Effect of austempering temperature on microstructure of ausferrite in austempered ductile iron

The effect of austempering temperature on the microstructure of ausferrite in austempered ductile iron was investigated. The results show that the grain sizes of retained austenite and acicular bainitic ferrite both become larger with the increase of austempering temperature. As the austempering temperature is 240°C, the crystallographic relationship between ferrite and austenite in ausferrite follows Greninger-Troiano relation. However, Nishiyama–Wassermann relation and Greninger-Troiano relation both appear in ausferrite austempered at 300°C. At this temperature, the point-to-point misorientations of individual ferrite needle austempered at 300°C are less than 1°, being less than those at 240°C. This means the ferrite needles at 300°C contain fewer defects. However, some poles of ferrite needles obviously deviate from their ideal positions, which mainly comes from some ends of ferrite needles.     

Formation of strain-induced martensite in austempered ductile iron

The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.     

Effect of austempering treatment on microstructure and mechanical properties of high-Si steel

In the present investigation, the influence of austempering treatment on the microstructure and mechanical properties of silicon alloyed cast steel has been evaluated. The experimental results show that an ausferrite structure consisting of bainitic ferrite and retained austenite can be obtained by austempering the silicon alloyed cast steel at different austempering temperature. TEM observation and X-ray analysis confirmed the presence of retained austenite in the microstructure after austempering at 400 °C. The austempered steel has higher strength and ductility compared to as-cast steel. With increasing austempering temperature, the hardness and strength decreased but the percentage of elongation increased. A good combination of strength and ductility has been obtained at an austempering temperature of 400 °C.     

Microstructure and toughness of CuNiMo austempered ductile iron

The effect of austempering on the microstructure and toughness of nodular cast iron (designated as CuNiMoSG) alloyed with molybdenum, copper, nickel, and manganese has been studied. Light microscopy (LM), scanning electron microscopy (SEM), and X-ray diffraction technique were performed for microstructural characterization, whereas impact energy test was applied for toughness measurement. Specimens were austenitised at 860 °C, then austempered for various times at 320 and 400 °C, followed by ice-water quenching. Austempering at 320 °C produces a microstructure consisting of a mixture of acicular bainitic ferrite and the stable carbon-enriched austenite. In this microstructure ε-carbides are also identified after austempering up to 5 h. Fracture mode is changed from ductile to brittle with the prolonged time of austempering at 320 °C. The highest impact energy (115 kJ) corresponds not only to ductile fracture, but also to the maximum value of the volume fraction of retained austenite. Only martensitic structure was observed during austempering at 400 °C, inducing brittle fracture and significantly low-impact energy (10–12 kJ).     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Study of wear behaviour of austempered ductile iron

An investigation was carried out to examine the influence of austempering temperature on microstructural parameters and the wear behaviour of austempered ductile iron. Ductile iron was austenitised at 900 °C for 30 min and austempered for 2 h at 260, 280, 300, 320, 350, 380 and 400 °C. Resulting microstructures were characterised through optical microscopy and X-ray diffraction. Wear test was carried out using a pin-on-disc machine with sliding speed of 289 m min⁻¹. Coarse ausferrite microstructure exhibited higher wear rate than fine ausferrite microstructure. At high austempering temperature large amounts of austenite was instrumental in improving the wear resistance through formation of deformation induced martensite. Study of the wear surface under scanning electron microscope showed that, under dry sliding condition, wear occurred mainly due to adhesion and delamination. Wear rate was found to be dependent on the yield strength, austenite content and its carbon content.     

The Influence of Austempering Conditions on the Machinability of a Ductile Iron

Austempering conditions such as temperature and time and their influence on austempered ductile iron machinability were analyzed. Austenitization at 910°C for 90 min and austempering into molten salt bath at 300°C, 360°C, and 420°C for 30, 60, and 90 min each were performed. Microstructures were analyzed by optical microscopy and hardness measurements. Samples were further machined in a lathe for machinability tests. The lathe was instrumented considering power and cutting time and machinability evaluation performed referring to cutting force and material removal. Microstructures at 300°C for 30 min showed ausferrite with retained austenite and martensite. Retained austenite decreased and acicular ferrite sheaves appeared at 60-min austempering time. Mixed bainite was also present at 90-min austempering. Ausferrite and retained austenite were observed in all austempering periods at 360°C, whereas at 420°C only bainite and fine pearlite were present. Hardness increased with increasing temperature at 30-min austempering and decreased with increasing time. However, an exception was observed at 420°C. The highest machinability performance was achieved at 360°C at 60-min austempering, and the lowest performance at 420°C at 90-min austempering.     

Effect of Heat Treatments on Austempered Ductile Iron

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.     

Relationships between microstructure and properties of unalloyed compacted graphite cast irons

Austempered cast irons have been the subject of much attention in recent years because of their excellent mechanical properties. The hardness, ultimate tensile strength and dynamic elastic modulus are presented for a commercially available unalloyed compacted iron (C.E. 4.31) and correlated with different matrix microstructures (as-cast, ferritized, normalized and austempered). For this study, two isothermal temperatures for the austempering treatment were chosen: 400°C and 300°C. The influence of a ferritizing treatment prior to normalizing and austempering has been evaluated, the results indicating that no advantages are obtained with this additional treatment. The influence of microstructure on properties and on the resulting fracture surfaces in tensile tests are discussed.     

Influence of austenitising temperature on the formation of strain induced martensite in austempered ductile iron

The present work was taken up to study the influence of austenitising temperature on the formation of strain-induced martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel, 0.3 wt.% molybdenum and 0.5 wt.% copper was subjected to austempering treatments which consisted of three austenitising temperatures, namely 850, 900 and 950 °C, and three austempering temperatures, namely 300, 350 and 400 °C. Tensile tests were carried out under all the heat-treatment conditions and strain-hardening behaviour was studied by applying Hollomon equation. Microstructures were studied by optical microscopy and X-ray diffraction. It was found that increasing austenitising temperature increased the tendency for the formation of strain-induced martensite at all the austempering temperatures.     

Role of pre-formed martensite on transformation of austempered ductile iron

A commercial ductile iron is treated by a novel austempering process to obtain a good combination of strength and ductility. The samples are austenitised at 890°C for 10 min, then quenched into patented quenching liquid, and austempered in an electric furnace at 220°C for 5, 10, 30, 60, 240 and 600 min, respectively, finally air cooled. The bending test and the tensile test are conducted and microstructural features are analysed on the austempered ductile iron. The optimum mechanical property is achieved at 220°C for 240 min. Main reason for high strength and ductility is the formation of a fine structure consisting of multiple phases of pre-formed martensite and lath bainitic ferrite with film retained austenite.     

Microstructure and mechanical properties of austempered AISI 9255 high-silicon steel

The present investigation is focused on evaluating the microstructure and mechanical properties of American Iron and Steel Institute 9255 high-silicon steel austempered at different temperatures and durations. Material characterisation was done using a scanning electron microscope and an X-ray diffractometer. Results show the bainite microstructure over a temperature range of 280–400°C. Bainite structure gains coarseness at higher temperatures at 360 and 400°C. A significant improvement in the tensile properties was observed for all austempered specimens; with a maximum tensile strength of 1852?MPa and elongation up to 35%. An excellent strain hardening response was observed from the samples austempered at temperatures of 360 and 400°C. Tensile properties were found to be superior at 15?min of austempering duration for all austempering temperatures.     

Influence of austenitising temperature on austempering of an Mn-Mo-Cu alloyed ductile iron Part 1—Austempering kinetics and the processing window

X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austempering kinetics of an alloyed ductile iron of composition (wt-%) Fe-3·49C-2·33Si-0·42Mn-0·25Cu-0·23Mo-0·035Mg at austempering temperatures of 300, 350, 375, and 400°C and austenitising temperatures of 870 and 920°C. The stage I reaction during austempering occurs in two steps, the first in the eutectic cell and the second in the intercellular area. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have a lesser effect on the stage II reaction. Decreasing the austenitising temperature produces a more uniform austempered microstructure and reduces the amount of martensite in this structure. These changes move the processing window to shorter austempering times and increase the temperature at which the processing window closes.

MST/3390     

Microstructural properties and hot pressing of rapidly solidified hypoeutectic low alloy white cast iron powders

Abstract

Hypoeutectic low alloy white cast iron powders were produced using a rapid solidification technique. The morphology and microstructural properties of these powders were investigated with respect to cooling rate and particle size. The density of hot pressed compacts as a function of parameters such as hot pressing time and pressure is described. It was found that retained austenite in the form of cells or dendrites was the main constituent of the powders. At 720°C the powder particles can be hot pressed into high density compacts that have a fine cementite–ferrite microstructure. These ultrafine grained compacts exhibited good superplasticity at elevated temperatures. An elongation to failure of 300% was observed.

MST/1682     

Influence of molybdenum on austempering behaviour of ductile iron Part 4 – Austempering behaviour of ductile iron containing 0·45%Mo

Abstract

Austempering kinetic measurements and mechanical property measurements are presented for a ductile iron of composition Fe–3·56C–2·77Si–0·25Mn–0·45Mo–0·43Cu–0·04Mg (wt-%) after austenitising at 870°C and austempering at 400, 375, 320, and 285°C. The austempering kinetic measurements show that increasing the Mo content of the iron, for example, to increase hardenability, does not delay the austempering reaction significantly and the processing window is open for all the austempering temperatures studied. The mechanical properties determined for austempering temperatures of 400 and 375°C show that the higher ductility grades of the austempered ductile iron standards can be satisfied as predicted by the open processing windows. The ductility of the 0·45%Mo austempered iron is reduced compared with that measured in 0·13%Mo and 0·25%Mo irons austempered under the same conditions. This is attributed to an increased amount and continuity of intercellular carbide as the Mo content increases.     

A study on microstructure and toughness of copper alloyed and austempered ductile irons

In this study, ductile irons with and without 1 wt% copper alloy were austempered to become austempered ductile irons (ADIs). Microstructure, impact toughness, and fracture toughness were evaluated to determine how both the copper alloying and austempering treatments influenced the toughness properties of ductile irons. The results show that, because copper increases the retained austenite content in ADI, the Cu-alloyed ADI has better impact toughness and fracture toughness (K_IC value) than does the unalloyed one. In particular, the impact toughness and the fracture toughness of ADI could be efficiently improved by treating the Cu-alloyed ductile iron at a higher austempering temperature (360 °C) to obtain more retained austenite in its microstructure.     

Austempering heat treatments of ductile iron using molten metal baths

This work aims to evaluate the use of two different zinc–tin and zinc–aluminum molten metal baths on austempering heat treatments performed in ductile cast iron. Samples were extracted from as-cast standard Y-blocks for austempering heat treatments. The samples were heated for austenitization at 910°C for 90?min and further cooled in two different molten metal baths for austempering: zinc–tin and zinc–aluminum alloys at 370 and 400°C, respectively, for 30, 60 and 90?min. The Zn–50?wt% Sn hypoeutectic alloy and the Zn–5?wt% Al eutectic alloy were chosen for the molten metal baths. After heat treatments, the samples were analyzed by optical and scanning electron microscopy, Brinell hardness, Vickers microhardness, Charpy impact and tensile tests, and fracture mode analysis. The results indicated the viability of using Zn–Al and Zn–Sn molten metal baths as a substitute of molten salts. When the austempering temperature was increased from 370 to 400°C, the hardness, tensile strength, and elongation decreased, while impact energy increased. The ideal processing parameters were obtained for austempering at 370°C for 60?min, where the austempered ductile cast iron presented a microstructure completely formed by finer ausferrite.     

Transformation characteristics of ductile iron austempered from intercritical austenitizing temperature ranges

In the present work, the transformation characteristics of ductile iron austempered from intercritical austenitization temperature ranges were investigated. For this purpose, an unalloyed ductile cast iron containing 3.50 wt% C, 2.63 wt% Si and 0.318 wt% Mn were intercritically austenitized (partially austenitized) at various temperatures and then rapidly transformed to a salt bath held at the 365 °C for austempering for various times to produce dual matrix structure with different ausferrite volume fractions in ferrite matrix. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular intercritical and austempering heat treatment temperature and time. It was demonstrated that the total volume fraction of transformed phases was approximately constant for all austempering times after rapidly transforming samples from a particular intercritical temperature to austempering temperature. It was found out that the new ferrite (It is also called epitaxial ferrite) introduced into the intercritically austenitized structure during austempering and its content was dependent on the intercritical austenitizing temperature and austempering time.     

The relationship between fatigue strength and microstructure in an austempered Cu-Ni-Mn-Mo alloyed ductile iron

Rotating bending fatigue measurements are reported for an austempered ductile iron containing 3.5 wt% C, 2.6 wt% Si, 0.48 wt% Cu, 0.96 wt% Ni, 0.27 wt% Mo, and 0.25 wt% Mn. The iron was austenitized at 870, 900 and 950°C and then austempered at 370 and 400°C for times between 30 and 240 min to obtain various austempered microstructures. The correlation between fatigue strength and austempered microstructure represented by the parameter XγCγ, where Xγ is the amount of high C austenite and Cγ its C content is examined. It is shown that fatigue strength increases as XγCγ increases. The highest fatigue strength is obtained with an ausferrite structure; the presence of martensite and/or carbide in the structure reduces the fatigue strength. Lower austenitizing temperatures increase the fatigue strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Comparative study of fracture toughness of austempered ductile irons with upper and lower ausferrite microstructures

Abstract

A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised by optical microscopy and X-ray diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (σ_y) volume fraction of retained austenite (X_y) and the carbon content of the retained austenite (C_y). Experimental results showed excellent agreement with the prediction of the model that <disp-formula><graphic href="splitsection9-m1.tif"/></disp-formula> is proportional to σ_y(X_yC_y)^½.