Viera Homolová

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.

Experimental and thermodynamic studies of phase transformations in Cr–V low alloy steels

Abstract Phase transformations in three Cr–V low alloy steels at 773 and 853 K were studied experimentally in a transmission electron microscope (TEM). The theoretical calculations were carried out for ternary Fe–Cr–C and quaternary Fe–Cr–V–C systems corresponding to the investigated steels. The calculations of phase equilibria were performed using the thermodynamic database program thermo-calc . M3C in 1Cr and 1Cr–0.3V steels, M7C3 in 1Cr–0.3V steel, and MC in 1Cr–0.3V and 1Cr–0.5V steels were identified. The formation M3C precedes the formation of M7C3. The M7C3 carbide in all investigated steels and the MC carbide in steels containing vanadium were determined by thermo-calc as equilibrium phases. The comparison of experimental and computational results indicates that the investigated steels have not achieved equilibrium even after aging for 5000 h at 773 and 853 K.

Ageing Effects on Microstructure, Mechanical Properties, and Fracture Behaviour of 9Cr-1.5Mo-1Co-VNbBN Martensitic Steel Welded Joint for High Temperature Application

This paper deals with long-term ageing effects of 9Cr-1.5Mo-1Co-VNbBN (CB2) steel weldment on its impact toughness, creep rupture behaviour, and hardness in relation to microstructure and fracture characteristics. The weldment was studied in PWHT state and after isothermal expositions at 625°C for 10000 and 30000 hours. Microstructure evolution was studied using analytical scanning and transmission electron microscopy. Charpy V-notch impact toughness tests were performed for all heat-treated states with a notch location in distinct weld regions such as weld metal (WM), heat-affected zone (HAZ), and base material (BM). The overheated HAZ region exhibited the lowest impact toughness as a result of severe welding induced microstructure degradation. Creep tests were performed at 625°C in the stress range between 80 and 120 MPa. At the highest applied stress, creep fracture occurred in WM, whereas at lower stresses the failure position shifted towards fusion zone at WM/HAZ interface. The hardness profiles experienced significant scattering due to weld microstructural heterogeneity. The major fracture mechanisms involved transgranular quasi cleavage and intergranular creep cracking in impact and creep loading conditions, respectively.

Experimental Study of Phase Composition of B-Fe-Mn-V Alloys and Thermodynamic Calculations of Phase Equilibria in the B-Mn-V and B-Fe-Mn-V Systems

Phase compositions of B-Fe-Mn-V alloys were studied by several experimental methods (DTA measurement, X-ray diffractions, and scanning electron microscopy). Besides the experimental study of the quaternary system, thermodynamic modelling of the ternary B-Mn-V system by the Calphad method and thermodynamic calculations for the quaternary B-Fe-Mn-V system were performed. Calculations for the quaternary system are based on the ternary subsystems (B-Mn-V, B-Fe-V, B-Fe-Mn, and Fe-Mn-V). Boron is modelled as an interstitial element in all solid solutions of vanadium, manganese, and iron. Very good agreement between experimental results and thermodynamic calculations was achieved. The created thermodynamic database is suitable for thermodynamic calculations of phase diagrams for all the ternary subsystems and also for the B-Fe-Mn-V quaternary system.

Experimental Investigation of Isothermal Section of the B-Cr-Fe Phase Diagram at 1353 K

The isothermal section of the B-Cr-Fe ternary system was studied experimentally at 1353 K. X-ray diffraction and scanning electron microscopy equipped with EDX analyzer were used for determination of phase equilibria and composition of the coexisting phases in the B-Cr-Fe model alloys after long-term annealing (1500–2205 h). Two iron borides FeB and Fe2B, six chromium borides Cr2B, Cr5B3, CrB, Cr3B4, CrB2, and CrB4 and also iron and chromium solid solutions (α(Fe,Cr), α(Cr,Fe), γ(Fe,Cr)) and β-rhombohedral B were observed in the alloys. High solubilities of the third element in binary borides and no ternary phase were found. Based on the experimental results, isothermal section of the B-Cr-Fe system at 1353 K was determined.

Prediction of Ternary Fe-B-Cr Phase Diagram

Fe-B-Cr ternary system and its binary subsystems have been studied with aim to develop database of parameters for various thermodynamic calculations of complex systems with boron (for example modified ferritic and austenitic steels for energy industry). All corresponding binary phase diagram were calculated with software THERMO-CALC. Prediction of ternary phase diagram for the Fe-B-Cr system was modelled with using binary data of corresponding subsystems. The prediction was compared with available literature experimental results of phase analysis. The experimental results were used for modifying of the phase diagram prediction by Calphad-method. In future the modified prediction is going to be compared with our experimental results of phase analysis of prepared model alloys to receive most reliable phase diagram of the system.