Papers by Keyword: Residual Austenite

Abstract: The inter-critical heat affected zone (ICHAZ) appears to be one of the most brittle sections in the welding of high-strength micro-alloyed steels (HSLA). Following repeated heating cycles in in with temperature ranging Ac₁ /Ac₃, the ICHAZ will face with an evident toughness and fatigue behavior reduction especially due to martensite-austenite constituent (MA) formation. Microalloying in high strength steels causes the generation of some phases in the matrix able to increase the mechanical properties of the joint. In this paper we report an investigation related to 1000 ppm vanadium addition in the welded joint of a structural S355 steel. The inter-critical zone of ta double pass welded joint is here reproduced by dilatometer, with second peak temperature ranging 720°C-790°C. The residual austenite dependence on inter-critical temperature is analyzed and related to the hardness behavior.

Abstract: This article is devoted to establishing the relationship between the state of the material, phase transformations, and the formation of the microstructure of structural steels widely used in mechanical engineering, as well as for the manufacture of welded sealed bulk structures including capsules for hot isostatic pressing.

Abstract: The microstructure of the advanced low carbon steel with a superior hardenability was studied. The steel contained the following main alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The dilatometer investigation of the steel under consideration revealed the only phase transformation occurring during continuous cooling (0.1...30 °C/s), which started at the martensite start temperature M_s. It was shown that the isothermal treatment of the studied steel led to the bainite formation above and below M_s. The temperature of the bainite morphology shift was determined.

Abstract: A study of the low-carbon steel with high hardenability was carried out. The steel contained the following alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The quenching – partitioning treatment of the studied steel was implemented. The microstructure of the steel consisted of the tempered martensite laths, bainite and martensite-austenite regions. The amount of the residual austenite and the carbon concentration in the residual austenite were estimated. The possibility of the quenching – partitioning treatment of the carburized steel was shown.

Abstract: Quenching and partitioning produces advanced high-strength steels that utilise transformation-induced plasticity for improved strength and deformability. Microstructures of these steels consist mainly of tempered martensite and carbon-enriched retained austenite. A novel processing route of direct-quenching and partitioning (DQP) facilitates carbon partitioning from supersaturated martensite to untransformed austenite directly from the quench-stop temperature in a decelerated cooling that simulates slow cooling of a coiled strip. A major advantage of DQP steels is that they keep both the costs and emissions down by inexpensive alloying and energy-efficient processing. In this study, we investigate the microstructures of 0.2C and 0.4C laboratory hot-rolled DQP steels with comparison to a direct-quenched variant with high-resolution transmission electron microscopy as the main research technique. We show that the structures of DQP steels have frequent nanotwinned regions and can contain three different crystal structures with characteristic length scales ranging from few nm to ~200 nm. This is in remarkable contrast to the traditional lath-martensitic microstructure of the as-quenched material. Density functional theory calculations provide further insight into these findings with the calculated results of energetics, and show that carbon helps in stabilising the newly found omega phase. These results give further insight to the aspects that must be considered when assessing their effect on essential mechanical properties like strain hardening and toughness.

Abstract: Ways to increase the abrasive wear resistance of high-chromium steel depending on changes in the temperature of heating for quenching and cold treatment are studied in this paper. It was found that during quenching from temperatures of 850-1000 °C, martensite is formed in the structure of steel H12МFL, which provides high hardness: however, maximum abrasion resistance is not achieved in conditions of abrasive wear. An increase in the heating temperature for quenching to 1170 °C leads to a decrease in the initial hardness, which is due to the dissolution of carbides and an increase in the amount of residual austenite, but this is accompanied by a significant increase in wear resistance in abrasive wear. Residual austenite, obtained as a result of high-temperature hardening (from 1170 °C), is metastable and, in the process of wear, becomes a deformation-induced martensite. This gives the steel maximum wear resistance due to its high frictional hardening ability. A further increase in the temperature of heating for quenching above 1170 °C is inexpedient, since it leads to grain growth. Additional possibilities for increasing abrasive wear resistance consist of the cold treatment of high-carbon steels because of an increase in the amount of cooled martensite and an increase in the initial hardness. Cold treatment of the test steel after high-temperature quenching with cooling to minus 70 °C for 20 min and low tempering at a temperature of 200 °C for 2 h allows for further increases to the abrasion resistance by 25% due to the formation of 15% high-carbon chromic martensite cooling and initial hardness up to 60 НRC, with the preservation of 20% of residual metastable austenite and carbides.

Abstract: Austempered ductile cast irons (ADI) have received great attention in last years because their combined properties of good ductility, high strength and fracture toughness, good fatigue strength, good wear properties and low production cost. Such combination of properties can be reached because of their microstructures consist of a mixture acicular ferrite (bainite), residual austenite with a high carbon content and nodular graphite. In this work, the effect of austempering heat treatment on the microstructure of a commercial alloy to produce three different grades of ADI, with different strength level, is analyzed. Microstructure characterization has been performed using techniques of optical microscopy, scanning electron microscopy and x-ray diffraction. Mechanical properties were evaluated from tensile and impact tests at room temperature. In addition, the residual stress due to heat treatment was evaluated. The results of this study show that there is a strong relationship between austempering temperatures and mechanical properties. The highest tensile and yield strength obtained were 1599 and 1427 MPa, respectively, for the sample austempered at 280°C. The sample austempered at 320°C presented the highest Charpy absorption energy (99,90 J) and highest volume fraction of austenite (27%).

Abstract: The effect of heat treatment on transformation of residual austenite in bearing steel is studied, by adding the cryogenic treatment into the normal heat treatment process. The results indicate that the residual austenite content is decreased and the hardness is improved obviously, when putting the cryogenic treatment at -70°C or lower directly behind the quenching process. While when the tempering is added between quench and cryogenic treatment, the temperature of cryogenic treatment must be much lower than -70°C to offset the thermal stability of residual austenite, which is given by tempering.

Abstract: TRIP (Transformation induced plasticity) steel has a good combination of high strength and high plasticity which depend on the micro phase transformation and staking fault development greatly. C atom was typical alloying agent of austenite and plays an important role in austenite behavior, especially for staking fault nucleation. As a micro materials behavior, molecular dynamics simulation was carried out to discuss the effect of C atom on the staking fault nucleation. From the simulation result we can find that carbon influence the staking fault nucleation greatly, with the increasing of the number of C atoms, strain for staking fault form decreased, system with 4 C atoms staking fault formed when strain was 7.5% and for system without C atoms there are no staking fault with local tension strain up to 10%. Under the same deformation, stacking fault distribution was uniform for the system with 1 carbon and become uneven with the increasing of the C atom.

Abstract: The reliability reprezents the main criterion, which imposes itself in the definition of the reliability and the competition of the bearins. The rotation precision and the functional role of the bearings are realized if is a good dimensional stability and proper mechanical properties during the running. To fulfill these conditions it is necessary to establish the technological parameters of the processes of primary heat treatment, hot or cold plastic deformation, intermediate heat treatment, mechanical processing, final heat treatment below 0°C or in ultrasonic field. The comparative evaluation of different variants of heat treatment classic or nonconventional (cryogenic or ultrasonic) which are applied on bearing steels from the point of view of reliability have revealed significant increases of the values of real and median reliability for non-conventional heat treated steels, [.