Ausferritic Structure Research Articles

Concept for development of economically alloyed ausferritic (bainitic) ductile cast irons

The distribution of the castings production in the world, whose total output is more than 112 million tons, is considered. A steady trend to the replacement of rolled alloy steel, which is used for critical engineering products, by Austempered Ductile Iron (ADI) is outlined. The latter is ductile cast iron with globular graphite, which is austempered (isothermally quenched) to produce ausferritic (bainitic) structure. The dynamics of prices for main alloying elements in ADI (molybdenum, nickel and copper, whose total amount in the alloy reaches 5 %) is considered and their steady growth is outlined.The research goal is to develop a concept of economical alloying of ADI (the total content of Mo, Ni and Cu should not exceed 2 %) retaining sufficient hardenability and substantially decreasing the cost price. The goal is attained by decreasing the content of the most expensive alloying element, molybdenum, to a minimal level along with the use of microalloying and applying one of the methods of hot plastic deformation to the casting.The reduced content of the main ‘triad’ of alloying elements is compensated by microadditives of strontium or barium, vanadium, boron, niobium and the addition of misch metal. Several groups of economically alloyed cast irons are proposed: (1) low‑nickel and low‑molybdenum with increased content of copper (up to 1.8 %) and the addition of vanadium, (2) molybdenum‑free with microadditive of boron, (3) molybdenum‑free with microadditives of niobium and boron, and (4) molybdenum‑free minimally alloyed with nickel and copper.It is shown that plastic deformation, along with giving the product the required shape, affects the kinetics of structural‑phase transformations. and acts similarly to alloying with Mo and Ni, shifting the TTT‑curve to the right. Therefore, the ausferritic structure can be obtained at a lower cooling rate.The mechanical properties with such alloying and the use of plastic deformation are the following: ultimate tensile strength 1100–1500 MPa and elongation to failure 2–4 % for lower bainite; ultimate tensile strength 800–1100 MPa and elongation to failure is 4–7 % for upper bainite.

A Review on Heat Treatment of Cast Iron: Phase Evolution and Mechanical Characterization.

The isothermal heat treatment process has been identified as a unique process of fabricating exceptional graphite cast iron due to its remarkable mechanical properties, such as excellent machinability, toughness, and high level of ultimate tensile strength. Austempered ductile iron (ADI), ductile iron (DI), and gray cast iron (GCI), known as spheroidal cast irons, are viable alternative materials compared to traditional steel casting, as well as aluminum casting. The graphite nodules from the microstructures of DI, ADI, and GCI are consistently encompassed by acicular ferrite and carbon-saturated austenite in the matrix, forming a distinctive ausferritic structure. All these materials are extensively used in the fabrication of engine sleeves, engine blocks, valves, gears, and camshafts in the automobile sector. With relative motion and outward loads, these components are regularly exposed to surface contact. In this project, it was observed that austempering temperature and a shorter holding period could also be used to manufacture needle-like ferrite platelets for austempered ductile iron (ADI) and other graphite cast irons. To overcome the brittleness challenges and catastrophic failures encountered by applied loads in present-day applications, it is essential to comprehend the isothermal treatments, morphological behaviors, phase analyses, processing techniques, and mechanical properties needed to properly incorporate these materials into future designs. This review article provides detailed information on the characterization and relevant potential mechanisms of ADI, DI, and GCI.

Austenitization and formation of ausferrite structure in austempered ductile iron with dual matrix

The effect of austenitizing conditions on the microstructure and mechanical properties of an austempered ductile iron (ADI) with a dual matrix was investigated. Carbon diffusion plays a critical role in the phase transformation of austenitization. In initially pearlitic structures, the carbon diffusion distances involved during austenitization are smaller compared to those in ferritic structures. The study was carried out to examine the influence of temperature and times of the austenitization process on the maximum carbon content in austenite and then its effect on the processing window of ADI with the dual matrix. An alloyed ductile iron (3,6%C; 2,44%Si; 0,36%Mn; 0,9%Ni; 0,61%Cu; 0,11%Cr; 0,036%Mg; 0,015%S và 0,006%P) was fully austenitic at various temperatures 870 °C–930 °C. The minimum hardness reaches the value of 270 HB. An increase in austenitization temperature increases the carbon content dissolved in the austenite, which in turn, decreases the free energy controlling the transformation of austenite to bainite ferrite and high carbon austenite. Raising the austenitization temperature makes the process window shift toward the extending time. The carbon atoms must diffuse out of the ferrite needle with a diffusion distance equal to the ferrite layer thickness. At 360 °C austempered temperature, diffusion coefficient D = 4.60.10–17(m2/s). With a ferrite needle thickness of 1μm = 10–6 m, the diffusion time would be t = 5330 (s).

Development and properties of austempered low alloyed white cast iron

Abstract This study examined the response of low-alloy white cast iron to austempering heat treatment. In addition, it investigated the microstructure and mechanical properties of austempered low-alloy white cast iron. The low-alloy white cast iron specimens were austenitized at 900 °C, followed by quick quenching into a salt bath at 375 °C, and held there for 15 to 120 minutes for austempering heat treatment. Microstructural features were studied by optical, scanning electron microscopes, and XRD analysis. The mechanical properties were determined by hardness and unnotched Charpy impact toughness tests. As a function of those austempering times, a microstructural map was constructed to show how the transformation products develop, quantitatively. The experimental results showed that the austempering heat treatment produced a microstructure consisting of eutectic carbides + ausferritic structure in low-alloy white cast iron. It can be concluded that the low-alloy white cast iron can be austempered, similar to ductile cast irons. Improved hardness and impact toughness values have been obtained in austempered low-alloy white cast iron.

Ultrafine Ductile and Austempered Ductile Irons by Solidification in Ultrasonic Field

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules’ diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite.

Study of Transformation Kinetics for Austempered Gray Cast Iron

Austempered gray iron (AGI) with its unique ausferrite microstructure is an alternative material to traditional quenched and tempered gray iron with martensite microstructure due to its excellent mechanical properties. This research studies the process of ausferrite formation and kinetic characterization for gray cast iron. Specimens were austenitized at 832°C (1530°F) for 20 min and then held at different isothermal temperatures from 232°C (450°F) to 427°C (800°F) and isothermal times from 20 s to 120 min. The experimental results were analyzed using hardness tests and optical microscopy. Needle-like ausferrite structure was found while using austempering temperatures ranging from 232°C (450°F) to 371°C (700°F). The ferrite plates became coarse with increasing austempering temperature. Feather-like ausferrite was obtained at the austempering temperatures of 399°C (750°F) and 427°C (800°F). Transformation kinetics was utilized to analyze the ausferrite formation, and the activation energy of gray cast iron was found to be 39905.27 (J/mol) with a frequency factor of 6.06457 (1/s).

Wear behavior of composite strengthened gray cast iron by austempering and laser hardening treatment

The present study mainly investigated the phase transformation and wear performance of laser hardened austempered gray cast iron specimens. Ball-on-Plate reciprocating sliding wear tests with PAO4 base oil were carried out on untreated, quench-tempered, austempered, laser hardened austempered gray cast iron specimens. Micro hardness profiles and worn surfaces were analyzed for specific wear mechanisms. It was found that four different zones were formed beneath the laser hardened surface including laser hardened zone (Ledeburite, ≈67HRC), upper heat affected zone (Martensite, ≈69HRC), lower heat affected zone (Tempered Ausferrite, ≈36HRC) and substrate (Ausferrite, ≈39HRC). In the sliding wear tests, the laser hardened austempered gray cast iron specimens showed higher wear resistance than others, which could be associated with the benefits of ausferritic structure with high fracture toughness and laser hardened regions with high hardness. By observing the worn surface, grooves and micro-sized material removal occurred within the substrate. The surface of the substrate ploughed by the asperities of the ball and hard wear debris stopped on the edge of the laser hardened regions since ledeburite and martensite had high hardness and abrasive wear resistance. However, some micro-cracks and spalls were developed within the laser hardened zone. The results obtained from this study could be used as reference in future research and applications of laser hardened austempered gray cast iron.

Tribological Performance of Austempered and Tempered Ductile Iron

Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications.

Understanding the Machinability of Austempered Ductile Iron (ADI)

Guidelines for production milling, turning and drilling of the standard grades of austempered ductile irons (ADI) have been established. Electron Backscatter Diffraction (EBSD) characterization has clearly shown that severe plastic deformation in the machining-affected-zone, ahead of and beneath the cutting tool, will cause strain-induced martensitic transformation of the austenite in the ausferrite structure that inhibits machinability. This phenomenon is particularly of concern during finish machining where small depths of cut are strongly influenced by surface martensite from prior machining passes.

Austempering Experiments of Production Grade Silicon Solution Strengthened Ductile Iron

Austempered ductile iron provides a feasible way to produce high strength components. However, in heat treatments resulting in highest strengths some of the ductility is lost due to formation of bainitic carbides. The role of silicon in inhibiting the formation of iron carbides in as-cast ductile irons as well as its solution strengthening effect is well known and acknowledged in industry. The effect of silicon on austemperability, resulting microstructures, and mechanical properties of austempered ductile irons with silicon contents with 3.4-3.8 w-% was researched. Quenching and austempering heat treatments were carried out for production grade silicon solution strengthened ductile irons EN GJS 500-14. Results indicate, that it is possible to manufacture a fully ausferritic structure into a silicon solution strengthened matrix and indeed good ductility can be achieved in combination with ultimate tensile strength of 1600 MPa. Segregation of silicon reduces the solubility of carbon into the matrix especially close to the graphite nodules which reduce the stability of carbon stabilized austenite and leads into compromised machinability.

Influence of Casting Section Thickness on Fatigue Strength of Austempered Ductile Iron

The influence of casting section thickness on fatigue strength of austempered ductile iron was investigated in this study. ASTM A536 65-45-12 grade of ductile iron was produced, machined into round samples of 10, 15, 20 and 25 mm diameter, austenitized at a temperature of 820 °C, quenched into an austempering temperature ( $$T_{\text{A}}$$ ) of 300 and 375 °C and allowed to be isothermally transformed at these temperatures for a fixed period of 2 h. From the samples, fatigue test specimens were machined to conform to ASTM E-466. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) methods were used to characterize microstructural morphology and phase distribution of heat-treated samples. The fatigue strength decreases as the section thickness increases. The SEM image and XRD patterns show a matrix of acicular ferrite and carbon-stabilized austenite with ferrite coarsening and volume fraction of austenite reducing as the section thickness increases. The study concluded that the higher the value of carbon-stabilized austenite the higher the fatigue strength while it decreases as the ausferrite structure becomes coarse.

Comparative Studies on the Wear of ADI Alloy Cast Irons as Well as Selected Steels and Surface-Hardened Alloy Cast Steels in the Presence of Abrasive

AbstractThe paper presents the results of wear tests obtained for 4 groups of materials: surface-hardened alloy steels and alloy cast steels for structural applications, hard-wearing surface-hardened alloy cast steels, and austempered alloy cast irons. The wear tests have been performed on a specially designed test rig that allows reproducing the real operating conditions of chain wheels, including the rolling and sliding form of contact between elements. The chain wheels subjected to tests were operated with the use of loose quartz abrasive. This study presents results of measurements of material parameters, micro-structure of a surface subject to wear, as well as the linear wear determined for the materials considered. Based on the results, the following was found: the best wear properties were obtained for surface-hardened alloy steels and wear surface; strengthening of the ADI surface took place - most probably as a result of transformation of austenite into martensite; the uniformity of the structure of the materials affects the surface wear process. The study also indicated a significant degree of graphite deformation in ADI characterized by the upper ausferritic structure and its oblique orientation in relation to the surface, which resulted in a facilitated degradation of the surface caused by the quartz abrasive.

Effect of Austempering Heat Treatment Parameters on the Microstructure and Dry Sliding Wear Behaviour of AISI 9255 High Silicon Steel

The present investigation is focused to evaluate the dry sliding wear behavior of AISI 9255 high silicon steel austempered at different temperatures and durations. Here three batches of cylindrical test specimens were prepared from as-received high silicon steel and were austenitized at 900 °C for 30 minutes, followed by austempering heat treatment in a salt bath maintained at temperatures 300, 350 and 400 oC for durations between 1 to 4 hours. The samples after austempering were cooled to room temperature in open air. The microstructural analysis was done by using optical microscopy, scanning electron microscopy and x-ray diffraction (XRD) and also hardness test was done using micro vickers hardness tester and correlated to the specific wear rate of the austempered steel. Results indicate that specific wear rate decreases with increase in austempering time and increases with increasing temperature. Specific wear rate was found to be lowest at austempering temperature of 300 °C, which exhibits lower ausferritic structure having high hardness. At higher austempering temperature 400 °C, specific wear rate was observed to be high because of upper ausferritic microstructure having lower hardness. Results reveal that the material with high hardness shows high wear resistance i.e., the one austempered at 300 °C showed superior sliding wear resistance than the rest.

Forced-Air Cooling Quenching: A Novel Technique for Austempered Ductile Iron Production

This study produced austempered ductile iron (ADI) with ausferrite structure using forced-air cooling as the quenching method. ASTM A536 65-45-12 grade of ductile iron was produced, machined into flat samples of 5, 10, 15, 20 and 25 mm thickness, austenitised at a temperature of 820 °C for 1-h soaking period inside a muffle furnace, forced-air-cooled in an air quenching chamber to an austempering temperature (TA) of 300 °C and rapidly transferred into another muffle furnace maintained at 300 °C in order to austemper them for a fixed period of 2 h. Finally, the microstructural morphology and phase distribution of heat-treated samples were characterised using scanning electron microscopy (SEM) and X-ray diffraction (XRD) method. The SEM image with electron-dispersive X-ray analyses shows predominant carbon and iron peaks of high-carbon austenite and ferrite, respectively, while the XRD patterns predominantly consist of austenite (γ) and ferrite (α) phases which are the characteristic features of ADI. The study concluded that with the use of forced-air cooling as a quenching method, ADI of section thicknesses up to 25 mm with ausferrite structure is producible.

Ausferrite Flake Graphite Cast Iron at the Thermal Fatigue

The automotive brake (clutch) disks are produced almost exclusively from flake graphite cast iron. These disks must fullfil a variety of strictly controlled parameters - high wear resistance, hardness, resistance against thermal fatigue and good thermal conductivity. The microstructure is created by the IA graphite in pearlite matrix. The isothermal hardening to ausferrite structure (AGI) was made in order to improve standard flake graphite cast iron properties. New methods and procedures of non-destructive structuroscopy, magnetoinductive, ultrasound and magnetic spot methods were used to compare material properties of flake graphite cast iron and ADI. By these methods, elasticity modulus, strength and hardness were evaluated. The resistance against thermal fatigue was evaluated by the Eichelberg factor. In this contribution, Material parameters of standard flake graphite cast iron and AGI are compared. The heat treatment of brake disk matrix from AGI can considerably improve their material parameters, especially durability.

Influence of the Selected Factors on Thermal Fatigue of the ADI in an Aspect of its Suitability as the Material for Metal Moulds

Abstract The investigation results of the thermal fatigue resistance of two ADI grades (EN-GJS-1200-2 and EN-GJS-800-8) are presented in the paper. Tests were performed on the author’s research stand, by means of the resistance heating of samples acc. L.F. Coffin method. The thermal fatigue resistance is the basic criterion in assessing the material suitability for metal moulds. The maximal temperature (Tmax) of thermal cycles influence on their number, which the sample can withstand before cracking, were estimated. Structure transformations, hardness and strength changes of the austempered ductile iron (ADI) were analysed. It was found, that at cyclic heating to Tmax <500°C, the ADI retains its primary ausferritic structure, even after more than 10.000 thermal cycles. Such ADI behaviour predisposes it to be applied as a material for metal moulds used in the pressure die casting, it means mainly at casting Mg and Zn alloys as well as for small castings of Al alloys.

Effect of cast wall thickness, composition and austempering temperature on microstructure and properties of ductile cast iron

In this study, two ductile irons were prepared: the first cast was unalloyed with a chemical composition of 3·65C–2·58Si–0·31Mn–0·045Mg, and the second one was alloyed with 0·5%Cr. The solidification rate was changed by casting four different cast thicknesses of 5, 10, 20 and 30 mm. The austempering treatment was carried out by austenitising the samples at 900°C for 1 h and then rapidly quenched into two different salt baths with temperatures of 325 and 375°C for 1 h each. The austempering temperature of 325°C showed higher mechanical properties than of 375°C. An addition of 0·5%Cr enhanced the tensile strength and hardness on the account of impact toughness. Maximum abrasion resistance was reported for the iron containing 0·5%Cr and austempered at 325°C for the cast thickness of 5 mm. This is due to the formation of fine ausferrite matrix and existing Cr carbides imbedded in the matrix that resist well the abrasion resistance. Minimum abrasion resistance was obtained for the unalloyed iron austempered at 375°C due to the formation of coarse ausferritic structure and existence of a higher amount of retained austenite.

Investigating the machinability of austempered ductile irons with dual matrix structures

Abstract In this study, the machinability of austempered ductile iron with dual phase structure was investigated. Specimens were intercritically austenitized (partially austenitized) in the two phase region (α + γ) at various temperatures (810, 820 and 830 °C) for 90 min, then quenched in a salt bath held at austempering temperatures of 315 °C or 375 °C for 120 min and then air cooled to room temperature to obtain various ausferrite volume fractions and morphologies. Machinability tests were carried out at various cutting speeds (150, 180 and 210 m min−1), at a constant feed rate (0.30 mm rev−1) and depth of cut at 2 mm after austempering heat treatment. The experimental results showed that austempered specimens from the (α + γ) temperature range exhibited better machinability than as-cast samples. On the other hand, the cutting force was decreased with increasing proeutectoid ferrite fractions. The results also showed that continuity of ausferritic structure along intercellular boundaries plays an important role in determining machinability of austempered ductile iron with different ausferrite volume fractions. Another important result is that higher proeutectoid volume fraction has negative effects on surface roughness.

Wear Performance of Cu-Alloyed Austempered Ductile Iron

An investigation was carried out to examine the influence of structural and mechanical properties on wear behavior of austempered ductile iron (ADI). Ductile iron (DI) samples were austenitized at 900 °C for 60 min and subsequently austempered for 60 min at three temperatures: 270, 330, and 380 °C. Microstructures of the as-cast DI and ADIs were characterized using optical and scanning microscopy, respectively. The structural parameters, volume fraction of austenite, carbon content of austenite, and ferrite particle size were determined using x-ray diffraction technique. Mechanical properties including Vicker’s hardness, 0.2% proof strength, ultimate tensile strength, ductility, and strain hardening coefficient were determined. Wear tests were carried out under dry sliding conditions using pin-on-disk machine with a linear speed of 2.4 m/s. Normal load and sliding distance were 45 N and 1.7 × 104 m, respectively. ADI developed at higher austempering temperature has large amounts of austenite, which contribute toward improvement in the wear resistance through stress-induced martensitic transformation, and strain hardening of austenite. Wear rate was found to depend on 0.2% proof strength, ductility, austenite content, and its carbon content. Study of worn surfaces and nature of wear debris revealed that the fine ausferrite structure in ADIs undergoes oxidational wear, but the coarse ausferrite structure undergoes adhesion, delamination, and mild abrasion too.

Microstructures and Mechanical Properties of Austempered Fe-C-Si-B Alloy

A new type of Fe-C-Si-B alloy was developed. The investigation on microstructure details and the mechanical properties were performed for different austempering combinations. The results indicated that the Fe-C-Si-B alloy comprises ferrite, pearlite and interdendritic eutectic borides in as-cast condition. The distribution of eutectic boride with a chemical formula of M2B (M represents Cr, Fe, Mn or Mo) and is much like that of carbide in high chromium white cast iron. Pure ausferrite structure that is consisted of bainitic ferrite and carbon-riched retained austenite can be obtained by austempering treatment to the Fe-C-Si-B alloy. No carbide would precipitate in the structure and there is not any morphology change of borides. The hardness of the Fe-C-Si-B alloy decreases with increasing of the austempering temperature, and decreases greatly in at the early stages (within 5 to 10 min) of austempering transformation. The transformation is almost finished after about 5 to 10 min when thehardness of the alloy does not change anymore. The Fe-C-Si-B alloy is a new kind of wear resistance material with bright application prospect.