Abstract: Bainite is of considerable importance in the design of high strength steels. There are two types of morphologies, upper and lower bainite. In upper bainite, cementite forms between adjacent bainitic ferrite plates. In certain steels, however, the cementite reaction is suppressed so that carbon-enriched austenite remains untransformed between bainitic ferrite plates. In lower bainite, cementite also has the opportunity to precipitate within bainitic ferrite plates. In order to model the development of these microstructures, it is necessary to treat the simultaneous formation of both the ferritic and carbide components of the microstructure. A theory has been developed to do exactly this, enabling the estimation of the phase fractions, the cementite particle size and the transition from upper to lower bainite. The results have been compared against experimental data.

Abstract: The influence of a novel two-step austempering process on the microstructure and the near threshold fatigue crack growth behavior of austempered ductile cast iron (ADI) were investigated. Cylindrical tensile and compact tension (CT) specimens (for fatigue threshold tests) were prepared from an alloyed nodular ductile cast iron as per ASTM standards and were austempered by both the conventional single-step and the novel two-step austempering processes at four different temperatures. The near threshold fatigue crack growth behavior of these samples was examined in room temperature and ambient atmosphere. Tests results indicate this two-step austempering process has resulted in higher hardness, higher yield and tensile strengths for ADI but higher near threshold fatigue crack growth rate and lower fatigue threshold, as compared to the conventional single-step austempering process. Results also demonstrate that fatigue crack growth behavior of ADI in the near threshold region is influenced by microstructural parameters, such as volume fraction of austenite ( X γ ), carbon content in austenite ( C γ ), ferritic cell size ( d ) as well as total austenitic carbon, X γ C γ . SEM fractographs in all samples exhibit a combination of mechanisms, i.e. ductile striation with quasi-cleavage facet around graphite nodules.

Abstract: Austenitizing as a first stage of the heat treatment of castings to produce the cast iron grades like ADI, ACI, or AGI consists in holding of castings at a temperature co mprised within the range of 800-950 0 C to obtain an austenitic structure of the matrix as a point of departure for ausferritic structure. The temperature and time of austenitizing exert an important effect on the structure parameters and mechanical behaviour of mat erial obtained after the final treatment, which is austempering. A mathematical model of the process has been caharcterized, allowing for the temperature field in cas ting, the field of diffusion in the area of pearlite lamellae, and changes of diffusion coe fficient in function of temperature. A numerical program was developed by means of which the kinetics of austenite growth and cementite and ferrite fading in lamellar pearlite during austenitizing were determined, and a non-stationary field of carbon concentration in the examined system was computed. The temperature field in casting was also verified e xperimentally (a sample of 6x10x15 mm held in salt bath).

Abstract: Austempered ductile iron (ADI) has proved to be an excellent material as it possesses attractive properties: high strength, ductility and toughness are combined with good wear resistance and machinability. These properties can be achieved upon adequate heat treatment which yields the optimum microstructure for a given chemical composition. In this paper the results of an investigation the austempering of ADI alloyed with 0.45 % Cu for a range of times and temperatures are reported. The microstructure and fracture mode developed throughout these treatments have been identified by means of light and scanning electron microscopy and X-ray diffraction analysis. It was shown that the strength, elongation and impact energy strongly depend on the amounts ofbainitic ferrite and retained austenite. Based on these results, and optimal processing window was established.

Abstract: Austempered ductile iron (ADI) is an austemper-treated ductile iron with acicular ferrite and high-carbon austenite as the matrix of microstructure. In general, the austempering is isothermally treated about in the temperature range from Ms to 450 °C, thus the traditional case hardening of high temperature cannot be available to treat ADI. In recent years, physical vapor deposition (PVD) technique using lower processing temperature has been widely adopted to coat various films, such as diamond-like carbon (DLC), CrN, TiN and so on, on the engineering material for surface modification. In particular, DLC film possesses excellent mechanical properties such as high hardness and low friction coefficient. Electroless nickel (EN), another lower temperature coating process, has also a wide field of application such as the industrial components and machine parts. Thus, the purpose of this study is to investigate the effect of EN and PVD–DLC surface coatings on mechanical behaviors of ADI, especially the tensile and fatigue properties. Analyses of the resulting films were also performed for correlating the mechanical properties attained to the coating characteristics.

Abstract: Crack initiation and growth behavior of an austempered ductile iron (ADI) austenitized at 800 °C and austempered at 260 °C have been assessed under three-point bend fatigue conditions. Initiation sites have been identified as carbides remaining from the as-cast ductile iron due to insufficient austenization. The number of carbides cracking on loading to stresses greater than 275 MPa is critical in determining the failure mechanism. In general, high carbide area fractions promote coalescence-dominated fatigue crack failure, while low area fractions promote propagation-dominated fatigue crack failure. Individual carbides have been characterized using finite body tessellation (FBT) and adaptive numerical modeling (Support vector Parsimonious Analysis Of Variance (SUPANOVA)) techniques in an attempt to quantify the factors promoting carbide fracture. This indicated that large or long and thin carbides on the whole appear to be susceptible to fracture, and carbides that are locally clustered and aligned perpendicular to the tensile axis are particularly susceptible to fracture.

Abstract: Cost competitive machining of ADI after heat treatment has been a normal practice for about twenty years. In order to be successful with machining after austempering all the metallurgical processes, both in the liquid and solid states, must be carried out at the best level of available technologies, involving all necessary investments to ensure consistent and reproducible quality. A high nodule count and a narrow range of hardness are the first indexes to be monitored. This will ensure the safety of the casting design. Research programs on material properties and material design, together with the success of running applications, are increasingly indicative of the high potential of ADI as a benchmark material for engineering applications. Metallurgical processes applied to castings are based on the unique relationship of spheroidal grade and silicon. This relationship allows us to produce the intermediate structure in austempered cast irons, knows as “Ausferrite”. When compared with steels, ADI castings are less dense, less likely to crack and have excellent wear resistance. Being a multi-phase, high performance material, ADI’s process window is narrower than other conventional materials. For this reason, ADI processes require large investments in the foundry and heat treatment, with maximum integration between engineering design and machining operations.

Abstract: The embrittlement of austempered ductile iron has been studied as a function of aqueous solution composition and of applied potentials. The extent of the embrittlement does not show marked...

Abstract: The fracture behavior of copper-alloyed austempered ductile iron (ADI) was studied using metallography and fractography of selected samples. Three different grades of ADI were developed by austenitization at 900 °C for 60 min, followed by austempering for 60 min at either 270, 330, or 380 °C. The variation in austempered microstructure was determined by scanning electron microscopy of metallographically prepared samples, and structural parameters such as volume fraction of austenite, carbon content, and bainitic needle width were determined from the X-ray diffraction of powdered samples. The effect of austempering temperature on these structural parameters and on hardness, 0.2% proof stress, ultimate tensile strength (UTS), percent elongation, and impact strength was also studied. The fracture behavior under tensile and impact loading was determined by examination of the fractured surfaces and transverse cross sections near the fracture surface. The hardness, 0.2% proof stress, and UTS decrease and the impact energy increases as the austempering temperature is increased, and the morphology of the bainitic structure changes from lower to upper.

Abstract: Transformation-induced-plasticity (TRIP) –aided steels are a promising solution for producing lighter, crash-resistant car bodies, due to their high-strength and large uniform elongation. The influences of the austempering temperature and time on the microstructure mechanical properties and the transformation behavior of a Si-Mn TRIP steel are investigated in this paper.

Abstract: The investigations presented in this contribution were targeted to replace a gear group made up of SAE 8620 steel in army jeeps with microstructural controlled austempered ductile iron, in...

Abstract: The main research results about the mechanism of bainite formation are described as follows: (1) Bainite in steel is of ultra-fine structure; (2) Surface relief of bainite is produced only by the deformation of the growing bainitic ferrite; furthermore, surface relief of a single smallest structural unit is tent-shaped rather than invariant-plane-strain-typed; (3) 3-dimensional superledges appear on the broad face of bainite ferrite and the austenite (335) plane of the structural units of bainitic sheaves is reasoned to be a diffusional interface, which indicates the growth of lower bainite by a diffusion process; (4) The bainite carbide nucleates in the γ side of α/γ interface and grows toward austenite; (5) Sympathetic nucleation-ledgewise growth model of bainite suggested on the base of the experiment data was proved by theoretical calculation.

Abstract: Two ADI heats transformed at temperatures of 400 and 380 °C during temporal range from 2 minutes to 9 hours were studied in details, with emphasis on structure composition and mechanical properties (tensile and fatigue properties were determined). In the case of the shortest dwells the level of mechanical properties is influenced by martensite, which occurs in the structure as a result of subsequent cooling. UTS and yield stress increase slightly with the dwell of isothermal transformation while the values of elongation to fracture as well as of fatigue limit are very closely dependent on the amount of the retained austenite in the microstructure.

Abstract: The abrasion wear behaviours of Ausformed Austempered Ductile Iron (AADI) and conventional Austempered Ductile Iron (ADI) were investigated. The effects of both Ni content and rolling reduction during the ausforming process on the wear resistance of AADI and ADI were studied. The ausforming process created a finer and more homogeneous ausferrite structure that had a direct influence on the mechanical properties of AADI. Maximum hardness and tensile strength were obtained with 35% rolling reduction. On the other hand, ductility and impact strength were reduced with increasing rolling reduction during the ausforming process. AADI showed superior abrasion wear resistance because of its finer and harder structure.

Abstract: The influence of austempering treatment on the microstructure, retained austenite and fracture toughness of high silicon cast steel has been studied. The experimental results show that the retained austenite increases with increasing the austempering temperature, and the carbon contents in the retained austenite also shows a similar behavior. The fracture toughness of the steel firstly increases with increasing the austempering temperature, reaching a peak value of 63 MPa·m~(1/2) at about 360-385℃ and then decreases. The maximum fracture toughness can be obtained for this steel when the microstructure contains about 30%-35% retained austenite.

Abstract: This preliminary study demonstrates that surface engineered austempered ductile iron is a valid alternative gear material to carburised steel. The impact energy of ductile cast iron samples in various conditions, including as – cast, austempered at 240 o C and austempered and shot peened using both 0.7mm and 1.4mm steel balls, was measured. As expected the as-cast material exhibited nominal impact energy. This has been attributed mainly to the pearlitic matrix structure. Austempering generally increases the impact energy. The extent of the improvement is however dependent on the austempering temperature and the resulting microstructure. It was shown that the structure following austempering at 240 o C consists of fine acicular ferrite needles with high aspect ratio and relatively small quantities of retained austenite. These two features account for the comparatively low impact energy and high hardness values. Increasing the austempering temperature to 375 o C leads to structures containing coarser ferrite needled and up to 40% retained austenite. Various investigators have shown that the high impact energy of irons austempered in this temperature range is dependent on the percentage retained austenite content (1, 2) and stability of the latter (3) . It can be seen that a reduction in size of shot used for peening has the effect of moving the maximum subsurface compressive stress closer to the surface. This increases the surface stress, but results in a shallower compressive surface layer. High surface compressive stresses make it more difficult to initiate surface cracks and thus properties such as fatigue and impact energy are bound to improve (4) . In the case of high surface compressive stresses, the sub-surface becomes critical and thus the advantage of having a deeper affected zone. This may suggest that in order to obtain maximum benefit from peening duplex size shot should be used. Samples austempered at 240 o C and shot peened have higher impact energy values than as cast samples. This may suggest that the fatigue behaviour of gears austempered at the higher temperatures may be superior to those treated at lower temperatures whilst the tribological characteristics may be inferior. This is however not necessarily the case and needs to be investigated further.

Abstract: Recently, austempered ductile iron (ADI) has emerged as a new class of ferrous materials and represents a major achievement in cast iron technology [1]. The mechanical strength and impact toughness of nodular iron are provided by the precipitation of the graphite phase as spheroids surrounded by ferrite (bull’s-eye structure) in a continuous pearlite matrix. The quality of ductile iron increases with the number of the graphite spheroids. A high spheroids volume fraction, which is mainly controlled by the inoculation process, limits the chemical segregation during solidification and ensures the structural homogeneity of the component. In this work, a lower value of Young modulus was obtained when the graphite phase was taken into account in the self-consistent modelling. For 12% of graphite the theoretical Young modulus agrees with the measured one (mechanical tensile test). The volume fraction of graphite was confirmed independently by micrographic observation (14%). It can be concluded that the macroscopic behaviour of ADI steel can be modelled by the self-consistent approach in which the austeno-ferritic aggregate is represented by an effective matrix, while instead of the graphite spherical empty spaces are introduced. Using such an approach it was shown that in the elasto-plastic range of deformation, presence of graphite phase caused stress relaxation.

Abstract: It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.

Abstract: The relationship between the crack nucleation and stress-induced martensitic transformation in the retained massive austenites (RM-� ’s) of austempered ductile irons (ADIs) was examined in detail by carrying out tensile tests and scanning electron microscope (SEM) observations for an ADI material. The SEM observations revealed that cracks were not nucleated in the peripheral regions of graphite nodules but were nucleated in the RM-� ’s. Surface relief due to stress-induced martensitic transformation was observed near the cracks in the RM-� ’s, and it was also observed in the RM-� ’s in which cracks were not visible. For this reason, the cracks were concluded to be nucleated mainly in the RM-� ’s subjected to stress-induced martensitic transformation.

Abstract: The influence of aluminium on the microstructure of austempered ductile cast iron (ADI) has been studied. The presence of well distributed graphite in the matrix, and a relatively high volume fraction of retained austenite are responsible for the good mechanical properties of the Aluminium alloyed austempered ductile irons including excellent wear resistance, higher resistance to thermal shock, better graphitising tendency and higher resistance to oxidation at high temperatures. A number of specimens of different composition have been made by green sand-casting and gravity die-casting. The transformation to a bainitic microstructure during austempering under different conditions was then examined for the most successful experimental casts. Austenitising temperature of 920°C, and austempering temperatures of 350°C at different holding times have been used. Microstructures have been examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, it was found that isothermal transformation at 350°C for different soaking times gave a typical bainitic microstructure. Extension of isothermal transformation time leads to precipitation of carbides which also depends on the Al concentration.

Abstract: The aim of present work is to investigate the influencing factors on mechanical property stability of Cu-Mo-Ni alloyed austempered ductile iron (ADI). The results show that after austenitized at 900 ℃ for 2 h followed by austempered at 370 ℃ for another 2 h, the mechanical property of the alloyed ADI can reach the Germanite GGG-100 standard, i.e.σb≮1 000 MPa, δ≮5%, at 95% confidence level. And the satisfactory mechanical properties were obtained when the alloyed ADI was austenitized at 850 ℃ to 1 000 ℃ for 1-4 h, and austempered at 355 ℃ to 400 ℃ for another 1 h to 4 h. The microstructures, including nodule number, white bright zone content (martensite-containing interdendritic segregation zone) and retained austenite content, can significantly influence the mechanical properties of the ADI. In order to obtain the good combinations of strength and ductility, the volume fraction of white bright zone should be less than 5%, and the retained austenite contents maintain between 30 % and 40 %. The application of inoculation techniques to increase graphite nodule number can effectively reduce the white bright zone content in the structure.

Abstract: Prowadzone badania nad zeliwem ADI sprowadzają sie zwykle do wlaściwości objetościowych tego materialu. Wlaściwości warstwy wierzchniej nadal jednak pozostają zbyt malo rozpoznane. Dotychczas stwierdzono, ze ADI posiada unikalną ceche umacniania powierzchniowego przez zgniot lub obrobke mechaniczną. Podejmowane byly rowniez nieliczne proby innego rodzaju oddzialywania na ADI, np. przez proces PVD lub laserową modyfikacje powierzchni, przede wszystkim ze wzgledu na nietrwalośc korzystnej mikrostruktury tego zeliwa w temperaturach klasycznych obrobek powierzchniowych. Zabiegi te mialy jednak ograniczone zastosowanie. Poszukiwane są zatem nowe, lepsze, bardziej ekonomiczne metody modyfikacji wlaściwości powierzchniowych wyrobow z zeliwa sferoidalnego ausferrytycznego. W artykule przedstawiony zostal nowy sposob obrobki powierzchniowej polegający na polączeniu obrobki cieplno -chemicznej z zabiegami obrobki cieplnej prowadzącej do otrzymania zeliwa sferoidalnego ausferrytycznego. W artykule przedstawiono wyniki badan strukturalnych, twardości oraz odporności na zuzycie przez tarcie zeliwa sferoidalnego hartowanego izotermicznie w zlozach fluidalnych. Odpowiednie wlaściwości powierzchniowe u zyskano za pośrednictwem procesu wysokotemperaturowego wegloazotowania, ktory zostal adaptowany do cyklu obrobki cieplnej dla zeliwa ADI. Okazuje sie, ze w wyniku takiej polączonej obrobki cieplnej i cieplno-chemicznej mozna uzyskac wysoką twardośc i odpornośc na zuzycie zeliwa sferoidalnego oraz znacząco poprawic energochlonnośc cyklu jego obrobki cieplnej przez zespolenie dwoch procesow obrobczych.

Abstract: A fabrication method for forged parts with tensile strength of class 14.99. The fabrication method is primarily to treat alloy steel wires through the following processes: (1) spheroidized annealing process, (2) dipping for pickling and bonderizing, (3) coarse drawing, (4) softening annealing, (5) dipping for pickling and bonderizing again, and (6) fine drawing. Then performing the forging and tread-rolling processes. Finally, heating up and undergoing the austempering treatment process directly so that the microstructure of the parts will transform to Bainite, which has better mechanical properties. This fabrication method will produce quickly, with very stable quality, high toughness forged parts that has tensile strength of class 14.99, namely, having elongation ratio 9-14% under stress 140 kgf/mm2.

Abstract: The influences of isothermal quenching parameters on the microstructure and mechanical properties of austempered ductile iron(ADI)have been studied. The isothermal quenching process has been chosen by the orthogonal tests.and excellent mechanical properties have been obtained. The tensile strength of ADI for crank shaft≥950 MPa,elongation≥6%,hardness28～32 HRC.

Abstract: A low alloy Bainite ductile cast iron had been obtained by using a new heat treatment technique of the step austempering in room temperature machine oil.The effects of elements B,Cu and Mn on structure and mechanical properties of the Bainite ductile cast iron in above-mentioned process were investigated.The phenomenon of hardness lag of the alloyed Bainite ductile cast iron has been discussed.This phenomenon showed that after the step austempering,the hardness increased with the time.Both elements B and Mn could increase the hardness and reduce the impact strength;element Cu could increase the impact strength.Reasonable alloy elements could improve mechanical properties of the Bainite ductile cast iron.Essentially,hardness lag of the alloyed Bainite ductile cast iron resulted from solute drag-like effect.

Abstract: The continuous cooling transformation behavior and microstructure for a Mn-Cr gear steel were investigated at different deformation temperatures in recrystallization region of austenite using a Gleeble 1500 hot simulator. The experimental results show that the decrease of deformation temperature promotes the formation of polygonal ferrite and pearlite, leading to the increase of critical cooling rate obtained polygonal ferrite plus pearlite microstructure. A competitive mechanism exists between the acicular ferrite and bainite formations. The grain boundary allotriomorphic ferrite decorates the austenite grain surfaces with decreasing deformation temperature and cooling rate, which causes a transition from bainite to acicular ferrite. With decreasing deformation temperature, the stability of austenite against acicular ferrite reaction increases, resulting in the decrease in bainite finished temperature and increase of M/A island.

Abstract: Mechanical characteristics of Hi-Cr cast irons containing 16.8%Cr and 3.0%C were studied with various heat treatments. After as-cast Y-block ingots were annealed fully, the ingots were machined into cylinderical specimens with the size of 9mm in diameter and 20mm in length in order to investigate the effect of heat-treatments on mechanical characteristics of high Cr cast irons. All specimens were heat-treated by quenching- tempering, austempering and cyclic heat at the various temperatures(950, 1000, 1050 and ) respectively. The wear amount was measured for each heat-treated specimens against the counterpart of a hardened SKD11 steel at the following conditions; wearing velocity: 0.7 m/s, load: 100N and sliding distance: 70 km. After as-cast specimens were annealed, fine carbides were formed, which affected the hardness and the wear resistance of Hi-Cr specimens. High hardness and good wear resistance were appeared on the specimens treated at 950 and and the austempered specimens show excellent wear resistance as well as high hardness.