One of the concepts behind Generation IV reactors is a molten salt coolant system, where the materials for the reactor itself and for the primary and secondary circuit components are subjected to extreme chemical and thermal stresses. Due to the unavailability of these materials, a nickel-molybdenum alloy known as MoNiCr has been developed in the Czech Republic. This paper discusses the manufacturing process for the MoNiCr alloy, covering conventional casting technology, forming, powder atomization, additive manufacturing (AM) using the directed energy deposition (DED-LB) process, and final heat treatment. Special attention was given to the quality of the input powders for additive manufacturing, particularly regarding the optimization of the chemical composition, which significantly influenced the quality of the additively manufactured components. AM enables the realization of complex structural designs that are critical for energy applications, despite the high susceptibility of the MoNiCr alloy to solidification cracking. Through AM, a test body was successfully produced with a maximum defect rate of 0.03% and the following mechanical properties: a yield strength (YS) of 279 MPa, an ultimate tensile strength (UTS) of 602 MPa, and an elongation (El) of 51%.

In this study, sheets of experimental high-carbon low-density steels (LDSs) with a thickness of 1.7 mm were processed in a combined tool designed for press-hardening. Press hardening, also known as hot stamping or hot press forming, is a manufacturing process used to create car body parts with exceptional mechanical properties and safety standards. These components often require tailored properties, meaning different mechanical characteristics in various parts of the component. LDSs have a lower specific density than conventional steels, so their use would be particularly suitable in automotive applications. Combined tools achieve distinct mechanical properties within a single part through thermomechanical processing. Simultaneous forming and heat treatment create tailored zones of high strength and ductility within the sheet metal. The hardened zone provides crashworthiness, while the more ductile zone absorbs kinetic energy and converts it into deformation energy. Hot stamping enables forming complex geometries from high-strength sheets with limited cold formability, a capability that can also be exploited for the aluminium-alloyed LDS under investigation in this work. Three different high-carbon LDSs with differences in chemical composition were subjected to this experiment, and the hardness, microstructure, and mechanical properties of the two areas of each sheet were evaluated. The aim is to determine their suitability for processing by press hardening and to try to achieve tailored properties (i.e., differences in ductility and strength across one part) as in a typical representative of 22MnB5 boron steel, where a strength limit of 1500 MPa at 5% ductility is achieved in the cooled part and 600 MPa at 15% in the heated part. Tailored properties were also achieved in the investigated LDS, but with only relatively small differences between the two tool areas. The omega profiles were produced by press hardening without visible defects, and it was possible to process the steels without any difficulties.

Using positron annihilation spectroscopy (PAS), x-ray diffraction (XRD), dilatometry and transmission electron microscopy (TEM) methods, the formation and evolution of the twins, carbides and dislocation density in medium carbon spring steel 54SiCr6 after quenching and subsequent tempering were studied. Combined measurements of positron lifetimes and coincidence Doppler broadening spectroscopy revealed at tempering temperatures of 350 and 400 °C; the formation of the Cu-rich defects characterized by single component with positron lifetime τ2 130 ps, which is attributed to positron trapped on the defects that, obviously, are not dislocations or vacancies. The relative fraction of screw dislocations assessed using PAS was used to estimate the dislocation density by the modified Williamson–Hall method from XRD data. The change in the average distance between twins with increasing tempering temperature was studied using TEM. The evolution of retained austenite, carbides and cementite was characterized by XRD, TEM and dilatometry. A conclusion was drawn about the nature of defects characterized by a positron lifetime τ2 130 ps.

The tempering procedure of quenched 54SiCr6 spring steel was analyzed using continuous heating dilatometry, isothermal dilatometry, metallography, and hardness measurement. The dilatometry was performed on four different steel modifications with graduated Si content and with two levels of Cu. Metallography and hardness measurement were analyzed only on samples with one Si level. Two types of tempering procedures were compared in this experimental program. The first one was sim-ple one-step tempering, the other was a special procedure of strain assisted tempering (SAT), which includes double tempering and strain applications between tempering. A dilatometry analysis with the support of metallography contributes to the material behaviour explanation, which is considerably different in both processing cases.

The paper presents the possibilities of heat treatment of low-density structural steels usable for springs. Heats have been prepared with chemical compositions 0.7 wt% C and 1 wt% C, as well as 7 wt% Al and 5 wt% Al. Samples were prepared from ingots weighing approximately 50 kg. These ingots were homogenised, then forged, and hot rolled. Primary transformation temperatures and specific gravity values were determined for these alloys. For low-density steels, there usually needs to be a solution to achieve the required ductility values. At cooling rates of 50 °C/s and 100 °C/s, the kappa phase is not present. A SEM analysed the fracture surfaces for the presence of transit carbides during tempering. The martensite start temperatures ranged from 55–131 °C, depending on the chemical composition. The densities of the measured alloys were 7.08 g/cm3 and 7.18 g/cm3, respectively. Therefore, heat treatment variation was carried out to achieve a tensile strength of over 2500 MPa, with ductility of almost 4%. Hardnesses above 60 HRC were achieved for 1 wt% C heats using the appropriate heat treatment.

This work deals with the fabrication of one low density steel by mixing AISI S2 tool steel and AlSi10Mg powders using powder-based directed energy deposition (P-DED) technique. Two approaches of mixing powders were compared-continuous mixing during the process (in-situ) and mixing the powder prior to the process (premixed). The P-DED sample was characterised by a variety of techniques such as optical microscopy, scanning electron microscopy, electron backscatter diffraction, X-ray diffraction, and hardness measurement. Our findings demonstrate the successful achievement of steel with a 8 wt. % AlSi10Mg addition when two dissimilar powders are premixed, resulting in approximately 12% reduction in the density of S2 steel. Optimizing the powder feed rate and the ratio of AlSi10Mg powder contribute to an improvement of printability, eliminating materials separation, leading to a homogenous deposited part. Compared to the in-situ mixing case, the premixed process within the current process window generates a more homogeneous microstructure consisting of three phases: Ferrite, Fe3Al and Fe3AlC carbide. Whereas, the in-situ sample exhibits only two phases Ferrite and Fe3Al. The hardness of the premixed sample is found to be slightly higher compared to the in-situ sample.

The paper describes the effect of laser traverse speed during laser hardening on hardness and microstructure. The experimental material is hot work tool steel AISI H11 with samples sized 100x100x35 mm. The initial state of the material before laser hardening is quenched and tempered. The laser hardening temperature is constant at 1100 degrees C, selected laser traverse speed was 1, 2, 4, and 6 mm/s. A numerical simulation performed in DEFORM-3D software before the experiment showed tendencies of temperature displacement and expected course of hardness. Increasing traverse speed leads to decreased laser-hardened depth and decreased hardness drop in the heat-affected zone (HAZ). The experimental program confirmed the results of the numerical model. The differences in the microstructure were investigated by light (LM) and scanning electron microscopes (SEM), which revealed an evident difference between the surface area and the locality with the lowest hardness. Local differences from the perspective of presence of carbides were analysed by energy dispersive spectroscopy (EDS). This investigation was performed to optimize laser traverse speed to improve the subsurface hardness profile, which is essential for the lifetime and reliability of forging dies.

This research studies the influence of the copper alloying of medium-carbon steel on mechanical properties after quenching and tempering at 500 °C. The microstructure was characterised using SEM, EBSD, TEM, and XRD analysis. The mechanical properties were comprehensively investigated using hardness measurements, tensile and Charpy impact tests and solid solution, grain boundary, dislocation, and precipitation strengthening contributions were estimated. Higher yield strength for Cu-alloyed steel was confirmed at about 35–73 MPa. The precipitation strengthening contribution from Cu precipitates in the range of 11–49 MPa was calculated. The interaction between Cu precipitates and dislocations retards the decrease in dislocation density. Similar values of effective grain size of martensite crystals were measured for Cu-alloyed and Cu-free steel as well. Copper alloyed steel exhibited significantly deteriorated impact toughness, total plastic elongation, and reduction of area. The size of Cu precipitates ranged from 8.3 nm after tempering at 500 °C for 6 h to 13.9 nm after tempering for 48 h.

The present work aimed to study the properties of medium-carbon steel during tempering treatment and to present the strength increase of medium-carbon spring steels by strain-assisted tempering (SAT). The effect of double-step tempering and double-step tempering with rotary swaging, also known as SAT, on the mechanical properties and microstructure was investigated. The main goal was to achieve a further enhancement of the strength of medium-carbon steels using SAT treatment. The microstructure consists of tempered martensite with transition carbides in both cases. The yield strength of the DT sample is 1656 MPa, while that of the SAT sample is about 400 MPa higher. On the contrary, plastic properties such as the elongation and reduction in area have lower values after SAT processing, about 3% and 7%, respectively, compared to the DT treatment. Grain boundary strengthening from low-angle grain boundaries can be attributed to the increase in strength. Based on X-ray diffraction analysis, a lower dislocation strengthening contribution was determined for the SAT sample compared to the double-step tempered sample.

In this study, dilatometry and metallography were used to investigate the effect of silicon and copper alloying on the decomposition kinetics of 54SiCr6 steel during continuous slow cooling. It is different from the published literature for using the approach of the local activation energy of the austenite decomposition Ef and the local Avrami exponent n of the volume fraction of the transformed phase f to study the kinetics of austenite-pearlitic transformation in cooled 54SiCr steel at slow cooling rates. The Johnson–Mehl–Avrami equation was used to determine the dependence of the local activation energy for austenite decomposition Ef and the local Avrami exponent n on the volume fraction of the transformed phase f. The mechanism of the austenite decomposition was analysed based on the calculated values of n. Both the local and average activation energies were used to evaluate the alloying effect, and the results were compared with those obtained from other methods. The type of microstructure formed as a result of cooling at rates of 0.5 K/s, 0.3 K/s, 0.1 K/s and 0.05 K/s was determined. The effects of changes in the cooling rate and the content of silicon (1.5–2.5 wt.%) and copper (0.12–1.47 wt.%) on the dimension of nucleation and growth kinetics of the transformed phase were studied. It was revealed that the pearlite microstructure was formed predominantly in 54SiCr6 steel as a result of continuous cooling at slow cooling rates. It was also found that alloying this steel with copper led to a significant decrease in the value of Ef, as well as to a change in the mechanism of the kinetics of the austenite-pearlite transformation, which was realised in predominantly two- and three-dimensional nucleation and growth at a constant nucleation rate. At the same time, alloying this steel with silicon led only to a slight change in Ef. The results of the study of 54SiCr steel presented the dependence of the activation energy of transformation and the local Avrami exponent on the volume fraction of the transformed phase at a given cooling rate at different copper and silicon contents. In addition, the study provides insight into the mechanism of kinetics in cooled 54SiCr steel as a function of the cooling rate.

The 54SiCr6 high-strength low-alloyed steel with medium carbon content is studied in this work. Its excellent mechanical properties allow a wide range of applications as springs and vibration dampers. The high strength is usually achieved during heat treatment consisting of quenching and tempering. This manuscript represents several methods to further increase in mechanical properties. Firstly, the influence of Accelerated Carbide Spheroidization and Refinement (ASR) on microstructure before quenching and tempering was studied and enhanced plastic properties were measured. Secondly, a thermomechanical treatment before quenching increased the strength. Finally, the use of strain assisted tempering caused a further strengthening effect compared to conventional tempering. All these methods improve mechanical properties, some increase strength and others ductility.

The evolution and coarsening of Laves phase in 10Cr-3W-3Co-1Mo nitrogen free creep resistant steels at temperature 650 degrees C was quantified by use of scanning electron microscopy and image analysis. Influence of B was followed by comparison of two experimental steels with 90 ppm and 120 ppm of B. Results showed the refining influence of enhanced B content on the Laves phase evolution and the formation of thin Laves phase films due to higher B content. Quasistatic tensile tests at temperature 650 degrees C did not show detrimental effect of these films on the material plasticity.

This paper introduces a new alloying concept for low-density steels. Based on model calculations, samples—or “heats”—with 0.7 wt% C, 1.45 wt% Si, 2 wt% Cr, 0.5 wt% Ni, and an aluminium content varying from 5 to 7 wt% are prepared. The alloys are designed to obtain steel with reduced density and increased corrosion resistance suitable for products subjected to high dynamic stress during operation. Their density is in the range from 7.2 g cm−3 to 6.96 g cm−3. Basic thermophysical measurements are carried out on all the heats to determine the critical points of each phase transformation in the solid state, supported by metallographic analysis on SEM and LM or the EDS analysis of each phase. It is observed that even at very high austenitisation temperatures of 1100 °C, it is not possible to change the two-phase structure of ferrite and austenite. A substantial part of the austenite is transformed into martensite during cooling at 50 °C s−1. The carbide kappa phase is segregated at lower cooling rates (around 2.5 °C s−1).

Spring steels are typical materials where enhancement of mechanical properties can save considerable mass for transport vehicles, in this way the consumption of fuel or electric energy can be decreased. A drastic change in both the resulting microstructure and mechanical properties could be achieved due to the inclusion of strain into the tempering process after quenching. The strain assisted tempering (SAT) technology was applied, i.e., the process of quenching and following a sequence of tempering operations alternating with strain operations. After the first tempering, controlled deformation by rotary swaging was carried out with a strain of 17% (strain rate is about 120 s−1). Considerably higher strength parameters after SAT compared to conventional quenching and tempering (QT) technology were nevertheless accompanied by enhanced notch toughness at the same time by the decrease of elongation and reduction of area. However, by optimizing the process it is was also possible to achieve acceptable values for those parameters. Remarkable differences are visible in resulting microstructures of compared samples, which were revealed by metallographic analysis and X-ray diffraction measurement. While the standard microstructure of tempered martensite with transition carbides was observed after QT processing, carbideless islands with nanotwins occurred in martensitic laths after SAT processing.

Understanding the tempering behavior of medium carbon steels is mandatory if their mechanical properties are to be improved. For an optimal technology to be developed for this purpose, a substantial experimental basis is needed to extract quantitative information on the microstructure of the tempered material. This paper reports on the characterization of microstructural changes induced by tempering in medium-carbon steels alloyed with Si, Cr, Cu, and Mn using state-of-the-art experimental techniques. Complementarities among these techniques are highlighted. The evolution of transition carbides, cementite, and copper precipitates is described using data from X-ray diffraction, small and ultra-small angle neutron diffraction, transmission electron microscopy, and dilatometry observation. The effects of silicon, chromium, and copper on the mechanism of carbide and copper precipitation are discussed. The considerable changes found in the size and volume of copper precipitates correlate well with the difference in the yield stress between tempered steels with and without copper.

Requirements for mechanical properties of steels are constantly increasing, and the combination of quenching and tempering is the method generally chosen for achieving high strength in medium carbon steels. This study examines the influence of various silicon contents from 1.06 to 2.49 wt% and the addition of copper (1.47 wt%) on the behavior of 1.7102 steel starting with the as-quenched state and ending with the tempered condition at the temperature of 500 °C. The microstructure was characterized by SEM and TEM, the phase composition and dislocation density were studied by XRD analysis, and mechanical properties were assessed by tensile and hardness testing, whereas tempered martensite embrittlement was assessed using Charpy impact test and the activation energy of carbide precipitation was determined by dilatometry. The benefit of copper consists in the improvement of reduction of area by tempering between 150 and 300 °C. The increase in strength due to copper precipitation occurs upon tempering at 500 °C, where strength is generally low due to a drop in dislocation density and changes in microstructure. The increasing content of silicon raises strength and dislocation density in steels, but the plastic properties of steel are limited. It was found that the silicon content of 1.5 wt% is optimum for the materials under study.

The 54SiCr6 steel belongs to spring steels which excel high strength and at the same time reaches high values of reduction of area and sufficient value of elongation. Nowadays, new methods are searched and examined how to get better properties from materials, higher strength and toughness, longer fatigue resistance or better corrosion properties. In the case of silicon-chromium spring steels, innovative heat treatments are investigated such as quenching and partitioning which enables to achieve higher ductility of steel due to higher content of retained austenite. The way of modification of the chemical composition of the 54SiCr6 steel was chosen in combination with conventional heat treatment composed of quenching and tempering to get better properties. The materials with in-creased content of copper to 1.5 wt. % and silicon to 2.5 wt. % were prepared. The influence of alloy-ing elements on microstructure and mechanical properties was followed up to the tempering temper-ature of 400 °C.

In the present study, the corrosion performance of three candidate Ni-Mo-Cr alloys (HASTELLOY-N (R), GH3535 (R), and MONICA (R)) for Molten Salt Reactor (MSR) systems have been investigated. It is shown that the molten salt corrosion performance of these alloys is not only dependent on their chemical composition, but also on their microstructural characteristics (grain size, grain boundary length, and the presence of second-phase particles). Obtained results suggest that MONICR (R) alloy is the least affected by the exposure to the FLiNaK molten salt. This is attributed to its chemical composition, high-angle grain boundary length (grain size) and virtually no presence of large (M6C) carbides.