Abstract: The microstructure of austempered high silicon (AHS) steel before and after treating with a modifier containing titanium, vanadium and rare earth elements (so-called Ti–V–RE modifier) and austempered at different temperatures has been investigated. The plane strain fracture toughness of the steel in room temperature and ambient atmosphere has been examined. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and optical metallography and correlated to the fracture toughness of the steel. The results show that the primary austenite grains are refined, the dendritic austempered structure is eliminated, and the volume of blocky shaped retained austenite is reduced by the addition of Ti–V–RE modifier. Modification with Ti–V–RE modifier can prompt the bainitic ferrite transformation and reduce the volume fraction of retained austenite. High fracture toughness is obtained for AHS steel with the addition of Ti–V–RE modifier when austempered between 350 and 385 °C with a retained austenite of 30–35% and the carbon content in the austenite is about 1.9–2%. The fracture toughness of AHS steel by the modification treatment can increase 10–40% than that of unmodified, an optimum value of 85 MPa m1/2 was obtained when austempered at 385 °C.

Abstract: An unalloyed ductile iron containing 3.42 C wt.%, 2.63 Si wt.% and 0.318 Mn wt.% was intercritically austenitized in two-phase region (α + γ) at various temperatures of 795 °C, and 815 °C for 20 min and then quenched into salt bath held at austempering temperature of 365 °C for various times to obtain austempered ductile iron (ADI) with different ausferrite volume fractions distributed in soft proeutectoid ferrite matrix. Some samples were also conventionally austempered for comparison reason. For austempered samples, the tensile strength and hardness decreased with increasing the austempering time, whereas the elongation increased considerably. In addition, the abrasive wear behavior of as cast and ADI's with dual matrix structure was studied under different loads without lubrication. It was found that the wear resistance of ADI with dual matrix structure was superior to that of as cast iron. Among the austempered samples, the lowest weight loss was observed for conventionally austempered samples with its wholly ausferritic structure throughout the specimen while the highest weight loss was obtained for the samples having the lowest ausferrite volume fractions in those samples with dual matrix structures. Moreover, wear resistance increased with increasing ausferrite volume fraction and decreasing austempering time in all tested samples.

Abstract: Ultra-fine, carbide-free bainitic structure of plate thickness between 34 and 116 nm has been obtained by low temperature austempering process of two hypo-eutectoid steels with 0.42 and 0.57% C. Decreasing the carbon content results in accelerating the bainite transformation reaction together with decreasing the retained austenite content, which is known to be detrimental to the mechanical properties. Furthermore, lowering the carbon content below the eutectoid composition allowed intercritical annealing of the material which resulted in a wider window for heat treatment parameters and consequently in a spread field for mechanical properties. Dilatometric measurements were used to design the suitable heat-treatment parameters including an estimation of the required time frames for the cessation of the bainitic reaction. The structure was characterized using light optical microscopy (LOM), scanning electron microscopy (SEM) and X-ray diffractometry. In order to investigate the effect of the microstructure parameters on the materials mechanical properties, compression tests had been conducted at room temperature.

Abstract: The microstructures and mechanical properties of a high-Si (1.5 wt.%) steel produced by a novel process of quenching and partitioning (Q & P) were compared with those obtained using traditional heat treatments (i.e. austempering, intercritical annealing for dual phase, quench and tempering). Plate steel was included for exploration of the Q & P process in applications requiring strength and toughness (such as an API line pipe), where retained austenite may contribute to the overall toughness via the TRIP phenomenon at a crack top. The Q & P process is based on the partial transformation of austenite to martensite, followed by partitioning of carbon from martensite into austenite, which leads to an untypical microstructure. Retained austenite amounts up to 6 vol.% with a carbon content of up to 0.88 wt.% were achieved in 0.1% carbon steel using Q & P. Superior impact toughness at higher yield strength levels was found after Q & P compared to other traditional heat treatments with equivalent partitioning, austempering or tempering conditions.

Abstract: Primary austenite is the least investigated microstructural component of grey cast iron. It is the first phase to be nucleated in the liquid metal and it grows in a dendritic manner, followed by growth of the eutectic phase. This paper describes an investigation on three potential nucleating agents to promote the nucleation of primary austenite in grey cast iron under industrial conditions. A direct austempering after solidification (DAAS technique) was used to determine the number of primary austenite grains. A colour etching technique was used to reveal the primary austenite grains and the eutectic cells. The results show that properly chosen nucleation agents significantly promote the number of primary austenite grains. Furthermore, the authors show that the eutectic cell size and the morphology of the graphite are obviously influenced by the primary solidification in grey cast iron. The increased nucleation of austenite grains resulted in an increased number of eutectic cells.

Abstract: The present work describes studies about the influence of processing variables on the microstructure and properties of dual phase austempered ductile iron (ADI). The upper and lower critical temperatures of conventional ductile iron melt were determined. Heat treatments involving austenitising within the intercritical interval, followed by austempering, allowed microstructures to be obtained composed of different combinations of free ferrite and ausferrite. Mechanical and fracture toughness tests performed on samples with mixed structures showed interesting combinations of strength and toughness, in comparison with fully ferritic and fully ausferritic matrices, particularly when austempering was carried out at 350°C. The results of the critical crack size, expressed by the relationship (K IC/σYS)2, which indicates the relative toughness of the material, showed the best values for ferritic matrices with ∼20% ausferrite. This effect is attributed to the location of the ausferrite in the last to fre...

Abstract: In the present study, an unalloyed ductile iron containing 3·50%C, 2·63%Si, 0·318%Mn and 0·047%Mg was intercritically austenitised (partially austenitised) in the two phase region (α + γ) at temperatures of 795 and 815°C for 20 min, and then was quenched into a salt bath held at an austempering temperature of 365°C for various times to obtain various ausferrite volume fractions. Fine and coarse dual matrix structures were obtained from the two different starting conditions. Some specimens were also conventionally austempered from 900°C for comparison. Results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high carbon retained or stabilised austenite) was developed. The phase previously described as new ferrite (also called epitaxial ferrite) was observed to form following heat treatment only in specimens with coarse austenite dispersion after austempering from the (α + γ) temperature range. It was observed that the parent austenite dispersion present at th...

Abstract: Austempering is an important thermal processing operation, where strong and tough bainitic steel is produced in a single heat treatment [1, 2]. During the austempering process, the steel is first austenitized and then cooled rapidly just above the martensite start temperature until bainite nucleates and grows, usually until the transformation stops and then it is cooled to room temperature. Due to the sluggish solid-state transformation kinetics, industrial austempering necessitates isothermal holds of 2–24 h, depending on the size and composition of steel. In contrast to conventional isothermal processing, cyclic thermal processing has been shown to accelerate the kinetics of several phase transformations [3–5], with a significant beneficial impact on productivity and energy consumption of these energy intensive operations. This was attributed to the non-isothermal effects resulting from cyclic treatment. The non-isothermal effect on phase transformations has also been utilized to enhance the productivity of a modern batch annealing operation [6]. In the present work, the effect of cyclic processing on austempering kinetics has been compared with conventional isothermal processing for 1080 steel. Furthermore, the effect of cyclic frequency on the austempering kinetics has been studied. Austempering kinetics experiments were performed on 6 mm diameter cylindrical samples of a 1080 steel using a Gleeble 3500 thermo-mechanical simulator (DSI Poestenkill, NY). A diametrical dilatometer was mounted on the specimen to measure the diameter change during the thermal processing. The austempering experiments were performed in two cycles, where the first cycle provides the same initial microstructure prior to each experiment. After completing the first cycle, the cylindrical specimens were heated to the austenitizing temperature (850 C), held for 5 min and then cooled to different austempering temperatures, where the bainite transformation was monitored for the desired period of time, followed by cooling to room temperature. The cooling rate was sufficiently fast to avoid any transformation occurring before reaching the austempering temperature (Fig. 1). The isothermal experiments were carried out at austempering temperatures of 260 and 300 C, whereas the cyclic experiments were carried out between 260 C and 300 C at two different heating/ cooling rates of 1 and 5 C/min. The percentage of bainitic transformation as a function of time was computed from the dilation curve [2] in conjunction with the microstructural examination by optical microscopy and SEM. The normalized dilatation curves for isothermal austempering at 260, 300 C, and cyclic austempering between 260 C and 300 C for 1 C/min and 5 C/min are shown in Fig. 2. In this figure, only the portion of the curve corresponding to austempering is shown and the dilatation curve has been offset to start at zero. The plateau of the dilatation curve with respect to the time represents the end of the transformation to bainite. It must be noted that the end of the bainite transformation does not necessarily correspond to 100% volume fraction of bainite [1]. Previously, using the same type of Gleeble experiments, it was shown that the end of the transformation corresponds closely to the austenite carbon content corresponding to a T0 equal to the austempering temperature [2, 7]. Furthermore the transformation dilatation at the end of the V. Sista P. Nash (&) Thermal Processing Technology Center, IIT, 10 W 32nd St., Chicago, IL 60616, USA e-mail: philip.nash@iit.edu

Abstract: High strength steels can be produced by austempering of Si-containing steels. It is possible to achieve high toughness and good wear resistance in these steels. Surface hardening of this group of steels can further increase the surface hardness and wear resistance and in combination with high strength in the bulk, also the fatigue strength. Surface hardening by laser-hardening has been performed on steel 55Si7 after austempering of the steel in order to create a ferritic-austenitic carbide free microstructure. Tempering effects and hardness values have been studied. Optical as well as scanning electron microscopy has been used together with x-ray diffractometry in the characterization of the micro-structural changes. Wear resistance testing of austempered and laser hardened samples respectively of the Si-alloyed steel have been reported and also compared with that of the conventional Cr-alloyed steel. The results of the specific phase transformation from austenite to martensite during wear process will be reported.

Abstract: A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.

Abstract: The influence of austenitization and austempering parameters on the impact properties of copper-alloyed and nickel-copper-alloyed austempered ductile irons (ADIs) has been studied. The austenitization temperature of 850 and 900 °C have been used in the present study for which austempering time periods of 120 and 60 min were optimized in an earlier work. The austempering process was carried out for 60 min for three austempering temperatures of 270, 330, and 380 °C to study the effect of austempering temperature. The influence of the austempering time on impact properties has been studied for austempering temperature of 330 °C for time periods of 30-150 min. The variation in impact strength with the austenitization and austempering parameters has been correlated to the morphology, size and amount of austenite and bainitic ferrite in the austempered structure. The fracture surface of ADI failed under impact has been studied using SEM.

Abstract: Ultra high-strength low-alloyed TRIP-aided steels possess high impact toughness, fatigue strength and hydrogen embrittlement performance, as well as good formability. So, it is expected that the TRIP-aided steels are applied to some automotive parts such as not only center pillar but also driving parts, spring and bolts. In the present study, the effects of ausforming conditions (temperature and strain) after austenitizing on tensile properties and toughness of TRIP-aided steels with bainitic ferrite matrix were investigated to develop a new type of ultra high-strength forging steel. TRIP-aided steel with chemical composition of 0.2%C, 0.5%Si, 1.5%Mn, 1.0%A1, 0.02%Nb and 0.1 %Mo achieved large total elongation and high impact toughness by ausforming to 0-50% strain at 600-900°C, followed by austempering at 400°C for 500s. This was mainly caused by refined microstructure and stabilized retained austenite.

Abstract: The microstructural characteristics of the bainite constituent associated with TRIP steels were investigated in detail by means of transmission electron microscopy (TEM). Two Si–Mn steels were selected for the study, i.e. a 0.6C–1.5Mn–1.5Si and a 0.18C–1.55Mn–1.7Si (wt%). Following austenitizing treatment the high carbon steel was cooled directly to bainite reaction temperature, whereas the low carbon steel was subjected to an intercritical holding step, i.e. in order to form polygonal ferrite prior to bainite transformation. Partially reacted samples at the bainite stage were examined in particular to determine the tendency of carbide precipitation at different transformation temperatures. Cementite particles were found in most of the analyzed conditions despite the high Si content of the specimens. The orientation relationship between cementite and bainitic ferrite was determined to be different from the frequently observed Bagaryatski relationship. The formation of cementite was rationalized in terms of its thermodynamic affinity as well as from a kinetic viewpoint. The findings strongly suggest that the common perception of carbide-free bainite pertinent to TRIP steels is not necessarily a valid assumption and thus carbide precipitation should not be neglected in a kinetic model and for designing the processing parameters for this class of advanced high strength steel.

Abstract: The heat treatment conditions for the medium-carbon steel strengthened by plastic deformation-induced martensitic transformation of the retained austenite (Transformation Induced Plasticity or TRIP) were developed. After heat treatment according to an optimum variant, the structure of the investigated steel is ferritic bainite, with a 23% fraction of retained austenite showing thermal stability. It was found that the bainite contains a very small fraction of carbides. Their precipitation is hampered by a Si concentration of 1.35%. The retained austenite occurs as regular grains of 3 m in diameter or as films between bainite laths. On the basis of the tensile test, it was found that the steel is characterised by high strength properties (UTS = 1120 MPa, YS = 790MPa) and excellent ductile properties (TEL = UEL = 34%). The high strength and excellent ductile properties as well as the instantaneous work-hardening exponent n ~ 0.4 show that optimum utilisation of the TRIP effect improves the mechanical and technological properties of the steel.

Abstract: The investigation was carried out to examine the in fluence of temperature and times of austempering pr ocess on the maximum extend to which the bainite reaction can proceed and the carb on content in retained austenite. It should be note d that a small percentage change in the austenite carbon content can have a significant eff ect on the subsequent austempering reaction changin g the volume fraction of the phases present and hence, the resulting mechanical propert ies. Specimens were prepared from an unalloyed ductile cast iron, austenitised at 950 o C for 60 minutes and austempered by the conventional single-step austempering process at four temperatur es between BS and MS, eg., 250, 300, 350 and 400 o C. The samples were austempered at these temperatures for 15, 30, 60, 120 and 240 minutes and finally q uenched to ambient temperature. Volume fractions of retained a ustenite and carbon concentration in the residual a ustenite have been observed by using X-ray diffraction. Additionally, carbon conce ntration in the residual austenite was calculated u sing volume fraction data of austenite and a model developed by Bhadeshia based on the McLellan and Dunn quasi-chemical thermodynamic model. The comparison of experimental data with the T0, T0' and Ae 3' phase boundaries suggests the likely mechanism of bainite reaction in cast iron is displacive rather than diffusional. The carbon concentration i n retained austenite demonstrates that at the end o f bainite reaction the microstructure must consist of not only ausferrite but additionall y precipitated carbides.

Abstract: Austempered ductile iron (ADI) for components requiring high strength and/or ductility, which has a silicon content of 335 weight-% to 460 weight-%, and which is obtainable by performing an ADI-heat treatment using an austenitization temperature of at least 910oC

Abstract: Some special features of the kinetics and mechanism of the bainitic transformation in high-speed steel HS18-0-1 have been studied. Austempering has been performed at temperatures 300°C and 270°C for 10–360 min, after heating up to 1100°C, 1200°C, and 1280°C. Optical metallographic analysis, electron-microscope analysis, X-ray phase analysis and testing of Rockwell hardness have been carried out. The bainitic transformation at temperatures 270°C and 300°C takes place according to the scheme: α + A(c). The temperature of heating (1100°C, 1200°C, 1280°C) does not influence the mechanism of bainitic transformation. It changes the kinetic process parameters only.

Abstract: Effects of finishing rolling temperatures and reduction on the mechanical properties of hot rolled multiphase steel were investigated Thermo-mechanical control processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finishing rolling temperatures and reduction and various austempering times were applied The results showed that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes, and that the strain-induced transformation to martensite from the retained austenite can occur gradually when the steel is deformed during tensile test Mechanical properties increase with decreasing finishing rolling temperature and increasing amount of deformation The most TRIP (transformation induced plasticity) effect, and ultimate tensile strength (UTS), total elongation (TEL) and the product of ultimate tensile strength and total elongation (UTS / TEL) are obtained at 20 min

Abstract: In this investigation, austempered ductile cast iron (ADI) with a fully ferritic microstructure was produced by a novel thermo magnetic austempering process. This novel process consists of austempering of ductile cast iron in a high magnetic field of 20 T. Unalloyed ductile cast iron samples were initially austenitized at 927 °C (1700 °F) for 2 h and then quenched in a salt bath to an austempering temperature of 260 °C (500 °F). The samples were austempered at this temperature for 0.5 h in the presence of a strong external magnetic field of 20 T around the salt bath chamber and test samples. This resulted in a fully ferritic microstructure. The microstructure and mechanical properties of the resulting material have been characterized and compared with conventionally processed austempered ductile cast iron at the same temperature (260 °C) without magnetic field.

Abstract: The production of lightweight ferrous castings with increased strength properties became unavoidable facing the serious challenge of lighter aluminum and magnesium castings. The relatively new ferrous casting alloy ADI offers promising strength prospects, and the thermo-mechanical treatment of ductile iron may suggest a new route for production of thin-wall products. This work aims at studying the influence of thermomechanical treatment, either by ausforming just after quenching and before the onset of austempering reaction or by cold rolling after austempering. In the first part of this work, ausforming of ADI up to 25% reduction in height during a rolling operation was found to add a mechanical processing component compared to the conventional ADI heat treatment, thus increasing the rate of ausferrite formation and leading to a much finer and more homogeneous ausferrite product. The kinetics of ausferrite formation was studied using both metallographic as well as XRD-techniques. The effect of ausforming on the strength was quite dramatic (up to 70% and 50% increase in the yield and ultimate strength respectively). A mechanism involving both a refined microstructural scale and an elevated dislocation density was suggested. Nickel is added to ADI to increase hardenability of thick section castings, while ausforming to higher degrees of deformation is necessary to alleviate the deleterious effect of alloy segregation on ductility. In the second part of this work, the influence of cold rolling (CR) on the mechanical properties and structural characteristics of ADI was investigated. The variation in properties was related to the amount of retained austenite (γ r ) and its mechanically induced transformation. In the course of tensile deformation of ADI, transformation induced plasticity (TRIP) takes place, indicated by the increase of the instantaneous value of strain-hardening exponent with tensile strain. The amount of retained austenite was found to decrease due to partial transformation of γ r to martensite under the CR strain. Such strain-induced transformation resulted in higher amounts of mechanically generated martensite. The strength and hardness properties were therefore increased, while ductility and impact toughness decreased with increasing CR reduction.

Abstract: Effect of austempering on the transformation induced plasticity (TRIP) of hot rolled multiphase steel was investigated. Polygonal ferrite, granular bainite, and a large amount of stabilized retained austenite could be obtained in the hot rolled multiphase steel. Strain induced martensite transformation (SIMT) of retained austenite and TRIP effectively occur under straining owing to austempering after hot rolling, and mechanical properties of the present steel remain at a relatively high constant value for austempering at 400°C. The mechanical properties of the steel exhibited a good combination of tensile strength (791MPa) and total elongation (36%) because the stability of retained austenite is optimal when the steel is held for 20min.