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.     

The influence of depth of cut on the machinability of an alloyed austempered ductile iron

The volume fraction of high carbon austenite present in the microstructure of austempered ductile iron (ADI) is one of the important factors that influence the mechanical and physical properties of the alloy. Formation of martensite by TRIP (transformation induced plasticity) mechanism during the machining operation in which a large amount of stress is applied to the microstructure results in a decrease in machinability of austempered ductile iron which has affected the expansion of ADI in industry. In this article, the effect of depth of cut as a machining variable is assessed in an alloyed austempered ductile iron containing Cu, Ni and Mo. The measurements of mechanical properties including impact energy, tensile strength, hardness and microhardness along the cross-section of samples are reported for samples austenitized at 870 °C followed by austempering at 375, 340 and 300 °C. Results indicate that contrary to the behavior of many alloys, in austempered ductile iron, reducing the depth of cut will not improve the machinability. In the case of studied composition, cutting with depths of 0.5 and 0.1 mm had the best and worst results, respectively.     

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.     

Effects of pearlite formation on mechanical properties of austempered ductile iron

Abstract

Mechanical property measurements are described for a ductile iron alloyed with Mn, Mo, and Cu in the fully austempered condition, andfor irons with various amounts of pearlite introduced by isothermal transformation at 550°C after austenitising at 920°C for 120 min and before austempering at 370°C for 60 min. The ultimate tensile strength, 0·2% proof strength, elongation, impact energy, and hardness all decrease as the amount of pearlite in the structure increases. A smaller amount of pearlite can be tolerated in the alloyed iron compared with an unalloyed iron before itfails to satisfy the standard.

MST/2041     

Effect of Various Heat Treatments Particularly Austempering on Mechanical and Physical Properties of a High Mn Ductile Iron

The stable and metastable phase diagrams, microsegregation of carbon and alloying additions and the driving force for single and double austempering are reported for a ductile iron (DI) of composition: 3.5% C, 2.64% Si, 0.25% Cu, 0.25% Mo, 0.67% Mn, 0.007% P, 0.013% S, 0.04% Mg The variation with austempering time of the retained austenite volume fraction (VRA), Unreacted Austenite Volume fraction (UAV), austenite carbon content (CJ, UTS, elongation, and unnotched charpy impact energy is reported for single austempering at 400°C, 285°C, and a double austempering treatment (400°C, 120 min., then austempering at 285°C) after austenitizing at 920°C for 120 min. Finally mechanical and physical properties including strength, ductility, toughness, wear resistance, hardness, thermal conductivity, and electrical properties for the following heat treatments are compared:

ADI (upper bainitic structure) 870°C, 120 min.; 375°C, 120 min.ADI (lower bainitic structure) 870°C, 120 min., 285°C, 1 dayADI (Double austempered) 870°C, 120 min., 375°C, 120 min., 285°C, 1 dayAir cooled (mainly martensitic/ some widmanstatten ferrite): 870°C, 120 min. furnace cooled (50.7% proeutectoid ferrite/49% pearlite): 870°C, 120 min. Step-cooled (15% proeutectoid ferrite/ 85% pearlite): 870°C, 120 min., 650°C 30 min., air cooled As cast structure (39.5% proeutectoid ferrite/ 60.5% pearlite)The comparison shows that double austempering can be used with the high Mn DI to improve elongation and impact energy obtained by a single austempering. Control over the austenitizing and austempering temperatures and times in the double treatment can be used to change the relative improvements in elongation and impact energy. The results show that furnace cooled irons have the best elongation and physical properties.     

Investigation into mechanical properties of austempered ductile cast iron (ADI) in accordance with austempering temperature

This work concerns mechanical properties of an austempered ductile cast iron (ADI). Samples alloyed with copper and molybdenum were austenitized at 910 °C for 90 min and subsequently austempered in a salt bath over a range of temperatures from 350 °C to 410 °C to obtain favorable mechanical properties such as tensile strength, elongation, and fracture toughness. Those properties were compared from various austempering heat treatments.     

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 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.     

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     

A Study of Microstructures and Properties of High‐Carbon Si‐Cr Cast Steel Containing Rare Earth and Titanium

The microstructure, tensile and impact behaviour of high‐carbon Si‐Cr cast steel containing rare earth (RE) and titanium have been determined after austempering. The additions of RE and titanium refined the primary austenite grain size resulting in improving toughness. The addition of silicon handicapped the formation of carbide and carbide‐free bainitic ferrite and carbon enriched retained austenite could be obtained in the austempering structures of high‐carbon Si‐Cr cast steel, which had excellent mechanical properties and abrasion resistance. Moreover, the basic tendency of the mechanical properties of high‐carbon Si‐Cr cast steel influenced by the austempering temperature was that the hardness and tensile strength reduced and the impact toughness and fracture toughness increased with increasing temperature. The comprehensive properties were the best while austempering at 330^oC.     

Austempering of an Mn–Mo–Cu alloyed ductile iron Part 2 – Structure–mechanical property relationships

Abstract

Ultimate tensile strength, 0·2% proof strength, elongation, and impact energy measurements are reported for an alloyed ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·25Cu–0·23Mo–0·035Mg for austempering temperatures of 400, 375, and 350°C and a range of austempering times after austenitising at 920°C for 120 min. The ADI ASTM A897M:1990 standard is satisfied for an austempering temperature of 350°C but not at 375 or 400°C. This behaviour is discussed in terms of the influence of the unreacted austenite volume from the stage I austenitising reaction and the carbide product of the stage II austenitising reaction on the ductility. The present findings are predicted by the processing windows determined from the austempering kinetics.

MST/3393     

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.     

Processing of a new high strength high toughness steel with duplex microstructure (Ferrite + Austenite)

In this investigation a new third generation advanced high strength steel (AHSS) has been developed. This steel was synthesized by austempering of a low carbon and low alloy steel with high silicon content. The influence of austempering temperature on the microstructure and the mechanical properties including the fracture toughness of this steel was also examined. Compact tension and cylindrical tensile specimens were prepared from a low carbon low alloy steel and were initially austenitized at 927 °C for 2 h and then austempered in the temperature range between 371 °C and 399 °C to produce different microstructures. The microstructures were characterized by X-ray diffraction, scanning electron microscopy and optical metallography. Test results show that the austempering heat treatment has resulted in a microstructure consisting of very fine scale bainitic ferrite and austenite. A combination of very high tensile strength of 1388 MPa and fracture toughness of 105 MPa √m was obtained after austempering at 371 °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).     

Effect of modified austemper on tensile properties of 0·52%C steel

Abstract

The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austempering at 673 K and subsequent water cooling produced the same mixed structure of upper bainite and 10 vol.-% ferrite. However, this treatment yielded inferior mechanical properties to those obtained with the modified austempering treatment, independent of the test temperature. The results are described and discussed.

MST/3102     

Development a multi-layer perceptron artificial neural network model to estimate the Vickers hardness of Mn–Ni–Cu–Mo austempered ductile iron

The hardness of austempered ductile irons is relative to its microstructure, strength, ductility, machinability and wear resistance properties. Therefore, hardness measurement can be used as a simple tool to control the heat treatment, chemical composition and mechanical properties of ADI parts during the production process. The aim of this study is to develop an Artificial Neural Network (ANN) model for estimating the Vickers hardness of ADIs after austempering treatment. A Multi-Layer Perceptron model (MLP–ANN) was used with Mo%, Cu%, austempering time and temperature as inputs and the Vickers hardness of samples after austempering as the output of the model. A variety of samples were prepared in different conditions of chemical composition and heat treatment cycle. The obtained experimental results were used for training the neural network. Efficiency test of the model showed reasonably good agreement between experimental and numerical results, so the synthesized ANN model can estimate the hardness of the castings with a small error in the range of the experimental results standard deviation.     

Fatigue crack propagation of bainitic nodular cast iron

The effects of austempering temperature and isothermal transformation time on fatigue crack growth rate in a ductile iron with a bainitic structure have been studied. Crack growth rates in austempered samples were compared with those in materials with a ‘bullseye’ casting structure. Using scanning electron microscopy, the mechanism of the fatigue crack growth can be understood by observing the fracture surface of a fatigue specimen. X-ray diffractometry was used to determine the volume fraction of retained austenite. It can be concluded that the volume fraction of retained austenite, the fracture mode and the matrix microstructure are closely related to the fatigue crack propagation rate and the fracture mode.     

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.     

Relationship between structure and mechanical properties in high manganese alloyed ductile iron

Abstract

Measurements of ultimate tensile strength, 0·2% proof stress, elongation, and impact energy are reported for an alloyed ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg,for a range of austempering temperatures and times after austenitising at 920°C for 120 min. It is shown that the mechanical properties satisfy the high strength grades of the standard AST MA897 M:1990, but fail to satisfy the higher ductility grades because of poor ductility. This is attributed to overlapping of the stage I and II reactions and the occurrence of the transformation induced plasticity mechanism during deformation, particularly in irons austempered at higher temperatures.

MST/3054     

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.