THE EFFECT OF TEMPERING TEMPERATURE ON MECHANICAL-PROPERTIES AND MICROSTRUCTURE OF LOW-ALLOY CR AND CRMO STEEL

Two low alloy Cr and CrMo steels with similar levels of carbon, mangan ese and chromium have been studied to determine the effect of temperin g temperature on the mechanical properties and microstructure. The que nching and tempering of steels were carried out using a high-speed dil atometer. The steels were quenched at the average cooling rate of 30K s(-1) in the temperature range from 1123 to 573K by flowing argon and tempered at 673, 823 and 973K. The martensite of steels formed during quenching was of entire lath morphology with 2 vol% retained austenite . It was found that after tempering at 973K the Cr steel contained onl y orthorhombic cementite, while the CrMo steel contained the cementite and hexagonal Mo2C particles in the ferrite matrix. At the same tempe ring conditions, the CrMo steel shows higher 2 strength but lower duct ility as compared to those of Cr steel. It is shown that this differen ce results from finer prior austenite grain, substructure within matri x and precipitate dispersion strengthening, primarily by Mo2C. Transmi ssion electron microscopy (TEM) bright-and dark-field micrographs as w ell 2 as selected area diffraction pattern analysis of orientation rel ationship showed that the cementite precipitated from the ferrite matr ix. Fractography analysis showed that the morphology fracture surface was changed by increasing tempering temperature. Tempering at 973K obt ained ductile fracture by the microvoid coalescence mechanism.