The aim of the work was to determine the diagrams of phase evolution under equilibrium conditions and numerical simulation of austenite phase transformations under non-equilibrium conditions, as well as to determine CCT (Continuous Cooling Transformation) and TTT (Temperature Time Transformation) diagrams with the use of JMatPro software. The subject of the analysis were two newly elaborated multiphase steels assigned for production of forgings: steel A, containing of 0.165% C, 2% Mn, 1.11% Si and steel B, containing 0.175% C, 1,87% Mn, 1% Si, 0.22% Mo and Ti and V microadditions at a concentration of 0.031% and 0.022%, respectively. The performed simulation revealed that the investigated steels have similar critical temperatures under equilibrium conditions: A c1 ~ 680°C, A c3 ~ 830°C. The chemical composition of steel B and the interaction of Mo, Ti and V in particular, determine that diffusion transformations, i.e. ferritic and pearlitic, in the elaborated CCT and TTT diagrams are significantly shifted to longer times in relation to the position of these transformations in the diagrams for steel A. A distinct delay also concerns the bainitic transformation. Moreover, it was found that the M S temperature of steel B is slightly lower. The determined CCT and TTT diagrams are essentially helpful in the development of heat and thermo-mechanical treatment conditions for new steel grades.
This work presents the results of phase transformation kinetics during continuous cooling in newly developed high strength low-alloy steel (HSLA). Initial theoretical calculations for the determination of heat treatment parameters were conducted. To determine the structural constituents formed due to the austenite decomposition the dilatometry approach was used. The material was cooled down from the austenitization temperature of 1000 °C with cooling rates between 0.1 °C/s to 60 °C/s. Then, light and scanning electron microscopy investigations were carried out. The microstructure after cooling at rates between 0.1 °C/s up to 1 °C/s is mainly ferritic with some fraction of granular bainite. Increasing the cooling rate led to formation of a higher fraction of bainitic ferrite. At 60 °C/s the microstructure was mainly bainite with some fraction of ferrite. To determine the presence of retained austenite, color etching using Klemm solution was used. The results show that the increase of cooling rate decreases the amount of retained austenite in the microstructure of the steel. Hardness measurements were made to determine the changes in the mechanical properties as a function of the cooling rate.
The article presents the results of the research on the influence of heat treatment conditions on corrosion resistance of newly developed HSLA-type (High Strength Low Alloy) steel in selected corrosive environments. Laboratory tests were carried out with using a salt spray chamber, enabling the continuous spraying of brine mist (5% NaCl) during 96 h under high humidity conditions. Additionally, as part of corrosion experiments, tests were carried out using the gravimetric method, in which the intensity of corrosive processes was measured by the linear corrosion rate. The research conducted revealed that the best corrosion resistance was noted for steel with a high-temperature tempered martensite microstructure. Investigated 0.28C–1.4Mn–0.3Si–0.26Cr steel with Nb, Ti, and V microadditions can be used in offshore drilling constructions and production platforms exposed to salts present in sea water, chlorides, sulfates, carbonates, and bromides, among others.
This study investigated the effect of hot working conditions on changes in yield stress and the softening degree in the newly developed multiphase steel with Ti and V microadditions. The research was performed on the GLEEBLE 3800 thermomechanical simulator. In order to determine the σ-ε curves, continuous compression tests were carried out. The samples were plastically deformed at temperatures from 900 °C to 1100 °C at the rate of 0.1 s−1, 1 s−1 and 10 s−1. The activation energy of the plastic deformation was 375 kJ·mol−1. The analysis of the shape and course of the curves indicated that the decrease in strain hardening was mainly the result of the continuous dynamic recrystallization process. Two-stage compression with isothermal holding of the samples was also carried out between the two stages of deformation lasting from 1 s to 50 s. The structure of primary austenite was generated using the ARPGE software. The different size of austenite grain is the result of various thermally activated processes—when increasing the strain rate from 0.1 s−1 to 10 s−1, the average grain size of the primary austenite decreases from approx. 16 µm to approx. 6 µm. The time t0.5 needed to form 50% of the austenite fraction recrystallized at 1100 °C is approx. 4 s and extends to approx. 10 s with the reduction in the plastic deformation temperature to 900 °C. The time of complete austenite recrystallization tR, which varies from approx. 50 s to approx. 90 s in the tested temperature range, lengthens even more. The obtained results make it possible to develop thermomechanical treatment technology for the production of forgings from the tested multiphase steel.
The paper presents a detailed analysis of the MX-type interstitial phase precipitation process and a thermodynamic analysis of the Ti x V 1-x C y N 1-y carbonitride precipitation in austenite. The subject of research analysis was the newly developed HSLA-type steel containing 0.175% C, 1.02% Si, 1.87% Mn, 0.0064% N, 0.22% Mo, and microadditions 0.022% V and 0.031% Ti. Analysis of the process of precipitation of MX interstitial phases under thermodynamic equilibrium conditions proved that the fi rst phase that precipitates in the austenite of the tested steel is TiN-type nitride. The onset temperature of this phase was 1450 °C. Subsequently, carbides of the TiC-type, VN-type nitrides and VC-type carbides, for which the precipitation onset temperatures were 1180 °C, 870 °C and 775 °C will be released, respectively. The analysis of the precipitation process of the complex carbonitride in austenite under thermodynamic equilibrium conditions was based on the Hillert and Staff ansson model, developed by Adrian, with the use of the CarbNit computer program. The beginning of carbonitride precipitation with the stoichiometric composition Ti 0.985 V 0.015 C 0.073 N 0.927 occurred at the temperature of 1394 °C. At 850 °C practically all of the Ti is bound in the carbonitride of the stoichiometric composition Ti 0.883 V 0.117 C 0.378 N 0.622 . At the same temperature, a signifi cant part of microaddition V will be dissolved in austenite, which means that vanadium will have a lesser eff ect on the formation of a fi ne-grained austenite structure, but more strongly on the precipitation hardening of steel by the dispersion VN and V particles (C, N) released during the cooling of the products.
The paper presents the results of the effect of deformation parameters on the hot-deformation behavior of the newly developed multi-phase steel assigned for production of forged machine parts. Continuous and interrupted compression tests of the samples after the given deformation were carried out with the use of the Gleeble 3800 thermomechanical simulator. The samples were tested in the temperature range from 900°C to 1100°C at the rate of 0.1s -1 and 1s -1 . Basing on the analysis of the form and the course of curves obtained in the compression test, it was found that in the studied range of parameters of hot plastic deformation, the decrease of strain hardening of studied steel is caused by the process of dynamic recrystallization. This is also confirmed by calculation results of activation energy of plastic deformation process. Executed hot compression tests will contribute to establishing conditions of forging of newly developed multi-phase steel with the method of thermo-mechanical treatment.
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