Papers by Danail Gospodinov
METAL 2021 Conference Proeedings, 2021
As the tendency to lower the mass of welded constructions emerges in the last years, the use of h... more As the tendency to lower the mass of welded constructions emerges in the last years, the use of high strength steels finds a wide range of applications in industry, namely, in reliable highly loaded welded constructions. Following the commonly accepted rules, these steels are suitable for welding. The most used welding methods for welding of high strength steels are submerged arc welding and MAG welding. The mechanical properties of weldment of high strength steels will depend on several different welding details, such as cooling time in the range from 800 to 500 °C-t8/5, the used filler materials, welding protection, etc. Nevertheless, data on the influence of joint preparation, and specifically of the weld gap, are limited or absent. Joint preparation of high strength steels is typically performed according to EN ISO 9692-1:2013 and EN ISO 9692-2:2001. When the welding gap differs significantly from the standards recommendations, it is of great importance to research and to determine the influence of the weld gap on weldment properties and structure. This study introduces the findings on the influence of joint preparation on some mechanical properties of high strength steel S700MC weldments produced by submerged arc welding.
In this paper, the influence of low pressure conditions on the quenching ability of specialized v... more In this paper, the influence of low pressure conditions on the quenching ability of specialized vacuum quench oils in the cooling vacuum chamber is considered. It is demonstrated that the decrease of the chamber pressure causes a considerable reduction of the examined media cooling ability. It is proved that the cooling ability rate gradient is not the same for the different oils. That’s why the knowledge of oils’ cooling ability at atmospheric pressure conditions is not sufficient for the proper vacuum quenching oil selection.
This paper provides a classification of titanium and titanium alloys associated with their chemic... more This paper provides a classification of titanium and titanium alloys associated with their chemical composition and structural state after annealing. Described are their physical, mechanical and technological properties. A short analysis of areas of application of these materials is made.
In the present work, the process of water quenching of a steel cylindrical body imitating a stepp... more In the present work, the process of water quenching of a steel cylindrical body imitating a stepped shaft is considered. The cooling function was determined experimentally, based on which the heat transfer coefficient was determined according to an existing methodology. The results obtained are used as input data for simulation using the finite element method. Results are obtained for the cooling functions at different points in two sections of the shaft. The results of the simulation are compared with the CCT curves and with the measured hardness at these points.
Journal of Achievements in Materials and Manufacturing Engineering, 2020
Purpose: To present a technology for hardfacing of metal-cutting tools by arc welding in vacuum. ... more Purpose: To present a technology for hardfacing of metal-cutting tools by arc welding in vacuum. Design/methodology/approach: The experiments were carried out using an installation for arc welding in vacuum. Objects of research were metal cutting tools (lathe knives), made of high-speed steel HS6-5-2 on a base metal of structural steel C45. The structure, hardness and wear resistance after hardfacing and after a triple tempering at 560°C have been determined. The heat resistance of the obtained instruments has been examined. Findings: The microstructural analysis showed that the structure of the built-up layer consisted of martensite, retained austenite and carbides. This was confirmed by the values of measured hardness after welding which were about 63-64 HRC. The triple tempering led to an increase in hardness by 3-4 HRC. It was found that the built-up layers (cutting edges of tools) retain their hardness (HRC=63-65) up to a temperature of 615-620°C, which shows that the heat resi...
Journal of Achievements in Materials and Manufacturing Engineering, 2016
Purpose: The aim of this paper is to determine the conditions of steady arc ignition invacuum and... more Purpose: The aim of this paper is to determine the conditions of steady arc ignition invacuum and the effect some of the main operating parameters have on the life of tantalumfoilhollow cathodes.Design/methodology/approach: The experiments presented in this paper have beencarried out with equipment for vacuum hollow cathode arc processing similar to whatis used in the industry. In order to find out if steady arc ignition is possible, a two-levelfactorial experiment has been conducted helping to establish the corresponding regressiondependency between the factors examined and the starting parameter values. The evaluationof hollow cathode operational life is based on three criteria: indirectly, considering their erosion[μ]; establishing possible damage on their surface which leads to plasma-forming gas leakageand makes the arc unsteady; taking into account the critical change in the cathode shape.Findings: It has been established that when a multi-stage arc torch is using the necessar...
Procedia Engineering, 2011
Samples of Armco iron with electrolytic Fe coating were hardened by vacuum oxy-nitrocarburizing a... more Samples of Armco iron with electrolytic Fe coating were hardened by vacuum oxy-nitrocarburizing at low temperature. The carbon amount (CO 2) in the nitriding atmosphere (NH 3) was 10 vol. % at a pressure of 8.10 4 Pa and the process time was 7 h. The influence of the structural difference on the depth profile, hardness distribution and Xray diffraction pattern of the oxy-nitrocarburized specimens were performed. The indicated parameters of the modified surface of electrolyte-precipitated Fe were compared to those of recrystallized at low-temperature and fully annealed in vacuum and oxy-nitrocarburized specimens. The results confirmed formation of "and ε-phases in the compound layer in all samples in different proportions. A significant difference in the phase distribution between the electrolyte-precipitated Fe layer and the substrate was demonstrated due to the ultra fine grained structure of the iron.
The ultra fine-grained (UFG) metallic materials are widely investigated during the past years wit... more The ultra fine-grained (UFG) metallic materials are widely investigated during the past years with a view to the opportunities for various industrial applications of their improved mechanical and technological properties. Most of these materials are produced ...
The 7075 aluminum alloy is well known in practice and it is utilized for many applications in ind... more The 7075 aluminum alloy is well known in practice and it is utilized for many applications in industry, especially in aircraft industry. A considerable amount of articles has been published in regard to plastic flow of this material [1], [2]. Unfortunately, there is lacking of information about plastic flow curves of 7075 aluminum alloy in ultra-fine grained (UFG) condition. This fact unusually make difficult to simulate different forming processes of above-mentioned material.
This paper presents research on vacuum quenching oils at different pressure conditions during the... more This paper presents research on vacuum quenching oils at different pressure conditions during the cooling stage of the heat treatment process. The aim of this work is to reveal the influence of the pressure, agitation (laminar or turbulent flow), and oil temperature on the cooling oil. Medium and low viscosity oils are investigated. The research is novel because it expands knowledge of quenching oil behavior at low and high pressure conditions (from 1 mbar to 2.5·103 mbars). The findings are presented as integral diagrams (time-temperature and cooling rate versus temperature curves) and tables of the local cooling ability valuations. It is found that the pressure in the chamber is the main factor that influences the cooling ability of the examined media. The trend of pressure influence can be generalised as: the greater the cooling oil ability, the stronger the pressure value influence. In addition to the above, the possibility of routine change of the pressure makes it a prospective factor to control the cooling oil ability.
Samples of Armco iron with electrolytic Fe coating were hardened by vacuum oxy-nitrocarburizing a... more Samples of Armco iron with electrolytic Fe coating were hardened by vacuum oxy-nitrocarburizing at low
temperature. The carbon amount (CO2) in the nitriding atmosphere (NH3) was 10 vol. % at a pressure of 8.104
Pa and
the process time was 7 h. The influence of the structural difference on the depth profile, hardness distribution and Xray
diffraction pattern of the oxy-nitrocarburized specimens were performed. The indicated parameters of the
modified surface of electrolyte-precipitated Fe were compared to those of recrystallized at low-temperature and fully
annealed in vacuum and oxy-nitrocarburized specimens. The results confirmed formation of J’- and İ-phases in the
compound layer in all samples in different proportions. A significant difference in the phase distribution between the
electrolyte-precipitated Fe layer and the substrate was demonstrated due to the ultra fine grained structure of the iron.
The sheet metal forming operations are carried out performed under conditions of biaxial tension ... more The sheet metal forming operations are carried out performed under conditions of biaxial tension through hydraulic bulging is a w idespread method in the engineering practice both for manufacturing of specific spatial parts and for mechanical testing at complex stress state. In this article is presented an autonomous laboratory> unit for sheet metal testing by hydraulic bulging. Key words: plastic anisotropy, sheet metal testing, uniaxial tension
Abstract: The objective of this investigation was to define the influence of cold plastic deforma... more Abstract: The objective of this investigation was to define the influence of cold plastic deformation
on the mechanical and magnetic properties of X5CrNi18-10 austenitic steel. A sheet can easily be formed to
complex shapes, but the regions suffering severe deformation will transform to another phases and generate
a high strength. The major drawback of this approach and these materials in general, is the unpredictable
mechanical properties. To predict the properties after deformation and during service it is vital to foresee the
amount of martensite formed during certain conditions.
Key words: chromium–nickel austenitic steel, plastic deformation at low temperature, metallic sheets.
Materials Performance and Characterization, 2014
Ultra-fine grained aluminum alloys AA5005, AA7075 and commercial pure aluminum, fabricated by sev... more Ultra-fine grained aluminum alloys AA5005, AA7075 and commercial pure aluminum, fabricated by severe plastic deformation, are investigated in this paper. Flow stress curves have been obtained by cold compression tests. The shape of this curves are discussed and some comments and recommendations according their approximation and using in metal forming simulation software are given. Directions for additional scientific investigations are pointed.
Solid-state carburizing capacities In the world crises time there is a decrease in the amount
of ... more Solid-state carburizing capacities In the world crises time there is a decrease in the amount
of the metallurgical, machine-building and metal working production. This influence the working conditions in
the companies which turn from large-scale to single-case production. This is the reason for searching
adequate, even old-fashioned but economical methods in machine-building technologies.
The article deals with the solid-state carburizing capacities for treating constructional steels in respect
to increase their endurance and reliability. Carbon saturation of surface welded layers on steel substrates is
investigated. The afterward heat treatment is discussed.
Key words: solid-state carburizing, surface welded layers, heat treatment.
Acivating additions for solid-state nitriding and carburizing. The main component in the
nitridin... more Acivating additions for solid-state nitriding and carburizing. The main component in the
nitriding and carburizing solid powders is charcoal. Activating additions are supplemented to the powders in
order to intensify saturation and regulate the gases’ potentials in the retort during these thermo-chemical
processes. The article presents different literature data as well as many years experienced practical results
of solid-state nitriding, carburizing and other thermo-chemical treatmens carried out at “METALIKA-MIST” Ltd
- Ruse.
Key words: nitriding, carburizing, solid powders, activating additions.
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Papers by Danail Gospodinov
temperature. The carbon amount (CO2) in the nitriding atmosphere (NH3) was 10 vol. % at a pressure of 8.104
Pa and
the process time was 7 h. The influence of the structural difference on the depth profile, hardness distribution and Xray
diffraction pattern of the oxy-nitrocarburized specimens were performed. The indicated parameters of the
modified surface of electrolyte-precipitated Fe were compared to those of recrystallized at low-temperature and fully
annealed in vacuum and oxy-nitrocarburized specimens. The results confirmed formation of J’- and İ-phases in the
compound layer in all samples in different proportions. A significant difference in the phase distribution between the
electrolyte-precipitated Fe layer and the substrate was demonstrated due to the ultra fine grained structure of the iron.
on the mechanical and magnetic properties of X5CrNi18-10 austenitic steel. A sheet can easily be formed to
complex shapes, but the regions suffering severe deformation will transform to another phases and generate
a high strength. The major drawback of this approach and these materials in general, is the unpredictable
mechanical properties. To predict the properties after deformation and during service it is vital to foresee the
amount of martensite formed during certain conditions.
Key words: chromium–nickel austenitic steel, plastic deformation at low temperature, metallic sheets.
of the metallurgical, machine-building and metal working production. This influence the working conditions in
the companies which turn from large-scale to single-case production. This is the reason for searching
adequate, even old-fashioned but economical methods in machine-building technologies.
The article deals with the solid-state carburizing capacities for treating constructional steels in respect
to increase their endurance and reliability. Carbon saturation of surface welded layers on steel substrates is
investigated. The afterward heat treatment is discussed.
Key words: solid-state carburizing, surface welded layers, heat treatment.
nitriding and carburizing solid powders is charcoal. Activating additions are supplemented to the powders in
order to intensify saturation and regulate the gases’ potentials in the retort during these thermo-chemical
processes. The article presents different literature data as well as many years experienced practical results
of solid-state nitriding, carburizing and other thermo-chemical treatmens carried out at “METALIKA-MIST” Ltd
- Ruse.
Key words: nitriding, carburizing, solid powders, activating additions.
temperature. The carbon amount (CO2) in the nitriding atmosphere (NH3) was 10 vol. % at a pressure of 8.104
Pa and
the process time was 7 h. The influence of the structural difference on the depth profile, hardness distribution and Xray
diffraction pattern of the oxy-nitrocarburized specimens were performed. The indicated parameters of the
modified surface of electrolyte-precipitated Fe were compared to those of recrystallized at low-temperature and fully
annealed in vacuum and oxy-nitrocarburized specimens. The results confirmed formation of J’- and İ-phases in the
compound layer in all samples in different proportions. A significant difference in the phase distribution between the
electrolyte-precipitated Fe layer and the substrate was demonstrated due to the ultra fine grained structure of the iron.
on the mechanical and magnetic properties of X5CrNi18-10 austenitic steel. A sheet can easily be formed to
complex shapes, but the regions suffering severe deformation will transform to another phases and generate
a high strength. The major drawback of this approach and these materials in general, is the unpredictable
mechanical properties. To predict the properties after deformation and during service it is vital to foresee the
amount of martensite formed during certain conditions.
Key words: chromium–nickel austenitic steel, plastic deformation at low temperature, metallic sheets.
of the metallurgical, machine-building and metal working production. This influence the working conditions in
the companies which turn from large-scale to single-case production. This is the reason for searching
adequate, even old-fashioned but economical methods in machine-building technologies.
The article deals with the solid-state carburizing capacities for treating constructional steels in respect
to increase their endurance and reliability. Carbon saturation of surface welded layers on steel substrates is
investigated. The afterward heat treatment is discussed.
Key words: solid-state carburizing, surface welded layers, heat treatment.
nitriding and carburizing solid powders is charcoal. Activating additions are supplemented to the powders in
order to intensify saturation and regulate the gases’ potentials in the retort during these thermo-chemical
processes. The article presents different literature data as well as many years experienced practical results
of solid-state nitriding, carburizing and other thermo-chemical treatmens carried out at “METALIKA-MIST” Ltd
- Ruse.
Key words: nitriding, carburizing, solid powders, activating additions.