Key Engineering Materials Vol. 647

Abstract: Creep resistance of the steel depends on chemical and structural composition and structural stability. Therefore it is necessary to check microstructural changes in material during the long-term service, however the necessary material analyses are generally destructive. The submitted contribution describes non-destructive electrochemical method for detection of some microstructural changes taking place during creep exposures. The shape of polarization curves are correlated to the microstructure of CB2 steel, the most promissing (9-12) %Cr martensitic steel for the cast components. The dynamic polarization curves were measured using samples in as received conditions and after long-term creep tests at 650°C. Microstructure was analysed using light and electron microscopy. The microstructure of the CB2 steel changed during creep exposures. In polarization curves additional peak appeared in the main passivation region and the curve shape changed also in the secondary passivation region. The curve changed in dependence on precipitation and growth of Laves phase particles.

Abstract: Routine non-destructive examination of the steam pipe elbow after more than 240 000 hours of operation at elevated temperature revealed the extensive creep damage on the outer surface of the pipe elbow. Complex metallographic analysis made in this area confirmed creep damage as well as the non-uniform nature of the cavitation. The density of cavities continuously decreased from the outer pipe surface towards the inner surface, but also its density rapidly waned beyond the damaged area in both directions, along the circumference as well as the length of the elbow. The actual extent of the material degradation was then evaluated by testing of mechanical properties, Charpy-V and fracture toughness testing and the results were used in calculation of the residual life of the pipe elbow. It was shown that although the creep damage was perhaps one of the worst detected in Czech Republic, the cracks in the pipe elbow would spread by the stable growth until the half of the pipe wall thickness. This result thus confirms the possibility of creep life extension far beyond the limit criteria used so far.

Abstract: Today, the study of surface integrity became a growing and multidisciplinary branch of science. Its purpose is to gain such understanding of the surface which can be exploited in real-world applications and in commercial services. The importance of the surface was and continues to be the topic of a number of highly specialized papers. However, the surface has not been yet evaluated in terms of multiple factors in order to obtain a comprehensive picture of its condition and to reveal the interrelationship between the manufacture of the surface and its resulting properties. The paper gives a description of effects of special cutting tools on the structure of the sub-surface region. These structural changes will be explored by means of metallographic observation, hardness measurement and evaluation of residual stresses.

Abstract: In this work we demonstrate the application of stereology-based image analysis for the characterization of highly porous cellular ceramics (alumina foams) prepared by biological foaming with yeast and subsequent drying (80-105 °C) and firing (1570 °C). It is shown that the ceramics prepared usually have total porosities in the range 78-84 % and that the porosities made up by large pores (volume fraction of foam bubbles) are usually in the range 58-75 %. Further it is shown that the mean chord length and the Jeffries size, i.e. pore size measures related to the interface density and the mean curvature integral density, respectively, are relatively close to each other (usually 0.8-1.4 and 0.8-1.2 mm) with a ratio close to unity (0.9-1.3) and that the mean surface-to-surface distance of pores gives a realistic picture of the average pore wall thickness (usually 0.46-0.69 mm). Using a special processing variant (excess ethanol addition) it is possible to obtain microstructures with lower porosity (total porosity 68-70 %, foam bubble volume fractions 50-56 %) and smaller pore size (approx. 0.5 mm). Absolute errors are calculated using normalized deviations corresponding to 95 % reliability in the Student distribution and the standard errors for the quantities in question (both observed and estimated). Relative errors are found to be below 12 % when the number of measurements is of order 400-1000.

Abstract: The porosity is unwanted phenomenon mostly that is tried to eliminate. The pores are initiation site of fatigue fractions usually, they worse strength and ductility of materials, quality of machined surfaces and their following surface treatment.

Abstract: The aim of this contribution is to propose acceptable ultrasound diagnostic methods which should help discover products with excessive porosity of sintered sliding layer on bearing shells which causes low bearing durability by means of measuring of samples taken from bearing shells manufactured by commercial companies in Slovakia and Germany.

Abstract: The article deals with analysis of defects indicated in large forgings in the course of ultrasonic testing. The aim of the metallographic analysis of the defects was to establish whether and to what extent casting or filling powders took part in their emergence that had been used when casting the ingots from which the forgings were made.

Abstract: The paper focuses on the metallographic analysis of damaged belt-conveyor coupling segments made of high-alloy austenitic steel. The object in question is a weldment – a bar with a ring welded on it at either end. The subject of the analysis is the evaluation of the microstructure of the base material of the two components (weldment and weld metal), the quality of the execution of the weld joints, subsequent heat treatment of the weldment, and the overall state of the component part after operational loading. The suitability of the design solution of the whole weldment is also discussed. The problematic is solved with the aid of a metallographic analysis of the microstructure of the material, using light microscopy, scanning electron microscopy inclusive of the EDS microanalysis of chemical composition, and microhardness measurement.

Abstract: The work is aimed at a detailed analysis of impaired chain segments manufactured by welding and determining the cause of chain segment failure. The main interest was focused on the base material structure, the quality of weld joint, the heat treatment of the weldment, and the overall condition of the component after operating workload. The chain segments were investigated in terms of the microstructure by means of the light microscope, scanning electron microscope equipped with an energy dispersive microanalyser, and hardness measurement.

Abstract: The paper deals with the qualitative, quantitative, and phase analyses of particles that were found during the metallographic analysis of a cracked spring washer made of silicon spring steel 38Si7, with the aim of ruling out the client’s suspicion of possible initiation of graphitization in the course of the heat treatment. The problem is solved via metallographic analysis using light microscopy, scanning electron microscopy including EDS microanalysis of the chemical composition and EDS line analysis.