Investigation of intergranular corrosion welded joint of austenitic stainless steel by electrochemical methods

Sensitization degree of the austenitic stainless steel welded joints was investigated by electrochemical methods: the double loop electrochemical potentiokinetic reactivation (DL EPR) in H2SO4 + KSCN solution and by of the corrosion potentials measurement of the steel in the solution drop of HNO3 + FeCl3 6H2O + HCl. The welded joints, by an X-ray radiographic method on possible presence of the weld defects, were tested. Grain size of the base metal and the welded joints, by applying an optical microscopy were determined. The existence of compatibility between the results for different electrochemical methods is shown. Heat affected zone (HAZ) of the austenitic stainless steel welded joints has shown significant degree of sensitization. The corrosion potentials measurement method is simple, nondestructive method, that gives qualitative informations about degree of the sensitizations of the stainless steel. The double loop electrochemical potentiokinetic method gives quantitative evidence about susceptibility of the stainless steel to intergranular corrosion. This method can be applied with some adaptation in field environments

PLASTIC DEFORMATION AND HEAT TREATMENT OF THIN WALLED CENTRIFUGALLY CAST HIGH STRENGTH CrMoNb STEEL TUBES

This work deals with effects of hot plastic deformation process and subsequent heat treatment on structure and mechanical properties of centrifugally cast (CC) high strength CrMoNb steel tubes. Plate samples, taken from CC tubes, were homogenized and subsequently hot rolled. One serie of samples was soft-annealed (SA) and other series oil-quenched and tempered (QT) between 373-923 K. Primary and secondary structures, non-metallic inclusions in radial direction and prior austenite grain size were analyzed using bright-field and polarized light microsopy. Experimental mechanical properties of SA and QT samples were modeled by means of polynomial functions and correlated with structure properties

Analysis of floating-head heat exchanger bolts failure

As-received floating-head heat exchanger bolts were broken (BB) and deposite-coated. The aim was to estimate a cause of their failure. The new bolts of the same material were used as a reference material (reference bolt – RB). After visual and radiographic examination, their chemical composition, structure and room-temperature mechanical properties were determined and compared. Comparison was made with the values set by standard, as well. Afterwards, fractography was performed on fractured surfaces of tensile specimens and originally (during exploitation) BBs to try to get an impression about bolts failure mechanism. Qualitative analysis of deposite was employed in order to confirm was there any possible influence of surroundings during their failure in terms of corrosion-assisted cracking. Chemical composition of RB and BB materials was analyzed by use of spectrophotometry and structure properties with light optical microscope (LOM). Fractured surfaces of tensile specimens and of BBs, as well as deposite chemistry, were analyzed by use of Scanning Electron Microscopy with Energy Dispersive System (SEM-EDS). BBs had an approximately three times higher sulphur content and lesser manganese content, lower ductility and higher strength values comparing to those of the RBs. Generally, fracture surfaces of both, RB and BB tensile specimens have a similar rosette-like macro-appearance. The only difference is that the radial marks in the case of the RBs are rougher. The surface has a more fibrous area and shear lip presence. Fracture mode can be characterized as dimple rupture and micromechanism as microvoid coalescence. In the case of BB tensile specimens, the mixed presence of dimples and cleavage facets was noticed. The macrofractography of originally broken surfaces shows a rough and complex topography of fracture surfaces indicating on a possibility that bolts failure has been a result of complex loading conditions. Presence of sulphur- and chlorine-containing particles on the fracture surfaces of BBs and in deposite reveals a possibility that failure was environmentally-assisted

The analysis of damage threshold in ruby laser interaction with copper and aluminium

U radu su prikazani rezultati delovanja laserske svetlosti, talasne dužine λ = 694,3 nm (rubinski laser, Q-switch mod), na uzorke od bakra i aluminijuma. Cilj ispitivanja je bio da se odrede maksimalne gustine energije laserskog snopa koje mogu da se koriste u dijagnostičke svrhe (interferometrijska snimanja, lasersko skeniranje, itd), kao i u uklanjanju depozita, a da pri tome snop ne stupa u interakciju sa osnovnim materijalom. Odabrani uzorci od bakra i aluminijuma bili su dugi niz godina izloženi atmosferskim uticajima. Rezultati delovanja laserske svetlosti ispitivani su skenirajućim elektronskim mikroskopom. Istraživanja su pokazala da je bezbedna granica gustine energije rubinskog lasera za dijagnostičke metode za oba metalna uzorka do 20×103 J/m2.Nondestructive methods are dominant in diagnosing the status and protection of all kinds of contemporary industrial objects, as well as objects of industrial heritage. Laser methods open wide possibilities of research in the field of diagnosis and metal processing. This paper presents the results of laser radiation interaction (wavelength λ = 694.3 nm, Ruby laser, Q-switch mode) with metal samples covered with a deposit. The goal of the examination was to determine the maximum energy density of the ruby laser beam, that can be used in diagnostics purposes (interferometric methods; 3D scanning) and as a tool for safe removal of deposits, without interacting with the basic material. Microscopic examination performed with SEM coupled with EDX allowed the determination of the safe laser light energy density levels, which caused the removal of the deposite from the surface of the sample, without degradation of the surface. The energy density up to 20 kJ/m2 is the maximum allowed for diagnosis or deposit removal

Characterization and comparison of the carbides morphologies in the near surface region of the single- and double layer iron-based hardfaced coatings

The properties of the high-chromium white iron (HCWI) hardfaced materials, such as hardness, toughness and wear resistance, are affected by the morphology of their reinforcing phase, the Cr-carbides. In this work, the influence of the 1) welding parameters, 2) welding procedures and 3) chemical composition on the near-surface Cr-carbides morphology is presented. Additionally, the effects of the nucleo C® formula (Castolin Eutectic Ltd.) of unknown composition are evaluated. The hardfaced coatings are deposited by using the submerged metal arc welding (SMAW) process. The obtained near-surface structures are observed with scanning electron microscope with energy dispersive system (SEM-EDS). The structure morphological parameters are quantitatively determined by use of the ImageJ software. In the single-layer HCWI hardfaced coatings, larger heat input per unit length (≈ 4.1 kJ/mm) is leading to a formation of the hypoeutectic structures. An approximately two times lower heat input (≈ 1.8 kJ/mm) stimulates the solidification of the near-eutectic structures with presence of spheroidized eutectic and primary Cr-carbides (the Nucleo C effect). The double-layer coatings deposited with the ≈ 1.8 kJ/mm possess the hypereutectic structures with blade-like primary and needle-like eutectic carbides (the absence of the Nucleo C effect). The lower heat input (≈ 1.4 kJ/mm) and increase of the chromium and carbon content result in a formation of the hypereutectic structures with presence of the primary rod-like Cr-carbides embedded in the eutectic matrix with coarser and irregular eutectic Cr-carbides. The influence of the nucleo C® formula on the Cr-carbides morphology is more pronounced in the single-layer HCWI hardfaced coatings than in the double-layer coatings

The effects of aging on the precipitation of the W-rich phase in the matrix of the 92.5W-5Ni-2,5Fe powder metallurgy heavy alloys

The precipitation behavior of W-rich phase on the specific matrix locations of Co-free and Co-containing 92.5W-5Ni-2.5Fe heavy alloys is investigated during aging of the as-quenched samples. The aging treatment is performed between 600 °C (873 K) and 800 °C (1073 K) and for aging time up to 9 hours. The average chemical composition of tungsten heavy alloys was determined by using the atomic absorption spectrophotometry (AAS) and the Leco apparatus. The Energy Dispersive X-ray (EDX) analysis was employed to analyze the chemistry of the existing phases. The changes in structure as a function of aging time and temperature have been examined by using the light microscopy (LM) and the scaning electron microscopy (SEM). The macro- and micro-hardness measurements were performed, as well. The local changes in matrix structure and various morphologies of the precipitates are observed. The aging has caused only a minor changes of the micro-hardness of the existing phases (γ and W-rich) as well as the macro-hardness of the investigated alloys. In the case of the Co-free 92.5W-5Ni-2.5Fe heavy alloy, at lower temperatures and for shorter time of aging, there is a dominant presence of lamellar and Widmanstätten morphology of W-rich precipitates within the γ-matrix grains. At higher temperatures and for longer time, there is a significant precipitation of W-rich precipitates at the matrix grain boundaries (γ/γ). In the case of 92.5W-5Ni-2.5Fe heavy alloy micro-alloyed with Co, it is dominant precipitation of W-rich phase at the matrix grain boundaries (γ/γ), especially at higher aging temperatures and for longer aging time

Numerical and Experimental Study of Temperature Distribution on Thermal Plant Coal Mill Walls

This article presents results of a numerical and experimental study of the temperature distribution inside and on the walls of the ventilation mill in the Kostolac B power plant, Serbia. This is very important because heat exchange between gas mixtures, ventilation mill walls and the environment, influences coal powder temperature, the combustion process, and the emission of greenhouse gases. The most complex multi-phase model in the Euler-Euler approach has been used through the ANSYS FLUENT code, for the numerical simulation of temperature distribution. Two ventilation mill models have been considered, one with zero wall thickness and the other one with steel walls, with and without insulation. An infrared thermographic camera was used for monitoring the temperature distribution on the outside wall of the ventilation mill housing, before insulator reparation, while after the reparation measurements were carried out at the selected points with infrared thermometer. The experimental results were used for verifying the results of the numerical simulation method as well as for detecting the damaged insulation

The analysis of damage threshold in the ruby laser interaction with copper and aluminium

Nondestructive methods are dominant in diagnosing the status and protection of all kinds of contemporary industrial object, as well as object of industrial heritage. Laser methods open wide possibilities of research in the field of diagnosis and metal processing. This paper presents the results of laser radiation interaction (wavelength λ = 694.3 nm, Ruby laser, Q-switch mode) with metal samples covered with a deposit. The goal of the examination was to determine the maximum energy density, that can be used in diagnostics purpouses (interferometric methods, 3D scanning, i.e.) and as a tool for safe removal of deposits, without interacting with the basic material. Microscopic examination performed with SEM coupled with EDX allowed the determination of the safe laser light energy density levels, which caused the removal of the deposite from the surface of the sample, without degradation of the surface. The energy density up to 20 103 J/m2 is maximum allowed for the diagnosis or deposit removal