Effect of Laves Phase on Ductile-Brittle Transition of 12 Pct Cr Steel

The impact toughness of a martensitic 12 Cr-0.6 Mo-2.2 W-4 Co-0.8 Cu-VNb (in wt pct) steel subjected to tempering at 770 degrees C for 3 hours and aging at 650 degrees C for 643 hours was examined in the temperature range of -20 to 250 degrees C. The steel in the tempered condition exhibits a ductile-brittle transition temperature (DBTT) of 41 degrees C and a Charpy V-notch impact energy of 100 J/cm(2) at a temperature of 20 degrees C. The tempered steel becomes completely brittle at 0 degrees C when the absorbed energy falls to 8 J/cm(2). The aging leads to the precipitation of a Laves phase at boundaries of the tempered martensite lath structure (TMLS). This deteriorates the impact toughness, the DBTT increases to 20 J/cm(2), the impact toughness at a temperature of 20 degrees C decreases to 20 J/cm(2). Under impact loads, the Laves phase particles serve as additional nucleation sites for micro-crack/void that facilitates unstable crack propagation. As a result, the appearance of cracks with critical dimensions takes place after stable crack propagation over a very short distance, and embrittlement takes place at relatively high temperatures.