František Kováč

Cracking of isotropic particle–matrix system: globular shape particle

Abstract The paper deals with a calculation of internal stresses in an isotropic particle–matrix system of a solid state. The internal stresses originate during a cooling process as a consequence of different thermal expansion coefficients of a matrix and of a particle. The conditions of the globular shape particle and of the hemispherical shape formed crack are discussed.

Effect of VC Nano-Inhibitors and Dynamic Continuous Annealing on the Magnetic Properties of GO Steels

We report on novel approach for the abnormal growth of Goss grains that employs the system of VC nano-precipitates in combination with a phenomenon of the deformation induced grain growth. The laboratory slab of grain oriented steel was subjected to hot rolling with reduction of the thickness of sheet to 2.2 mm. The influence of coiling temperature on the distribution of VC particles was analyzed by TEM. The obtained results have confirmed the presence of VC nanoparticles with a typical size of 5-15 nm located preferentially in the vicinity of grain boundaries. Subsequently, the hot rolled strips were subjected to the cold rolling with the reduction ε ~ 84%, followed by primary recrystallization, temper rolling and final annealing in dynamic conditions at 1050 °C. This procedure led to evolution of the sufficiently strong {110} 〈001〉 Goss texture, which follows in the soft magnetic behavior comparable to that obtained in the longtime heat treated industrial GO steels. Thus, the proposed approach allows reach the equal materials quality at significantly shortened manufacturing time and lower energy consumption as compared to the conventional process of GO steel fabrication.

Magnetic Properties of Temper Rolled NO FeSi Steels With Enhanced Rotation Texture

In the present work, we have used an adjusted temper rolling process for development of particular textures {100} <; (0vw) in nonoriented (NO) FeSi steels. The main idea behind the improvement of soft magnetic properties relies on deformation induced grain growth and heat transport phenomena promoting the preferable formation of columnar grains with desired orientation. The vacuum degassed NO steels with silicon content 1% wt. taken from industrial line after final annealing were chosen as an experimental material. A columnar grained microstructure with pronounced intensity of rotating cube and Goss texture components was achieved during a dynamic final annealing. The obtained microstructural and textural state of sample leads to a significant reduction of coercivity. The coercivity values measured in DC and AC (50 Hz) magnetic field decreased from 70 A/m to 16 A/m and from 179 A/m to 65.4 A/m, respectively.

Stabilization of the Barkhausen Noise Parameters

The Barkhausen noise was measured in nonoriented Fe-3%Si steel with different average grain sizes. Air gaps between the yoke and the measured objects were also varied during the measurements. The change of the grain size was achieved by different combinations of cold rolling and heat treatment processes. The rise of the gap size degraded the level of the Barkhausen noise and caused that the parameters of the Barkhausen noise, such as the amplitude of the Barkhausen noises envelope, decreased. In order to suppress the influence of the air gap size on the measurement results, we analyzed the amplitude probability distribution of the Barkhausen noise and we found that the distribution at small levels of the voltage practically does not change with the grain size, but it increases with rise of the gap size. This change of the amplitude probability distribution with the gap size was used to correct the amplitude of the Barkhausen noises envelope. It was shown that the correction essentially increases the precision of the evaluation of the grain size at varying air gap. Similar technique can be used also to decrease the error of evaluation of other microstructural changes of ferromagnetic materials using the Barkhausen noise method, without measuring and feedback setting the defined waveform of the magnetic field in the sample. For this method to be applicable, several conditions should be met, especially the level of the Barkhausen noise should be essentially higher than the level of the disturbing noise.

Microstructure and texture development of Fe–3%Si GO steel during high temperature annealing

Abstract The one- and two-stage cold rolled Fe–3%Si grain-oriented electrotechnical steels were used as experimental material. Investigated steels were treated according to 6 different high-temperature annealing schedules. The relations between the regime of high-temperature annealing, state of secondary particles system at the onset temperature of secondary recrystallization and development of microstructure and texture during high-temperature annealing were investigated.

Effect of Microsecond Pulse Laser Modification on Electromagnetic Properties of Grain Oriented Silicon Steel

A microsecond pulsed laser beam was used to local magnetic domain modification of electrical grain oriented silicon steel. It was carried out using three different laser pulse regimes: a single pulse laser regime, a multipulse laser regime and a multipulse laser regime with modulation of laser pulses. The laser processing variables were pulse energy and and number of pulses. The samples were tested for nanohardness and coercivity before and after laser treatment. Light optical microscopy, scanning electron microscopy and magnetic force microscopy were used to observe the cross-sectional profile, surface of the samples, and magnetic domain visualization, respectively. The local laser treatment of grain oriented silicon steel surface has been studied in terms of its influence on the magnetic domains and coercivity. It was found that laser-modified samples showed coercivity improvement in comparison to the non-treated samples. The most significant improvement in coercivity was obtained in the modulated multipulse regime and negligible improvement in the single pulse laser regime. Three main effects responsible for the observed improvement were identified, namely: magnetic domain refinement, influence of number of laser pulses and shape of laser HAZ profile. The present work highlights on differences in the magnetic domain structure, microstructure of the laser modified material and basic electromagnetic and mechanical properties. In present study, the pulse laser surface processing was presented as a useful energy efficient alternative to other techniques e.g. mechanical scribing, electrical discharge scribing, plasma jet scribing, etc. The refined magnetic domains in electrosteels are responsible for the observed low coercivity, which indicates perspective application of the investigated laser modified steels in the power transformer cores with lower core losses.

Study of Microstructure and Texture Evolution in Grain-Oriented Steels Via Coercivity Measurements

The relationship between microstructure, phase transformation, and texture in grain-oriented (GO) steels on the one side and the magnetization processes on the other side is very complex. To contribute to a better understanding of this dependence, the magnetic measurements were carried out on the steels in combination with a microstructure, texture, and phase analysis. It was shown that the coercivity measurement technique can be successfully applied to reflecting of main microstructure and texture changes of GO steels. The mentioned changes taking place during application of particular thermomechanical treatments. Moreover, this measurement method also can be applied to detect phase transformations in the materials containing sufficient carbon content. Within this paper, it is shown that the measurement of coercivity is a powerful tool that can be successfully used to reflect such important phenomena as microstructure and texture development in the GO steels.

Textured nickel tapes prepared from commercially available material

Textured Ni tapes were fabricated from commercially available nickel pellets (98.5% Ni). Ingots produced by a melt process were cold rolled to 150–400 μm thick tapes. Texturing was achieved by annealing in a reducing atmosphere (Ar + 6.5% v/v H2). Sharp cubic biaxial textured Ni tapes were obtained by thermal treatment at 1000°C for 2 hours in a reducing atmosphere. The tapes were characterized by scanning electron microscopy, X-ray diffraction and by electron backscattering diffraction. The tensile strength, the thermal expansion behavior and the Vickers hardness for the cold rolled tapes and for the heat-treated tapes were measured.

Secondary recrystallization kinetics in GO electrotechnical steels

Abstract GO electrotechnical steel with AlN inhibition particles was used as experimental material. Samples taken out from production process after decarburizing annealing were anisothermic annealed with three different heating rates. Texture measurements as well as determination of residual grains orientation were carried out with the aim of establishing the most suitable heating rate for obtaining the sharpest Goss final texture.

Abrasive wear resistance of modified X37CrMoV5-1 hot work tool steel after conventional and laser heat treatment

Abstract The effects of two conventional heat treatments and one innovative processing by laser surface remelting of modified X37CrMoV5-1 tool steel on its abrasion wear resistance were investigated. Conventional heat treatments consisted of quenching from 990 °C, individually followed by two different tempering treatments to achieve secondary hardness either at 520 °C or 560 °C. Laser surface remelting was performed using optimized parameters of continuous laser beam scanning mode. The results showed that the highest wear resistance was obtained for almost carbide-free, surface remelted microstructure with the highest hardness due to microstructural refinement and martensitic transformation hardening. The observed differences in wear resistance among individual material states are discussed in relation to their microstructures, hardness, and wear mechanisms characteristics.