Martin Šebek

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.

Magnetic losses reduction in grain oriented silicon steel by pulse and continuous fiber laser processing

The present paper shows the impact of different laser scribing conditions on possible reduction of magnetic losses in grain oriented electrical steel sheets. The experimental Fe-3%Si steel was taken from industrial line after final box annealing. The surface of investigated steel was subjected to fiber laser processing using both pulse and continuous scribing regimes in order to generate residual thermal stresses inducing the magnetic domains structure refinement. The magnetic losses of experimental samples before and after individual laser scribing regimes were tested in AC magnetic field with 50Hz frequency and induction of 1.5T. The most significant magnetic losses reduction of 38% was obtained at optimized conditions of continuous laser scribing regime. A semi quantitative relationship has been found between the domain patterns and the used fiber laser processing.

Relationship of Microstructure and Mechanical Properties of Dual Phase Steel after Various Annealing Conditions

The development of ultrafine ferrite grain size has become one of attractive way how to improve the behavior of dual phase (DP) steels. The other possible way how to enhance mechanical properties of DP steels is to modify the chemical composition. Therefore object of our investigation was the dual phase steel with modified alloying (three times higher Cr content with addition of phosphorus). The dual phase steel was annealed in laboratory conditions in accordance with three specified annealing cycles: into intercritical region (780°C), into austenite region (920°C) and into austenite region (920°C) by subsequently cooling into intercritical region (780°C) with the hold at the temperature of 495°C. The obtained microstructure after selected annealing regimes consists of three phases (ferritic matrix, martensite and martensite/bainite grains) with different size and distribution. For studied annealing regimes were clearly defined mechanical properties such as: YS, UTS, elongation, n-parameter and ratio YS/UTS. It was defined the scheme of microstructure evolution on base of austenite grain size during the continual cooling process with defined three phases: 1) the hard martensite formed on the grain boundary; 2) the soft interior bainite and 3) the hard isolated martensite.

Preparation of Alumina Fibers by Needle-Less Electrospinning

In this study the needle-less electrospinning by means of “NanospiderTM“ (ELMARCO) as technology for the preparation of fine α-Al2O3 fibers with diameters of 0.5 - 1.5 µm is presented. The fabrication consists of three steps: i) preparation of spinning solution, ii) electrospinning of the prepared solution and collection of the composite fibers, iii) calcination of the composite precursor fibers. The electrospun fibers were prepared from polyacrylonitrile/N,N-dimethylformamide (PAN/DMF) polymer solution and Al(NO3)3.9H2O in ratio 1/10/1. Thereafter, the precursor fibers were calcined in the furnace at 900, 1100 and 1200 °C with a rate of 5 °C/min in air. The formation of crystalline phases, surface morphology and diameters of metastable and final alumina fibers were characterized using thermogravimetric analysis, X-ray diffraction analysis, the scanning electron microscopy and transmission electron microscopy. The precursor PAN/Al(NO3)3 fibers were amorphous. The thermal treatment leads to the phase transition from γ-Al2O3 to α-Al2O3 accompanied by removing of polyacrylonitrile (PAN). The fine porous microfibers composed of pure α-Al2O3 phase were prepared after calcinations at 1200 °C.

Microstructure Modification of Tool Steel X38CrMoV5-1 by Fibre Laser

In the present work, we investigate the influence of laser radiation on the evolution of microstructure and wear resistance of X38CrMoV5-1 tool steel in hardened and tempered condition. The microstructure of the laser surface hardened zone consists of fine and coarse grained carbides (M23C6, M7C3, MC or M2C) which are dispersed within martensite matrix. On the other hand, the laser melted microstructure is characterized by martensite, retained austenite and fine carbides precipitated in the inter-dendritic zone. The laser surface affected zone microstructure exhibits the enhanced hardness in the range from 857 to 775 HV in comparison with quenched and tempered conditions (720-655 HV) and decreases from the surface to the soft zone with a typical hardness of 530 HV. The wear resistance of the laser treated samples is investigated by “ball on disc” method, which shows a significant improvement as compared to that in quenched and tempered condition.