Seznam abstraktů 2015: SEKCE C - VÝROBKY Z OCELI A JEJICH VLASTNOSTI

Bimetals targeted for industrial applications are usually designed to combine the properties of two dissimilar metals e.g. high strength, high hardness and low cost of steels with chemical resistive titanium. In this work the structure of one particular, but for industries very interesting, bimetallic system - Cr/Ni stainless steel clad with titanium has been studied. The material was prepared by explosion welding, a method capable of joining a wide variety of similar or dissimilar materials. Our analysis is based on X-ray micro-diffraction experimentation utilizing hard monochromatic X-rays focused down to micro-meter size. In this way the bimetal in bulk form was analysed and microstructural differences between the joined materials and their interface were determined.

Residual stress is an elastic stress in remaining part after the various stages of production. If residual stress values in rails are known, mechanical properties of rails and their effects in railway can be predicted. For this purpose, residual stresses of rails which had same casting number were measured by cutting and X-ray diffraction method in this study. Five rails were used for measurement. Firstly, according to the EN 13674-1 which is railway rails standard, strain gauge that is an electrical apparatus were used and one meter long rails were cut. After cutting, Hooke law was applied and residual stress values of rails were found. The residual stress values were around between 115 MPa and 180 MPa. In addition to standard residual stress measurement, X ray diffraction method was used to compare between destructive and non destructive techniques. Five specimens taken from five rails in the appropriate size for X-ray device. In this method stress values were around between 200 and 400 MPa. After residual stress measurement, brinell hardness test was applied to all samples and microstructures of rails were examined by SEM. The results show that, residual stress values in cutting method were different from X-ray diffraction method and residual stress could effect the rails hardness and microstructure.

A failure analysis has been performed to investigate the root cause for the rejection of the H10 (DIN 1.2365) hot work tool steel of the Hatebur forging machine. The punch is subjected to intensive thermal shocks, cyclically variable loads and high pressure at high temperatures. Although several surface defects are found related to different degradation phenomena, the higher thermal expansion occurred at the surface, compared with the inner part of the die, makes thermal fatigue the main failure responsible. In fact, thermal conductivity of an alloy will depend upon temperature and microstructure and therefore time and of course chemical composition. Damage analysis including dimensional control, surface analysis and metallographic analysis is completed by ThermoCalc and JMatPro calculations done for the standard 1.2365 tool steel. These are very useful tools to know in advance which are the stability ranges of the phases present (at equilibrium and non-equilibrium conditions), the elements in solid solution, the nature of the carbides, CCT curves and some other features for a better understanding of the failure analysis of the described system.

The subject of the research was a study of an elastic-plastic behavior of the materials, S355N steel and Domex700MC steel, which are used in welded construction parts of car bodies, the Domex700MC steel is used with the intention of decreasing the weight of the parts. The solved problem is to determine the local static parametres of the steels without the possibility to use the standard mechanical testing. In order to assess the strength decrease, the critical area was experimentally simulated. The main aim of the paper is to ascertain strength differences in the heat affected areas originating as a result of welding. As the main tool for the assessment of the elastic-plastic response, the methodology based on an instrumented penetration test with the use of a cylindrical indenter was chosen. For validation of the used method, the standard tensile test was performed. The aim of the work is to state optimal methods for determining a yield strength and to find a changes tendency in these areas. The comparison these steels revealed different changes due to welding process. The substantial decrease of the yield strength of Domex700MC was observed, on contrary to a stable, or partially increased, yield strength of the S355N steel. Structural analyses revealed a different material response of the evaluated steels, according to a different hardness in the heat affected zone.

On the surface of a 17-4 PH stainless steel, powders from the cermet group were submitted, using the plasma thermal spray technique on a METCO SULZER installation. Thereafter, a part of the sprayed samples were remelted at the surface with the Nd:YAG laser beam, on a Trumpf HL 123P LCU installation. The cavitation behavior of the samples was tested using an ultrasonic vibratory cavitation erosion equipment. The examination at the optical microscope and scanning electron microscope of the marginal layer and of the cavitated surface revealed the formation of a dense microstructure, finished by remelting, which explains the increase by 133% of the erosion resistance compared with the structural state obtained through 1050 ºC solution treatment with air cooling.

This work contains a detailed description of the influence of continuous heating from as-quenched state on the microstructure development in 17-4PH steel. The temperature ranges of the phase transformations occurring during heating from the as-quenched state (tempering) were determined from dilatometric tests. An interpretation of heating dilatometric curves was also presented. The temperature ranges of the occurred transformations during tempering of investigated steel were drawn on CHT (Continuous Heating Transformations) diagram. Moreover, the microstructure development in investigated samples, reflecting the extension of the phase transformations during tempering, was discussed too. The microstructure of the investigated steel was examined by a light microscope Axiovert 200 MAT and the Zeiss scanning electron microscope with Tident XM 4 system of the EDAX Company – energy dispersive X-RAY (EDS) analysis and JEM 200CX transmission electron microscope.
In as-quenched state the microstructure of investigated steel consist of low carbon martensite, ferrite  banding as well as of not dissolved during austenitizing niobium secondary carbides of MC type. The changes in microstructure of the investigated steel during heating from the as-quenched state results from precipitation of transition iron carbides, transformation of small amount of retained austenite into martensite and mainly the Cu phase precipitation. There were no significant changes in the chemical composition as well as in the morphology of the ferrite .

Soft annealing is important and time demanding part of bearing steel treatment process. The experimental program of accelerated carbide spheroidisation deals with significant shortening of the time necessary for producing steel microstructure consisting of ferritic matrix and globular carbides. Globular carbide particles and the grain size of the matrix are significantly smaller after accelerated carbide spheroidisation process in comparison with conventional long-duration soft annealing. This fact ensure finer martensitic structure after hardening and this shows that the microstructure and properties of final hardened product are dependent on previous spheroidisation annealing. Finer carbides in structure enhance hardness and facilitates carbide dissolution during austenitisation. This effect enables quenching temperature lowering and it reduces the energy demand of the quenching process and mitigates the risk of residual stresses and distortion. The main objective of the present research was to identify the effect of the initial size of carbides and prior austenite grains upon the final microstructure and hardness of induction-quenched 100CrMnSi6-4 bearing steel. Microstructure evolution and hardness were monitored during austenitisation, quenching and subsequent tempering. Microstructure and properties after accelerated treatment were compared with those after conventional long-duration treatment.

Today’s advanced steels are required to possess high strength and ductility. One way of achieving a favourable ductility level and a very high ultimate strength is to stabilize retained austenite in martensitic matrix. Among heat treatment processes capable of producing such microstructure, there is the Q&P process (Quenching and Partitioning). It can produce microstructures consisting of martensite and a certain amount of retained austenite, which exhibit strengths above 2000 MPa and elongation levels of 10-15 %. For some processes, it is appropriate to depress the Ms and Mf temperatures and modify the ferritic and pearlitic transformations to occur at lower cooling rates. By choosing an appropriate steel chemistry, the retained austenite can be stabilized in martensitic matrix at lower temperatures than in the ordinary Q&P process. Four steels were selected for the present experimental programme. Several heat treatment sequences were tried with these steels and multiple parameters were varied: austenitizing temperature, cooling rate, quenching temperature and the carbon partitioning temperature.
Final strengths of the steels were in the range of 1750-2400 MPa and their A5mm elongation level was up to 10 %. The morphology and distribution of retained austenite dictate the resulting mechanical properties. The microstructures were studied using transmission electron microscopy. The distribution and morphology of retained austenite were examined using diffraction analysis (SAED) and bright and dark-field illumination. X-ray diffraction was employed to measure the volume fraction of retained austenite. The values were between 9 and 15 %.

This work presents results of the analysis of phases formed in a SUPER 304H steel during a medium-term static isothermal ageing (675°C/15000 h). The investigations of the precipitates were especially focused on the occurrence of sigma-phase because its formation leads to the serious embrittlement.
The evaluation and distribution of brittle sigma-phase were determined on macroscopic level by using light microscopy/colour etching. The microstructure was also investigated on microscopic level by scanning electron microscope Tescan Lyra 3 and JEOL JSM 5410 equipped by electron backscatter diffraction and by transmission electron microscope Jeol 2100F. This work also investigates the effect of sigma-phase formation on the impact strength of an aged state of SUPER 304H steel. From comparison reasons the microstructure and the mechanical properties of the initial state were also investigated. It was found that the formation of brittle sigma-phase in the aged and therefore degraded microstructure led to the significant decrease of the impact strength. The influence of brittle sigma-phase on long-term durability of the degraded steel regarding its insufficient impact strength is discussed.

This study focuses on the characterization and examination of the fracture mechanisms acting simultaneously on the fractured surface of a of stainless steel turbine blade. This objective is reached using a variety of experimental approaches to determine the material's properties (chemical composition, morphology and crystalline structure) and the microscopic observation of the fractured surface. Major morphological variations are observed (microfacets, microcupules and streaks). Correlations have been established leading to the conclusion that competing fracture mechanisms are involved. The main fracture mechanism is intergranular thermo-mechanical fatigue starting at the border of the blade. However, intragranular fracture by stress corrosion under load, and intergranular fracture by creep are also operative simultaneously in the center of the observed surface. Various explanations for the premature fracture behaviour of the turbine blade are discussed

Several corrosion mechanisms are relevant to the LP turbine blades, including pitting, stress corrosion cracking (SCC), corrosion fatigue (CF) and erosion-corrosion (E-C). Considerable work has been performed to understand the individual mechanisms and the environmental conditions that lead to each of them. However, little progress has been made in understanding the interactions between these damage processes. For example, little is known about the transition of corrosion pits to cracks and the early stages of crack propagation. Different forms of corrosion are not necessarily operating independently. In some cases, one form of corrosion leads to another. For example, pitting can lead to crack formation and SCC, and erosion can lead to stress concentrations and CF. The corrosion resistance of materials used depends on the service environment. Many factors play a role in the corrosion. The pH, the oxygen content, and the specifics of the steam chemistry all affect corrosion susceptibility and subsequent pitting rates and CGRs. Considerable effort has been put toward understanding how these factors influence corrosion. This paper provides a summary of the most recent literature on corrosion mechanisms in LP steam turbines. Literature from various sources has been reviewed and summarized. The primary sources of information were corrosion science and corrosion engineering journals and corrosion conference proceedings and several Electric Power Research Institute (EPRI) technical reports. The literature review is followed with a gap analysis, during which several avenues for future research are identified.

Hydrogen diffusion characteristics were studied in C-Mn-Si-P TRIP 800 steel containing 0.05% of phosphorus. The steel was submitted to two-step annealing: firstly, in the intercritical region, secondly, in the region of bainite transformation. The steel was also tested in states corresponding to 5 and 10% tensile deformation. Steel microstructure was studied by light microscopy, transmission electron microscopy and X-ray diffraction. Hydrogen diffusion characteristics were studied by means of electrochemical permeation method. Two build up transients corresponding to lower and higher charging current densities as well as a decay transient were recorded during experiments. In the initial state, steel microstructure consisted of ferrite, bainite and approx. 9% of retained austenite. The tensile deformation resulted in a decrease of retained austenite content and in a considerable increase of yield and tensile strength. Hydrogen diffusion coefficients of the studied TRIP steel varied from 1.6.10-7 cm2.s-1 to 1.3.10-6 cm2.s-1. The lowest values corresponded to the initial state – without deformation; the highest ones corresponded to 5% tensile deformation. For one state, the lowest values of hydrogen diffusion coefficient were observed during the first build up transient. During the 2nd build up transient corresponding to the higher charging current density, hydrogen diffusion coefficients increased markedly reflecting thus the fact that hydrogen trapping was less pronounced. For decay transients hydrogen diffusion coefficients were situated between values obtained for the 1st and 2nd build up transients. Very high sub-surface hydrogen concentrations (up to 35 ppm!) were calculated from the permeation data.

A necessity of ever-more efficient use of energy and fuel in cars and machines has triggered the compelling demands for various lightweight structural materials. Steel remains most competitive and most widely used as engineering materials due to its abundance and versatility. But, it is now facing challenges from alternative lightweight solutions such as fiber-reinforced-plastic, magnesium, aluminium and/or titanium alloys. This situation is driven by the low strength-to-weight ratio (specific strength) of iron and steel, and the desire to improve such mechanical properties with other materials. Recent studies on low-density steels have revealed the effectiveness of alloying aluminium in increasing the specific strength by reducing density. However, the formation of brittle intermetallic compounds such a s FeAl (B2) and Fe3Al (DO3), with increasing aluminium content to further reduce the density, leads to poor ductility. Early this year, in a paper published in Nature (nature.com/articles/doi:10.1038/nature14144), we have shown that an FeAl-type brittle but hard intermetallic compound (B2) can be effectively used as a strengthening second phase in high-aluminium low-density steel, while alleviating its harmful effect on ductility by controlling its morphology and dispersion. However, the phase transformation mechanism during annealing of FCC/B2 dual-phase sheet steel is not clear. In this contribution, we report the evolution of microstructure during annealing of cold-rolled FCC/B2 dual-phase steel, trying to clarify the phase transformation mechanism based on the results of XRD, SEM and TEM. Finally, we correlate the microstructural changes with those of the mechanical properties.

Fe-25Cr-5Mo-0.82C alloy was synthesized in an arc furnace in high purity argon atmosphere and crystallized on water-cooled copper mould. The cross section of the ingot was examined by light microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD) and its Vickers hardness determined. The microstructure is composed of primary and secondary dendrites Fe-Cr-Mo solid solution with equiaxed and elongated morphology, depending on the nature of the crystallization. In the interdendritic regions complex M23C6 carbides are present. The average hardness of the alloy is 264±5 HV10.

A series of high entropy alloys (HEAs), CrxCuxMnFeNi, was produced by a vacuum arc-melting method. The multi-element systems can crystallize as a single phase, despite containing multiple elements with different crystal structures which often exhibit unusual properties. For this reasons they are significant interest of this materials. Their phases, microstructures and base mechanical properties at room temperature were investigated. It has been found a two face-centered cubic (FCC) crystal structure forms in these alloys with higher Cr chemical composition and a single solid-solution phase with a body-centered cubic (BCC) and two face-centered cubic (FCC) crystal structure forms in these alloys with higher Cu chemical composition. The different hardness between those alloys was observed.

This paper assesses the possibilities of graphitic cast iron parts and components of hydraulic turbines. The experimental program evaluated the behavior of spheroidal and vermicular graphite under conditions which simulated challenging work environment hydraulic machines. Five different graphitic irons allowed to assess the influence of the chemical composition, structure, basic metal matrix and character of graphite (shape, size and distribution) to the overall degradation of the material. The intensity of the damage was evaluated by testing the erosive wear, corrosion tests and metallographic analysis. Mechanical properties were verified by a tensile test, impact bending at room temperature and hardness test. The results confirmed the assumption that in achieving the appropriate structure, can spheroidal graphite cast respect predominant degradation mechanism of specific water machines and can be a suitable alternative to frequently used stainless steels.

Eutectoid or high carbon steel wire rods are widely used for the production of steel cable, metal cord, springs, and other wire products. In this regard, the main microstructural objective is to obtain pearlite with fine interlamellar spacing and also to control the mechanical strength. The best mechanical properties and wire drawability (diameter reduction) cabability are achieved by traditional lead patenting process at the temperature range of 480–600oC. However, due to some disadvantages such as low safety, environmental pollution and high investment costs, the application of this heat treatment method has been limited. In this research; the cooling behavior of 0.72 % carbon steel wire upon patenting using a lead bath and polymeric aqueous solutions were investigated. The experimental results showed that the high-carbon steel wire can complete the fine pearlite transformation in the aqueous solution, but the transformation temperature is higher and the transformation time is longer than that observed for lead patenting because of the continuous cooling transformation process in the fluid used as an alternative to molten lead.

The aim of present study is to investigate the effect of fusion zone (FZ) size on the mechanical performance of dissimilar DP780 dual phase steel and low carbon steel (LCS) joints. The results showed that the mechanical efficiency of the DP780/LCS joint is related to the FZ size. Regarding to FZ size two different modes of pullout and interfacial failure were perceived. It was noticed that the conventional recommendation of d=4t1/2 (t=sheet thickness) for sizing spot weld is not proper to determine the transition form interfacial to pullout failure in tensile-shear test. A simple analytical model was used to estimate the FZ size to ensure pullout failure in tensile-shear test.

Delamination or laminar flaws occur by inclusions such as Manganese Sulphur (MnS), hydrogen flakes, etc., in steels at manufacturing. A laminar flaw is a subsurface flaw parallel to the rolling direction of the plate, where the applied stress is typically parallel to the rolling direction. The definition of the laminar flaw is provided only in the ASME Boiler & Pressure Vessel Code Section XI. In addition, the ASME Code provides the allowable area of the laminar flaws and the combination rules for multiple laminar flaws.
Besides, a large number of laminar indications were detected in nuclear power reactor vessels. The observed indications were caused by hydrogen flaking induced during the manufacturing process. The ASME Code did not previously consider such a large number of laminar flaws and these laminar flaw angles are larger than the angles of the normal laminar flaw definition. The large tilted laminar flaws were named as quasi-laminar flaws. The ASME Code Section XI Committees have recently developed a rule that provides a grouping methodology for a large number of quasi-laminar flaws.
This paper introduces the definitions of laminar and quasi-laminar flaws and allowable area of laminar flaws provided by the ASME Code Section XI. Furthermore, this paper describes the grouping methodology for laminar and quasi-laminar flaws for flaw evaluation.

Steel 34CrMo4 is world widely used steel type for the variety of products, for example forgings, castings, shafts, bars etc. Desired mechanical properties of mentioned steel can be achieved by various treatments such as in a metallurgical process by titanium, niobium, vanadium, zirconium or nitrogen microalloying. Another suitable way how to achieve desired mechanical properties is use of different heat treatment modes due to excellent hardenability and temperability of steel 34CrMo4 or combination of these methods in thermomechanical forming. To achieve the best combination of mechanical properties, the grainsize is the one of the most important microstructural parameters. In this article, the development of 34CrMo4 steel grainsize together with the microstructure after four treatments of the material including as hot rolled state, after normalization annealing, normalization annealing with subsequent quenching and tempering and as final treatment of quenching and tempering were evaluated in relation to achieved values of mechanical properties after each treatment. Except the grainsize and microstructural investigation, as it was mentioned, also the evaluation of mechanical properties was a part of the experimental procedures. This procedure revealed that the most favorable and balanced mechanical properties (tensile strength of 1172 MPa vs. 61 J.cm-2 of CVN (at - 50 °C) and elongation of 16.1 %) in case of the treatment based on the quenching and tempering processes applied after the hot forming were achieved. Other treatment types also showed favorable results of mechanical properties however always lower in comparison with the above mentioned treatment.

For many years, low alloy CrMo(V) creep-resistant steels are applied for parts in the construction of energy and chemical structural units working under creep conditions (temperature about 550 °C and pressure about 15 MPa). These steels most often include creep-resistant steel 13CrMo4-4 (ČSN 15 121), 14MoV6-3 (ČSN 15 128), 10CrMo9-10 (ČSN 15 128). The main problem of steam pipe-lines made of these creep-resistant steels is a residual lifetime, because these parts are at presence in most Czech power plants at the end of their projected lifetime (2,5 . 10^5 hours).
This article deals with a lifetime assessment of one steam piping bend made of ČSN steel 15 128 (16MoV6-3) which was operated in conditions of creep damage in Czech fossil fuel power plant until its creep deformation obtained a limit of 1 percent. The overall condition of the bend was assessed using replica and cross section metallography including electron microscopy and by evaluation of mechanical properties in as-received state and after additional laboratory aging at 600 °C for up to 5 000 hours in air conditions. As the final result, the residual lifetime by using mathematical approaches and standards was estimated.

Creep test to the rupture of the dissimilar weld joint made of COST FB2 and COST F martensitic steels was carried out at temperatures ranging from 550°C to 650°C in the stress range from 70 to 220 MPa. Creep rupture strength was evaluated using Larson-Miller parameter. Assessment of microstructure, changes of precipitates and dislocation substructure were correlated with the creep strength. Critical zones of creep damage were determined. At lower temperatures and higher stresses the weld joint ruptured in the base material of COST F steel unaffected by welding, while at higher temperatures and lower stresses rupture occurred in the intercritical heated and fine-grained parts of heat affected zone (HAZ) of steel COST F. The processes of recovery in connection with precipitation of new particles of Laves phase and their coarsening is the main causes of decrease in creep rupture strength in the HAZ of steel COST F.

Active thermography is an advanced experimental procedure, which uses a non-contact measurement of tested materials thermal response after their external excitation. The material can be excited by a direct heating or using thermo-physical effects. Information regarding surface inhomogeneities, defects or material properties can be obtained by an evaluation of the measurement. Thermo-mechanical testing is one of the active thermography methods. It uses a cyclic mechanical loading for the tested material excitation. Material thermal response can differ based on intensity and mechanism of the loading and depending on effects, which can the loading accompany. Results of a thermographic measurement during a mechanical-cyclic tension test are presented and discussed in this contribution. Possibilities, methods and limitations of thermographic measurement during mechanical tests are discussed. A standard fatigue test results and a thermographic fatigue evaluation results were compared. The performed experiments showed that thermo-mechanical effects could be usable for materials fatigue properties evaluation.

The fatigue properties and microstructure of 08Ch18N10T steel used for production of the interior of a pressure water reactor VVER 440 were investigated. The influence of the service conditions was simulated using thermo mechanical strain controlled low cycle fatigue tests in the range from 150°C to 270°C. The cyclic stress behaviour was investigated at different total strain amplitudes. The cyclic test to rupture and fractographic analysis were carried out. Selected tests were interrupted at different stages of fatigue life and changes in microstructure were evaluated. Dislocation density was determined using transmission electron microscopy and X-ray diffraction. Dislocation rich and dislocation poor regions appeared as a result of cyclic loading. Vein structure and successive formation of persistent slip bands and dislocation cells were observed similar to those in FCC metal monocrystals fatigued at room temperature. Dislocation density increased considerably at the beginning of cyclic loading; maximum was reached at 0.05 of life time. The following gradual decrease was related to rearrangement of dislocations into the walls. Simultaneously the nature of the dislocations changed. A majority of the screw dislocations occurred in as received conditions and at the beginning of cyclic loading. The fraction of edge dislocations increased with the increasing number of cycles. Distance between persistent slip bands and size of dislocation cells were related to the distance between striations on the fracture surface.

The work describes obtaining experimental cast samples and rolled products of a new environmentally friendly free-machining stainless steel with no lead additions and containing low-melting and low-boiling elements like bismuth and calcium to improve machinability. Quaility characteristics of the steel (mechanical properties and non-metallic inclusions content) is in accordance with existing industrial standards for stainless steels. At the same time, the studied steel has lower by 0.001–0.002 % nitrogen concentration in rolled product, finer austenite grain (ASTM grain size number 6–7), more uniform distribution of free-machining additions along the height and cross-section of ingot or billet, better macrostructure (no macroscopic inclusions of addition elements). Machinability of the steel is 10–15 % greater than that of currently used analogous stainless steels.

In this work, the microstructure and mechanical properties of 100CrMnSi6-4 bearing steel after austempering heat treatment has been studied. Steel samples have been subjected to austenitisation followed by quenching and isothermal annealing at various temperatures in the bainitic transformation zone. After heat treatment the microstructure of the samples was investigated by means of transmission electron microscopy (TEM). It was revealed, that depending on parameters of the isothermal process two kinds of bainitic microstructure were formed: lower bainite at 260C and nanobainite at 320C. The mechanical properties of the heat treated samples have been investigated by use of: hardness test, Charpy impact test and static tensile test. It was found, that the 100CrMnSi6-4 steel with the nanobainitic microstructure exhibited higher strength along with the relatively high ductility and impact strength as compared to the values reported for steel with lower bainite. However for steel with lower bainite the hardness reached higher value than in nanobainitic steel.

Fracture surfaces of X70 steel DWTT broken samples are analyzed using statistical methods and fractal concepts. Applying self-affine fracture profile construction method and the first return probability method the Hurst exponent of ductile fracture areas has been found. Increasing percentage of ductile fracture on broken DWTT sample surface the Hurst exponent grows. Modeling of the DWTT fracture surface area by triangle net the statistical distribution of deviations between normal vectors of two neighboring elementary triangles has been discovered. This distribution behaves as Gaussian mixture. The mean value of deviation angles is lower for ductile than for brittle fracture regions whereas scatter of deviation angles are about the same for the both modes of cracking. Calculated fracture surface map of ductile fracture probability highly corresponds to real distribution of ductile and brittle fracture areas on fracture surface of the broken sample. Newly developed statistical methods of the fracture surface treatment of DWTT broken samples is useful tool for objective evaluation of ductile fracture percentage.

The variation of mechanical properties has been one of the continuing problems in hot-coils from the beginning of history of hot strip mill. In order to fulfil strength requirement, steel mill has used micro-alloyed high strength steel, containing titanium and niobium content. The addition of microalloying elements is economical in increasing the strength of the hot coils but, it is promoting anisotropy of mechanical properties and developing tensile properties variation further when hot rolling process conditions is not optimized properly. Steel coils for ERW casing and tubing pipes had been used with low carbon HSLA for better toughness but, there had been several typical difficulties such as surface defect at coil width edge due to peritectic range of carbon, lower tensile strength occasionally, and higher deviation of strength through a coil length. Huge efforts have been made to improve all the above mentioned weak points and then medium carbon HSLA steel has been developed with the optimized HSM processing control. Medium carbon showed less surface defect and the amount Ti and Nb contents were precisely selected with considering nitrogen content. Slab reheating temperature and retention time were optimized with consideration of dissolution kinetics of Ti and/or Nb content. Finishing temperature of rough rolling was defined based on recrystallization stop temperature. Moreover finish rolling temperature and coiling temperature were controlled tightly to reduce deviation of mechanical properties. All the details will be explained by the actual test results throughout coil length in this paper.

The microstructure of sub-zero treated Vanadis 6 tool steel has been examined with reference to the same material without the application of sub-zero treatment. Changes in the microstructure were recorded using light microscopy, scanning electron microscopy and transmission electron microscopy. The obtained results infer that the amount of retained austenite decreases with increased duration of sub-zero treatment. In other words, martensitic transformation manifests indications of time dependency in certain extent. The retained austenite is located mainly along the boundaries of martensitic domains. The martensite is refined due to sub-zero treatment. The carbide phases in as-quenched and/or sub-zero treated Vanadis 6 steel can be divided into eutectic phases (MC-carbides), secondary carbides (M7C3-phase) and small globular carbides that were identified as alloyed cementite. Sub-zero treatments refine the carbides and increase their number, particularly the small globular carbides, but it does not alter their nature. In a similar way to the martensitic transformations, the formation of small globular carbides manifests clear symptoms of time dependency, e.g. the duration of sub-zero treatment has an effect to the number of these particles.

AISI T1 high speed steel was modified with the replacement of 5% W by 1% V with and without extra carbon addition. The solidification reactions, temperature of precipitation starting of precipitated carbides were evaluated and compared with those of the standard AISI T1 steels.
In this work, thermo-dynamical equilibria have been calculated for high speed tool steel (HSS), AISI T1 composition belonging to the multicomponent system Fe–C–Cr–W–Mo as well as for its three different variant containing vanadium.
The experimental results show that the substitution of tungsten with vanadium and/or carbon in HSS 18-0-1 steel has influence mainly the three main solidification reactions during solidification of investigated steels from the melts. In addition, the substitution of tungsten with vanadium and/or carbon in HSS 18-0-1 steel has influence mainly the stability of gamma austenite and temperature of precipitation beginning of MC and M6C type carbides which have great influence on the chemical composition of precipitation.

The wear resistance of the material and the wear mechanism are considered as the most important property of tool materials. Diverse microstructures of tool materials may results in the wear mechanism changes. Therefore, the aim of this study was to determine the relationship between microstructure and wear resistance of the hypereutectoid steels. Material used in investigations was 90CrV6 hypereutectoid steel. The material was heat treated on the basis date from CCT diagrams. After heat treatment the tribological tests was performed by the use of T-05 tribological test machine. Metallographic analysis of the microstructure and microanalysis of the friction area for each samples after tribological test were performed. Analysis of the material microstructure and wear mechanism allow to define the relationship between the microstructure and wear resistance of the samples. For samples with perlitic matrix the friction coefficient increased simultaneously with their hardness. The increase of hardness for samples resulted in the decrease of average friction coefficient amplitude. Increase of the bainit volume fraction results in the decrease of average friction coefficient. The average friction coefficient changes were related with different wear mechanism for the bainitic microstructure. On the friction area of samples with bainitic microstructure the microridging mechanism was observed. For the marthensitic microstructure the wear was the biggest. The decrease of the wear resistance was probably related with the spalling of marthenit plates.

Subject of this article are the results of gradient tubes analysis (material of the shell: 347H, material of the body: 11CrMo9-10) based on Amec Foster Wheele specifications requirement. The aim of the analysis was the determination of mechanical and technological properties of gradient tubes in order to install the gradient tubes in a commercial boiler. The NDT, outer shell thickness measurement, chemical analysis, tensile testing at 20°C and at elevated temperatures, flattening test, leak tightness test and metallographic examination of longitudinal and cross sections were used to prove the quality of gradient tubes. For this contribution, the gradient tubes (38mm outer diameter, 5mm wall thickness which austenitic shell is ~ 0,8mm thick) made by BENTELER were used.
The work was done within the project GRAMAT supported by EU Research Fund for Coal and steel (www.gramat.org). The projects main aim is to acquire knowledge necessary to develop new cost-effective manufacturing technology of boiler tubes made from semi products with through thickness gradient chemical composition, tailored to carry both creep loading (low alloyed body) and fireside corrosion (high alloyed shell). Gradient tubes for the project are manufacturing in production facilities in BENTELER – Germany and PODBREZOVA IRONWORKS – Slovak Republic from semi products manufactured by unique casting technique (ZDAS – Czech Republic); and to be tested by Amec Foster Wheeler – Finland in selected biomass boiler.

The hereby work presents the influence of the additional annealing prior to heat treatment on the final properties of the steel.
Often companies despite ordered materials in softened conditions performed further annealing. In many cases it is just a waste of energy and money because there is already an adequate condition to perform heat treatment.
In hereby work two widely used materials were selected 100Cr6 and 145CrV6 steels. Two sets of samples were prepared first in the delivery state and second after additional annealing carried out in the laboratory furnace. For each steels a typical heat treatment simulation was performed on the L78 R.I.T.A. (Rapid Intensive Thermal Analysis) dilatometer made by Linseis. Critical temperatures and thermal expansion coefficient were determined and compared. The microstructures for each set of samples before and after heat treatment were compared. Obtained results allowed to determine the suitability of an additional soft annealing process.

Favorable combinations of high strength and good formability of Dual-phase (DP) steels made them a potential candidate for the fabrication of automobile body frame structures. Because the primary method that is used in welding of automotive structures is resistance spot welding (RSW), then there is increasing demand to better understanding of failure mode of DP steels in RSW. The aim of this research is to investigate the microstructure and mechanical properties of DP980 dual phase steel joints. The microstructural results showed that the fusion zone (FZ) consisted of mostly martensite. On the other hand the heat affected zone (HAZ) could be divided to three distinct areas with different hardness. The outer part of the HAZ relatively consisted of tempered martensite with the hardness lower than the base metal (BM). The tensile-shear test results showed that there is a transition from interfacial to pullout failure mode in DP980 resistance spot welds by increase in nugget size.