Abstract: A change in the mechanical properties of a carbide-free bainitic steel was observed during prolonged holding at austempering temperature after termination of the bainitic transformation. To determine the origin of the property change, the microstructure was investigated by correlative electron microscopy. Although the retained austenite content remains the same during prolonged holding, its morphology changes from thin films separating the individual bainitic sub-units to a more globular structure. Since films of austenite contain a higher C concentration, the blocky austenite becomes gradually enriched in C during this morphology change. The more homogeneous distribution of the C after prolonged austempering leads to higher deformability as a result of a more pronounced TRIP effect.

Abstract: The volume fraction and morphology of the microstructural constituents are extremely important when discussing the impact toughness of nanostructured bainitic steels. On the other hand, microstructural features can be influenced by plastic deformation of austenite prior to isothermal bainitic transformation. This paper aims to investigate the effect of 10% ausforming process before austempering heat treatment on impact toughness of nanostructured low temperature bainitic steels austempered at 300 °C. Results indicate that the volume fraction and morphology of high carbon retained austenite within the microstructure are important factors affecting the impact toughness energy. However results showed that the role of bainitic sheaves cannot be ignored due to their valuable effect of arresting/deflecting the cracks. It has been found that bainitic sheaves grow at more limited crystallographic variants in ausformed samples which can influence the value of the impact energy absorbed during straining the samples.

Abstract: The use of modern casting materials allows the achievement of higher product quality indices. The conducted experimental studies of new materials allow obtaining alloys with high performance properties while maintaining low production costs. Studies have shown that in certain areas of applications, the expensive to manufacture austempered ductile iron (ADI) can be replaced with ausferritic ductile iron or bainitic nodular cast iron with carbides, obtained without the heat treatment of castings. The dissemination of experimental results is possible through the use of information technologies and building applications that automatically compare the properties of materials, as the machine learning tools in comparative analysis of the properties of materials, in particular ADI and nodular cast iron with carbides.

Abstract: The bainite/martensite multiphase microstructure was studied in 0.22C-2.0Mn-1.0Si-0.8Cr-0.8(Mo+Ni) (wt%) bainitic steel subjected to austempering between 325 °C and 400 °C to elucidate the toughening mechanism and the determining role of martensite/austenite (M/A) constituents obtained through various heat treatment. Steel austempered at higher temperature had lower strength and poor toughness, because of the wide bainitic ferrite lath and blocky M/A constituents. Bainite with similar crystallographic orientation to lath martensite was formed and exhibited a typical K–S relationship. The samples austempered below martensite-start (Ms) temperature exhibited an excellent impact toughness at room temperature, due to the superfine sub-plates in lower bainite blocks. Meanwhile, air-cooled toughness at low temperature was superior to the steel austempered at 325 °C, because of its finer effective grain size. The relationship between microstructure and mechanical properties was studied using a combination of SEM, TEM and crystallographic analysis.

Abstract: The impact of bainite transformation during initial quenching and partitioning steps on the microstructural evolution was studied in a Fe-0.4C-2.0Mn-1.7Si-0.4Cr (wt%) steel. By optimizing quenching cooling rate and partitioning time, the final microstructure comprised of initial-quenched bainite, carbon-depleted martensite, bainite formed during partitioning, and final-quenched martensite, together with retained austenite. High volume fraction of retained austenite with desired carbon-content was obtained by prolonging the partitioning time to 2700 s The initial-quenched bainite, bainite formed during partitioning, and martensite provided carbon atoms to austenite, leading to the formation of retained austenite with different degree of stability. Consequently, a good combination of strength and elongation (ultimate tensile strength: 1688 MPa, total elongation: 25.2%) was obtained.

Abstract: The paper presents a short review on the past research and recent trends in the development of rail and wheel steels using bainitic morphology from the angle of mechanical and wear performances. The enhancement in mechanical properties as well wear resistance of the bainitic rail and wheel steels is very promising. In addition, the mechanical behaviour of a bainitic rail steel and a wheel steel along with their linearly reciprocating dry sliding wear characteristics in comparison to the existing pearlitic rail and ferritic–pearlitic wheel steels used in Indian Railways is discussed. The steels were subjected to austempering heat treatment at different temperatures and durations in the bainitic region as determined from the TTT diagrams of the respective steels. Bainitic steels show better mechanical properties and wear resistance due to the typical morphology of bainitic microstructural.

Abstract: We elucidate here the mechanistic contribution of the application of quenching and partitioning (Q&P) concept to a high carbon Mn-Si-Cr steel in obtaining a multiphase microstructure comprising of martensite/austenite and nanostructured bainite (bainitic ferrite and nanometer-sized film-like retained austenite) that exhibited tensile strength of 1923 MPa and total elongation of 18.3%. The excellent mechanical properties are attributed to the enhanced refinement of blocky austenite islands obtained by the Q&P process. The austenite was stabilized by both carbon partitioning from martensite and bainite transformation. Compared with conventional heat treatment to produce nanostructured bainite, the total time is significantly reduced without degradation of mechanical properties.

Abstract: The microstructure and mechanical properties of a heat-treated biomedical ASTM F90 Co-20Cr-15W-10Ni (mass pct) alloy were investigated. The alloy was heat-treated at temperatures of 673 K to 1623 K (400 °C to 1450 °C) for holding times of 0.6 to 259.2 ks. The formation of M23X6-type (M: metallic element and X: C and N) and η-phase (M6X-M12X type) precipitates was detected in the heat-treated alloys using a combination of electrolytic extraction and X-ray diffraction. The formation of precipitates deteriorated the ductility of the heat-treated alloys. On the other hand, both the tensile strength and elongation were improved by heat treatment at low temperatures of 673 K to 873 K (400 °C to 600 °C). In this heat-treated alloy, the density of stacking faults was higher than that of the as-received alloy, and the formation of the e-phase was detected.

Abstract: Austempered ductile iron (ADI) is a group of ductile irons offering the design engineers remarkable mechanical properties. It exhibits an excellent combination of high strength, ductility, toughness, fatigue strength, and exceptional wear resistance that is unavailable in other grades of cast iron. Austempered ductile iron is almost twice as strong as the regular ASTM grades of ductile iron, whilst still retaining high elongation and toughness characteristics. In addition to the exceptional wear resistance and fatigue strength, it enables designers to reduce a component's weight and costs for equivalent or improved performance. Therefore, ADI has become an attractive and economic substitute for forged steel and cast steel in many engineering applications. This led to marked interest in ADI in the past few years with considerable research work to understand the effect of processing parameters on its characteristics and mechanical properties. The objective of this paper was to review works that have been conducted over the past years on the effects of process variables, mechanical properties, benefits, and applications of ADI.

Abstract: The application of a deep cryogenic treatment during the heat-treatment processes for different types of steels has demonstrated a significant influence on their mechanical and tribological properties. A great deal of research was conducted on steels, as well as on other kinds of materials, such as hard metal, gray cast iron, aluminum, aluminum alloys, etc., but not on austempered ductile iron (ADI). In this research the influence of a deep cryogenic treatment on the microstructure and abrasive wear resistance of austempered ductile iron was investigated. The ductile cast iron was austempered at the upper ausferritic temperature, deep cryogenically treated, and afterwards tempered at two different temperatures. The abrasion wear resistance was tested using the standard ASTM G65 method. The microstructure was characterized using optical microscopy, field-emission scanning electron microscopy, electron back-scattered diffraction, and X-ray diffraction in order to define the microstructural changes that influenced the properties of the ADI. The obtained results show that the deep cryogenic treatment, in combination with different tempering temperatures, affects the matrix microstructure of the austempered ductile iron, which leads to an increase in both the abrasion wear resistance and the hardness.

Abstract: A novel process, quenching to a low temperature far below M s followed by a short austempering treatment (Q-A-T), was carried out on a low-carbon martensite steel. Results show that a notably improved toughness without losing strength was obtained on the Q-A-T specimen as compared with the oil-quenched specimen. Microstructure analyses reveal that the Q-A-T specimen consisted of low-carbon martensite, high carbon–low temperature bainite and retained austenite. The fast quenching process effectively refined the martensite plate and the austempering process introduced the high strength phase–low temperature bainite, both of which are responsible for the improved mechanical properties.

Abstract: This study aims at identifying the relationship between the shrinkage cavities and the solidification structure in spheroidal graphite cast iron. Cast samples specially designed to contain shrinkage cavities were used. The solidification macrostructure was revealed using the Direct Austempering After Solidification method, while the solidification microstructure was revealed by using colour etching. At the midsection of the pieces, the shrinkage cavities and the solidification structure were observed jointly. The study showed that the classification of shrinkage porosity found in literature does not correspond to the ductile iron solidification model recognized by most of the scientific community. Early solidification models, and therefore shrinkage formation mechanisms, were proposed in instances when there was not a thorough knowledge of the morphology of the solid phases during solidification. Nowadays, defects formation mechanisms can be described with higher accuracy. Therefore, an updated classifica...

Abstract: In this paper, the wear rate of ferritic and pearlitic ductile iron, as well as three types of austempered ductile iron (ADI materials), austempered at 300, 350 and 400 °C is investigated. Two wear parameters were varied, wear load (0.5, 1.3, 2 kg) and grit paper abrasive grain size (P240, P500, P800). The hardness of ductile iron and stress-assisted phase transformation of retained austenite into martensite (SATRAM) phenomenon in ADI materials were found to play major roles in wear behaviour. The pronounced SATRAM phenomenon was detected for the most severe wear parameters. Another important factor in the occurrence of SATRAM phenomenon is the presence of metastable, low carbon-enriched retained austenite. As a consequence, the wear rate of ADI austempered at 400 °C is nearly equivalent to ADI austempered at 300 °C at medium and high loading with the coarsest abrasive paper grit tested (P240).

Abstract: Ductile iron with austenitic-ferritic matrix (ADI), due to the specific heat treatment, shows an advantageous combination of engineering properties. The superior wear resistance and strength-to-weight ratio, together with high stress resistance and good ductility, make such material suitable for cost- and weight-efficient automotive structural applications. In this paper, results of milling experimental trials carried out on ADI 1200 are discussed. Tests were performed using uncoated and titanium aluminium nitride (TiAlN) or titanium carbo-nitride (TiCN)-coated cutting inserts obtained from both recycled and traditionally prepared tungsten carbide-cobalt (WC-Co)-based materials. Results show that satisfactory milling performance was obtained even by using recycled cutting materials, particularly when coated.

Abstract: Phase fractions and austenite carbon contents in austempered ductile iron samples with three different nickel contents were determined by in situ neutron diffraction. The samples were austenitized at 1178 K (905 °C) for 30 minutes and austempered for 3.5 hours at temperatures between 523 K and 723 K (250 °C and 450 °C) using a mirror furnace. Based on the in situ neutron diffraction studies, plateau times were derived, which determine the end of stage I reaction. The austenite contents increase for higher austempering temperatures when the austempering times are selected properly, considering the accelerated phase transformation at higher temperature. Appropriate austempering times were derived for austempering temperatures between 523 K and 723 K (250 °C and 450 °C). Increased nickel contents lead to higher austenite phase fractions. Moreover the retarding effect of nickel on the phase transformation was quantified. The plateau values of phase fraction and the according austempering times were converted to TTT diagrams. The evolution of the austenite carbon content shows a maximum at 623 K (350 °C) austempering temperature. This can be explained by temperature-dependent carbide precipitation and carbon diffusion into lattice defects. Fine carbides within the ferrite could be found by preliminary APT analysis.

Abstract: The invention discloses carbide-containing austempered ductile iron and its preparation method. Chemical composition of the ductile iron comprises, by weight, 3.3-3.8% of C, 1.5-2.5% of Si, 0.2-0.7% of Cr, 0.4-1.0% of Mn, 0.2-0.6% of Mo, 0.02-0.08% of V, 0.2-0.5% of Ni, 0.15-0.30% of Cu, 0.015-0.030% of Mg, 0.02-0.05% of RE, S being less than or equal to 0.080%, P being less than or equal to 0.080% and the rest of Fe. The preparation method comprises the following steps: preparation of raw materials and intermediate frequency furnace smelting; spheroidizing and inoculation of an iron liquid; and austempering heat treatment. By adding a trace amount of V element and properly reducing content of Si, a fine and dispersive carbide structure is obtained. The structure can greatly raise wear resistance of the ductile iron. Meanwhile, by adding a few amount of Ni, impact property of the ductile iron is enhanced.

Abstract: The effect of various versions of quenching and partitioning (Q&P) and austempering plus tempering (A&T) processes on the combined properties and microstructure of a 0.2C–0.8Si–2.2Mn bainitic steel has been investigated. Results show that the steel exhibits a higher value of product of strength and elongation (PSE) than that reported before with similar compositions. The one-step Q&P process at 230°C and A&T process at 450°C can result in a toughness higher than 80 J cm−2 and a relatively high PSE (above 29.8 GPa%). The alloy design of this steel is suggested to be beneficial for industrial production because there is a big window for similar PSE. The long-partitioning time (1 h) has good effect on combined properties.

Abstract: This study investigates low-temperature toughness of the offshore steel with two different austempering heat treatments. Toughness of upper bainite is significantly lower than that of lower bainite. Low impact toughness of the upper bainite is caused by the presence of martensite–austenite (MA) and increasing the amount of low-angle lath boundaries in upper bainite package. EBSD equipped in the SEM demonstrates an effective approach to analyze the misorientation angle of lath boundaries in upper bainite packet.

Abstract: Abstract The aim of the present research is to check an analytical model of the kinetics of bainite transformation that will enable the producers of ADI to optimise the microstructure and mechanical properties and minimise the expensive and extensive experimental trials. A combination of thermodynamics and kinetic theory was used successfully to estimate the evolution of bainite as a function of temperature, time, chemical composition and austenite grain size and predict the processing window in austempered ductile iron using a bainite transformation model developed previously for high silicon steels. The results of the present research show that the bainitic model developed for high silicon steels is applicable for calculations of the processing window for ADI.

Abstract: This investigation focuses on the study of the solidification mechanism of compacted graphite cast iron (CGI). The solidification macrostructure was revealed in cast samples using a special technique known as direct austempering after solidification (DAAS). The microstructure was revealed by colour etching. The results were compared with earlier investigations of the solidification of spheroidal (SGI) and lamellar (LGI) graphite irons, and show that, similarly to other free graphite cast irons, the solidification of CGI is dominated by the presence of relatively large grains of austenite that can be observed with bare eyes. The CGI cast samples show a typical ingot structure, containing columnar and equiaxed grains, with a narrow columnar to equiaxed transition. The microstructure analysis showed that a dendritic substructure and a large number of eutectic colonies form the grains. Microsegregation is located inside the grains, mostly between secondary dendrite arms. The results indicate that the ...