Anna Výrostková

Carbide reactions and phase equilibria in low alloy Cr–Mo–V steels tempered at 773–993 K. Part I: Experimental measurements

Abstract Carbide reactions and phase equilibria of 2.4Cr–0.7Mo, 2.6Cr–0.7Mo–0.14V, 2.6Cr–0.7Mo–0.3V, and 2.6Cr–1.0Mo–0.3V (wt%) steels tempered at 773–933 K for a maximum of 1000 h were investigated. Carbide particles extracted in carbon replica were analysed by means of electron diffraction and energy-dispersive X-ray spectroscopy. Thermodynamic calculations were used to complement and make accurate the experimental results. The time–temperature diagrams of carbide phase stability were constructed for all investigated steels. Here the M 3 C, M 2 C, M 7 C 3 and MC carbides in various combinations are lined up in sequences, the carbides M 23 C 6 and M 6 C form separate areas. The shape of the M 6 C area is similar to that of the Laves phase in medium alloy steels. In the investigated steels three types of phase equilibria were found: ferrite+M 7 C 3 , ferrite+M 7 C 3 +MC and ferrite+M 7 C 3 +M 6 C+MC.

Carbide reactions and phase equilibria in low-alloy Cr–Mo–V steels tempered at 773–993 K. Part II: Theoretical calculations

The coexistence of carbide phases in thermodynamic equilibrium was studied by means of theoretical calculations and compared with experimental results. The study was carried out for systems corresponding to experimentally used low alloy Cr steel (approx. 2.5 wt%) with different Mo (0.70--1.0 wt%) and V (0.02--0.32 wt%) contents. The carbon content in experimental materials was about 0.1 wt%. Theoretical calculations were realized by the PD-pp software and the modeling of thermodynamic equilibria in the Fe-Cr-Mo-V-C system for the concentration corresponding to the experimental materials based on the Hillert-Staffansson sublattice model was carried out. The equilibrium phase coexistence of carbidic phases with b.c.c. ferrite was calculated in dependence on temperature. It was found that M{sub 23}C{sub 6} carbide is thermodynamically stable for low temperatures (up to 800--850 K) and then gradually replaced by M{sub 7}C{sub 3} for higher temperatures. M{sub 6}C is stable up to 890--950 K and dissolves completely. MC carbide was found to be thermodynamically stable for all temperatures for systems with vanadium content above 0.1 wt%. Good qualitative agreement between the theoretical and experimental results was reached.

Influence of thermal-deformation history on evolution of secondary phases in P91 steel

Abstract The influence of thermal-deformation history on the evolution of secondary phases in P91 steel annealed at 853, 893, and 923 K was studied by means of transmission electron microscopy. Calculations of phase equilibria for a Fe–Cr–V–Mo–Ni–Nb–Mn–N–C system corresponding to the investigated steel were also carried out. The calculations were performed using the thermodynamic database program Thermo - Calc. M 23 C 6 and MX phases were identified experimentally in all conditions investigated. Laves phase was only found in conditions annealed for longer times at 853 and 893 K and in the non-deformed condition annealed at 923 K per 1000 h. The thermodynamic calculations revealed M 23 C 6 and MX as equilibrium phases at 853, 893, and 923 K. The discrepancy between experimental and calculated results concerning the Laves phase indicates that either the investigated steel has not achieved equilibrium after aging for 5000 h at 853 and 893 K or the thermodynamic model for this phase should be modified. Also the temperature dependencies of the Cr/Fe ratio for M 23 C 6 carbide showed opposite tendencies for originally deformed and non-deformed conditions.

Effect of tempering on development of carbide particles in 2.7Cr-0.6Mo-0.3V steel

To study the influence of tempering conditions (823–1058 K, 9–3600 ks) on the phase transformations and changes of morphology, size and chemical composition of carbide particles in 2.7Cr-0.6Mo-0.3V steel, the methods of, electron diffraction and EDXS/STEM have been used. Three carbide types: M3C, M7C3 and MC have been identified altogether, the last two of which are equilibrium ones under given conditions. Diagrams describing changes in chemical composition of carbide particles and the average size during tempering have been constructed. In the development of carbide chemical composition three stages have been recorded. The carbide particles grow continuously with the increasing of time and/or temperature of tempering.

Influence of long-term isothermal exposures upon M7C3 carbide changes in 2. 7Cr-0. 6Mo-0. 3V steel

This paper reports that although carbide particles represent only a small fraction in the microstructure they largely determine the mechanical properties of low alloyed steels and reflect the microscopic changes taking place in their surroundings. From the view points of mechanical properties and fracture behavior, the morphology, size, and location of carbide particles in the microstructure are of great importance. It is generally known that dispersive particles are effective obstacles against plastic flow in the matrix and they are responsible for the strength and the brittleness increase. One- and two-dimensional particles act as stress concentrators and hence the sites with preferred crack nucleation. The shape, size, and distribution of carbide particles and also their structural stability depend on various factors, the most important of which are: chemical composition of the steel; thermal and deformational history of the steel; and carbide type (given by crystallography, stoichiometry, and chemical composition).