precipitation processes, especially in high alloyed grades. A detailed TEM characterisation of precipitation processes in a 13Cr6Ni2.5MoTi supermartensitic steel proved that single tempering at temperatures of 600 degrees C and 690 degrees C was accompanied by precipitation of three minor phases: MX (TiX), M23C6 and Laves phase (Fe2Mo type). Precipitation processes were more intensive at 690 degrees C. Volume fractions of MX and M23C6 phases were low. Laves phase precipitation was intensive and particles of this minor phase grew fast. However, thermodynamic calculations using the Thermocalc software suggest that Laves phase is not an equilibrium phase in the steel under consideration.