The tempering of iron-nitrogen martensite; Dilatometric and calorimetric analysis
Abstract
The aging behavior of iron-nitrogen martensitic alloys (0.8 to 7.0 at. pct N) between -190 °C and 450 °C was investigated by quantitative analysis of the corresponding changes in volume and enthalpy. Martensitic specimens were prepared by gaseous nitriding of pure iron in a mixture of NH3 and H2 and subsequent quenching in brine and liquid nitrogen. Both X-ray diffraction analysis and metallography (light microscopical analysis and microhardness measurement) were used for interpretation of the structural changes. Analysis of the transformation kinetics was achieved by employing a range of heating rates. At least five different stages of structural change could be distinguished, which were quantitatively analyzed in terms of their effects on volume and enthalpy: (1) transformation of retained austenite into martensite (between-160 °C and -40 °C); (2) segregation and “ordering” of nitrogen atoms (below 100 °C); (3) precipitation of incoherent α' nitride (between 100 °C and 220 °C); (4) conversion of α' nitride into γ' nitride (between 220 °C and 290 °C); and (5) decomposition of retained austenite (between 240 °C and 350 °C). Differences with the tempering behavior of analogous iron-carbon martensites were discussed.
- Publication:
-
Metallurgical Transactions A
- Pub Date:
- January 1990
- DOI:
- 10.1007/BF02656420
- Bibcode:
- 1990MTA....21...13C