INFLUENCE OF CARBON CONTENT ON MICROSTRUCTURE AND TEMPERING BEHAVIOR OF 2-1 4 CR 1 MO STEEL/

Transmission electron microscopic studies aimed at elucidating the eff ect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0. 06 % and 0.11 %) were cooled in flowing argon gas (AC) from an austeni tization temperature of 1 323 K and tempered at 823, 923 and 1023 K fo r times ranging from 2 to 50 h. The tempering behaviour at these tempe ratures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to pe ak hardness and the saturation level of hardness at long tempering tim es. Based on a detailed study, using analytical electron microscopy, o n the morphology, crystallography and microchemistry of secondary phas es, the factors governing the observed variations in tempering behavio ur are related to the difference in the dissolution rate of bainite, n ucleation of acicular M2C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote sof tening were identified as M7C3, M23C6 and M6C, whereas hardening is ma inly imparted by M2C.