J. Janovec

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.

Equilibrium grain boundary segregation of phosphorus in 2.6Cr-0.7Mo-0.3V steels

The equilibrium grain boundary segregation of phosphorus in two 2.6Cr-0.7Mo-0.3V steels with 0.06 and 0.005 Wt% of C tempered at 823, 853 and 913 K was investigated. Auger electron spectroscopy was used to establish the grain boundary concentrations of the P, C, Cr, Mo and V. Chemical compositions of the carbides were determined by means of EDX/STEM. The standard McLeans theory and Guttmanns theory of co-segregation were used to analyze the experimental data. The values of [Delta]H[sub P], [Delta]H[sub Mo], [Delta]G[sub P][sup 0], Delta G[sub Mo][sup 0], and [alpha][sub MoP] were calculated. The value of interaction coefficient calculated according to Guttmanns theory is low to suppose the influence of molybdenum upon the P segregation. The more expressive changes in the grain boundary concentrations of P and C in relation in tempering temperature were found out in the steel with lower C content.

On kinetics of phosphorus segregation in Cr-Mo-V low alloy steel

The low carbon, low alloy Cr-Mo-V steels with stabilized microstructures are widely used in electric power plants and petroleum industries because of their good serviceability of elevated temperatures. However, the phenomenon of temper embrittlement after thermal exposure at 623 to 873 K was stated. There are time-temperature relationships between the embrittlement of steels and grain boundary impurity segregation. Temperature dependence of embrittlement can be explained by the theory of equilibrium segregation. In practice, the exposure time is usually less than that required for equilibrium segregation. Therefore, in order to understand the stages preceding the equilibrium state, a study of segregation kinetics is important. This paper deals with the segregation kinetics of phosphorus in the 2.6Cr-0.7Mo-0.3V steel at 853 K.

Some aspects of intermetallic phase precipitation in a 12% Cr-steel

A 12% Cr-steel was investigated by TEM after aging at 773 K for up to 22,000 h. In addition to the carbides M{sub 23}C{sub 6} and MC an intermetallic phase was observed after aging for 22,000 h. This phase contained mainly Mo, Fe and Cr and additionally Si and P. These particles occurred in three different morphologies (mosaic, lamellar and compact) and were mainly found to be located close to the carbides. The chemical composition was similar to that of the Laves-Phase (Fe,Cr){sub 2}Mo, but the precipitates showed five-, six- and two-fold symmetries and were therefore classified as quasicrystals having icosahedral symmetry.

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.

Thermodynamics of phosphorus grain boundary segregation in 17Cr12Ni austenitic steel

Abstract The paper presents results of thermodynamic analysis of phosphorus grain boundary segregation in 17Cr12Ni austenitic steel annealed for 1000 h at 923, 973 and 1073 K. With respect to values of compensation temperature τP=930 K and segregation enthalpy ΔHP0=−14.1 kJ/mol the analyzed interfaces were considered to be special grain boundaries.

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).

Role Of M23C6 Carbide Precipitation and Phosphorus Grain Boundary Segregation in Intergranular Fracture and Corrosion of 18Cr-12Ni Austenitic Stainless Steel

Abstract The effects of intergranular precipitation (size of precipitates) and phosphorus grain boundary segregation on fracture and corrosion behaviours of the 18Cr-12Ni steel were investigated. In the investigation, techniques of light microscopy, SEM, TEM, AES, EDX, electron diffraction, and the Huey test were used. A good correlation was found between the average size of intergranular particles and the portion of intergranular fracture for samples impact loaded at 173 K. The equilibrium values of phosphorus grain boundary concentration showed the close correlation with the values of weight loss (penetration depth) measured during the Huey test.

EVOLUTION OF SECONDARY PHASES IN Cr-V AND Cr-Mo-V LOW ALLOY STEELS

Abstract The evolution of secondary phases in Cr-V and Cr-Mo-V low alloy steels was studied. The steels with two levels of chromium (0.9 and 2.5 wt%) were aged in the range of 100 to 5000 hours at 773, 853, 953 and 993 K. In the investigation, a limited transmission electron microscopy experimental program was combined with credible thermodynamic predictions (Thermo-Calc). It revealed the positive influence of higher chromium bulk content on the stability of molybdenum rich M6C carbide. In the steels with a lower bulk chromium content (0.9 wt%), the M7C3 carbide with a slight iron dominancy was identified. Molybdenum was considered to accelerate the M6C precipitation. It was predicted thermodynamically that an increase in the bulk vanadium content favours the phase equilibrium: ferrite + MX (X = carbon + nitrogen). On a étudié l’évolution des phases secondaires des aciers faiblement alliés au Cr-V et au Cr-Mo-V. On a endurci par vieillissement des aciers à deux teneurs différentes en chrome (0.9 et 2.5% en poids) pour une durée de 100 à 5000 heures à 773, 853 et 993 K. Dans cette étude, on a combiné un programme expérimental de microscopie électronique à transmission et des prédictions thermodynamiques crédibles (Thermo-Calc). L’étude a montré l’influence positive d’une plus grande teneur massique en chrome sur la stabilité du carbure M6C riche en molybdène. Dans les aciers à plus faible teneur massique en chrome (0.9%), on a identifié le carbure M7C3 avec légère domination de fer. On considère que le molybdène accélère la précipitation de M6C. On a prédit par la thermodynamique qu’une augmentation de la teneur massique en vanadium favorisait l’équilibre de phase: ferrite + MX (X = carbone + azote).