Mária Dománková

Influence of Thermal History on the Hot Ductility of Ti-Nb Microalloyed Steels

The hot ductility of Ti-Nb microalloyed steel has been investigated to evaluate the sensitivity to surface crack formation during the continuous casting process. Tensile samples were subjected to different thermal treatments and were tested at deformation temperatures ranging from 650°C to 1000°C using a strain rate of 10-3s-1. It has been found, that the investigated steel evinced poor ductility over almost the whole testing temperature range characterized by marked grain boundary cracking, irrespective of which thermal cycle has been utilized or whether the samples have been melted or only reheated. Microstructural examinations and supplementary thermo-kinetic computer simulations revealed distinct Ti-Nb precipitation throughout the microstructure being responsible for the deteriorated materials hot ductility.

Electron Beam Welding of steel sheets treated by nitrooxidation

The paper describes the nitrooxidation process in a fluid environment applied to low-carbon steel sheets. The influence of this process on structural changes, weldability, mechanical properties and formability is also presented. Specimens welded by electron beam welding were examined by visual inspection, microstructure analysis and microhardness analysis. Visual inspection showed irregularities in weld bead formation, significant porosity and excessive spatter. In order to improve the appearance of the weld bead, it was necessary to use a triple welding pass and electron beam oscillation. The structural microanalysis of electron beam-welded specimens revealed a polygonal ferrite structure with the presence of upper bainite. The heat-affected zone was approximately three times wider in comparison to laser beam welding. No grain coarsing was observed in the high temperature heat-affected zone. In comparing electron beam welding and laser beam welding of nitrooxidation-treated materials, it can be claimed that electron beam welding is not a suitable welding method for this type of material.

Effect of Multiple Local Repairs on Microstructure and Mechanical Properties of T24 Steel Welded Joint

The effect of multiple local repairs on the microstructure and mechanical properties of the T24 steel welded joints was studied. T24 steel tubes were butt-welded by the GTAW method. Peripheral welded joints were made in four locations of the tube. In order to simulate the repair procedure, the welds were cut off from the root and the first local repair was performed. Other two local repairs were carried out in the same way. After each local repair, the microstructure and mechanical properties of the joints were evaluated. The results of the mechanical tests demonstrate that only two local repairs can be performed on the T24 steel peripheral welded joint. After the third local repair, impact energy of the welded joint was lower than required value, which is attributed to the coarser martensite and the coarser carbide precipitates formed in the heat-affected zone, compared with the weld metal.

Aging Precipitation Behaviour of Cr-Mn-N Austenitic Stainless Steels

This work is focused on the austenite decomposition in Cr-Mn-N austenitic stainless steels. The experimental steels was solution heat treated (1100°C/30 min. followed by water quenching) and then annealed in the temperature range from 650 to 900°C for holding time 5 min. to 100 hours for the study of precipitation. The precipitation behaviour during isothermal aging was investigated using light optical microscopy, transmission electron microscopy, selected area diffraction and energy-dispersive X-ray spectroscopy. The results show that the phase composition of the precipitates depends on the chemical composition of the experimental steel. The critical temperature of precipitation is 825°C with the corresponding time 100 s.

Welding of Austenitic Stainless Steel AISI 304 by Fiber Laser

Paper deals with the welding of AISI 304 thin sheets with the thickness of 1 mm by fiber laser. Statistical software MiniTab was used for planning the experiment. The influence of laser power, welding speed and focal point position on width and depth of weld bead was examined. Output power, welding speed and focus point position were used as variable factors for DOE. Samples were prepared by standard metallography. Weld bead width and depth were evaluated. Statistical results were afterward verified by welding reference samples with material the thickness of 0.25 mm. The difference between real width of weld bead and predicted width obtained from statistical software was 8.26 %. In case of weld depth measurement, the difference was 23 %.

Application of Metallographic Methods for Thermal Ageing Evaluation on Samples from Primary Piping of NPP

During a long-term operation of nuclear power plants (NPP), the changes of structural material properties occur. To ensure the safe and reliable operation, it is necessary to monitor and evaluate these changes mainly on components from primary circuit of NPPs. One of the dominant ageing mechanisms of NPP components besides the radiation embrittlement and the fatigue loads is the thermal ageing. The thermal ageing is the temperature, material and time dependent degradation mechanisms due to long-term exposure at the operating temperature of 570 K.This paper describes the project for thermal ageing monitoring at primary piping in NPP Bohunice Unit 3. There are summarized the results obtained from evaluation of original primary piping material.

Identification of Carbides in Sub-Zero Treated Vanadis 6 Ledeburitic Steel

Sub-zero processing is an add-on step of heat treatment that includes cooling down, holding at low temperature and re-heating up to room temperature. The aim of this process is to improve some mechanical properties of high-carbon steels. In this research, the Cr-V ledeburitic steel Vanadis 6 produced by powder metallurgy was used. As-received material as well as that after heat treatment contains various types of carbides, which are different in terms of both the nature and the origin. During the austenitizing, a part of the carbides (eutectoid, secondary) undergo dissolution in the austenite while other carbides (eutectic, secondary) remain in the material undissolved. The application of sub-zero treatment leads to increase in the volume fraction of small globular carbides, with a size below 0.3 μm. The volume fraction of secondary carbides was found to be constant, irrespectively to the use of sub-zero processing, but it decreases with increase of the austenitizing temperature. The nature of the carbides was found to be practically unaffected by the sub-zero treatment and it can be described as follows: The eutectic carbides are the vanadium rich MC-phase and the secondary ones are the M7C3. Regarding the nature of small globular particles, their nature is not doubtless because the electron diffraction fixed them as alloyed cementite but the EDX analysis revealed very high chromium content in the particles.