M. Sozańska

Examination of hydrogen interaction in carbon steel by means of quantitative microstructural and fracture descriptions

The relations between the quantitative microstructural characteristics and the resistance of carbon steels to hydrogen-induced cracking (HIC) were studied for plates used in the oil and refinery industry. The width of the pearlitic bands and the degree of banding were considered if the testing of the resistance to HIC was performed in accordance with the NACE TM 0284 standard. The role of the degree of banding was important while that of the width of pearlitic bands was negligible. Additional hydrogen embrittlement testing of tensile specimens oriented in longitudinal and through-thickness directions revealed that hydrogen strongly increased the anisotropy of mechanical properties. These changes could be correlated with the geometric characteristics of nonmetallic inclusions (MnS) and the pearlitic bands in different metallographic sections. A quantitative description of fracture surfaces has been made by means of a profilometric method.

Phase Identification in Nickel-Based Superalloys Using EBSD/SEM and Electron Diffraction in STEM

Aero engine turbine blades made of nickel-based superalloys are critical components in flight safety. Therefore, it is very important to make sure that the chemical composition, phase composition and microstructure are suitable. However, due to their chemical compositions, superalloys are prone to many transformations and the formation of deleterious phases, which deteriorate the mechanical properties. Hence, investigations concerning the structural stability and phase identification—especially topologically close-packed phases (TCP)—are necessary. Because the volume fractions of these phases are generally small, phase identification should be performed by nanodiffraction techniques in a scanning transmission electron microscope (STEM) and electron backscatter diffraction in a scanning electron microscope (EBSD/SEM). These methods complement each other, but each of them is characterized by different difficulties and limitations. In this paper we present the possibilities and limitations of phase identification in single crystal CMSX-4 superalloy after long thermal exposure.

Use of quantitative metallography in the evaluation of hydrogen action during martensitic transformations

Experiments based on image analysis method and electrochemical potentiostatic tests were performed with particular attention to the influence of hydrogen on the stabilisation of austenite of Fe–32 wt.%Ni alloy. This alloy exhibits great hydrogen absorption capacity (25 wt. ppm) and this presence modifies the martensitic transformation essentially at temperatures near MS (−40°C). In the whole range of the quenching temperatures, the difference between the volume fraction of martensite, measured on specimens with hydrogen or without hydrogen, is constant. It was found that the value of the interfacial area of the martensite plate groups SV is the best parameter describing the interfacial area between martensite and austenite. It was also found that there is a general relationship, expressed by a linear function, between this morphological parameter and an anodic current I.

Characteristics of the Structure of Welded Joints in Sanicro 25 Steel

Sanicro 25 stainless steel (X7NiCrWCuCoNbNB25-23-3-3-2) is a preferred material for structural elements of boilers with supercritical and ultracritical parameters. Welded joints are critical sites that are vulnerable to failure. Hand-welded and automatically uniform welds of Sanicro 25 are evaluated in material studies, and the structure and basic mechanical properties of welded joints are examined. The proposed welding technology for Sanicro 25 ensures proper microstructure, and has obtained quality class B certification.

Comparison of the Investigations of Photonic Crystals Using SEM and Optical Technics

All over the world the investigations of nanophotonic structures called photonic crystals (PCs) are performed. These crystals have potential applications in optoelectronics, e.g. optical filters, antireflective surface coatings, lossless frequency selective mirrors. In Institute of Physics at Silesian Technical University the opal photonic crystals consisting of monodisperse spherical particles, that have diameters of several hundred nanometers, are produced using colloidal self-assembly technics. The main aim of this work is the comparison between pieces of information on morphology of photonic crystals that can be obtained from electron microscopy and from the angular characteristics of optical transmittance and reflectance. The morphology of the samples is characterized by scanning electron microscopy (SEM). Nanosphere diameters are established from statistical analysis of SEM images. The optical properties, which are determined by the photonic band structure, are studied by means of light transmission and reflection measurements. There is a relationship between the wavelength position of transmittance minimum or reflectance maximum and the diameter of the nanospheres. The size of nanospheres obtained from optical measurement results were compared with data obtained from SEM images.

Growth morphology of single-crystal grains obtained by directional crystallisation of an Al–Cu–Fe alloy

Quasicrystalline as well as crystalline faceted single grains of four phases were obtained during directional crystallisation of an Al–Cu–Fe alloy by the Bridgman technique. The monoclinic λ phase, Al13(Cu, Fe)4, dominating at high temperatures formed single-crystal lamellae 0.5 mm to 1 mm thick. A second type of attractive morphological form exhibiting flux dissolution terraces was observed on spherical single crystals of β phase Al(Fe, Cu). Rectangular, hexagonal and octagonal shaped dissolution terraces were revealed at the positions of two-, three- and four-fold symmetry axes, respectively. A single quasicrystalline ψ phase, Al6Cu2Fe, exhibited icosahedral symmetry with growth forms of a dodecahedron with pentagonal facets. The flux dissolution of the β phase apparently plays an essential role in a peritectic reaction leading to quasicrystalline ψ phase formation. Polygonal single grains of ω phase Al7Cu2Fe exhibiting tetragonal symmetry formed the fourth type of thermodynamically stable growth forms. Single grains of the ω phase crystallised in the form of pellets with an octagonal cross-section. The growth morphology of the stable phases was investigated by scanning electron microscopy. The chemical composition of the growth forms described was confirmed by X-ray microanalysis using a scanning electron microscope, whereas the phase composition was determined using electron selected area diffraction and X-ray powder diffraction.

Analysis of Stray Grain Formation in Single Crystal CMSX-4 Superalloy

Abstract Modern single crystal (SX) turbine blades are fabricated by directional solidification using a grain selector. The grain selection process was investigated by numerical simulation and verified by the experiment. A coupled ProCAST and cellular automaton finite element (CAFE) model was used in this study. According to the latest literature data, we designed the grain selector. Simulation confirmed an optimal grain selection efficiency of the applied selector geometry. The obtained experimental results reveal the possibility of stray grain formation in SX castings with a designed selector, in contrast to the simulation results.

Application of EBSD method for the investigation of microstructure and crystallographic orientation in RE2Zr2O7 TBC

Modern aero engine turbine blades made of nickel-based superalloys are covered by thermal barrier coatings (TBC) for thermal and oxidation protection. A new generation of TBCs consist of a bond coat (thin layer of MCrAlY, where M may be Ni, Co, Fe) followed by a ceramic top coat of RE2Zr2O7 (RE - rare earth element). In this paper we present the possibility of the electron backscatter diffraction (EBSD) method for characterisation of the microstructure and crystallographic orientation of a new TBC consisting of a Gd2Zr2O7 top coat and a NiFeCrAlY bond coat after long thermal exposure (1100 °C, 500 h). During thermal exposure, a thermally grown oxide (TGO) layer forms at the bond coat/top coat interface. The TGO is mainly composed of Al2O3. But, there is a possible reaction between Gd2Zr2O7 and Al2O3, leading to Gd-Al-O phases. Phase composition plays an important role in controlling the stress evolution, TGO deformation and crack propagation. Application of SEM-EDS-EBSD techniques allows direct characterisation of the chemical composition, phase composition and crystallographic orientation in the ceramic top coat and TGO layers. This paper presents the possibilities of using the EBSD method for phase identification (Gd2Zr2O7, spinel Ni(Al,Cr)2O4, GdAlO3 and other phases) and orientation analysis of grains in the TGO layer.

Determination of Failure Causes of a Steam Turbine Casing

The paper presents results of research and failure analysis undertaken to determine failure causes of a steam turbine casing. After 130,000 hours of service the crack in a outer shell of the turbine casing was found. The inner shell of the casing was made of cast steel grade G21CrMoV5-7, and the outer shell of grade G20CrMo4-5. Following research were performed in order to determine causes of the casing failure: chemical analysis; microstructure examinations with the use of light microscope, scanning electron microscope (SEM); mechanical properties examinations using the Charpy impact test, and Vickers hardness test; fracture mode evaluation with SEM.

Evaluation of Hot Corrosion Resistance of Directionally Solidified Nickel-Based Superalloy

The turbine blades made of directionally solidified nickel-based superalloys are exposed to combination of high temperature and aircraft environment, in which appear corrosive elements like sulphur, sodium and vanadium (hot corrosion). Corrosion resistance of superalloys is mainly dependent on their structure and chemical composition. Therefore, it is important to be aware of the correlation between the hot corrosion and changes in chemical composition and morphology of a surface of the material. The following paper presents the influence of sulphur on the microstructure of directionally solidified nickel-based superalloy. The research was carried out in Na2SO4 environment at two temperatures of 850oC and 900oC (below and above the melting point of salt, 884°C). The results show scale morphology on material surface and changes in chemical composition of surface of nickel superalloy.