Conventional long-term creep test (CCT) to the rupture and so called accelerated creep test (ACT) of the dissimilar weld joint made of FB2 and F martensitic steels and of the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa. Assessment of microstructure development and changes of hardness was correlated with the creep strength. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centers for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and cause failure of samples. The aim of this paper is to compare results and possibilities of the “standard” methods and advanced scanning electron microscopy performed by instrument equipped with a concentric backscatter electron detector (CBS). Filtering of the signal enables improving and/or diminishing of selected type of contrast caused by various types of particles of secondary phases. The images were used as an input data for image analysis and developments of microstructures during CCT and ACT were compared. Results have shown that specimens after ACT contains significantly lower content of the Laves phase.
Specimen cleaning and drying are critical processes following any metallographic preparation steps. The paper focuses on automation by reason of absence of the process repeatability during manual sample handling. An etchant or electrolyte results in inhomogeneous surface quality because the solution runs off the specimen surface during its removal from the beaker. High-quality specimen cleaning is absolutely crucial for the acquisition of the specimen suitable for characterization by a scanning electron microscope operated at very low landing energies of the primary electrons (SLEEM). The SLEEM technique is a powerful tool for the characterization of advanced steels, as described by many scientific papers. The SLEEM requires the specimen absolutely free of water and any organic residues on the surface. This work presents a novel unique apparatus enabling automatic specimen cleaning and drying after the etching or electropolishing processes. Automation reduces the influence of dependent variables that would be introduced into the process by the metallographer. These variables include cleaning time, kinematics, and motion dynamics, but the process can also be affected by variables that are not obvious. Performed experiments clearly demonstrate our in-house designed apparatus as a useful tool improving efficiency and consistency of the sample cleaning process. The high quality of the specimen surface is verified using a light optical microscope, an electron scanning microscope, and above mentioned SLEEM technique.
The paper aims to demonstrate a modern scanning electron microscope (SEM) as a powerful tool for visualization of the secondary phases in TRIP steel. The TRIP steel specimens prepared by various metallographic techniques were imaged by the SEM and the secondary phases presence was confirmed by an electron back-scattered diffraction (EBSD) technique. The chemical polishing by 5 % HF in H2O2 for 10 seconds results in selective etching for each individual phase, as confirmed by an atomic force microscopy (AFM) and hybrid AFM-in-SEM techniques. The phases are easily distinguishable in the SEM micrographs created by the low energy high take-off angle signal electrons. The proposed sample preparation technique together with special SEM imaging conditions enables us accurate analysis of distribution of secondary phases within the TRIP steel matrix and moreover, the retained austenite is distinguishable from the martensite phase.
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