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The analysis of the literature data on the properties of iron and steels carried out in this work indicates numerous anomalies of physical and mechanical properties near a temperature of ≈200°C. Based on the thermal effect, changes in magnetic properties, anomalies of shear modulus, oxidizability, heat capacity, diffusion, Lorentz number, the transformation in iron at ≈200°C is justified, which may be the cause of a decrease in the magnetization of perlite, sorbitol and martensite, as well as the performance of high-hard low-tempered steel having almost the same hardness after quenching. The study was carried out on samples of practically pure iron (0.008 % C). The experiment (metallographic examination, X-ray diffraction analysis, metal deformation resistance (Gleeble-3800 installation) was carried out every 20 -40°C. The experimental results indicate significant changes in the structure, as well as extreme values of the mechanical properties of iron (0.008 % C) and steels near a temperature of ≈200°C, which makes it possible to declare the transformation in iron and its derivatives -steels at this temperature. Recognition of the transformation at this temperature allows us to explain: the reason for the decrease in magnetization of perlite, sorbitol and martensite, the maxima of impact strength and hardness of iron, the disappearance of the yield point on the tensile curve of riveted steel after aging, the nature of irreversible tempering fragility (the nature of the latter, as well as reversible is still debatable), etc.
The analysis of the literature data on the properties of iron and steels carried out in this work indicates numerous anomalies of physical and mechanical properties near a temperature of ≈200°C. Based on the thermal effect, changes in magnetic properties, anomalies of shear modulus, oxidizability, heat capacity, diffusion, Lorentz number, the transformation in iron at ≈200°C is justified, which may be the cause of a decrease in the magnetization of perlite, sorbitol and martensite, as well as the performance of high-hard low-tempered steel having almost the same hardness after quenching. The study was carried out on samples of practically pure iron (0.008 % C). The experiment (metallographic examination, X-ray diffraction analysis, metal deformation resistance (Gleeble-3800 installation) was carried out every 20 -40°C. The experimental results indicate significant changes in the structure, as well as extreme values of the mechanical properties of iron (0.008 % C) and steels near a temperature of ≈200°C, which makes it possible to declare the transformation in iron and its derivatives -steels at this temperature. Recognition of the transformation at this temperature allows us to explain: the reason for the decrease in magnetization of perlite, sorbitol and martensite, the maxima of impact strength and hardness of iron, the disappearance of the yield point on the tensile curve of riveted steel after aging, the nature of irreversible tempering fragility (the nature of the latter, as well as reversible is still debatable), etc.
The goal of the study is to reveal the impact of change in the structural state of steel 07Kh3GNMYuA after heat treatment on the values of the critical stress intensity coefficient (K1c) obtained at a temperature of 50°C and on the velocity of ultrasound wave propagation, as well as to determine a correlation between them for rapid assessment of the crack resistance using acoustic characteristics. The mechanical characteristics of the material and the critical stress intensity coefficient K1c were obtained on the test machine «Inspekt 100 Table». The tangent method is used for determination of K1c. Three samples per K1c value were used in the experiment for a three-point bending scheme at the operating temperature T = –50°C. Acoustic parameters were measured using the echo-pulse method. The results of ultrasonic scanning of heat-strengthened samples made of steel 07Kh3GNMYuA demonstrated the possibility of non-destructive quantitative evaluation of the critical stress intensity coefficient. New data on the mechanical properties of steel 07Kh3GNMYuA and on the correlation between the velocity of longitudinal elastic waves and the values of the critical stress intensity coefficient of structures were obtained. Deviation of the calculated values of K1c obtained using acoustic measurements from the experimental values does not exceed 10%. The proposed model, which explains change in the acoustic characteristics of steel 07Kh3GNMYuA on the basis of phase changes occurring in the steel structure upon tempering, provides conducting of similar studies for other modes of heat treatment and other steel grades. The method is a low labor- and time-consuming procedure for determination of the mechanical characteristics of the products made of steel 07Kh3GNMYuA, since it does not require the manufacturing of samples and their testing. The developed procedure can be proposed for manufacturing application, as the main or additional method for evaluation of the mechanical parameters of materials after various modes of heat treatment.
Abstract—The influence of quenching and tempering on the structure, phase composition and mechanical properties of high-strength Fe–0.34 C steel with 1.77 wt % Si is considered. The tempering at temperatures up to 500°C has virtually no effect on the structural characteristics of packet martensite formed during quenching. At tempering temperatures in the range of 200–400°C, the precipitation of transition η-carbide occurs, which leads to an increase in the yield strength to 1490 MPa and impact toughness to 35 J/cm2. The determined temperature of the brittle-ductile transition after tempering at 200°C is about –50°C. A decrease in the impact toughness and a decrease in the proportion of ductile fracture with a decrease in the test temperature is accompanied by a transition from transgranular to intergranular fracture. The precipitation of cementite particles along the boundaries of laths and blocks is observed after tempering at 500°C. This leads to a decrease in the yield strength, while the impact toughness of the steel remains unchanged.
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