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The paper deals with the problematic of weakening of the original austenite grain boundaries which often takes place during the processing of heavy forgings. This phenomenon occurs predominantly in regions of locally coarsened grains during the long dwell times at temperatures of forging or primary heat treatment and can cause development of intercrystalline fracture even before the final heat treatment of forgings. Samples cut from discarded forging made from steel 26NiCrMoV 14-5 were at first laboratory processed by regime of austenitization at 1200 °C/2 hours and oil quenched. Quenched samples were then tempered at the temperatures in range (200 - 700) °C for 1, 10 and 100 hours. Microstructural changes of monitored steel and its selected mechanical properties (impact energy and hardness) in the individual stages of processing were evaluated. Microstructure and fracture surfaces were observed using light microscopy, scanning electron microscopy. Precipitation changes were evaluated using transmission electron microscopy of thin foils and replicas. The transformation and decomposition of initial martensitic structure related with the toughness decrease at temperatures 300 °C and 400 °C was monitored. Significant increase of impact energy was observed at temperatures of 600 °C and 700 °C where minimum amount or no cleavage at all was monitored. Nevertheless, the slightly decreased impact energy does not reach critical value of the intergranular rupture initiation during the forging process.
Keywords: Microstructure, tempering, embrittlement, precipitation© This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.