Controlling the Content and Morphology of Phase Constituents in Nanobainitic Steel Containing 0.6%C to Obtain the Required Ratio of Strength to Plasticity

The phase composition of nanobainitic steel 0.56–0.60%C, 1.68–1.95%Mn, 1.58–1.80%Si, 1.30–1.47%Cr, 0.57–0.75%Mo is described in this paper. The phase composition is controlled in order to obtain diversified mechanical properties for specific applications, such as armor plates. The effect of temperature and time of isothermal heat treatment on both the microstructure and the mechanical properties of the steel were determined. Dilatometric studies, as well as measurements of volume fraction and size distribution of retained austenite were carried out. Analysis of the kinetics of isothermal transformation in the temperature range of 200–225 °C for times of up to 144 h were also carried out, and the parameters of the production process of the steel were determined. A microstructure consisting of nanolathy carbideless bainite and blocky and lathy retained austenite, providing tensile strength of at least 2000 MPa, yield strength of at least 1300 MPa, and total elongation of at least 10% has been found.

Shaping Microstructure and Mechanical Properties of High-Carbon Bainitic Steel in Hot-Rolling and Long-Term Low-Temperature Annealing

Based on the research results, coefficients in constitutive equations, describing the kinetics of dynamic, meta-dynamic, and static recrystallization in high-carbon bainitic steel during hot deformation were determined. The developed mathematical model takes into account the dependence of the changing kinetics in the structural size of the preliminary austenite grains, the value of strain, strain rate, temperature, and time. Physical simulations were carried out on rectangular specimens. Compression tests with a flat state of deformation were carried out using a Gleeble 3800. Based on dilatometric studies, coefficients were determined in constitutive equations, describing the grain growth of the austenite of high-carbon bainite steel under isothermal annealing conditions. The aim of the research was to verify the developed mathematical models in semi-industrial conditions during the hot-rolling process of high-carbon bainite steel. Analysis of the semi-industrial studies of the hot-rolling and long-term annealing process confirmed the correctness of the predicted mathematical models describing the microstructure evolution.

Effect of Hot Deformation on Phase Transformation Kinetics in Isothermally Annealed 3Mn-1.6Al Steel

The kinetics of ferritic transformation and the corresponding microstructural evolution in 0.17C-3.1Mn-1.6Al-0.04Nb-0.22Mo-0.22Si medium-Mn steel during isothermal annealing was investigated in dilatometric studies. The material was subjected to thermal and thermo-mechanical treatments aimed at obtaining, by the austenite → ferrite transformation, a sufficient fraction of ferrite to stabilize the retained austenite by C and eventual Mn partitioning. The samples were isothermally held for 5 h in a temperature range from 600 to 750 °C to simulate simplified temperature conditions of an industrial coiling process following hot rolling. Some of the samples were plastically deformed at a temperature of 900 °C before isothermal holding in order to study the effect of hot deformation on the kinetics of phase transformations. After the dilatometric investigations the material was subjected to light and scanning electron microscopy to reveal relationships between the holding temperature, deformation and microstructure evolution. Hardness tests were performed to assess the mechanical behavior. A significant effect of manganese in slowing down diffusional transformations during the cooling of steel was found. The influence of austenite deformation on the kinetics of austenite to ferrite transformation was noted. The plastically deformed samples showed an accelerated start of ferritic transformation and the extension of its range. During dilatometric tests, low-range dynamic ferritic transformation was recorded, which was also confirmed by the microscopic tests.

Thermoanalytical and dilatometric studies of the Al2O3–Cu–Mo hybrid composite

The present research is focused on the characterization of the composites from Al2O3–Cu–Mo system. The composites were prepared by slip casting method and subsequent sintering of green bodies in a reduced atmosphere. Two series of samples with different volume content of metallic powders were produced in the work: 10 vol% and 15 vol% of metal content with respect to the total solid-phase content. The sintering process of the composites was analyzed in detail. The linear shrinkage of the composites was measured. The shrinkage curves were obtained by dilatometry test in a heating mode. The composites were characterized by DTA/TG analyses and thermal conductivity. The composites were characterized by XRD, SEM, and EDX. The hardness was measured by Vickers hardness tester. Moreover, the Brazilian test was performed to determine the tensile mechanical properties of the composites. Fractography investigation was carried out as well. Dilatometric tests showed that the increase of the metallic phase volume in the ceramic matrix causes a decrease in the starting temperature of densification. Simultaneously, dilatometric experiments indicated that an increasing amount of metallic particles into the ceramic matrix increases the temperature of maximum densification and decreases the total shrinkage of the composites during sintering. The DTA/TG showed the characteristics of the dispersant decomposition to the atmosphere during thermal treatment and increase of mass connected with the oxidation of metals. Fractography results revealed good adhesion between Al2O3 matrix and the metallic phase. The observation allowed to conclude that the Al2O3 matrix surface is characterized by the brittle fracture mechanism.

The phase transitions in selective laser-melted 18-NI (300-grade) maraging steel

Dilatometric studies in 18-Ni steel components fabricated by selective laser melting technique were carried out to determine the influence of heating rate on transitions occurring during the heating cycle. SLM components have been examined in controlled heating and cooling cycles. For analysis, heating of the analysed materials was carried out at heating rates of 10, 15, 20, 30 and 60 °C min−1. During the heating process, two solid-state reactions were identified—i.e. precipitation of intermetallic phases and the reversion of martensite to austenite. A simplified procedure based on the Kissinger equation was used to determine the activation energy of individual reactions. For precipitation of intermetallic phases, the activation energy was estimated 301 kJ mol−1, while the martensite to austenite reversion was determined at the activation energy 478 kJ mol−1.

Investigation of the decay kinetics of austenite with continuous cooling K76F steel for wide-gauge railway rails

The aim of the work is to study the kinetics of decomposition of supercooled austenite with continuous cooling of steel for railway rails with a high content of manganese and micro-alloyed vanadium. The relevance of the work is the need to determine the possibility of producing rails from K76 F steel according to foreign standards, in particular EN 13674-1-2011. During the research, a thermo-kinetic diagram was constructed with different cooling rates for samples of K76F steel heated by the heating mode for quenching the rails in an induction unit at the heat treatment site at Azovstal Iron and Steel Works. Dilatometric studies have shown that when samples are heated at a rate of 300С/min in steel for railway rails, polymorphic  -transformation begins at temperature 7350С (Ас1), ends at 7600С (Ас3). It is established that the temperatures of the beginning and completion of the formation of ferrite, perlite and bainite decrease with increasing cooling rate. With an increase in the rate of cooling, the morphology changes and the dispersion of perlite increases - from the middle plate of frequent perlite to highly dispersed sorbitol structures and quenching troostite. It was concluded that in the studied steel grade K76F with 0.80%С, 0.25%Si, 0.97%Mn, 0.055%V, it is impossible to achieve hardness above 400НВ without the formation of bainite. For the manufacture of high-strength rails of category R400HT EN 13674-1-2011, it is necessary to create steel of a new chemical composition. It is shown that it is possible to develop grounded reinforcing heat treatment mode and to increase the operational reliability of rails by defining critical points and studying the kinetics of decomposition of supercooled austenite with continuous cooling of steel.

The Microstructure and Phase Composition of 35CrSiMN5-5-4 Steel After Quenching and Partitioning Heat Treatment

The aim of the study was to characterise the microstructure of 35CrSiMn5-5-4 steel which was subjected to a new heat treatment technology of quenching and partitioning (Q&P). The parameters of the treatment were chosen on the basis of computer simulations and dilatometric studies of phase transformations occurring in steel. The transmission electron microscopy (TEM) observations of steel microstructure after the Q&P treatment revealed the presence of martensite as well as significant amount of retained austenite in form of layers between the martensite laths. The rod-like carbides in the ferritic areas were also observed, which indicates the presence of lower bainite in steel. It was found that the retained austenite content measured by means of TEM was about 28% for partitioning at 400°C and 25% for partitioning at 260°C. These results are in good agreement with the phase composition calculated theoretically as well as those determined experimentally by use of dilatometric tests.