Tempering behavior of a low nitrogen boron-added 9%Cr steel

The effect of tempering temperature on microstructure and mechanical properties was studied in a low nitrogen, high-boron, 9%Cr steel. After normalizing and low-temperature tempering, cementite platelets precipitated within the martensitic matrix. This phase transformation has no distinct effect on mechanical properties. After tempering at 500 degrees C, M23C6 carbides appeared in the form of layers and particles with irregular shapes along the high-angle boundaries. Approximately, 6% of the retained austenite was observed after normalizing, which reduced to 2% after tempering at 550 degrees C. This is accompanied by reduction in toughness from 40 J/cm(2) to 8.5 J/cm(2). Further increase of the tempering temperature led to spheroidization and coagulation of M23C6 particles that is followed by a significant increase in toughness to 250 J/cm(2) at 750 degrees C. Three-phase separations of M(C,N) carbonitrides to particles enriched with V, Nb and Ti were detected after high-temperature tempering. (C) 2016 Elsevier B.V. All rights reserved.