Seznam abstraktů 2019: SEKCE C - VÝROBKY Z OCELI A JEJICH VLASTNOSTI

Additive manufacturing (AM) is a modern technology for the production of complex shaped structures, which has been already used for applications in medicine or automotive industry. This technology treats mainly with traditional materials such as aluminum alloys, titanium alloys and steels. One of the main techniques of AM is selective laser melting (SLM) based on consolidation of metallic powders by laser beam. In this work, we present the effect of SLM processing on the microstructure of a simple material - pure iron. We will show that in SLM iron a hierarchical microstructure is produced consisting of (i) large, nearly equiaxed grains separated by high-angle grain boundaries; (ii) micrometer-sized subgrains with low-angle grain boundaries which are located inside the majority of these grains; and (iii) nanometer-sized cells appearing inside the subgrains, mutually separated by dislocation walls. The presence of nearly equiaxed ferritic grains in iron is related to its solid-state phase transformations. Theoretical analysis of the orientation relationships of the grains, which are produced during repetitious α-γ-α transformations in SLM iron, enabled us to develop a model of the resulting microstructure. This model explains the occurrence of nearly equiaxed grains instead of columnar ones which might be expected to appear during the SLM process. The equiaxed grains occur namely during quick up-and-down temperature changes thus inducing quick α-γ-α transformations when the laser is repeatedly scanned over the studied location. During these transformations, the initially formed columnar grains are broken and refined. Our contribution documents that new hierarchical microstructures can be produced via AM.

Nowadays, automobile emissions are one of the leading causes of air contamination. In this regard, the weight reduction of the vehicles structures and parts is vital, while the passenger’s safety is the first issue. Therefore, high demand for Advanced High Strength Steels (AHSS), is constantly increasing in the automotive industry, which can adequately fulfill both safety and emissions issues through reduction in thickness. A new generation of AHSS covers these demands owing to excellent forming and mechanical properties. Accordingly, Quenching and Partitioning (Q&P) steels have been developed with a special microstructure composed of an initial fraction of martensite and a slight fraction of stabilized retained austenite. The aim of the present work is to assess the effect of different amounts of manganese in Q&P steel under various heat treatment cycles relevant to an industrial process. A 0.2C-1.5Si base steel with different amount of Mn, namely 1.5, 3.5 and 5.5 (in wt%) were selected to promote different amounts of retained austenite after Q&P process. In order to determine the critical transformation temperatures such as Ac1, Ac3, Ms and Mf, which are needed to design Q&P thermal cycles, dilatometric experiments were carried out. The results are presented according to the chemical composition, the final microstructure and the hardness obtained in each condition.

During thermomechanically controlled processing (TMCP) of carbon steels, controlled rolling in no-recrystallization regime followed by directing quenching results in excellent mechanical properties, besides enabling cost-saving in respect of energy costs normally incurred during reheating and quenching procedure. The effects of three different combinations of Cr with 0.06 wt.% of Nb on the microstructures and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed steel plates have been investigated to obtain balanced mechanical properties. Laboratory-scale ingots were cast, hot rolled and direct quenched into 12 mm thick plates as per an experimental plan. Tensile properties, impact toughness, hardness and hardenability were studied. In addition, coarse grained heat affected zone (CGHAZ) simulations were performed in a Gleeble simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t8/5) of 5 s and 15 s.

Quenching and partitioning (Q&P) processed steel is a promising material for automotive application. It shows not only high yield strength originated from martensitic matrix but also moderate ductility due to considerable amount of retained austenite stabilized by carbon partitioning. However, it shows poor hole expansion ratio due to presence of blocky type retained austenite which is formed when the volume fraction of retained austenite is high. To minimize the amount of blocky type austenite, isothermal heat treatment was applied prior to conventional Q&P process; it is named as bainitic based quenching and partitioning (BQ&P) process. Blocky type austenite was replaced with nanometer-sized film type austenite by applying BQ&P process. The nanometer-sized film type austenite which was formed during BQ&P process was highly stabilized by the film type morphology and the high carbon concentration. As a result of high mechanical stability of retained austenite, BQ&P processed steel showed improved tensile properties compared to the Q&P processed steel by exhibiting transformation induced plasticity (TRIP) even at high strained state. The film type austenite can also effectively hinder crack propagation during the hole expansion test, therefore the BQ&P processed steel showed better hole expansion properties than the Q&P processed steel.

The contribution deals with a thermographic method for a determination of fatigue limit of a cyclically loaded sample. Infrared thermography is a method for a noncontact temperature measurement. Quantitative and high accuracy thermographic measurement requires knowledge of optical properties of the measured sample surface. Thermographic paints with high emissivity are often used for these purposes. These paints also should fulfill additional requirements on mechanical loading properties if they are used for cyclic loading tests. Results of several thermographic high-emissivity paints testing are presented. Known fatigue limit testing methods were verified and improved in terms of our research. A novel 2D approach to the thermographic fatigue limit evaluation improves a visualization of the tested part violation. Experiments with different samples, using different loading frequencies and different coefficient of loading force asymmetry were performed to validate the thermographic fatigue testing method. The results of validation tests and a procedure for an automatic online evaluation of fatigue limit thermographic measurement are introduced.

Thermo mechanical treated steel API 5LX65 has high mechanical characteristics and is intended for the welding of main pipelines for the oil and natural gas transportation. The pipes made of this steel have a diameter of 42 '' (1066.8mm) and a wall thickness of 31.75mm. They were welded by narrow gap MAG welding selecting a filler metal for the root layer and another filler metal for the filling layers. Testing of the plastic deformation behavior of the welded joints was done by cold bending tests on both specimens in which the longitudinal axis of the welded joint is perpendicular to the axis of the support rollers and parallel to the direction of application of the force as well as on the notched specimens. The results obtained were corroborated with macro - and micro fractographic investigations

Dual phase steel has been significantly used in the automotive industry for the outer body parts. It has high strength, good formability and high toughness. Passive safety during a crash is significant, which drives this experimental work. In this research work, dynamic strength and fracture behaviour of the dual phase steel DP500xxx under varying high strain rate has been investigated using pendulum hammer impact testing machine and Electrohydraulic testing machines. The Strain rate sensitivity of the steel and microstructural phase changes are studied at higher strain rates. Thin sheets of dual phase steel are tested with from quasi-static strain rate up to 103 s-1. Fracture mechanism is observed after the impact-tensile tests and strain hardening effect is measured using local hardness measurement. Measurement of local hardness gives the spread of the plastic deformation and nature of strain hardening.

Quenching and Partitioning method of steel heat treatment (Q&P) [1-4] consists of two steps: partial martensitic quenching (Q) followed by short annealing i.e. partitioning step (P). During partitioning carbon atoms diffuse from supersaturated martensite laths or plates into surrounding untransformed austenite. This process leads to the stabilisation of retained austenite in steel after martensitic transformation. As a result, a two-phase microstructure is obtained. It consists of tempered martensite and stable retained austenite. The presence of stabilised austenite in steel improves the ductility of steel due to the TRIP effect (Transformation Induced Plasticity). The aim of this study was to design the parameters of Quenching and Partitioning (Q&P) process of X46Cr13 steel and to investigate the influence of the process parameters on the microstructure and on mechanical properties of this steel. The parameters of Q&P treatment were determined with the use of dilatometric measurements. To determine phase composition after Q&P process, magnetic measurements were conducted. The microstructure of martensite and austenite as well as the presence of carbides were examined through Transmission Electron Microscopy (TEM). Changes in hardness after various variants of Q&P treatment were also analysed.

A low pressure superheated hydrogen steam system has been used to accelerate the oxidation kinetics while keeping the electrochemical conditions similar to those of the primary water in a pressurized water reactor. A special corrosion apparatus was built in cooperation with University of Manchester and Škoda JS for the purpose. The initiation is investigated using a Constant Extension Rate Test (CERT) with flat tapered specimens made from Type 316L austenitic stainless steel in the vapour environments at 350 °C. Two different surface finishes were used: as-ground and as-polished. Constant extension rate tests with a test rate of 2×10-6 ms-1 were performed on 4 specimens - at room temperature and at an elevated temperature of 350 °C and as a more oxidizing environment was chosen R = 1/6, on the contrary as a more reducing environment was chosen R = 6, where parameter R represents the ratio between the oxygen partial pressure at the Ni/NiO transition and the oxygen partial pressure. Post-test evaluation using SEM was applied.

Metal Matrix Composites (MMC) combined with the concept of Functionally Graded Materials (FGM) offer potential to adjust the material properties based on the local load conditions. In this study, a sinter-forging process was deployed to produce an MMC wear component, consisting of a steel matrix and Fused Tungsten Carbide (FTC) particles, for later use in a hot forging tool. A low alloyed carbon steel powder and a high-speed steel powder (similar to HS 6-5-2) were processed. A graded hard phase concentration was generated by stacking layers of increasing hard phase concentration (max. 10 vol.%), before the pressing of the powder specimens. After pressing, the specimens were sintered and then forged to near full density. Metallographic imaging was used to display the hard phase gradient and to identify the microstructure of the components. The dissolution of the tungsten carbides during sintering was also investigated by Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDX). The sinter-forged components showed a dense microstructure with embedded hard phases. The introduced gradient structure was not altered by the forging operation and the components seem promising for the planned use as a wear component in hot forming tools.

The effect of optimizing the weld current and the post weld current used in resistance spot weld were studied to investigate the effectiveness of the welded joints when welding a thin material onto a thicker part through microstructural and mechanical properties analysis. All welded specimens were pulled at room temperature (20 ºC) and sub-zero temperature (-46 ºC) to test the strength of the welded joints. No full button failure was observed in both the samples pulled at room temperature and the ones at -46ºC after optimisation. The fusion zone was observed to mainly consist of martensitic phase due to rapid quenching whiles the HAZ composed of islands of martensite within a ferrite and bainite matrix. The base 817M40 metal remained fully ferritic after the welding processes. The hardness of the recorded phases showed good correlation with the microstructures observed, whereby the fusion zone (FZ) was measured to be 600HV, the base metal (BM) 320 HV and the heat affected zone (HAZ) 450 HV. A good fusion of the weld was observed with an increase in the welding current. Good stretch of the weld nuggets across the interface between the two opposing surfaaces was observed in all the samples.

The microstructure and mechanical properties of Vanadis 6 ledeburitic tool steel, which was subjected to sub-zero treatment in liquid helium, has been examined with reference of the same material after conventional room temperature quenching. It has been found that the sub-zero treatment considerably reduces the amount of retained austenite, and increases the number and population density of carbides. As a result of SZT, the hardness, the flexural strength and the fracture toughness of the material are improved, except minor cases.

Influence of sub-zero treatment and tempering on the microstructure of Cr-V ledeburitic steel Vanadis 6 has been examined. The samples were austenitized at the temperature of 1050 °C in a vacuum furnace, held at the final temperature for 30 min. and nitrogen gas quenched (5 bar). In SZT, conventionally heat treated specimens were cooled down immediately after quenching from the room temperature to the temperature of liquid helium (-269 °C), kept there for 17 h and then re-heated to the room temperature. Double tempering was performed at the temperatures from the range 170 - 530 °C, whereas each tempering cycle was realized with a hold of 2 h. Typical heat treated microstructure of ledeburitic steels consists, besides of the martensitic matrix with certain amount of retained austenite, of several types of carbides - eutectic, secondary and small globular carbides. In sub-zero treated steel the amount of retained austenite is significantly reduced. The population density of small globular carbides increases as a result of sub-zero treating as - possibly due to high internal stresses in martensite and material efforts of their relaxation. Tempering of the material resulted in decrease in population density of small globular carbides with increasing the tempering temperature. The hardness of sub-zero treated material is higher than that of conventionally quenched one. Also, this tendency is preserved when the steel is low-temperature tempered. On the other hand, the hardness of conventionally quenched steel becomes higher than that of SZT one when tempered at the temperature of secondary hardening.

The carbide in X90CrMoV18 martensitic stainless steel governs both hardness and fracture toughness, which are the key performance of tool steels. Optimizing these opposite properties results in longer tool life by increasing the wear resistance. Deep-Cryogenic Treatment (DCT) has been introduced in this research to fully transform the Retained Austenite (RA) into martensite and/or secondary carbide. The precipitation of M7C3 (Primary Carbide; PC) increases only the hardness while M23C6 (Secondary Carbide; SC) improves the toughness. Hence, the increase in M23C6/M7C3 carbide ratio will enhance the tool life and the role of DCT in this regard is our research focus. The carbide type, size and shape are observed by Scanning Electron Microscopy (SEM) and Light Optical Microscopy (LOM). The synchrotron Grazing Incident X-Ray Diffraction (GIXRD) is applied for investigating the change in c/a ratio of martensite matrix and carbide identification. The synchrotron X-ray Absorption Spectroscopy (XAS) is applied to the quantification of precipitation. The latter is quantified by XAS with Linear Combination Fit (LCF) of the Extended X-ray Absorption Fine Structure (EXAFS) spectra. A suitable DCT condition increases 4.2 % in the hardness while surface toughness is raised to 150 %. The carbon atoms in martensite matrix are forced out from the lattice resulting in larger amount of SC. The average diameter of the carbide decreases by 15 % after DCT.

The small punch (SP) creep testing technique was applied to the service-exposed 2.25Cr-1Mo steel, which had been actually used as a boiler piping in the fossil power plant for long periods of time, to investigate the applicability of this technique to the remaining-life assessment. The SP creep tests were carried out at the temperatures of 580 and 650°C and under the load range from 150 to 400 N using small disk-type specimens removed from the piping. In order to compare the results obtained from the SP creep test with those from the uniaxial creep test, the SP load (F) was converted to the stress (σ) with the load/stress conversion coefficient (F/σ), which was determined by the high temperature small punch test result, that is, the central displacement to the maximum load. The F/σ based on the central deflection at the minimum creep deflection rate, which has been recently proposed in Europe, was also applied to the conversion. The experimental results revealed that there was no significant decrease in creep rupture strength in the service-exposed steel, although its rupture lives were shorter than those of non-serviced one at the high stress region. It was also found that the load/stress conversion coefficients were 2.5-2.7 irrespective of conversion methods, the SP creep rapture data were well correlated with the uniaxial ones using both methods.

This paper deals with influence of austenitizing and tempering temperature on microstructure, substructure and mechanical properties of creep resistant 9CrNB steel. Austenitizing temperature of 1050 °C was used with holding time of 15 min (state N1) and 45 min (state N3). Subsequently, the samples in these conditions were tempered at 790 °C/60 min (state S1 and S3). Microstructure after austenitization is formed by lath martensite and low bainite. The microstructure after tempering was formed by tempered martensite and lower bainite, with darker and lighter areas. This is due to the non-homogeneous distribution of precipitates. It can also be seen, that fine particles of precipitates were found at the martensite and low bainite lattice interfaces. It is likely to be a carbide particles, whereby excluded this particles occurred during the tempering on the original grain boundaries of austenite. The substructure was analysed in the following austenitizing conditions: 1050 °C/15 min and 1050 °C/45 min and their subsequent tempering at 790 °C/60 min. The precipitates were identified by selective electron diffraction analysis. The mechanical properties were evaluated after austenitization and tempering conditions.

Special steel H13 is most frequently used for manufacturing of the tools used for metal forming, metal machining, and metal cutting. Local metalworking industries complain that H13 steel does not provide a better combination of wear resistance and toughness. The main objective of the present study was, therefore, set to evaluate the toughness and wear properties at different vacuum heat treatments. After manufacturing the specimens of the required geometry was given different hardening, tempering and nitriding heat treatment cycles using vacuum furnaces.
Results indicated that with increasing the tempering stage the volume fraction of chromium carbide precipitates increased, which increased the wear resistance of H13 steel. Nitriding stage further augmented the wear resistance. Encouraging results in terms of minimum wear rate (1.2x10-7cm2/sec) with 14 Joule toughness has been achieved in HTTNN (hardening followed by double tempering and double nitriding stages) sample, which showed that the hardening and double tempering followed by double nitriding heat treatment is the best heat treatment for H13 tool steel. Results, in the present investigation, suggest that cooling rates after nitriding treatment play a significant role in the embrittlement of steel substrate, so it must be studied carefully.

One of the intensively studied issues regarding hydrogen embrittlement of advanced high strength steels (AHSS) is the absorption and diffusion of hydrogen produced by atmospheric corrosion. Scanning Kelvin Probe (SKP) is a tool sensitive enough to detect low amounts of hydrogen formed during atmospheric corrosion. In this work, SKP was used to study the effect of the presence of corrosion products on hydrogen uptake and permeation through AHSSs specimens. Corrosion was initiated on one side of the specimen contaminated with a corrosion activator, while Volta potential mapping on the opposite side allowed for in-situ hydrogen detection. Hydrogen release indicated by low potential areas in the maps corresponded to the locations of corrosion products on the opposite side. This shows that hydrogen entry is linked to the presence of red rust.

This paper describes a methodology of small sample test technique for fracture toughness determination in the upper shelf region. The effect of size and strain rate has been investigated and described. Standard and small sized samples for 3-point bend test were produced and tested from several steels. The results are promising for further usage in many fields, where only limited amount of material is available, like residual life assessment, additive manufacturing industry, local properties determination in case of heterogenic material, etc. In addition, competitive methods like small punch test will be discussed as well.

The work is designed for the establishment of experimental techniques for replicating of highly-active neutron irradiated samples in SUSEN hot-cells. Using the most sensitive devices for dimensions and defects measurements of irradiated materials is limited due to the negative effect on their operation lifetime and accuracy. Therefore, new replication techniques are needed for the long time operation of the hot cells and its equipment which are sensitive to gamma radiation. The replicas of irradiated material, that are made using manipulators inside the hot-cell, are made of the polymers that are not activated during the operation, so that they can be easily used for the standard measurement procedures. This work describes 3D dimensions measurement and reconstruction of the irradiated materials and evolution of the replication methodology.

The aim of the work was to determine the influence of the quantity and distribution of carbides on the bainitic transformation process that takes place in 90SiCrMn6-4 steel. The kinds of possible carbides that can be formed in steel and the kinetics of their precipitation were examined by the use of computer simulations. Various thermal treatments, leading to obtaining of different size and distribution of carbides in steel, were carried out. The obtained microstructures were studied by the use of Light Microscopy (LM) and by Transmission Electron Microscopy (TEM). The pre-heat-treated steel samples with different carbide size and distribution were subjected to austempering processes. The kinetics of bainitic transformation in samples with different carbide size and distribution were analysed by means of dilatometric tests and the resulting microstructures were examined by use of TEM. The obtained microstructure was composed of carbide free bainitic matrix in which carbides were observed. Thickness of the bainitic ferrite plates and austenite layers, in samples with different carbide distributions, was measured. Finally, hardness of the heat treated samples was determined.

Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing technology in which a defined metal powder thickness is selectively melted with a laser, according to the geometry of the part being produced. The layer-by-layer approach allows manufacturing functional complex shaped components, with high structural integrity at low cost. This technique has been proven to produce near net-shape parts up to 99 % relative density and has viable economic benefits. However, the typical build envelope for this type of machines is of 300x350x250 mm, thus the manufacturing of large-dimension parts is unachievable. The goal of this project was to develop a customized LPBF machine with a build envelope of 1020 mm x 1020 mm x 520 mm that is able to produce high quality parts. Through fine-tuning of the processing parameters, the machine has produced samples with 316L stainless steel which exhibit relative densities above 99 %. Besides the samples, the machine has also successfully built large-dimension parts. The results obtained are a positive indicator towards the ultimate goal of zero-defect manufacturing.

Austempered ductile cast iron (ADI) has come out as an important engineering material in recent years. It is because of its excellent mechanical properties such as high strength with good ductility, fracture toughness, fatigue strength and good wear resistance. In this investigation, ductile cast iron with predominantly pearlitic as-cast structure was processed by two-step austempering process. All samples of ductile iron were first austenitized at 900°C for 1 hour. After austenitizing, samples were quenched in salt bath at 250 degrees centigrade. The salt bath temperature was then raised to 400 degrees centigrade. The samples were soaked at that austempering temperature for 2 hours, 6 hours and 10 hours. Effect of this variation of time of austempering on microstructure and mechanical properties of ADI was studied. Structure-property correlation was established.