Dimitar Stavrev

Dimitar Stavrev
Technical University of Varna | TUV · Department of Materials Science

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35
Publications
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26
Citations

Publications

Publications (35)
Article
The study and application of the materials that are stable in the temperature range up to 1000°C are necessary to repair forming dies operating in this range. Nickel-based alloys can be used for this purpose. The structural state of a nickel alloy layer deposited onto a KhV4F tool steel and then heat treated is investigated. KhV4F tool steel (RF GO...
Article
The structure and fracture behavior of a layer of maraging alloy deposited by laser surfacing on steel 3Kh3M3F is studied in the initial condition and after precipitation hardening at 550 and 600°C. Microhardness is measured in layer cross sections, and fractures after surfacing and aging are analyzed in an electron microscope.
Article
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The present paper deals with investigation of the possibility for application of laser welding-on technology for repairing the mold cavities of die casting dies. The surface of the samples, made of 32CrMoV12-28 (DIN) steel, was welded on by additional material – maraging alloy with chemical composition 0.02С–19.3Ni – 0.4V–2Cr-14.5Co–4.7Mo–0.25Al–0....
Article
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Abstract The aim of this study was to investigate the in-depth structure transformations of the laser-melted layers of the steels mostly used in tool production – T1 and D2 steels. The experiments of laser surface melting of T1 and D2 steels were done with continuous wave CO2 laser (wavelength λ = 10.6 μm). The sample’s microstructure was investiga...
Conference Paper
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Abstract: The paper studies the mechanism, kinetics and morphology of phase changes primarily at heating by concentrated energy flows (CEF). The objects of study have been a medium carbon low-alloy steel 37Cr4 (DIN), a high-carbon tool steel X210Cr12 (DIN) and grey cast irons with flake and spheroidal graphite (GG 15, GG 30, GGG 50 - DIN) treated b...
Book
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INTRODUCTION The application of concentrated energy for treatment of the materials takes an important place in the fundamental research of physics in the second half of 20th century, as the 1960 years are the period with significant discoveries. The intensive scientific and scientific-applied investigations, leading to the final goal - industrial...
Article
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Purpose: Purpose of the present paper is to investigate the fractographical peculiarities of the layers welded-on the surface of 30CrMoV12-28 (DIN) tool steel by laser with additional material of marraging alloy. Design/methodology/approach: The operation was accomplished on an AL 200 automatic weld-on machine with an Nd:YAG pulse resonator providi...
Article
The aim of this study was to investigate the in-depth structure transformations of the laser-melted layers of the steels mostly used in tool production–T1 and D2 steels. The experiments of laser surface melting of T1 and D2 steels were done with continuous wave CO 2 laser (wavelength λ = 10.6 μm). The sample’s microstructure was investigated, and t...
Article
Full-text available
The surface hardening by induction heating has many advantages over other methods of conventional thermal processing. It is known that this method is applied to achieve high hardness in the surface layer maintaining a resilient core. The implementation of the required thermal cycle depends both on the characteristics of the material undergoing proc...
Article
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Abstract: The paper presents a methodology based on the finite element method for analyzing the structural heterogeneity during thermal treatment by high frequency induction surface heating of details made of ferromagnetic Fe-C alloys. A multiphysics problem is analyzed combining the external electromagnetic effects with the heat distribution insid...
Article
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High-temperature gas corrosion of steel X6CrNiMoTi17-12-2 (1.4571) used for internal structure components of chemical reactors is studied. The condition of individual regions of the structure in various stages of damage is investigated.
Article
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This research aims investigation of the structural changes of austenite-ferrite steel 105MA (1.4892) subjected to the exploitation impact in the cement furnace. The chains were made of the investigated steel, working in the furnace high temperature zone. Their function is to transport and dry up the technological slurry (37% water) in temperatures...
Article
Full-text available
The surface hardening by induction heating has many advantages over other methods of conventional thermal processing. It is known that this method is applied to achieve high hardness in the surface layer maintaining a resilient core. The implementation of the required thermal cycle depends both on the characteristics of the material undergoing proc...
Article
Full-text available
This work aims to investigate the dimensions' accuracy in different schemes of laser beam movement during the laser cutting process. Samples with different thickness were made of DC01 EN 10130 steel. On each sample 32 round holes were cut in 4 rows by different laser beam movement schemes – linear, zigzag and random. The tests were made by pulse CO...
Article
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The present paper deals with the microstructure and hardness distribution in width and in depth of the surface layer of steel Ch18N10T GOST (AISI 321, EN X6CrNiTi 18-10) after surface melting by continuous wave CO2 laser. Light microscopy, XRD analysis and Vickers hardness testing (HV5 and HV0,05) have been used in our research. Phase analysis show...
Article
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The present work deals with a new method, technology and equipment developed for “in depth surface hardening” of large rotary details and tools, made of constructional, carbon, alloyed or low-alloyed tool steels. According to the technical requirements the details and tools have to possess hardened layer with specific shape, depth in the range 20-3...
Article
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Present paper deals with the structure transformations and hardness changes of the surface layer of Armco‐Fe after combined Plasma‐Arc Treatment (PAT) and Surface Plastic Deformation (SPD). The SPD was realized by rotational‐progressive movement of spherical contra‐body. Optical microscopy, TEM and durometric investigations were used. The proces...
Conference Paper
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The results of the technological investigation for hardening of large traversing wheels for mining-metallurgical equipment are presented in this paper. The details' dimensions are: diameter 630-2050 mm, traversing part width (height) 180-190 mm, hardened area of the working surface 6023-21532 cm 2 , weight 380-3400 kg. They were made of steel GS 42...
Conference Paper
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The present paper deals with the microstructure transformations, hardness changes and thermal fatigue micro-cracks in the melted zone of steels AISI H11 and H21 after laser surface treatment and thermal cycling. The tested samples are processed by a continuous wave CO 2 laser, operating in a mode providing melting of the surface layer. After that t...
Article
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The paper deals with the structure features of Fe-C alloys quenched by means of laser, electron beam and plasma arc. The martensite and residual austenite obtained are highly inhomogeneous. Their morphology and distribution depend both on the initial state before quenching and on the kinetics of the temperature changes. Four different structures of...
Article
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The present paper deals with the structure and properties of two types of tool steels with high chromium content (12% Cr) hardened by means of Concentrated Energy Fluxes (CEF). The treatment conditions are chosen to ensure liquid state transformations. The melted zone of the surface layer features a quasi-ledeburite structure, consisting of inhomog...
Conference Paper
Full-text available
In present work a new method, called by the authors " in depth surface hardening " is suggested, tested and implemented into production. It is applicable predominantly for large rotary details and tools as gears, traversing wheels, rolling disks etc. The technological process consists of volume heating in furnace, gas-flame or combined heating, nex...
Conference Paper
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Abastract: The corrosion damage of the main loaded construction elements of the chemical reactors was investigated. The details were made of steel 316Ti AISI (1.4571 or EN X6CrNiMoTi17-12-2). The reactors ensure NO for production of HNO 3 by nitrogen oxidization. The main reactions perform in the gas space around the construction elements in temper...
Article
Full-text available
In this paper is carried out a combined surface strengthening treatment of high chromium tool steels. Metallographic, X-ray and durometric analysis investigate the changes in the steel structure and its properties after surface plasma-arc and surface deformation treatment. During the high speed crystallization on the surface layer is formed fine de...
Article
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In laser treatment the graphite phase in the gray irons exerts significant influence on the structural changes in the surface layer. Ferrite, ferrite-pearlite and pearlite gray irons with lamellar, nestlike and globular graphite forms have been investigated. The samples were treated by continuous (CO 2) and pulse (Nd:YAG) laser. The graphite inclus...
Conference Paper
Full-text available
The present paper deals with the thermal fatigue micro-cracks in the laser-hardened layers of steels AISI L6, Н21 and Н11. The thermal cycling tests were carried out by heating in a molten aluminum alloy– Tmax=680 o C and cooling in water – Tmin=20 o C. The parameters of the maximum – l max and average – l av length of the micro-cracks in the cross...
Article
An affecting of concentrated energy fluxes on graphite inclusions and carbides in steel and cast iron is studied. It is shown experimentally that the processes of superspeed heating and cooling result in variation of graphite and carbides with formation of metastable phases. Microstructural analysis of gray iron and Kh12 steel revealed a nonequilib...
Article
Plasma-arc surface treatment is a rather simple and effective method for hardening steels. The increase in the resistance to external effects depends on the treatment parameters, the composition of the treated alloy, its initial structure, and the preliminary and post-heat treatment. The present work is devoted to the effect of the regime of plasma...
Article
Four steels (U8, 40, 45 and KhVG) were chosen to be investigated for the effect of plasma-arc surface treatment on structure and properties. Investigation results showed an increase in hardness and wear resistance of all steels. It is recommended that preliminary toughening be used to prevent crack initiation during plasma processing. A conclusion...
Article
The thermal effect of plasma-arc flow on surface of steel is investigated. The hardening was done by plasma-arc torch using direct arc. The theorethical examination of the thermal processes was made by mathematical model based on non-linear, two-dimensional heat conduction equation with corresponding initial and boundary conditions. The temperature...
Article
1. By plasma surface fusion in gray irons with lamellar and spheroidal graphite it is possible to obtain a hardened case with a hardness of not less than 580–600 HV and a structure characterized by high inhomogeneity, which consistas basically of quasiledeburite, cementite, carbides, martensite, residual austenite, bainite, sorbite, and graphite in...

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