AGING AND TEMPERING BEHAVIOR OF IRON-NICKEL-CARBON AND IRON-CARBON MARTENSITE

The aging at room temperature (RT) and the tempering behavior in the t emperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pet Ni and 2.35 at, pet C) was investigated prin cipally by using X-ray diffractometry to analyze changes in the crysta lline structure and differential scanning calorimetry to determine hea ts of transformation and activation energies. These techniques also we re used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pet C) in the tem perature range 298 to 773 K. Analysis of the structural changes reveal ed that in both FeNiC and FeC the following processes occurred: (1) fo rmation of carbon enrichments and development of a periodic arrangemen t of planar carbon-rich regions up to 423 K; (2) precipitation of epsi lon/eta transition carbide and transformation of a part of the austeni te into ferrite under simultaneous enrichment with carbon of the remai ning austenite (between 423 and 523 K); (3) decomposition of the retai ned austenite into ferrite and cementite between 523 and 723 K (only p artly for FeNiC); (4) precipitation of cementite between 523 and 723 K ; and (5) for FeNiC, reformation of austenite from ferrite and cementi te above 773 K. A short comparative discussion concerning the first st age of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martens ites is given.