Effect of Tempering on Microstructure and Mechanical Properties of Boron Containing 10%Cr Steel

The effect of heat treatment on the microstructure and the mechanical properties of a 10%Cr steel with 0.008% boron was examined. The microstructure and the mechanical properties of this steel subjected to the normalizing were studied after tempering under different conditions. The layers of retained austenite are located along the lath boundaries. The formation of M-23(BC)(6) phase having film-like shape takes place on interface boundaries of retained austenite/martensite during tempering at 525 degrees C. As a result, the steel exhibits brittle fracture with a low value of Charpy V-notch impact toughness of 6 J/cm(2). Particles of the M-23(B.C)(6) phase are highly resistant against the spheroidizing. The tempering at 770 degrees C only leads to the coagulation of these particles; when the fraction of M-23(B.C)(6) phase significantly decreases while the fraction of M23C6 carbides increases. The tempered at 770 degrees C steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm(2). The effect of boron additives on the phase composition and the brittleness of high-chromium steels is discussed.