Papers by Keyword: Metastable Austenite

Abstract: The effect of quenching from 900°C (20 min exposure) and different tempering in the 250-650°C (for 1 hour) interval, as well as additionally preliminary carburization for 8 hours at 930°C, followed by a similar heat treatment on abrasive and shock-abrasive wear of low-carbon manganese (10-24%Mn) steels, phase composition and mechanical properties was studied. It was confirmed that an increase in the manganese reduces the abrasive wear resistance and increases the impact-abrasive wear resistance. The expediency of carburization of low-carbon manganese steels is shown in order to obtain the residual austenite in the structure which amount and stability must be optimized in relation to specific abrasive impact characterized by the dynamic ratio with taking into account the chemical composition.

Abstract: The structure and mechanical properties of the corrosion-resistant metastable austenitic steels 03Kh14N11К5М2YuТ and 03Kh14N11КМ2YuТ have been investigated. The steels have been deformed by tagging them at room and negative temperatures. It has been established that the amount of martensite and the strength properties are higher at a negative temperature than at room temperature with an equal degree of strain. The investigated austenitic steels are strain-metastable. The higher the strain degree and the lower the deformation temperature, the greater the amount of strain-induced martensite and, correspondingly, the higher the strength properties. Martensite is not observed upon cooling to the temperature of liquid nitrogen.

Abstract: The application of hardening heat treatment process at high temperatures (1100-1170 °C) for high-chromium steels of martensitic-carbide class 95Kh18 and Kh12MFL has been studied. Metallic substrate consisted of high-carbon martensite and residual metastable austenite with some traces of carbide has been obtained. Experiments have shown the resulting structure gains high frictional hardening capacity upon the application of heat. Sufficient amount of cooling martensite can be traced in the analyzed steel after high-temperature quenching (cooling up to the temperature of-70°С). Being combined with residual metastable austenite, it provides the increase of abrasive wear resistance by 25% compared with high temperature annealing. The influence of tempering temperature on hardness and abrasive wear resistance of analyzed steels 95Kh18 and Kh12MFL has also been determined.

Abstract: Mechanical properties of all metals depend predominantly on the type and morphology of their microstructure. Microstructure attributes can be altered by various heat treating and thermomechanical treatment procedures. One of the advanced techniques profoundly affecting the microstructure evolution is semi-solid processing. It can produce unconventional microstructures even in conventional steel types. Moreover, subsequent heat treatment can also deliver a wide range of microstructures and correspondingly varied mechanical properties. In the present experimental programme, the X210Cr12 ledeburitic tool steel was studied. Its initial annealed microstructure consisted of ferritic matrix, chromium carbides and globular cementite particles. The semi-solid processed structure, on the other hand, contained polyhedral austenite grains embedded in carbide-austenite network. The austenite volume fraction exceeded 95 %. This microstructure was then altered by subsequent heat treatment or thermomechanical treatment. The experimental programme comprised three stages. At the first stage, the effects of the rate of cooling from the semi-solid region to the ambient temperature on the nature and morphology of the ledeburitic network and the austenitic grain size were explored. The second stage was aimed at the impact of tensile and compressive deformation applied after transition through semi-solid state on the microstructure evolution and, in particular, on grain size. Once suitable processing conditions and parameters were identified, the treatment led to a recrystallized austenitic microstructure with an average grain size of less than 3 μm. As high volume fractions of austenite were obtained, the third stage involved exploring the effects of thermal exposure. The stability of austenite and the decomposition of austenite into other microstructure constituents were mapped. Metallographic observation revealed a resulting wide range of microstructures from fine pearlite to martensite, depending on the heat treating schedule.

Abstract: The microstructure evolution laws of ultrafine grained metastable automobile steels was studied in this paper by laser confocal scanning microscope, EBSD, XRD and TEM. Results showed that, the matrix organizations of hot-rolled steel were lath martensite and deformation ferrite, and there were a little of retained austenite film and lath between the lath martensite. After heat treatment, the matrix organizations of steel were ultrafine ferrite and retained austenite. The retained austenite transformed into martensite and ε-martensitic in the deformation process, and the strength and plasticity of steel were improved. A lot of retained austenite were obtained in the annealing process. The TRIP effects by the large fraction of metastable austenite and the ultrafine grain size add to the test steel with high strength and high plasticity.

Abstract: This work addresses an urgent problem which is saving alloying elements (Ni, Mo, Nb, V, W and others) -which very scarce in Ukraine and other countries- while increasing the physical, mechanical and operational properties of the new designed and developed wear-resistant cast irons. Optical microscopy, x-ray structure analysis, transmission and scanning (fracture) electron microscopy methods, different-thermal, magnetometric and x-ray analyses, as well as hardness and micro-hardness tests, impact energy, and abrasive wear tests in environments of cast-iron shots were employed in the research. The work summarizes the controlling conditions for forming various metastable phases by including deformation (or strain) induced phase transformation (DIPT) and through realization in the process of testing. New wear-resistant economical alloyed cast irons with a metastable austenite-carbide and austenite-martensite-carbide structure are developed, which are being strain- hardened under impact-abrasive wearing operation conditions due to the realization of process DIPT in the superficial layer. These new cast irons are intended for manufacturing parts of different equipment (protective plates of sinter machines, furnaces, tracks for transferring raw materials at iron and steel works, etc.). These cast irons do not contain expensive and deficit alloying components, like nickel, molybdenum, vanadium, and more expensive similar materials. Simultaneously they possess enhanced impact-abrasive wear resistance.

Abstract: In this study a C-Mn High Strength Low Alloy steel (HSLAs) was processed by quenching and austenite reverted transformation during annealing (ART-annealing), which results in an ultrafine grained duplex microstructure characterized by scanning electron microscopy equipped with electron back scattered diffraction, transmission electron microscopy and x-rays diffraction (SEM/EBSD, TEM and XRD). Microstructural observation revealed that the full hard martensitic microstucture gradually transformed into ultrafine grained duplex structure with austenite volume fraction up to 30% at specific annealing conditions. Mechanical properties of this processed steel measured by uniaxial tensile testing demonstrated that an excellent combination of strength (Rm~1GPa) and total elongation (A5~40%) at 30% metastable austenite condition in studied C-Mn-HSLAs. This substantially improved strength and ductility were attributed to the strain induced phase transformation of retained austenite dispersed throughout the ultrafine grained microstructure. At last it is proposed that ART-annealing is a promising way to produce high strength and high ductility steel products.

Abstract: In this study Quenching and Partitioning (Q&P) as proposed by Speer was applied to improve the ductility of C-Mn high strength Low Alloy steel (HSLAs). Microstructural observations revealed a multiphase microstructure including first martensite, fresh martensite and retained austenite in the Q&P processed steel. During tensile process, the austenite volume fraction gradually decreased with strain increasing, suggesting the phase transformation induced plasticity for the Q&P processed steel. Ultrahigh strength about 1300-1800MPa and tensile elongation about 20% were obtained after Q&P processing at specific conditions, which is significant higher than that of ~10% of conventional martensitic steel. The the product of tensile strength to total elongation increased from 25 to 35GPa% with increasing carbon content in studied steel. This improved mechanical properties were related to the ductility contribution from TRIP effects of the retained austenite and strength contribution from the hard martensitic matrix. At last it was turned out that the Q&P process is a promising way to produce ultrahigh strength steel with relative high ductility under tailored heat treatment conditions for different micro-alloyed carbon steel.