Seznam abstraktů 2018: SEKCE C - VÝROBKY Z OCELI A JEJICH VLASTNOSTI

Ledeburitic steels based on Cr- or Cr-V alloying belong to the most important cold-work tool materials for large series manufacturing. To enable high production stability, the tools must withstand various types of degradation processes. On the first side, it concerns the plastic deformation and wear. Therefore, the materials should have high hardness, tensile and/or compressive strength and wear resistance. Alternatively, the materials have to resist the brittle fracture, i.e. they must manifest sufficiently high toughness and fracture toughness. The review paper deals with the effect of heat treatment procedures on the microstructures of Cr- and Cr-V ledeburitic steels. Particular attention is paid to the effect of the austenitizing and quenching, tempering and sub-zero treatment on the main microstructural features of the steels, and their relation to the main mechanical properties.

The article briefly describes methods of cryogenic heat treatment. The progressive heat treatment procedures applied on the test knives are also mentioned. The knives were made of tool steel 1.2362 – 5%Cr supplied by Moravia steel a.s. Testing the knives was done in operation in cutting machines. The result of the tests was the increased life of knives processed by cryogenic heat treatment, compared to the classically heat-treated knives. The mechanical properties and the amount of wear were evaluated on the knives. The microstructure was also evaluated using an electron microscope.

Cold rolled and batch annealed 51CrV4 spring steel grade is preferred in many industrial areas such as automotive component and metalware production. Product quality, ease of manufacturing, final mechanical and microstructural properties are affected by processing route and process parameters. Design of this process route and its parameters is essential to meet the requirements of different end applications of 51CrV4 steel. In this study, various batch annealing profiles are employed on cold rolled steel to observe the effects of soaking temperature and soaking duration on microstructural and mechanical properties of material. Ideal time-temperature profiles, which provide a homogeneous microstructure, optimized spheroidization of carbides and hence the required mechanical poperties of different specifications are developed. Mechanical testing results also agree well with alterations in annealing parameters.

Two cast and heat-treated NbC and TaC – strengthened superalloys have been developed for a precision casting of spinner discs for glass wool industry. In this work constant load creep tests in tension were carried out in argon atmosphere at three testing temperature 900, 950 and 1000°C and at the initial applied stresses ranged from 40 to 80 MPa. All the tests were continued until the final fracture. The following creep testing creep specimens were prepared for microstructural and fractographic examinations by means of light and scanning electron microscopy. It was found that carbide precipitation is the primary strengthening mechanism in the cobalt-base superalloys under investigation and the amount, morphology and type of carbides have the decisive effect on the creep properties including creep damage and fracture processes. A mutual comparison of creep characteristics of the investigated superalloys under comparable creep loading conditions showed that NbC-strengthened superalloy exhibited longer creep life than TaC-strengthened one. By contrast, NbC-superalloy exhibited a more brittle character of creep fracture than TaC-strengthened superalloy. Therefore, the purpose of this study was to investigate in more details the effect of the creep damage evolution and creep fracture mechanisms and modes. It was found that different creep fracture plasticity of both superalloys could be explained by different way of development of creep damage and fracture. Whereas in NbC-strengthened superalloy the dominating damage is the interface decohesion between carbide and the matrix, in the case of TaC-strengthened alloy is the critical damage mechanism very localized destruction mostly M23C6 carbides.

Presented data were obtained within European FP7 project MATISSE effort to model the softening and relaxation behaviour of Grade 91 steel under creep-fatigue loading. The supporting test effort was focused on strain controlled low cycle fatigue tests with different tension hold times. Inside of this article the methodology for evaluation was processed and consulted. Achieved results were compared together with another workplace abroad.

This work deals with properties and microstructure of heat resistant chromium steel P92. This steel is considered to be one of the best modified 9-12 % Cr steel for the construction of coal power plants with ultra-super-critical steam parameters. High creep rupture strength of steel P92 is determined by its chemical composition and by microstructure as well. Optimal microstructure of steel P92 is ideally composed of homogeneous martensite and fine dispersion of secondary particles.
During the experiments one P92 heat with an occurrence of more than 20 % delta ferrite was produced. The results of this heat are confronted with mechanical and creep properties of another heat without delta ferrite in the microstructure. Conclusions relate to the influence of delta ferrite on mechanical and creep properties of P92 steel.

The paper deals with four temperatures of high temperature austenitization and two durations of microstructure and heat-resistant 9CrNB steel substructure. The microstructure consists of lathe martensite and bainite, and the presence of delta ferrite was demonstrated at specific austenitizing temperatures. Transmission electron microscopy showed that the substructure of all conditions was formed by martensite and bainite laths. They were identified rod-shaped respectively needle-shaped particles extracted into carbon replicas from these laths. Carbide particles M3C-type were found by using diffraction analysis. Rod-shaped particles were excluded at the boundaries of the delta-ferrite grains, and these particles are MC type carbide particles according to the electron diffraction analysis. The presence of delta ferrite grains at the austenite grain boundary effectively prevents their migration during austenitization and thus limits the growth of the mean austenite grain size in the austenitisation temperature range of 1190 °C to 1230 °C. The presence of a larger amount of oval particles of the precipitate was identified at the austenitizing temperature of 1070 °C. It is very likely that they are NbC carbide particles according to the diffraction analysis. These particles should prevent a significant increase in austenite grain. This type of particles is observed only sporadically at a temperature of 1150 °C that did not produce more delta-ferrite grains. The loss of the braking effect of NbC carbide particles on the migration of austenite grains caused the formation of significantly larger austenite grains at these austenitizing temperatures compared with the austenitizing state at 1070 °C.

This paper document the microstructure changes of steel SUPER 304H caused by high temperature overheating (1140°C, 15 hours). The experimental material was firstly isothermally laboratory exposed about 30 000 hours at temperature 650°C and then subject to high temperature overheating. Main focus is given to microstructural changes and precipitation processes. The article summarized influence of those changes to practical applicability of steel SUPER 304H for ultra-super critical coal power plants. Base material, laboratory expose state and overheated state ware compared. For comparison was used combination of color etching and optical microscopy method, electron scanning microscopy and energy dispersive spectroscopy analysis.

Crack initiation of the ferritic martensitic steel T91 and austenitic steel 316L are currently under study. This is due to the evaluation ongoing in the EC for materials compatibility in Heavy Liquid Metals environment, and, in particular, the issue of Liquid Metal Embrittlement. Slow Strain Rate Tensile (SSRT) tests were performed with flat specimens in PbBi at 300°C with oxygen content in the range of about 10-6 wt.% down to 10-12 wt.% and, for comparison in air, up to the point of maximal stress (without or with minimum necking). Tests were performed with flat tapered specimens, which were meant to create a uniform variation of stress along the gauge length, with the maximum stress concentrated in the narrowest cross-section area. The cracking mode of T91 changed due to environment influence when compared to the testing in air, however the austenitic 316L seemed unaffected, even though at the lowest oxygen content, the crack numbers and morphology changed. Post-tests examinations were carried out with a Scanning Electron Microscope.

Steel 34CrMo4 is commonly used for a variety of high strength application in many industrial areas. Some of such applications are exposed to environment containing H2S and high humidity that can cause the SSC issue. Among such applications, high strength tubes, pipes and also high pressure steel cylinders belong. To prevent any possible failure of above mentioned applications, variety of destructive methods can be applied e.g. tensile testing, CVN testing and microstructural observations. However, such procedures demand the component extraction with subsequent samples preparations. All of these methods are highly cost inefficient and take significant amount of time. As an alternative method to detect a possible initiation and propagation of SSC, the AE testing and observation takes place. This method is one of a few possible in-situ methods to precisely distinguish early phases and propagation phases of such degradation process. This paper is focused on in-situ observation of entire SSC during simulated accelerated test on 34CrMo4 of strength reaching to 1215 MPa.

Growing efforts for weight and gas emission reduction in automotive industry led to development of advanced high strength steels (AHSS), which show a very good strength-to-ductility balance. However, AHSS can be prone to hydrogen embrittlement, which leads to the degradation of mechanical properties, mainly ductility, in the presence of hydrogen. It can limit the utilization of AHSS for the car body construction. Therefore, it is crucial to assess the risks associated with hydrogen absorption to AHHS during corrosion processes.
In this work, a relatively new technique for hydrogen detection was used. Scanning Kelvin probe force microscopy (SKPFM) enables the measurement of Volta potential difference with very high spatial resolution. Hydrogen was found to influence Volta potential difference due to reduction of Fe3+ species leading to changes in the Fe3+/Fe2+ ratio, which can be detected by SKPFM. SKPFM measurements of changes in Volta potential difference were performed on two grades of AHSS in order to study hydrogen entry and absorption as a result of atmospheric corrosion.

Nitriding is the one of well-known processes which increases material hardness and wear resistance. In the significant way morphology of nitride layer influent on material surface properties. Different parameters of a nitriding process could change the nitride layer morphology, such as thickness, morphology, volume fraction and type of nitrides. In the present paper the analysis of the pitting wear of 30CrMoV9 nitriding steel is presented. The tribology tests, microstructure analysis, microhardness and electron microscopy observations of the wear track ware performed. Main analyzed parameters ware the pits shape and its correlation to the phase compositions of the nitride layer. The results of the performed investigations allow to state that for samples with the nitrocarbides layer at samples surface, the pitting cracks were initiated at the material surface. Due to the fatigue character of wear cracks propagated through the nitride layer and change the propagation direction in the subsurface area. Samples without the visible layer of carbonitrides at the surface was characterized by the initiation of the pitting wear beneath the surface in the regions with the lower hardness. Pitting initiation at subsurface area resulted in the quicker formation of the pits, more intense delamination and degradation of the nitride layer than in the case of samples with the carbonitrides layer formed at material surface.

Intercritical batch annealing requires special steel alloys to obtain lower AC1 temperatures for austenite transformation and to retard the cementite precipitation at slow cooling rates. In other words, insights on the effects of parameters such as annealing temperature, isoholding time, cooling rate etc., are necessary to design the dual phase microstructure. In this study, the effects of intercritical batch annealing temperature and time on the phase transformations were studied for a 0.2C-2.3Mn-1.0Cr steel. Light and scanning electron microscopy was performed to investigate the effects of intercritical temperature and time on phase transformations. Dilatometer experiments were conducted at temperatures from 650 °C to 710 °C for 2-7 hours to develop similar microstructures of hot spot and cold spot of batch annealing coils. It was found that dual phase microstructure can be obtained after batch annealing. Microstructural investigations agree well with dilatometry results.

Since it is difficult to obtain accurate Time – Temperature – Transformation (TTT) data for steels where the incubation time is quite short, and / or the transformation rate is slow at tested temperature, extracting isothermal kinetics from non-isothermal data become important. The method proposed by Rios has been widely used. However, accurate and large amounts of data are required to use Rios's method which is almost impossible to achieve. In the present paper, a modified Rios's method is proposed to obtain isothermal transformation kinetics from continuous cooling transformation data. Based on additivity rule and John – Mehl – Avrami - Kolmogorov (JMAK) equation, the interpolating function of Q-T (Cooling rate vs Temperature) curve, which is key to extracting kinetic parameters, is determined by small amounts of data. Using experimental data obtained by dilatometry, the kinetic parameters n and k are successfully extracted. The reliability of the proposed method is verified by comparing calculated and measured transformation data.

Grain boundary segregation is a phenomenon studied both experimentally and theoretically for decades. The interest in this phenomenon is evoked not only by its close relationship to temper embrittlement but also by its ability to stabilize nanocrystalline structures by solute segregation in the concept of Grain Boundary Engineering.
A full description of the exact temperature and solute concentration dependence of grain boundary segregation requires reliable values of segregation enthalpy and -entropy as well as solute interaction coefficients. However, experimental studies of all these values are rather limited, and theoretical studies are usually restricted to segregation energy at zero Kelvin. It is obvious that such a database does not allow a full description of the grain boundary segregation in most systems of interest.
In this contribution we present a semi-empiric method enabling prediction of all required thermodynamic parameters - ideal enthalpy, ideal entropy and real binary interaction (Fowler) coefficient - which, in combination, describe fully the segregation of any segregant at any temperature and at individual grain boundaries. This method is based on the relationship between the segregation enthalpy and the solid solubility limit, and on the enthalpy-entropy compensation effect. The data for numerous solutes in alpha iron are predicted and they are compared to available data in the literature.

In the present study, state-of-the-art 3D APT technique was employed to systematically study the effect of thermo-mechanical treatment on the formation of nano particles during austenite to ferrite transformation in a Ti-Mo microalloyed steel of composition .03C-1.52Mn-0.2Si-0.03Al-0.08Ti-0.22Mo (wt%).
Industrial coiling was simulated using Servotest TMTS 500 KN (Thermomechanical simulator). Through 3D APT, it was possible to successfully characterise (Ti,Mo)C nano clusters of size 1-2 nm which had formed at the initiation of the austenite to ferrite transformation (within 30s). These nano clusters were rich in C and Ti and contained less than 50 atoms. As the transformation progressed these these nano clusters grew into nano precipitates of size range 2-5nm and contained more than 50 atoms. The variation in the composition of the nano particles with the progress of transformation was a very interesting finding. This is the first time that nucleation and growth of nano particles in Ti-Mo microalloyed steel was studied systematically using 3D APT. In future APT can be employed to study the effect of processing parameters such as strain, strain rate and coiling temperature on nano particles. An interesting study on the coarsening behaviour of the nano particles during tempering has also been taken up.

100Cr6 steel is most widely used material for bearing applications. Carbide size distribution is crucial due to the detrimental influence of coarse carbides on fatigue life. Dissolution behavior of (Fe,Cr)3C carbides in bearing steel was investigated to achieve homogeneously dispersed fine carbides in the final microstructure of bearing parts. Austenitization treatments were performed at different temperatures between 840 ºC and 1050 ºC. Image analyzing studies carried out to determine undissolved carbide volume fraction and carbide size distribution using ImageJ software. Mean diameter of carbides in initial soft annealed condition used for establishing a DICTRA model. DICTRA simulation results were used in comparison to experimental data as well as predict alloying element distribution near carbide – austenite interface. Experimental and computational results for carbide volume fraction were compatible.

Bearing steels are employed for the manufacture of bearing in applications that require higher hardness and wear resistance. Bearing parts are conventionally soft annealed and subsequently hardened at temperatures between A1 and Acm to have a partial austenite with undissolved carbides. Some coarse carbides may remain in microstructure with tempered martensite and finer carbides. In this study, an alternative heat treatment cycle to conventional method is investigated. Spherical carbides are fully dissolved by pre-austenization at different temperatures and followed by a pre-quenching before the conventional hardening and tempering treatments. Thus, carbides are freshly formed at hardening temperature rather than dissolving of coarse carbides before the quenching step. All heat treatments are applied by Baehr dilatometer. Kinetics of spherical carbide dissolution and austenite formation are investigated at different temperatures of the pre-austenization treatment. The results from the dilatometer curves are supported by microstructural characterization. During the austenization stage, the dilatometric curves indicate different increase rate of volume. It has been observed that in the hardening step, the extension is continuously maintained in conventional method, while the pre-austenitized and quenched samples exhibited shrinkage by nucleation of new carbides.

Manufacturing of the tensile test specimens with minimal surface defects and close tolerances are the key factors to achieve desirable and accurate mechanical properties. In addition, faster and cost-effective manufacturing methods with reliable results are also critical requirements. This study focuses on determination of the effects of various manufacturing methods on tensile properties and surface conditions produced during different machining operations. For this purpose, tensile test specimens of S700MC steel were prepared in rolling directions of 0°, 45°, and 90° using wire electro-discharge machining (WEDM), abrasive water jet cutting (AWJC), plasma cutting (PC), laser cutting (LC), and milling operations. The results were discussed in terms of specimen dimensions, surface roughness, (Ra and Rz) and tensile properties in detail.

Developement and maintainance of materials for nuclear power plants and aerospace usage is an important area of the materials science. It includes the study of various aspects of materials damage as well as an improvement and developement of new observation and testing techniques. New research facility for materials study in Rez allows to perform mechanical testing and microstructural analysis using novel techniques specially developed for irradiated materials. New hot cells complex allows to opearate with irradiated samples up to 300 TBq of 60Co. In this work, the instrumentation part is described and first experimental results are presented.

This article is devoted to the influence of copper on the bainitic transformation. Copper addition hinders proeutectoid ferrite formation and thus is favourable for bainitization. Copper is also one of the alloying elements used for the precipitation in ferrite matrix. The precipitation is usually induced by the formation of supersaturated copper solution in ferrite and subsequent heating. Normal way to achieve this treatment is quenching and tempering.
However, copper precipitation can be also induced during continuous cooling, if the austenite decomposition temperature is sufficiently high. Isothermal quenching into salt bath is a possibility to obtain both transformation strengthening and precipitation strengthening in one-step process.
Steel with 0.2 wt.% of carbon and different Mn and Cu content was examined. Samples were austenitised and cooled in salt bath of different temperatures; from 320 to 420 °C. Mechanical properties were measured by tensile test and hardness measurement. Metallography analysis was performed to compare structure of the samples.

Testing of material fatigue properties belongs to important material examinations. A standard fatigue measurement is mostly a time-consuming and expensive experiment, among others due to a number of samples needed. Infrared thermography is a method for a noncontact temperature measurement. Thermographic methods can detect small temperature changes caused by transformations between mechanical and thermal energy during a cyclic loading. This allows a use of the thermographic methods for fatigue testing. This contribution is focused on a development of an online thermographic method for fatigue limit evaluation during a sample cyclic loading. A novel 2D approach of fatigue limit evaluation is introduced. It brings a pixel wise view, which advantages compared to a standard 1D evaluation are demonstrated. It is presented a good correlation of results obtained by a standard Wöhler curve based evaluation and by thermographic methods.

Many parts for automobiles and home electric appliances are made of steel sheets. In sheet metal forming processes, the knowledge of technological values as yield or tensile strength and r-value enables more precise control of processes and results in many benefits such as an improvement in yield efficiency in machining processes and a decrease in loss of material. The current paper shows possibility of determination of all considered properties with the use of Micro-Tensile Test (M-TT) technique. Previously developed and verified testing procedure of M-TTs for bulk materials is applied here for steel sheet made of DC01 characterization. To assess the applicability of M-TT for sheet characterization, standard and M-TT tests were performed in 3 orientations: 0°, 45° and 90°. Moreover, for thickness influence evaluation, micro-specimens were machined with several different thicknesses.

the problems of additional recovery of iron and its oxidized compounds from utilized steel-smelting slags are considered in the article. It is shown that there is a noticeable amount of the Uzbekistan, after allocation of magnetic components in dump slags. Various methods are investigated for their allocation, and gravitational enrichment on a concentration table is recognized as the most optimum technology.

Possibility of FeSiCr alloys production, with low content of carbon, in industrial conditions depends upon various factors like temperature of the solution, refining time, chemical compositions and others. In order to identify the evolution and behavior of solid inclusions in the solution, laboratory tests for alloys at high temperatures of 1400 - 1500 °C have been carried out. For identifying the trajectory of inclusions, numerical simulations, in industrial conditions, have been performed for velocity and temperature distribution in a ladle furnace. Microscopy identification of selected inclusions, important from industrial refining viewpoint, has also been performed. The paper summarizes results of theoretical calculations and laboratory tests for ultra low carbon (ULC) FeSiCr alloys.