Austempering and austempered ductile iron microstructure in copper alloyed ductile iron

The variation in the austempered microstructure, the volume fraction of retained austenite, X_λ, the average carbon content of retained austenite, C_λ, their product X_λC_λ and the size of bainitic ferrite needles with austempering temperature for 0.6% Cu alloyed ductile iron have been investigated for three austempering temperatures of 270, 330, and 380 °C for 60 min at each temperature after austenitization at 850 °C for 120 min. The austempering temperature not only affects the morphology of bainitic ferrite but also that of retained austenite. There is an increase in the amount of retained austenite, its carbon content, and size of bainitic ferrite needles with the rise in austempering temperature. The influence of austempering time on the structure has been studied on the samples austempered at 330 °C. The increase in the austempering time increases the amount of retained austenite and its carbon content, which ultimately reaches a plateau.     

Tensile properties of copper alloyed austempered ductile iron: Effect of austempering parameters

A ductile iron containing 0.6% copper as the main alloying element was austenitized at 850 °C for 120 min and was subsequently austempered for 60 min at austempering temperatures of 270, 330, and 380 °C. The samples were also austempered at 330 °C for austempering times of 30–150 min. The structural parameters for the austempered alloy austenite (X _γ ), average carbon content (C _γ ), the product X _γ C _γ , and the size of the bainitic ferrite needle (d _α ) were determined using x-ray diffraction. The effect of austempering temperature and time has been studied with respect to tensile properties such as 0.2% proof stress, ultimate tensile strength (UTS), percentage of elongation, and quality index. These properties have been correlated with the structural parameters of the austempered ductile iron microstructure. Fracture studies have been carried out on the tensile fracture surfaces of the austempered ductile iron (ADI).     

The influence of nickel and copper on the austempering of ductile iron

In the present investigation, the effect of alloying elements on the austempering process, austempered microstructure, and structural parameters of two austempered ductile irons (ADI) containing 0.6% Cu and 0.6% Cu/1.0% Ni as the main alloying elements was investigated. The optical metallography and x-ray diffraction were used to study the changes in the austempered structure. The effect of alloying additions on the austempering kinetics was studied using the Avrami equation. Significantly more upper bainite was observed in the austempered Cu-Ni alloyed ADI than in Cu alloyed ADI. The volume fraction of retained austenite (X _γ), the carbon level in the retained austenite (C _γ), and the product X _γ C _γ in an austempered structure of Cu-alloyed ADI are higher than in Cu-Ni-alloyed ADI. The austempering Kinetics is slowed down by the addition of Ni.     

Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material. The results of the present investigation indicate that for this low manganese austempered ductile iron (ADI), upper ausferritic microstructures exhibit higher fracture toughness than lower ausferritic microstructures. Yield and tensile strength of the material was found to increase with an increase in austempering time in a lower bainitic temperature range, whereas in the upper bainitic temperature range, time has no significant effect on the mechanical properties. A retained austenite content between 30 to 35% was found to provide optimum fracture toughness. Fracture toughness was found to increase with the parameter (XγCγ/d)^1/2, where Xγ is the volume fraction of austenite, Cγ is the carbon content of the austenite, and d is the mean free path of dislocation motion in ferrite.     

Effect of austenitizing conditions on the impact properties of an alloyed austempered ductile iron of initially ferritic matrix structure

The effect of austenitizing conditions on the microstructure and impact properties of an austempered ductile iron (ADI) containing 1.6% Cu and 1.6% Ni as the main alloying elements was investigated. Impact tests were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850,900, 950, and 1000°C for 15 to 360 min and austempered at 360°C for 180 min. Results showed that the austenitizing temperature, T_γ, and time, t_γ, have a significant effect on the impact properties of the alloy. This has been attributed to the influence of these variables on the carbon kinetics. The impact energy is generally high after short t_γ, and it falls with further soaking. In samples austenitized at 850 and 900°C, these trends correspond to the gradual disappearance of the pro-eutectoid ferrite and the attainment of fully developed ausferritic structures. In initially ferritic structures, the carbon diffusion distances involved during austenitization are large compared to those in pearlitic structures. This explains the relatively long soaking periods required to attain fully ausferritic structures, which in spite of the lower impact energy values, have a better combination of mechanical properties. Microstructures of samples austenitized at 950 and 1000°C contain no pro-eutectoid ferrite. The impact properties of the former structures are independent of t_γ, while those solution treated at 1000°C are generally low and show wide variation over the range of soaking time investigated. For fully ausferritic structures, impact properties fall with an increase in T_γ. This is particularly evident at 1000°C. As the T_γ increases, the amount of carbon dissolved in the original austenite increases. This slows down the rate of austenite transformation and results in coarser structures with lower mechanical properties. Optimum impact properties are obtained following austenitizing between 900 and 950°C for 120 to 180 min.     

Impact Properties of Copper-Alloyed and Nickel-Copper Alloyed ADI

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

Microstructure and Mechanical Properties of Austempered Medium-Carbon Spring Steel

Changes in microstructure and mechanical properties of medium-carbon spring steel during austempering were investigated. After austempering for 1 h at 290 °C or 330 °C, the bainite transformation stabilized austenite, and microstructure consisting of bainitic ferrite and austenite could be obtained after final cooling; the retained austenite fraction was smaller in the alloy austempered at 290 °C because carbon redistribution between bainitic ferrite and austenite slowed as the temperature decreased, and thereby gave persistent driving force for the bainite transformation. The products of tensile strength and reduction of area in the austempered alloy were much larger in the austempered steel than in quenched and tempered alloy, mainly because of significant increase in reduction of area in austempered alloy.     

Control of austenitic transformation in ductile iron aided by calculation of Fe-C-Si-X phase boundaries

To control austenite transformation of ductile iron, thermodynamics procedures were used to calculate the Ae₃, the Gr/γ (A_cm), and the A₁ phase boundaries of high Mn and Ni-Cu-Mn alloyed iron as a function of austenitization temperature. The results of calculation show that segregation of Mn in the intergraphite regions reduces the carbon content of austenite at the Ae₃ phase boundary to the lowest value. If one ignores the effect of substitutional alloying elements on the nucleation of austenite, the austenite should first nucleate in the cell boundaries and then grow to the graphite nodules. In addition, the calculated results show that the A₁ temperature is the lowest in the intercellular region of a high Mn alloy. Therefore, if the austenitization temperature is not sufficiently high, only those parts of the matrix that have the A₁ phase boundary below the austenitization temperature transform to austenite, and dual formation of the α and γ phases will occur. By using the procedure introduced in this study, the volume fraction of each phase can be evaluated by calculating the A₁ phase boundary as a function of intergraphite distance. In the case of Ni-Cu-Mn alloy, Ni stabilizes austenite, which lowers the Ae₃ phase boundary. In this alloy, carbon content of austenite at the Ae₃ phase boundary is lower near the graphite nodule and higher in the intergraphite regions. However, the variation of carbon content of austenite at the Ae₃ phase boundary in the matrix of this alloy is much lower than in the high Mn alloy.     

Effect of austempering temperature on machinability of Cu–Ni–Mo alloyed austempered ductile iron for heavy section parts

Abstract

The machinability of an austempered ductile iron with a suitable chemical composition for heavy sections has been assessed. Austempering heat treatment was carried out at three temperatures, 300, 340 and 375°C, after austenitising at 870°C for 100?min. Drilling tests, tool wear and surface roughness measurements were used to evaluate the machinability. Drilling operation failure, severe tool wear and the poorer surface roughness of the specimens austempered at lower temperatures indicated that austempering at higher temperatures clearly resulted in better machinability. The machinability of testpieces austempered at 375°C, which contained higher fractions of retained austenite, was superior to that of testpieces autenitised at lower temperatures, indicating that hardness is an important factor in assessing machinability in addition to high carbon austenite content.     

Effects of austempering conditions on the microstructures and mechanical properties in Fe-0.9%C-2.3%Si-0.3%Mn steel

In this study, the effect of the microstructure and mechanical properties of austempered high-carbon (0.9 %C) high-silicon (2.3 %Si) cast steel were investigated. The specimens were austenitised for 60 min. at 900 °C, and austempered at 260 °C, 320 °C, and 380 °C for periods of time ranging from 30 min to 240 min. After receiving this heat treatment, the mechanical properties were measured using both a tensile test and hardness test. To analyze the microstructure, an optical microscope was used and an X-ray diffraction (XRD) analysis was carried out. In this study, high carbon high silicon cast steel without graphite and with higher tensile strength (1300 MPa to 2200 MPa) and elongation (∼25 %), when compared to austempered ductile cast iron (ADI), was developed. When the austempering temperature was at 260 °C, the microstructures were low ausferrite, but at 380 °C, an upper ausferrite structure was formed. As the austempering temperature increased from 260 to 380 °C, the ultimate tensile strength and hardness decreased, but the elongation and retained austenite volume fraction increased. In addition, the microstructures were coarser.     

Improvement in fracture toughness of austempered ductile iron by two-step austempering process

Abstract

Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m^1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m^1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness.     

Influence of nodule count on residual stresses and distortion in thin wall ductile iron plates of different matrices

Surface characteristics of thin wall ductile iron (TWDI) parts, such as residual stresses, distortion and dimensional changes produced during casting and heat treatment, are relevant variables when it comes to production processes design.This work focuses on the effect of nodule count on residual stresses (RS) and of TWDI plates distortion. As-cast, ferritised and austempered samples were employed. The role played by two typical austempering temperatures (280 and 360 °C) and three significantly different nodule counts (265, 1200 and 1700 nodules (nod)/mm²) are discussed by establishing microstructural changes, i.e., microstructure fineness, retained austenite volume fraction (V_γ%), and austenite carbon content (C_γ%). Besides, residual stresses profiles below the surface and dimensional changes are determined.Results show that all samples display compressive RS on the surface and neighbouring layers. Samples with high nodule count austempered at 280 °C lead to higher compressive RS and distortion as well as to an increase in the RS field below the surface. As-cast and ferritised plates exhibit noticeably lower compressive RS values.     

Hardness and Tensile Properties of Austempered Aluminium—containing Ductile Iron

Abstract

Changes in the mechanical properties of austempered aluminium-containing ductile iron obtained by varying the austenitising temperatures, 850–950°C, and austempering temperatures, 300–425°C and times, 5–300 min, can be related to the corresponding effects on austempered microstructures. The largest effects on the properties are associated with the changes in the morphology of the bainitic constituents and the volume of the retained austenite, both of which are controlled mainly by the austempering temperature. In comparison, smaller effects are caused by microstructural changes related to the variation in the austenitising temperature or austempering time.     

Effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron (CADI) grinding balls

The effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron(CADI) grinding balls were systematically investigated in this work. The microstructures were oberserved by optical metallography and analyized by X-ray diffraction. The surface residual stress measured by the cutting method is mainly composed of thermal stress and phase transformation stress. The thermal stress in grinding balls was determined by ANSYS simulation technique, and the surface phase transformation stress was obtained by subtracting the simulated surface thermal stress from the measured surface residual stress. Results show that all microstructures consist of ausferrite, white-bright zones(mixture of martensite and austenite), nodular graphite, and carbides. The distribution of ausferrite shows uniform. With the increase of austempering temperature, the volume fraction and carbon content of austenite increase, whereas the amount of white-bright zone decreases. In addition, the surface residual stress increases with the increase of austempering temperature. Only the tension exists at the austempering temperature of 200 ℃, and the pressure exists at the austempering temperature of 220-260 ℃. The thermal stress changes from the tension on the inside with the radius of 0-35 mm to the pressure on the outside with the radius of 35-62.5 mm, and the stress balance state presents at the radius of 35 mm. It is also found that the transformation stress is related to the content of carbon-rich austenite, and will reduce by 5.03 MPa accompanied with 1 vol.% increase of the austenite. The thermal compressive stress and the transformation tensile stress on the surface both decrease with the increase of the austempering temperature.     

Formation of an <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript> superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L1₀ superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.     

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.     

Adjusting the linear expansion coefficient Lin Fe-Ni-Co-Ti invars by aging and phase hardening

The N30K10T3 and N40K10T3 invars with the Curie points θ_C ≈ 200°C and θ_C ≈ 310°C and the martensite temperatures M _s ≈ −80°C and M _s < −196°C, respectively, have been studied. The two alloys were hardened by quenching in the range of temperatures from 100 to 750°C. In addition, the first alloy was hardened by a combination treatment including phase-transformation-induced hardening and aging. The method of phase hardening consisted in the use of a forward (γ → α) and a reverse (α → γ_ph) martensitic transformations. It has been shown that the temperature dependences of the linear expansion coefficient and the dependences of the hardness on the temperature and time of aging are considerably different for both alloys upon decomposition of the supersaturated solid solution. Both the ordinary and the double aging have been studied.     

The Strain-Hardening Behavior of Partially Austenitized and the Austempered Ductile Irons with Dual Matrix Structures

In the current study, an unalloyed ductile iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.%, and 0.047 Mg wt.% was intercritically austenitized (partially austenitized) in two-phase regions (α + γ) at different temperatures for 20 min and then was quenched into salt bath held at austempering temperature of 365 °C for various times to obtain different ausferrite plus proeutectoid ferrite volume fractions. Fine and coarse dual matrix structures (DMS) were obtained from two different starting conditions. Some specimens were also conventionally austempered from 900 °C for comparison. The results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high-carbon austenite (retained or stabilized austenite)) has been developed. Both of the specimens with ∼75% ausferrite volume fraction (coarse structure) and the specimen with ∼82% ausferrite volume fraction (fine structure) exhibited the best combination of high strength and ductility compared to the pearlitic grades, but their ductility is slightly lower than the ferritic grades. These materials also satisfy the requirements for the strength of the quenched and tempered grades and their ductility is superior to this grade. The correlation between the strain-hardening rates of the various austempered ductile iron (ADI) with DMS and conventionally heat-treated ADI microstructures as a function of strain was conducted by inspection of the respective tensile curves. For this purpose, the Crussard-Jaoul (C-J) analysis was employed. The test results also indicate that strain-hardening behavior of ADI with dual matrix is influenced by the variations in the volume fractions of the phases, and their morphologies, the degree of ausferrite connectivity and the interaction intensities between the carbon atoms and the dislocations in the matrix. The ADI with DMS generally exhibited low strain-hardening rates compared to the conventionally ADI.     

Effect of Retained Austenite on Properties of Austempered Ductile Iron

Austempered ductile iron (ADI) exhibits a favourable combination of strength and toughness, and has been used as a substitute for quench-tempered or carburise-quenched steel. A characteristic feature of bainite transformation of cast iron, as opposed to carbon steel, is that precipitation of carbide is suppressed by the high concentration of silicon. Thus, a favourable structure, consisting of bainitic ferrite and retained austenite without carbide, can be provided by the optimum austempering treatment. Such microstructure and the mechanical properties of the iron are significantly affected by the conditions of the austempering treatment and the chemical composition. In this study, several grades of ductile iron were austempered under various conditions. The relationship between the impact strength, the quantity of retained austenite and the isothermal transformation curve was investigated. The stability of the retained austenite is considered important, because ADI contains a large amount of retained austenite which contributes to the improvement of ductility and toughness and which may transform to martensite when held at low temperature or subjected to stress. In this study, the stability of the retained austenite at low temperatures was examined by holding or stressing to establish the relations between transformation and temperature, stress and strain.

When the austempering time is short, the untransformed austenite partially transforms to martensite during air cooling, due to the lower carbon content, resulting in lower impact strength. As the austempering time increases, the untransformed austenite is stabilised by carbon-enrichment and there is little transformation to martensite, resulting in a large amount of retained austenite and higher impact strength. When the austempering time becomes much longer, the carbon-enriched austenite decomposes, presumably to bainitic ferrite and carbide, decreasing impact strength. In increasing the silicon content, precipitation of carbide in bainite is suppressed and both the maximum impact value and the content of retained austenite increase. The decreasing rates after the maxima through an additional isothermal holding becomes smaller.

By holding at temperatures down to –40°C, the decrease in retained austenite and the increase in hardness are both small. The retained austenite is stable under stress lower than that required to cause plastic deformation. Compressive stress hinders the martensitic transformation, because the transformation is accompanied by volume expansion.     

Effect of the austenite stability on the physicomechanical properties of invars strengthened by combined treatments

This work is devoted to the investigation of an N30K10T3 invar alloy with metastable austenite (martensite point M _s ≈ −80°C) and an N40K10T3 invar with stable austenite (M _s < −196°C). The Curie points of the alloys are θ_C ≈ 200 and 310°C, respectively. Effects of aging of preliminarily deformed invars on the hardness, thermal-expansion coefficient, stress-corrosion resistance, and coercive force have been studied. It has been demonstrated that these properties of quenched alloys can be affected by both deformation and decomposition of the supersaturated solid solution. In the metastable alloy, the coefficient of linear expansion depends on temperature and aging time; no such dependence is observed in the stable alloy.